LWR with High Burnup Fuel | 1
LWR with High Burnup Fuel
Current light water reactors are typically designed to achieve burnup about 50 GWd/tU.
With newer fuel technology, and particularly the use of advanced burnable absorbers, these
same reactors are now capable of achieving up to 60 GWd/tU. Some studies shows that in
the near future, even with the present enrichment limit (5 wt %), fuel burnup could be
extended near to 70 MWd/kg. Further increase in fuel burnup is impossible without the
relaxation of the present enrichment limit (5%).
As can be seen, there has been a clear trend over the last decades toward increasing fuel
burnup in light water reactors. In nuclear power plants, high fuel burnup is desirable for:
Reducing the number of fresh nuclear fuel assemblies required and thus reducing
spent nuclear fuel assemblies generated. Both aspects lead to improvements in the
economics of fuel cycle.
Reducing duration of refueling outage.
High burnup results in a lower mass of spent fuel discharged per unit of electricity
generated. This can reduce spent fuel handling and transportation.
A benefit (crutial aspect for some operators) is that loading of the “high” burnup
assemblies in the periphery reduces the neutron flux on the pressure vessel.
Reducing the potential for diversion of fissile material from spent fuel for nonpeaceful.
On the other hand, there are signals, that increasing burnup above 50 or 60 GWd/tU leads
to significant engineering challenges and even it does not necessarily have to lead to
economical benefits.
See also: Etienne Parent. Nuclear Fuel Cycles for Mid-Century Deployment. MIT. 2009.
Higher burnup fuels require higher initial enrichment to sustain reactivity. Since the
amount of separative work units (SWUs) is not a linear function of enrichment, it is more
expensive to enrich higher enrichments. For instance, to produce one kilogram of uranium
enriched to:
5% U-235 requires 8.9 SWU if the tails assay is 0.20%
4% U-235 requires 6.3 SWU if the tails assay is 0.20%
But there are also operational aspects of high burnup fuels that are associated especially
with reliability of such fuel. Main concerns associated with high burnup fuels are:
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LWR with High Burnup Fuel | 2
Increase burnup places additional demands on fuel cladding, which must withstand in
the reactor environment for longer period.
The longer residence in the reactor requires a higher corrosion resistance.
Higher burnup leads to higher accumulation of gaseous fission products inside the fuel
pin resulting in significant increase in internal pressure.
Higher burnup leads to increased radiation induced growth, that can lead to
undesirable changes in core geometry (fuel assembly bow or fuel rod bow). FA bow
can result in an increased control rods drop time due to friction between control rod
and bowed guide tubes.
These factors must be considered, because they can lead to a decrease in operational
reliability
High burnup fuel generates a smaller volume of fuel for reprocessing, but with a
higher specific activity.
Strategic management and decision making in respect of the middle part of nuclear fuel
cycles is a very specific problem of power engineering. It is obvious also such evaluation
must contain nuclear calculations and business-economic evaluation, thus they cannot be
carried out separately. Detailed nuclear calculations are necessary, because they can
exclude many promising strategies. On the other hand they do not provide any information
about benefits or costs. A strategic decision making must be carried out on the basis of
business-economic evaluations.
Special reference: Technical and economic limits to fuel burnup extension, Proceedings of a
Technical Committee meeting, IAEA, 11/2009
References:
See above:
Reactor Operation
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