395 C H A P T E R 10 Nuclear Power Plants* 10.1 Introduction

395 C H A P T E R  10 Nuclear Power Plants* 10.1 Introduction
C H A P T E R 10
Nuclear Power Plants*
10.1 Introduction
Nuclear power is universally controversial. Many would say that it is also universally
needed–as an alternative or supplement to power generated by fossil fuels.
The combustion of fossil fuels produces carbon dioxide, now notorious for the
threat of global warming. Nuclear power plants produce neither carbon dioxide nor
oxides of sulfur and nitrogen, as does the burning of fossil fuels. Thus nuclear power
reduces the global production of carbon dioxide and other pollutants, and helps to
alleviate many of the pervasive problems of fossil fuel supply.
Petroleum is least available in regions of widest use; natural gas is, for the time
being, plentiful and sought after by all; and widely abundant coal has come to be
regarded as the great Satan of air pollution. Water power is important, but it offers
limited possibility for growth. Solar energy, while promising, is far from being a
mainstay of the world’s energy supply. Thus sources other than fossil fuels and nuclear
power offer little hope to become major suppliers during our lifetimes.
Nuclear power, in stasis for many years, may make a comeback. Engineers have
been quietly working on new and safer designs for nuclear power plants, and the
political climate may be swinging slowly back in favor of nuclear power. According to
references 31 and 34, there were 434 operating nuclear plants producing 17% (350,000
megawatts) of the world’s electricity in 1998. Regardless of one’s position towards it,
nuclear power is a major factor in world power production.
Knowledge of nuclear power is not American, French, Indian, or British; it is
virtually universal. Nuclear power plants such as those shown in Figure 10.1 are
producing power in many nations around the world. Blockage of the growth of nuclear
power in the United States did not prevent the development and extensive use of
nuclear-generated electricity in France or Canada. Should developing countries with
minimal fossil resources not use nuclear power? Should a country that has seen the
terror wrought by nuclear weapons be denied the benefits of electricity from the
nucleus? Can attempts to halt the growth of nuclear power stop the proliferation of
nuclear weapons? These and many other issues around nuclear power, which obviously
extend far beyond the bounds of engineering, are much too broad to be pursued here.
Nuclear power is controversial, but it is here; it is important and likely to remain so.
*Thanks to Dr. Andrew A. Dykes for his valuable inputs and comments on this
This chapter is concerned largely with nuclear fission power reactors. Most nuclear
power plants use steam cycles that differ little from those of fossil fuel plants except for
the source of heat for the steam generator and for steam supply conditions. Steam
turbine cycles were considered in some detail in Chapter 2. Thus this chapter will focus
primarily on (a) the characteristics of nuclear reactor steam supply systems and, to a
lesser extent, on (b) the climate for future nuclear development. In preparation for this
study let us first review relevant aspects of atomic and nuclear structure.
10.2 Review of Atomic Structure
Atoms and Molecules
Somewhat more than a hundred elements are known and are thought to be the building
blocks of everything in the universe. The atom is the basic unit of structure for each
element. An important connection between the microscopic world of the atom and the
macroscopic world of experience is given by Avogadro's number. A gram-mole of any
element has Avogadro's number (6.023 x 1023) of atoms.
The atom may be considered as consisting of a positively charged nucleus at its
center and one or more negative charges around the nucleus called electrons that make
the atom electrically neutral. The electron is the fundamental unit of negative charge. It
may be viewed as a particle which is much smaller than the nucleus and which orbits
around the nucleus as a planet orbits the sun, or it may be viewed as a diffuse electron
cloud around the nucleus. Still another concept is that of a particle whose location is
not known but which is more likely to be in some places than others, its position being
given by a probability distribution. We will not concern ourselves with the rationales
for these views.
Thus atoms consist of nuclei surrounded by electrons. The sizes of atoms are
conveniently measured in Angstroms (10-8 cm). The nucleus typically is of the order of
10 –5 Angstroms. Thus the volume of the atom is largely due to the size of the outer
electron’s orbit or to the atom’s electron cloud.
Molecules are collections of atoms held together by electromagnetic forces between
the nuclei and the electrons. Atoms and molecules can exist in a variety of energy states
associated with their electron distributions. These microscopic states and their
macroscopic influences are dealt with theoretically in the fields of quantum mechanics
and statistical thermodynamics.
Molecules and atoms can interact with each other to form different molecules in
ways that are controlled by their electron structures. These interactions, called
chemical reactions, have little to do with the nucleus. In Chapter 3, we considered
aspects of these reactions that are relevant to combustion. The magnitudes of the
energy associated with these chemical changes, while of great importance in thermal
engineer-ing, are so small that they have no significant influence on the nuclei of the
reacting atoms. Thus the nuclei may be thought of as merely going along for the ride
when a chemical reaction occurs.
An electrically neutral particle, however, can penetrate an atom's electromagnetic
field and approach the nucleus, where it interacts via short range but powerful nuclear
forces. Then the electrical forces holding the nucleus together may be overcome,
resulting in changes in the nucleus. In these cases the interatomic forces are largely
irrelevant and are overpowered by nuclear events. It is these changes that are the
concern of this chapter.
The Nucleus
The nucleus, for our purposes, may be thought of as being made up of integral numbers
of protons and neutrons. The proton is a particle with a positive charge of the same
magnitude as that of the electron, so that pairing a proton with an electron produces
exact electrical neutrality. Thus protons account for the charge of the nucleus, and a
like number of electrons ensures the electrical neutrality of the atom. Compared with
the electron, the proton is a massive particle, having a mass which is about 1800 times
the mass of the electron. Thus the hydrogen atom, which consists of one proton in the
nucleus and a single electron in orbit around the nucleus, is electronically neutral and
has a mass only slightly larger than that of the nucleus.
Atoms larger than the hydrogen atom have more than one proton in the nucleus and
have one or more neutrons as well. A neutron, as the name suggests, is an electrically
neutral particle with a mass only slightly larger than that of the proton. As components
of the nucleus, protons and neutrons are called nucleons, and are thought of
interchangeably with respect to mass because their masses differ so little from each
The number of protons in an atom of an element is called the atomic number of the
element. Thus hydrogen has an atomic number of 1. The atomic number of a given
element is unique to that element. Thus we could identify the elements by their atomic
numbers rather than by their names if we wished. Elements are ordered in the periodic
table in part by their atomic numbers.
The mass number of an element is the number of nucleons in an atom of that
element and is therefore the sum of the number of protons and neutrons in the nucleus.
Atoms of a given element that have differing mass numbers are called isotopes of the
element. A given isotope of an element is sometimes designated by a notation that
includes the element's chemical symbol, its mass number, and its atomic number. For
example, the most common isotope of uranium is denoted as 92U238, where 92 is the
atomic number of the element uranium and 238 is the sum of the number of protons
and neutrons in the isotope nucleus. The isotopes are also sometimes simply identified
by their name or symbol and mass number, such as U-235 or Uranium-235. Other
significant examples are the isotopes of hydrogen, deuterium, 1H2, and tritium, 1H3,
which have, respectively, one and two neutrons accompanying the proton. These
isotopes are sometimes written 1D2 and 1T3 to reflect their commonly used names. The
form of water, H2O, in which the isotope deuterium replaces hydrogen is commonly
called heavy water, D2O, because of the added mass of the extra neutron in each
nucleus. It will be seen later that heavy water has characteristics that are advantageous
in some nuclear processes.
10.3 Nuclear Reactions
Just as chemical fuels undergo chemical reactions that release energy, nuclei may also
participate in energy-releasing nuclear reactions. When this happens atoms of the
reacting elements are converted to atoms of other elements, the sort of transmutation
sought by the alchemists of the past.
Mass-Energy Equivalence
Two nuclear processes are known to be capable of releasing energy on a scale large
enough to influence the personal and business lives of humans. The reality of both
processes has been amply demonstrated by the production and detonation of the atomic
and hydrogen bombs. Both processes owe their energy release to the annihilation of
matter consistent with the famous Einstein formula, E = mc2, which asserts the
convertibility of mass to energy. Because the speed of light is so large, the equation
shows that the annihilation of a small amount of mass yields a large quantity of energy.
These energy releases are usually measured in MEV, millions of electron-volts. The
electron-volt, EV, is defined as the energy required for an electron to pass through a
potential difference of one volt. The reactions of individual nuclei typically produce
particles with energies measured in MEV. On the other hand, as indicated earlier, the
most energetic of chemical reactions releases much less energy, only a few EV per
As a result of our encounter with Einstein's energy-mass relation we must adjust
our philosophical position on the conservation laws of mass and energy and think
instead in terms of conservation of mass-energy. Mass and energy may be thought of as
different forms of the same thing. Any change in the mass of an isolated system must be
accompanied by a corresponding change in system energy. This in no way influences
the discussions in previous chapters, because changes in mass are entirely insignificant
in chemical and other nonnuclear processes.
Fission and Fusion
The process known as nuclear fusion occurs in nature in the stars, including our own
sun. Since the Second World War, scientists have been attempting to achieve the
conditions for fusion in the laboratory. Because it can use heavy water from the sea as
a fuel, controlled thermonuclear fusion offers the hope of vast quantities of power for
many centuries in the future.
Fusion occurs when light atoms interact to form a heavier atom in reactions such as
D2 + 1D2 ; 2He3 + 0n1 + 3.2 MEV.
Here, two deuterium atoms collide to form helium-3 and a neutron while releasing 3.2
MEV of energy. Other fusion reactions exist that provide comparable amounts of
energy. Using precise atomic masses measured with mass spectrometers, we can
determine the differences of the masses of the reactants and products in this reaction.
Application of the Einstein relation to the mass loss yields the same energy release (in
this case 3.2 MEV) as is obtained by energy measurements. Thus the energy yield of
known nuclear reactions may be determined with only mass measurements.
For over fifty years, researchers have pursued the goal of achieving controlled
thermonuclear fusion on a scale suited for commercial power production. Since the
reactants are two positively charged nuclei, they must have high kinetic energies to
overcome their mutual repulsion. These high energies imply a gaseous state with
enormously high temperature, a condition known as a plasma. Because solid materials
cannot exist at plasma conditions and plasmas would be cooled by the presence of
solids, magnetic confinement of plasmas has been one approach to achieving a
thermonuclear plasma. Large experimental devices called Stellerators, Tokamaks, and
mirror machines have been built to help solve the problems inherent in achieving largescale fusion reactions and in stably confining the associated thermonuclear plasma.
While progress continues, controlled thermonuclear fusion remains, and will likely
continue, in the research stage for many years. It will therefore not be considered
further here.
Whereas nuclear fusion annihilates mass by forming larger atoms from light atoms,
fission is a process of breaking massive atoms into two large, more-or-less equal-sized
atoms, with an accompanying mass loss and energy release. While controlled fusion
remains elusive, nuclear fission has been producing electrical power on a commercial
scale for decades. The remainder of this chapter therefore deals with the fundamentals
and commercial use of nuclear fission.
10.4 Fundamentals of Nuclear Fission
Nuclear fission involves the breakup of certain massive elements resulting from collisions with neutrons. Uranium U-235, for instance, the isotope of uranium with 235
nucleons, forms an highly excited isotope with 236 nucleons upon capture of a neutron,
as in Figure 10.2(a). These excited U-236 atoms are unstable and break up into a
variety of pairs of large atoms shortly after they are created. The many such fissions
occurring in a reactor may be expressed as an average reaction:
U235 + 0n1 ; 92U236; F1 + F2 + 2.47 0n1 + 203 MEV
where F1 and F2 represent fission fragment elements, as in Figure 10.2(b). On the
average, 2.47 neutrons are released in the process, one of which must initiate another
fission to sustain a chain reaction, as illustrated schematically in Figure 10.3. In
addition alpha, beta, and gamma radiation is released. One of the many reactions that
participate in the average reaction above creates xenon and strontium fission fragments
and two neutrons:
U235 + 0n1 ; 92U236 ; 54Xe139 + 38Sr95 + 2 0n1 + energy
U-235 reactions exemplified by the Xe-Sr reaction create diverse fission fragments and
small integral numbers of neutrons that average to 2.47 and release energies that
average to 203 MEV.
Over 80% of the 203 MEV of energy released by the average U-235 fission
reaction is the kinetic energy of the fission fragments associated with their large mass
and high velocity. Because of their high energy, the fission fragments become ionized
and ionize nearby atoms as they tear their way through surrounding materials. The
fission fragments, however, travel only a very short distance, for interactions cause
them to lose much of their energy to the surrounding solid. Thus most of the fission
energy appears immediately as internal energy and therefore locally high temperature of
the surrounding materials. It is, therefore, necessary that the fuel be cooled
continuously to counter the fission heat generation to avoid temperature buildup and
possible melting. The heated coolant is then provides the energy input for a
thermodynamic (usually steam) cycle.
As the fission fragments come to rest, their presence changes the character of the
reactor materials. Non-fissionable material exists where fissionable and other atoms
once resided. In time these materials may decay radioactively, forming still different
substances and releasing additional heat. But, importantly, they absorb neutrons
without offering the possibility of fission. Thus the fission fragments are said to poison
the reactor.
Fission Reactor Design Considerations
While most of the fission heat generation is due to the fission fragments, reactor design
focuses on the actions of neutrons. This is because neutron captures produce fissions,
fissions produce more neutrons, and, as we will see later, fissile material may also be
produced using neutrons. Thus neutrons are the currency of the reactor and may be
used constructively or wastefully. The manner in which the neutrons in a reactor are
used is called the neutron economy. The reactor designer pays close attention to all of
the details of the neutron economy. One of these details is the distribution of the kinetic
energy of the neutrons, that is, the fractions of the neutrons that lie in given energy
Almost all commercial power reactors (as opposed to experimental reactors) are
thermal reactors in which fission is caused by thermal neutrons. A thermal neutron is a
neutron that is in thermal equilibrium with the surrounding atoms. This implies that
they have energies on the order of 0.02 EV. Because they move relatively slowly,
thermal neutrons are much more likely to cause fission of U-235 than are higher-energy
neutrons; therefore, they are the choice for most power reactor designs. However, the
neutrons created by the fission reaction are not thermal neutrons. The neutrons created
by fission have kinetic energies that range from about 1 to 10 MEV. They are called
fast neutrons because of this high kinetic energy.
Thus, if a chain reaction is to be sustained in a thermal reactor, it is necessary for
the fast neutrons to be slowed, or thermalized, to much lower energies so that they can
cause fissions before they are absorbed in nonproductive captures in reactor material or
before they escape from the reactor. They do this by colliding with certain other nuclei
in the core, put there for that purpose. These nuclei are called moderators. A good
moderator is a light element that, on collision with a neutron, is speeded up by the
collision, and thereby extracts energy from the neutron without absorbing it.
The moderator concept may be understood by considering what happens when a
billiard cue ball hits another ball head-on. The cue ball stops, and the second ball carries
away the kinetic energy. However, with balls of differing mass, if the second ball is
heavy the cue ball bounces off without losing energy, whereas light balls are propelled
at high speed and extract significant energy from the cue ball. In neutron collisions with
atoms, it is the lightest atoms that extract the most energy.
Ordinary water, heavy water, graphite, and beryllium are all used as moderators
because they are light and are poor absorbers of neutrons. Hydrogen and deuterium are
the moderators in water. The oxygen in the water is not an effective moderator but has
low absorption and therefore does not interfere significantly with the moderation
It is evident that conservation of neutrons is a prime consideration in any reactor
design. The number of neutrons per unit volume, or neutron density, in a reactor is an
important design parameter. Neutrons diffuse about in the reactor as they are scattered
and slowed by moderator atoms. Because they have no charge, they are uninfluenced
by electromagnetic fields and therefore may travel further than charged particles.
Four events can influence local neutron densities as they pass through the
surrounding reactor core:
1. The neutrons can be captured by a fissionable atom and produce a fission.
2. They can be absorbed non-productively by fission products, structural materials,
or nonfissioning fuel.
3. They can escape through the walls of the reactor.
4. They can be absorbed in nuclei that create more fuel.
The last possibility will be considered in Sections 10.5 and 10.8.
Events 2 and 3, absorption and escape, result in nonproductive waste of neutrons in
the neutron economy. The necessity that enough events of type 1, rather than types 2
and 3, occur to sustain the chain reaction suggest several important considerations for
reactor design:
+ The reactor should be large enough that only events of types 1, 2, and 4 take
place and hence that escape of neutrons from the reactor is rare. Since reactor size
is dictated primarily by the cooling requirements imposed by nuclear heat
generation, this usually follows automatically from the design process. In addition,
positioning moderating material such as water at the boundaries of the reactor as a
reflector, to deflect escaping neutrons back into the reactor, may allow a more
compact design, in some cases.
+ Materials to be used in the reactor design are selected so that type 2 events are
minimized. The gradual buildup of poisons must also be considered in designing for
the change in reactor performance with time between refuelings.
+ The reactor should be designed to minimize the amount of structural materials in
the active fuel region, to reduce the frequency of type 2 events.
+ Neutron-absorbing materials may be moved into and out of the reactor to change
the average neutron density for reactor control purposes.
A reactor can function over a range of neutron densities. When the neutron density
and power levels are constant, the reactor is said to be critical. This implies that the
number of neutrons producing fission at one instant is the same as at a later instant, a
situation depicted in Figure 10.4. Since high neutron densities produce more fissions,
each of which produces 203 MEV in the case of a U-235 thermal reactor, heat
generation, and thus power output, increases with neutron density.
Let's assume that 2.5 fast neutrons are produced per fission in a critical thermal
reactor and track the activities of 100 fast neutrons created in an instant. In a critical
reactor, about 40 of these fast neutrons must thermalize and undergo fission to produce
another 100 fast neutrons. Figure 10.4 shows that 2 of the original 100 fast neutrons
produce (fast) fissions and 5 more neutrons immediately so that there are 103 fast
neutrons diffusing around. Of these, about 10 escape from the core or are absorbed in
non-fissile materials while slowing down. Of the remaining 93 thermal neutrons, about
3 escape from the core and about 43 are absorbed in non-U-235 materials while
diffusing around at thermal energies. The remaining 47 captures by U-235 result in 7
non-fission captures and 40 fissions. These 40 fissions, in turn, produce 100 fast
neutrons for the next generation in a reactor operating at critical. This sequence
representing a single generation in a critical reactor is repeated over and over by an
enormous number of neutrons. By slight changes in the reactor configuration during
operation (for instance, by addition or removal of a small amount of poisons), the
neutron economy can be adjusted to increase or decrease the number of neutrons in the
next generation and thus change the reactor operating condition.
A parameter, k, called the multiplication factor, is defined as the ratio of the
number of neutrons in one generation to the number in the preceding generation. Thus
k = 1 for a critical reactor. If k is less than or greater than 1, the neutron density is
decreasing or building and the reactor is said to be subcritical or supercritical,
A reactor is designed to have a maximum value of k > 1 so that it may be brought
up to a desired power level and maintained there over the duration of the fuel cycle.
Once the reactor approaches the desired power level, control actions are taken to
adjust the value of k to 1, bringing the reactor to critical and stabilizing its operation.
The control action requires the introduction of additional neutron-absorbing material or
the reduction of the amount of moderator in the reactor, to reduce the rate of buildup
of the neutron density to zero. As fissionable material is depleted and poisons build up,
control material is gradually withdrawn from the reactor to maintain critical operation.
When the reactor can no longer maintain critical at its design power level with no
control material present, it must be refueled.
The presence of a nuclear heat source is the major difference between fossil fuel
and nuclear plants. In that connection, there are several important factors that must be
considered in the design of a nuclear power plant that are not factors in conventional
power plant design and operation. First, the entire amount of fuel needed to operate a
nuclear power plant for up to two years is loaded into the plant at one time. The rate
at which power is generated must then be maintained by controlling the neutron chain
reaction over the wide range of reactor operating and fuel depletion conditions that can
arise between refueling operations. This calls for detailed planning and analysis, both
before and throughout the operating cycle.
Second, because the products of fission are highly radioactive and their rate of
decay cannot be controlled, the heat from radioactive decay of fission products after
shutdown amounts to as much as 7% of full power output. Consequently, provision
must be made in the thermal design to remove this heat under all credible operating and
accident conditions. (The reactor at Three Mile Island melted over an hour after the
nuclear chain reaction was terminated, because the operators misinterpreted their
instruments and turned off the emergency systems that were removing the decay heat.)
The nuclear decay process is not self-limiting and has no maximum temperature, as
with chemical reactions. If the heat generated is not removed from the reactor core, the
core will melt and be destroyed.
Third, if radioactive materials from the reactor core find their way to the
environment, they can be hazardous to nearby life. Although the proper cooling of the
core will ensure that these materials will remain contained inside the fuel assemblies, a
defense-in-depth approach to safety must be employed to make the possibility of a
release extremely remote.
Table 10.1
Nuclear Fuels
Fertile Fuels
Fuels Fissionable by Thermal Neutrons
10.5. Nuclear Fuels
Uranium-235 is the only material that is both fissionable by thermal neutrons and found
in nature in sufficient abundance for power production. Other fissile fuels are uranium233 and plutonium-239 (Table 10.1) which are created from thorium-232 and uranium238, respectively, by absorption of neutrons. Substances from which fissionable fuels
are created, called fertile fuels, are transmuted into fissionable fuels in a reactor by
extra neutrons not needed to sustain the fission chain reaction. Fertile fuel used in this
way is said to have been converted. The resulting fissile materials may be processed to
make new fuel elements when sufficient quantities have accumulated. Some of the
converted material may be consumed directly by fissions during reactor operation.
The composition of uranium ore is about 99.3% U-238 and 0.7% U-235. Because
of the ore’s small percentage of U-235, it is difficult to design a water-cooled, thermal
reactor that uses natural uranium. Therefore the power reactors in the United States
and most other parts of the world are thermal reactors that employ uranium enriched to
between 2% and 5% U-235. Such reactors use ordinary (light) water for both cooling
and moderation and are therefore commonly called light-water reactors.
Uranium enrichment is an expensive and difficult process because it involves
separation of two isotopes of the same element, which rules out most chemical
methods. Thus processes that rely on the small mass difference between U-235 and
U-238 are usually used. The gaseous diffusion process involves conversion of a
uranium compound processed from the ore to gaseous uranium hexafluoride, UF6. The
gaseous UF6 flows in hundreds of stages of diffusion through porous walls that
eventually produce separate UF6 gas flows containing enriched and depleted uranium.
The enriched UF6 then is processed to UO2 powder which is sintered into hard ceramic
fuel pellets such as those shown in Figure 10.5. The pellets are sealed in long cylindrical
metal tubes for use in the reactor.
Newer enrichment processes currently available or under development include: high
speed centrifugal separation; which also relies on the uranium isotopic mass difference;
a separation process that relies on differences in chemical reactivity between the
isotopes; and laser enrichment, which relies on ionization of uranium by an intense light
beam with subsequent chemical or physical separation of the ions (ref. 8).
10.6 Light-Water Power Reactors
Power reactors active in the United States today are light-water reactors. They are
designed so that the core is both moderated and cooled by highly purified water and
therefore must use a fuel that fissions with thermal neutrons.
Water has many advantages in thermal reactors. From a neutron point of view,
H2O is an extremely efficient moderator. As we know from its extensive use in
conventional power plants, water has excellent heat transfer characteristics, and the
technologies of its use in steam power plants are well established.
Water has disadvantages as well. To maintain its excellent moderation and heat
transfer capabilities, it must remain a liquid. Thus water reactors are currently limited to
producing hot liquid or steam with little superheat. Moreover, boiling temperatures
suited to an efficient plant require very high pressures, as in fossil fuel plants. Thus
water-cooled reactor cores must be encased in pressure vessels that operate with high
temperatures nearby. In addition, they must endure, for the design life of the plant, the
severe environment resulting from the fission reactions. Finally, and perhaps most
importantly, should reactor pressure integrity be lost while the reactor is operating, the
liquid water will flash to steam, losing much of its heat transfer advantages. All of
these factors contribute significantly to the challenges that an engineer faces in the
thermal and mechanical design of light-water reactors.
There are two major types of light water reactors (Figures 10.6 and 10.7), which
are differentiated primarily by the thermodynamic conditions of the water used to cool
uranium fuel elements in the reactor vessel. The boiling water reactor (BWR) operates
at a pressure that allows boiling of the coolant water adjacent to the fuel elements. The
water in the pressurized water reactor (PWR) is at about the same temperature as in
the BWR but is at a higher pressure, so that the reactor coolant remains a liquid
throughout the reactor coolant loop. In addition to their use in utility power reactors,
PWRs are used in American nuclear submarines.
Boiling Water Reactors
A schematic of the layout of the General Electric BWR/6 system is shown in Figure
10.8. There the turbines, condenser, pumps, and feedwater heaters studied in Chapter 2
appear in a familiar configuration. Water boils inside the reactor core, producing
slightly radioactive steam that passes directly to the steam turbines. The radioactivity in
the steam, however, has a half-life of only a few seconds. The carryover of radioactivity
to the turbine-feedwater system is virtually nonexistent, and experience has shown that
components outside the reactor vessel (turbine, condensate pump, etc.) may be
serviced essentially as in a fossil-fueled system. Some other reactor designs, such as the
pressurized water reactor to be considered later, have an additional separate water
loop, as seen in Figure 10.7, that isolates the turbine steam loop from the reactor
coolant to provide further assurance that the turbine-feedwater system components
remain free of radioactivity.
Fuel Assemblies
Uranium appears in most boiling water reactors in the form of sintered cylindrical
pellets of uranium dioxide (UO2) (Figure 10.5), about 0.4 inches in diameter and about
0.4 inches long. These pellets are stacked inside of long sealed zirconium alloy
(zircaloy) tubes called fuel rods. Fuel rods, in turn, are mounted in an eight-by-eight
array in a fuel bundle, as seen in Figure 10.9. The fuel bundle and the fuel channel that
surrounds the fuel rods comprise a fuel assembly, as shown in Figure 10.10.
The lower-tie-plate nose piece seen there, together with the fuel channel, directs the
coolant water flow over the fuel rods inside of the fuel assembly.
The fuel assemblies, mounted vertically in the core, are designed to minimize
operating stresses on the fuel rods. For example, Figure 10.10 shows that the fuel rods
are spring loaded so they are free to expand in the axial direction in response to
changes in reactor operating temperatures.
The Reactor Assembly
The core of a BWR/ 6 1220-MWe reactor (MWe stands for electrical generator power
output, as opposed to MWt for reactor thermal output) has 732 fuel assemblies and 177
control rod assemblies in an approximately 16-ft-diameter circular array, as shown in
Figure 10.11. Cooling water receives heat from the fuel rods by forced-convection and
two-phase nucleate boiling as it flows upward through the fuel assemblies. As the water
cools the fuel assemblies, it also thermalizes the fast neutrons that diffuse through the
Water that bypasses the fuel assemblies and flows upward on the outside of the fuel
channels is confined by the cylindrical core shroud, as seen in Figure 10.11. This flow
cools the channels and the neutron absorber control rods. The control rods are
mounted in cruciform assemblies, as seen in Figure 10.12. The control rod assemblies
fit vertically in a fuel module between 4 fuel assemblies in the core, as diagrammed in
Figure 10.13. The control rod assemblies move vertically between the fuel bundles to
change the effective multiplication factor to compensate for changes in reactor
operating conditions due to buildup of poisons over time. The stainless-steel-clad
control rods contain boron carbide (B4C), which absorbs neutrons and hence tends to
terminate fission processes when in place in the core. The control rod assemblies are
moved up and down through the core to change the rate of absorption of neutrons
when significant changes in power level or adjustments to account for fuel burnup are
required, or in the event of an emergency shutdown.
The positions of the control rods are adjusted by hydraulic drives located below the
core. The rods are fully inserted in the core during a shut down making the
multiplication factor less than 1. Most of the control rods are fully out of the core
during critical operation. When all of the rods are out of the reactor the multiplication
factor slightly exceeds 1, which allows the neutron density and power level to increase.
The bottom-entry fuel rod drives in the GE BWR are an unusual feature in reactors.
Their location below the reactor simplifies the refueling process which is carried out
from above in most reactor designs.
The quality of the steam leaving the top of the core is approximately 11%, indicating that most of the water is still liquid and must be recirculated through the core for
additional heating. The liquid water leaving the top of the core and the steam
separators flows downward outside the core shroud, driven by the jet pumps located
between the shroud and the reactor vessel wall, as shown in Figure 10.14. The jet
pumps, in turn, are driven by recirculation pumps located outside the reactor vessel.
The jet pumps induce the downward flow outside the shroud by momentum transfer to
the slower-moving liquid.
Vapor bubbling up through the fuel assemblies leaves the core and passes upward
with the liquid. The flow is turned by stationary vanes in the steam separator, where
the higher angular momentum of the liquid separates it from the vapor. The separated
liquid flows to the outside of the reactor vessel where it is recirculated outside the core
The steam passing through the separator is further dried in the steam dryer
assembly before it leaves the reactor vessel as a slightly superheated or saturated vapor.
The turbines are specially designed to operate with saturated vapor at the throttle and
small amounts of liquid within. Liquid is separated from the wet steam leaving the HP
turbine. Steam tapped from the HP-turbine-throttle steam line is used to reheat the HP
exit steam before its entry to the LP turbines. The low HP-turbine-throttle conditions
of 550°F and 1040 psia lead to a plant thermal efficiency of about 32%.
In the boiling water reactor, control is primarily achieved by adjustment of the rate
of recirculation through the reactor by the recirculation and jet pumps shown in Figures
10.11 and 10.14. Change in the rate of water recirculating through the core changes
both the onset of boiling and the volume fraction of steam in the cooling channels, and
thus the amount of moderator in the core at a given time. This allows significant
adjustment of reactor power output without control rod movement. For example,
increasing the recirculation and jet pump speeds sweeps bubbles away faster, increasing
moderation and raising the power level until the additional boiling restores the proper
void fraction for critical operation at a higher power level.
As the coolant passes upward through the fuel assemblies, heat from the fuel rods
produces vapor bubbles via nucleate-boiling heat transfer. Nucleate boiling is
characterized by the local formation of bubbles of vapor that break away from the fuel
rod surface, causing vigorous, agitated, fluid motion with resulting high heat transfer
rates. This may be contrasted with film boiling, in which a stable vapor layer covers the
tube surface, resulting in low heat transfer rates. Film boiling occurs at high surface-tobulk fluid temperature differences. It is crucial that a departure from nucleate boiling,
DNB, be avoided, because reduced heat transfer coefficients and cooling rates produce
drastic increases in tube temperatures, leading to fuel melting or zircaloy fuel rod
burnout. Thus the ratio of maximum heat flux to the critical heat flux for DNB is a
major thermal design parameter for the water reactor. The reactor fuel rod heatgeneration rate and flow-channel convective cooling are designed to maintain
tube-to-fluid heat fluxes well below the unstable transition range between nucleate and
film boiling. The maximum UO2 fuel temperature based on the maximum design fuel
rod heat-generation rate of 13.4 kW/ft is approximately 3400°F, whereas the fuel
melting temperature is about 5100°F.
The boiling water reactor, like other reactors, has numerous active and passive
safety systems. A thick pressure vessel, for instance, surrounds the reactor core. An
even thicker concrete containment structure surrounds the pressure vessel to confine
anything that may escape from it. An emergency core cooling system (ECCS) senses
overheating of the core and supplies a flood of water to take away the heat generated
by the fuel elements. These and other safety systems clearly reduce the danger of
accidents but also increase the cost and complexity of plant operation.
Pressurized Water Reactors
The pressurized water reactor, PWR, is currently the predominant reactor type in the
world. It is a light-water reactor that uses slightly enriched U-235 as fuel. Figure 10.15
shows the three reactor containment buildings of the Oconee PWR Nuclear Station.
A major difference between BWRs and PWRs is that the pressure of the PWR
coolant is above the saturation pressure (it is subcooled liquid) through the entire
cooling loop so that there is no possibility of bulk boiling in the core. As shown in
Figure 10.7, separate steam generators receive heat from the reactor liquid cooling
loop, thus preventing radioactive material from entering the turbine power loop.
Another difference is that control rods are at the top of the PWR and can drop by
gravity into the reactor when necessary. Figure 10.16 shows a sectional view of the
reactor building of the Oconee plant. The stairs and landings give some idea of the size
of the equipment within the containment. Figure 10.17 gives a less cluttered view of the
major components. Table 10.2 shows that, for PWRs, the turbine loop is at a lower
pressure than the liquid in the reactor loop and therefore produces an outflow of steam
to the turbine throttle with about 50 Fahrenheit degrees of superheat.
We have seen that, in contrast to the BWR, which circulates the reactor coolant
through the turbine, the PWR uses two loops: a primary loop that cools the reactor
core, and a secondary loop, heated by the primary loop, that provides steam for the
turbines, as diagrammed in Figure 10.18. The primary loop is contained completely
within the reactor containment building (Figure 10.16) and is designed so that the
water that cools the core is completely isolated from the environment. The secondary
loop executes a steam turbine cycle similar to those of conventionally fueled power
plants, with the exceptions that the steam is generated by heat transported from the
reactor and that the maximum turbine inlet temperature and superheat are limited by
the maximum temperature in the reactor core.
The Babcock & Wilcox steam generator is a counterflow shell-and-tube heat
exchanger, as seen in Figure 10.19. The primary water enters the top of the unit and
flows downward through thousands of small-diameter tubes to provide a large heat
transfer surface. Feedwater is piped into the bottom of the shell side of the steam
generator and is first heated, then boiled by heat from the hot tubes containing the
primary water flow. As the steam rises, it encounters the hotter portions of the primary
tubing, reaching 50 Fahrenheit degrees of superheat at the top of the steam generator.
An important thermal design parameter of the steam generator is its size. A larger
unit increases the heat transfer area from the primary loop, but it also increases the
capital cost of the plant, both in the cost of manufacturing the generator itself and in
the larger size required of the primary containment. This must be balanced with the cost
savings, lower heat exchanger effectiveness and consequent lower thermal efficiency
achieved, if the heat transfer area is made smaller.
The pressurizer (Figure 10.17) controls the pressure in the primary loop and serves
as a surge tank to accommodate the reactor coolant water’s expansion and contraction
with temperature changes. It is a large tank designed to contain saturated water and
steam, each occupying about one-half of the tank volume. As shown in Figures 10.17
and 10.18, the pressurizer is connected to the primary loop at the outlet of the reactor
through a single pipe. The temperature of the pressurizer contents are controlled with
internal electric heaters and water sprays. The resulting saturation temperature
establishes the operating pressure of the reactor.
An important part of the PWR thermal design is sizing the pressurizer tank to
permit the primary system to respond to all possible transients without bursting the
coolant pipes. Recalling that water is an almost incompressible fluid that expands when
heated, if the reactor power rises and heats the primary water to a higher temperature,
the expanding water will flow into the pressurizer, compressing the steam bubble.
Pressure sensors will detect the increased pressure and open spray valves at the top of
the pressurizer to condense some of the steam, thus restoring a lower operating
pressure. Conversely, if the primary loop temperature declines, heaters in the bottom
of the pressurizer will turn on, creating more steam to fill the volume formerly occupied
by the contracting water and thus increasing the reactor temperature and pressure.
Although there is no bulk boiling in a PWR core, heat transfer to the coolant is by
subcooled nucleate boiling. In subcooled nucleate boiling, steam bubbles are formed
on the surface of the cladding. As the bubbles expand they become detached from the
tubes and immediately collapse as they are swept into the coolant channel. The lateral
motion in the coolant achieved by this action is an extremely effective mechanism to
transport the energy from the cladding into the coolant stream. With subcoolednucleate-boiling heat transfer, the cladding surface temperature will stay within 10°F of
the water's saturation temperature while providing the very high rates of heat transfer
needed to remove the fission energy from the fuel rods.
An extremely important design limitation is the critical heat flux at which steam
bubbles form and grow so fast that they coalesce to form a vapor film over the clad. At
this point the heat transfer undergoes a departure from nucleate boiling to film boiling,
and the clad no longer touches liquid water. Since metal to vapor heat transfer is much
more inefficient than nucleate boiling, the fuel rod temperature rises in order to transfer
the accumulating heat that is created by fissions. Unfortunately, film boiling heat
transfer coefficients are so low that the high surface temperatures needed to achieve a
steady state are sufficient to melt the clad and severely damage the fuel rods. For this
reason the Nuclear Regulatory Commission has established safety factors for the ratio
between the critical heat flux and the maximum heat flux expected in a reactor under its
most severe overpower transient conditions. In its application for an operating license,
every nuclear power plant must be able to demonstrate through engineering analysis
that it will maintain the required safety factor under all credible overpower and
undercooling transients.
The Babcock and Wilcox PWR uses U-235 enriched to about 3% in the form of
UO2 fuel pellets (Figure 10.20) encased in a zircaloy-clad tube similar to the fuel rods
described for the BWR. Table 10.2 shows that a 1300-MWe plant has 205 fuel
assemblies with 54,120 fuel rods. Figure 10.21 shows fuel assemblies for an electric
utility PWR and for the Nuclear Ship Savannah. Each fuel assembly for electric power
generation is fitted for the inclusion of sixteen control rods, as seen attached to an
activating spider in the left fuel assembly photo. Positioning of the fuel assemblies in
the core and those assemblies containing control rods are shown in Figure 10.22. There
are sixty-nine control rod assemblies in Oconee Unit 1, of which sixty-one are for
control of power level; the remaining eight contain poisons in the lower part of the rods
for shaping of the reactor power distribution. The control rod neutron-absorbing
material, silver-indium-cadmium, is encased in stainless steel.
10.7 The CANDU Reactor
The CANadian Deuterium Uranium, CANDU, reactor is a reactor of unique design that
utilizes natural uranium as a fuel and heavy water as a moderator and coolant. These
reactors produce a substantial saving due to the absence of fuel enrichment costs, but a
large chemical plant is required to supply the quantities of heavy water required. The
Pickering station near Toronto, an Ontario Hydro plant shown in Figure 10.23, uses
eight CANDU reactors to generate 4800 MWe and has been generating power since
One of the important and unique features of the CANDU reactor is that, whereas
light-water reactors are shut down for refueling annually, CANDU reactors are
refueled daily. The pressurized heavy-water-cooled fuel bundles are horizon-tally
oriented in individual fuel channels inside the unpressurized "calandria," as
diagrammed in Figure 10.24. Each bundle may be individually accessed, rearranged in
the calandria, or replaced using special fuel handling equipment while the reactor is
operating. Heavy water at atmospheric pressure in the calandria surrounds the fuel
channels and moderates the reactor. Thus, in an emergency, the reactor can be shut
down by draining the calandria to remove the moderator, thereby depriving the fuel of
thermal neutrons. The pressurized heavy-water loop draws hot coolant from the fuel
channels through headers to supply heat to steam generators as in a PWR. A lightwater loop passing through the steam generator in turn supplies steam to the
turbine-feedwater loop. A turbine hall at the Pickering Station is shown in Figure
The largest cylindrical structure seen in Figure 10.23 is a vacuum building that
connects with each of the reactor buildings. In the event of an emergency, escaping
steam and radioactive materials would be drawn by vacuum into the structure. A coldwater spray would condense the steam to limit any pressure buildup.
The thermal efficiency of a CANDU reactor plant is only about 29%, but the
CANDU reactor uses a larger fraction of U-235 in uranium ore than other reactors and
also makes better use of the U-238 to Pu-239 conversion process to extend fuel
Moreover, statistics show that, among large reactors, CANDU reactors have
outstanding reliability records, with annual capacity factors (the ratio of annual
electrical energy output to maximum possible annual output) as high as 96% and
cumulative capacity factors as high as 88% (ref. 14).
10.8 Fast Reactors
Reactors may be designed to fission with fast neutrons, but these fast reactors must
be more compact than thermal reactors so that the fast neutrons may produce fissions
quickly before they are absorbed or moderated by surrounding materials. They are
designed with structural materials that are poor absorbers and moderators of neutrons,
such as stainless steel. The core of a fast reactor must contain a fissionable fuel of
about 20% enrichment to compensate for the lowered probability of fissioning with
high-energy neutrons.
Because of their high fuel density, fast reactors have a high power density that
poses a difficult cooling problem. One solution is the use of a liquid metal as coolant.
Liquid metals such as sodium and potassium have excellent heat transfer characteristics
and do not interfere significantly with neutron functions.
The choice of fuel used for thermal and fast reactors depends on the fuel’s fission
probability and the net number of neutrons produced per neutron absorbed. Most
effective for thermal reactors is U-235, whereas Pu-239 is most suitable for fast
reactors. In fissioning with fast neutrons, Pu-239 emits almost 20% more neutrons than
does U-235. These additional neutrons are extremely important for making breeding
practical, as will be discussed shortly. Because Pu-239 must be created from fertile
U-238, a plutonium reactor can use fuel processed from fuel produced in a uranium
thermal reactor or in another plutonium reactor. This would occur in the core of a
plutonium fast reactor and in a blanket of U-238 surrounding the core, where
additional plutonium is created using neutrons that escape from the core.
Figure 10.26 diagrams two steps in a chain reaction in a fast reactor where a plutonium atom is created for each one consumed. Note that each fission must produce a
minimum of two neutrons for this reaction to continue. As a practical matter, more
than two neutrons are required for complete replacement because of non-productive
neutron captures.
A reactor that transmutes a fertile fuel to a fissionable fuel is called a converter. The
conversion efficiency is the ratio of the number of new fissionable atoms produced to
the number of atoms consumed in the fission. Thus the conversion efficiency of the
reaction shown in Figure 10.26 is 1, because a new fissionable atom is created for each
atom consumed. In this case there is no net fissionable fuel consumption.
Fast Breeder Reactors
The cover of a 1971 U. S. Atomic Energy Commission booklet (ref. 7), (see Figure
10.27) shows a notebook page that poses and answers the following question: Johnny
had 3 truckloads of plutonium. He used 3 of them to light New York for 1 year. How
much plutonium did Johnny have left? Answer: 4 truckloads. This neatly emphasizes
the point that if unproductive neutrons in the process shown in Figure 10.26 were to
transmute a U-238 atom to Pu-239 occasionally, the chain reaction would produce
more fissionable fuel than it consumes. The reactor would have a conversion efficiency
greater than 1. A reactor that does this is called a breeder reactor. Thus, the breeder
reactor provides a means of increasing the world supply of fissionable fuel as it
generates power. While it may appear that the breeder provides something for nothing,
it actually only takes advantage of the possibility of conversion of the fertile fuels
through more efficient use of fission neutrons.
The breeder reactor is of great importance because it would allow the use of the
vast store of U-238 in uranium ore that remains as a by-product of the U-235
enrichment process to provide fuel for current LWRs. This supply of U-238 has the
potential to provide fuel for many years without further uranium mining. Fission of
U-235 is currently the only natural large-scale source of neutrons. The continued use of
low-conversion-efficiency reactors could preclude the eventual use of much of the
energy resource of the U-238 in uranium ore.
One possibility for the design of a breeder reactor is a liquid-metal fast breeder
reactor, LMFBR, which has the characteristics described briefly in the preceding
section. The development of such a reactor involves careful design of its neutron
economy and the development of a system of fuel reprocessing and nuclear waste
storage. These topics will be considered in the next section.
In 1971, President Nixon set the development of a breeder reactor as a national
goal and established a program for the development of an LMFBR pilot plant in
Tennessee to be known as the Clinch River Breeder Reactor, CRBR. The vocal
opposition of segments of the American public to all forms of nuclear power, coupled
with well-known nuclear accidents, bureaucratic and construction delays, and concern
for diversion of nuclear fuels to foreign military or terrorist uses, brought the
development of new nuclear plants in the United States, including the CRBR, to a
standstill. While some other nations, notably Japan, France, and Russia, continue to
support extensive nuclear establishments and research, the path to the development of
breeder reactors has been slow. The French, in particular, have developed experience
with their 233-MW Phénix reactor, in operation since 1974, and an advanced 1200MW LMFBR called the Super-Phénix, which went into operation in 1986. In 1994, the
status of Super-Phénix was changed from production to a research facility; and in 1998
the French government directed that it be shut down, well before its planned 2015
shutdown (refs. 24 to 26). In Japan, delayed reporting of a minor sodium leak during
commisioning of the Monjo breeder in 1995 produced a national turmoil that had
serious repercussions for the Japanese breeder program. (Ref. 27).
10.9 Innovation in Reactor Design
During the hiatus in new domestic nuclear plant orders in the United States since the
late 1970s, the industry has been working to incorporate new ideas and recommendations, based on utility operating experience, into the design of plants that will be more
capable of winning public, government, and industry acceptance.
Popular opinion notwithstanding, the safety record of nuclear power plants in the
United States has been a good one. No one has been killed in a nuclear accident in a
U.S. power plant. Few other industries can approach that record. Nevertheless, serious
and expensive accidents have taken place at home and abroad; and the threat of
catastrophic accidents, while extremely remote, remains.
Designs are being considered by industry, the U.S. Department of Energy (DOE),
the Electric Power Research Institute (EPRI), and the Nuclear Regulatory Commission
(NRC) that would provide built-in systems and safeguards that are passive, rather than
active, that must be depended on to perform when a malfunction occurs (ref. 15).
Providing for natural conduction/convection cooling of the core in case of an accident,
rather than depending on pumps to force water through the core, is one of such
features under consideration. The General Electric Company, with an international
team, is developing a 600 MWe simplified / small boiling water reactor (SBWR) that
incorporates this approach (ref. 28).
Nuclear plant incidents sometimes result from failure of an operator to interpret
instrument data and act thereon or from instrument failure. Redundant instrumentation
is provided to avoid such problems, but operators do not always analyze the readings
correctly or make the right decisions. If the plant is designed to minimize the necessity
of such actions, the likelihood of accidents can be significantly reduced. Thus, much
effort focuses on reducing the complexity of reactor control. Designs have been
produced that appear to accomplish this to a significant degree. The NRC must, of
course, conduct hearings and evaluate the adequacy of any design before a plant is
One of the problems of the power industry is the lack of standardization. Every
American power plant is different from every other because of the number of reactor
suppliers and power plant engineering firms and the diverse requirements of the
different utilities that buy them. One of the obvious reasons for the reliability of the
CANDU reactors is that they are planned and constructed by Ontario Hydro and
Atomic Energy of Canada, largely for Ontario Hydro. American suppliers are also
striving for standardization, but resolving the differences between differing requirements and standards is a formidable problem. An advanced boiling water reactor
(ABWR) program with an NRC certification (ref. 29) addresses these issues by
providing a standardized design that places pumps within the reactor envelope,
eliminates much external piping, and incorporates other passive safety features.
Reference 23 indicates that this system benefits from a new NRC licensing process in
which safety issues are resolved, with full public participation, before construction
begins. ABWR planning calls for a forty-eight-month construction schedule once a site
has been approved.
The long-term trend has been for utilities to seek larger and larger plants to take
advantage of the economies of scale. When the annual rate of growth of the electrical
power industry dropped drastically in the 1970s to 2–3% due to conservation and other
factors, many of the utility requirements for large reactors became less critical or
disappeared. Interest has appeared in smaller reactors that can be constructed
economically and on a timely basis. One hope is that better quality control could be
exercised, and better economics result if small reactors were built, entirely or in
modular fashion, in a factory and shipped to and installed at the power plant site, rather
than erected there. Some of theses issues are addressed in the ABWR and SBWR
programs (ref. 23).
10.10. Nuclear Reprocessing and Waste Disposal
Spent fuel is fuel that has resided in a reactor for a year or more, has been depleted of
much of its fissionable material, and includes a buildup of radioactive fission products.
When a reactor is shut down for refueling, spent fuel assemblies are removed and
partially spent assemblies may be repositioned in the core to obtain further fuel
utilization. Ideally, the spent fuel would be reprocessed to reclaim the unused and
newly created fissionable materials for use in new fuel rods, and the high-level waste
would be isolated to minimize the volume of highly toxic wastes. Even without breeder
reactors, current spent fuel inventories can provide ample Pu-239 to support
reprocessing. In the United States, by law, fuel assemblies are being stored indefinitely
in reactor-fuel storage pools in nuclear power plants until legal decisions are made on
whether reprocessing will take place and on the final disposition of nuclear waste.
Unfortunately, these decisions are among the most politically difficult ones of our
10.11 Concluding Remarks
Nuclear power engineering has become so hopelessly intertwined with politics that any
speculation on the future of nuclear power must simultaneously consider both technical
and political realities. Even the continuation and expansion of nuclear power generation
in the United States is in question.
A major concern for many years has been the availability of fissionable resources
for an expanding nuclear power generation industry. It has been widely recognized that
U-235, as the sole natural neutron resource, places limits on extensive nuclear
development. For that reason, countries with nuclear generation capability have
planned on developing breeder reactors to greatly extend that neutron resource. It is
possible that the continuing long-term use of reactors that do not provide for efficient
conversion of the fertile fuels may result in the waste of this enormous energy resource.
While the American nuclear power industry languishes, foreign development of
nuclear power continues in some areas. The eventual recovery of the American nuclear
power industry may depend on the success of international nuclear development. The
absence of CO2 production by nuclear plants, and its global warming implications, gives
an added incentive for international perpetuation and development of the industry.
Expansion of the nuclear power industry and development of a breeder reactor
nuclear power economy inevitably entails the development of nuclear processing and
hazardous waste disposal facilities and reactor fuel recycling. These activities arouse
concerns about the possible diversion of nuclear materials for the production of
weapons, (refs. 13 and 17–19). The tradeoffs among nuclear power, coal, and other
energy conversion alternatives are also becoming more prominent as concerns over
global warming and atmospheric pollution intensify. Well-thought-out and consistent
government policies and regulation, as well as international cooperation, would give
welcome direction to the power generation industry. Reference 16 observes that
success in nuclear programs in other countries seems to correlate with limiting public
intervention, and it questions whether the democratic institutions of the United States
are consistent with the growth of the American nuclear power industry.
Bibliography and References
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5. Glasstone, S., and Sesonske, A., Nuclear Reactor Engineering. New York: Van
Nostrand Reinhold, 1981.
6. Steam / Its Generation and Use, 39th ed. New York: Babcock and Wilcox, 1978.
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8. Nero, Anthony V. Jr., A Guidebook to Nuclear Reactors. Berkeley, Calif.:
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10. Judd, A. M., Fast Breeder Reactors, An Engineering Introduction. New York:
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10.1 Consider the collision of a particle of mass m and velocity v with a stationary
particle of mass M. Write energy and momentum equations for the collision.
Derive an equation for the ratio of the final to the initial kinetic energy of the
original moving particle in terms of the masses of both particles. Use the result to
show why light atoms are used as moderators.
10.2 Evaluate the assertion of the title of reference 12 that 100 grams of uranium equal
290 tons of coal. Assume coal to be represented by pure carbon. Could you make
the title of the book more accurate? If so how?
10.3 Estimate the core thermal power and thermal efficiency of a 1220-MWe boiling
water reactor that has 46,376 fuel rods in a 150-in.-high core with a maximum
fuel rod linear energy density of 13.4 kW / ft and a fuel rod peak-to-average
power release of 2.2. If there are 748 fuel assemblies, what is the average number
of fuel rods per assembly? Estimate the number of thermally inactive rods in a
reactor with an eight-by-eight fuel assembly array.
10.4 Assuming the fuel temperature to be 295K, calculate the energy of a thermal
neutron using 3kT/ 2 where k is the Boltzmann constant.
10.5 Study the literature and then discuss the details of the nuclear processes by which
neutrons convert U-238 to fissionable fuel. Include nuclear reaction equations.
10.6 Study the literature, then discuss the details of the nuclear processes by which
neutrons convert thorium to a fissionable fuel. Include nuclear reaction equations.
10.7 Sketch and label a PWR steam generator flow diagram and a T-s diagram for the
two flows through the PWR steam generator, using the data from Table 10.2.
10.8 Determine the ratio of the reactor-loop flow rate to the steam flow rate from the
data for the PWR given in Table 10.2.
10.9 Sketch a T-h diagram and determine the pinchpoint temperature difference for a
PWR steam generator that has, respectively, 572.5°F and 630°F reactor inlet and
outlet temperatures and steam generator inlet and outlet temperatures of 473°F
and 603°F.
10.10 Discuss the changes in the neutron economy diagram (Figure 10.4) due to (a)
insertion of control rods, (b) withdrawal of control rods, (c) increase and (d)
decrease in recirculation flow rate in the boiling water reactor.
10.11 List from BWRs, PWRs, CANDUs, and LMFBRs those power reactors that are
(a) Thermal reactors
(b) Fast reactors
(c) Heavy-water reactors
(d) Light-water reactors
(e) Natural uranium reactors
(f) Enriched uranium reactors
(g) Plutonium reactors
10.12 Identify those power reactors that use moderators and those that do not.
10.13 Which power reactor (a) uses light water as coolant and has separate reactor
coolant and steam loops, (b) uses heavy water and natural uranium, and (c) uses
plutonium as fuel and liquid metal as coolant?
10.14 Based on data in Table 10.2 for the Oconee Unit 1 PWR, estimate the turbineloop thermodynamic conditions (temperature and pressure) and power delivered
by a simple Rankine-cycle steam turbine with 85% efficiency.
10.15 Compare the reactor temperature rise shown in Table 10.2 for the Oconee Unit 1
with an analysis based on heat transfer data given in the table. Discuss your result.
10.16 Compare the reactor temperature rise shown in Table 10.2 for the 1300-MWe
PWR with an analysis based on heat transfer data given in the table. Discuss your
10.17 Based on the data given in Table 10.2, determine the heat transfer rating for the
Oconee Unit 1 PWR steam generator, and evaluate the average reactor-loop heat
transfer loss rate and fraction. Discuss your result.
10.18 Based on data in Table 10.2, estimate the turbine-loop thermodynamic conditions
(temperature and pressure) and power delivered by a simple Rankine-cycle steam
turbine with 85% efficiency for the 1300-MW PWR.
10.19 Based on the data given in Table 10.2, determine the heat transfer rating for the
1300-MWe PWR steam generator, and evaluate the average reactor-loop heat
transfer loss rate and fraction. Discuss your result.
10.20 Develop a thermal design for a 2000-MWe pressurized water reactor core with
0.4 in. diameter, 15-ft-long fuel rods having an average linear power output of 16
kW/ft. Assume an average film coefficient of 4500 Btu/hr-ft2-R. Prepare a report.
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