NUCLE A R FORENSIC S INTERN ATION A L
TECHNIC A L WORK ING GROUP
ITWG GUIDELINE
ON CHARACTERISTIC PARAMETERS OF
URANIUM DIOXIDE (UO2) FUEL PELLETS
EXECUTIVE SUMMARY
Low enriched uranium dioxide (UO2) in form of ceramic pellets is typically used in commercial nuclear power
reactors as fuel. In order to find out the intended use of UO2 pellets, and subsequently to narrow down the
possible production facility, UO2 pellets contain a few characteristic parameters (i.e. signatures) that can be
helpful in the origin determination. Such signatures include e.g. dimensions, 235U enrichment and impurities.
The following guideline will shortly describe each of the signatures and discuss about the information they
may provide for nuclear forensic investigations.
This document was designed and printed at Lawrence Livermore National Laboratory in 2016 with
the permission of the Nuclear Forensics International Technical Working Group (ITWG).
ITWG Guidelines are intended as consensus-driven best-practices documents. These documents
are general rather than prescriptive, and they are not intended to replace any specific laboratory
operating procedures.
1. INTRODUCTION
Fuel pellets were found out of regulatory control
frequently in the 1990s when the phenomenon of illicit
trafficking of nuclear materials started. Therefore, a
lot of effort was put in studies on their characteristic
parameters with a view to trace back the origin [1].
Fuel for commercial power reactors is produced by
compacting uranium dioxide (UO2) powder to cylindrical
pellets (about 1 cm × 1 cm) and sintering them at high
temperatures to produce ceramic nuclear fuel pellets
with a high density and well defined physical properties
and chemical composition. A grinding process is used
to achieve a uniform cylindrical geometry with narrow
tolerances. Such fuel pellets are then stacked and filled
into the metallic tubes (cladding made typically of
zircalloy) composing fuel rods. The finished fuel rods are
grouped into fuel assemblies that are used to build up
the core of a power reactor (Fig. 1). UO2 pellets are (or
have been) produced at least in Argentina, Brazil, Canada,
China, France, Germany, India, Japan, Kazakhstan, Korea,
Pakistan, Romania, Russia, Spain, Sweden, UK and USA.
2. CHARACTERISTIC PARAMETERS
OF UO2 PELLETS
There are number of characteristic parameters (i.e.
signatures) in UO2 fresh fuel pellets that can give
information about their intended use (i.e. reactor type)
and, consequently, about their production place. The
most prominent “signatures” are:
•
Dimensions (e.g. diameter, length, central hole)
•
Markings
•
Microstructure
•
235
•
U isotopic composition
•
Additives and impurities
•
Age
U enrichment
In the following each of the “signatures” and their
importance are described shortly.
Fig. 1. Fuel fabrication process.
(Source: World Nuclear
Association)
10 mm
I T WG-I N F L-U O F P-v1_2016_09
Fig. 2. Various UO2 fuel pellets.
(Source: ITU)
1
2.1. DIMENSIONS
2.2. MARKINGS
Typically every reactor type (e.g. HWR, PWR, FBR) requires
fuel pellets of tailored dimensions (Fig. 2; Table 1). Pellets
are typically cylindrical in shape and may contain well
defined geometrical features such as “dishing” and
“chamfer” or “central hole” (Fig. 3).
It should be noted, though, that dimensions (encountered
in the specifications) are associated with narrow
uncertainties, less so for the length.
In some cases pellets may bear markings, e.g. dots or
numbers, at the ends of the pellets (Figs. 2 and 4). For
instance, the numbers on the pellets shown below were
intended to represent the nominal 235U enrichment of
the pellet (Fig. 4a) and the press number used to press
the pellet (Fig. 4b). Markings are comparative signatures
and can best be interpreted for forensic purpose with the
support of a national nuclear forensic library.
Table 1. Typical dimensions (in mm) of uranium fuel pellets in some common reactor types [1,2]
Reactor type
Model (Name)
Diameter
Length
Central hole
PHWR
Candu 6 (Cernavoda in Romania)
12.16
16
-
PHWR
Candu (Pickering in Canada)
14.3
19.8
-
PHWR
(Rajasthan in India)
12.2/14.34
13.4/17.2
-
PWR
VVER-440 (Loviisa in Finland)
7.6
10
1.2
PWR
VVER-1000 (Kalinin in Russia)
7.55
11-12
2.2-2.4
PWR
(Philippsburg 2 in Germany)
9.11
11
-
PWR
CP2 (Gravelines in France)
8.19
13.3
-
BWR
BWR 5 (Fukushima Daini in Japan)
10.4
10.3
-
BWR
BWR 2 (Nine Mile Point 1 in USA)
9.55
9.6
-
LWGR
RBMK-1000 (Chernobyl in Ukraine)
11.5
15
-
LWGR
RBMK-1500 (Ignalina in Lithuania)
11.5
12-15
2.0
AGR
(Heysham in UK)
14.5
15
6.4
Fig. 3. a) Fuel pellet with a
central hole, dishing and
chamfer; b) Measurement
of the dimensions of
the same fuel pellet
by Scanning Electron
Microscopy (SEM).
(Source: ITU)
5 mm
Fig. 4. Markings on pellets.
(Source: ITU)
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2.3. MICROSTRUCTURE
2.5. U ISOTOPIC COMPOSITION
Microstructure of pellets (e.g. grain size, porosity and
surface roughness) can be indicative of the process used
for precipitation, solidification, sintering and grinding. For
instance, sintering temperature and use of additives can
be used to achieve different grain sizes (Fig. 5). Surface
roughness, on the other hand, indicates the grinding
process used, e.g. the so-called “wet grinding” produces
slightly smoother surface compared to the “dry grinding”
process [1].
Besides the 235U enrichment of the fuel which is a
key parameter, also the uranium minor isotopes can
constitute to a useful nuclear forensic signature. Firstly,
the 236U and 232U in the fuel show that the uranium has
been exposed to neutrons, thus being previously in
a reactor and afterwards reprocessed (and possibly
enriched again). Secondly, as not all fuel fabrication
facilities produce fuel from reprocessed uranium, it can
limit the number of possible manufacturers.
2.4. 235U ENRICHMENT
2.6. ADDITIVES AND IMPURITIES
Depending on the type of reactor and the fuel
configuration, the requirement for 235U enrichment may
vary. In the first approach we can identify three groups of
enrichment level (which may overlap):
Several commercial fuels use additives, such as
gadolinium, erbium or boron, as burnable poison to
control the reactivity of the fuel. The amount of the
burnable poisons in fuel is considerably higher than if
they were found as impurities.
•
Heavy water moderated reactors such as CANDU use
natural uranium as fuel.
•
Graphite moderated reactors such as RBMK-1000 and
RBMK-1500 use fuel enriched between 2.0% and 2.6%,
depending on the reactor model and additives of
burnable neutron poison [1].
•
Light water moderated reactors such as VVER-440
and VVER-1000 have used typically enrichments of
3.6% and 4.4%, respectively; however the newer
models utilise somewhat higher enrichments. Other
type light-water reactors (PWR and BWR) have 235U
enrichment up to 5% [2].
Impurities are detected in fuel in trace levels and they
originate from various sources, including U ore (e.g. REE
pattern in natural U fuels), process piping, reagents, etc.
Thus, they may be indicative about the process used to
produce the fuel. For example, some fuel manufacturers
use aluminium stearate as a mold release agent. Use of
aluminium stearate can elevate the residual Al in the fired
fuel pellet by 30-60 ppmw.
Contaminants on the surface of the pellets may provide
information about pellet processing (sintering, grinding,
etc.). For example, trace amounts of Mo can be left on
pellets after sintering, from the Mo trays used to hold the
pellets during the sintering process.
Within pellet inhomogeneity of the 235U enrichment
(e.g. grain to grain variation) may be indicative of the
production process (powder blending, co-milling).
Fig. 5. Surface
morphology of two
different types of
fuel pellets by SEM.
(Source: ITU)
I T WG-I N F L-U O F P-v1_2016_09
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2.7. AGE
4. REFERENCES
The radioactive decay of uranium isotopes allows
establishing the “age” of the material. As the method
is based on certain assumptions (see ITWG guideline
INFL-ADPD on age dating), the “age” is referred to as the
model age. This is understood to describe the production
date of the material. It should also be noted that the final
separation of the parent/daughter pair for a UO2 fuel can
take place in various stages depending on the process
used before conversion the material to UO2 powder.
Therefore, it can be e.g. during the enrichment, the
hydrolysis or the chemical conversion. The model age,
even if determined with low uncertainty, should be used
thoughtfully when put in the context of a technological
production process. One must remember that several
sources of material can be combined to make up the final
fuel pellet (co-milling, powder blending). This will lead
to an “average” age of the material for bulk age dating.
In addition, many manufacturers add some U3O8 to the
powder mix before pressing into pellets, so as to increase
the O/U ratio prior to sintering (“U3O8 Add-Back”). The
U3O8 is made from scrap UO2 and will usually have an age
slightly older than the UO2 used in the pellets. This can
bias the model age several months older than the age
since conversion of UF6 to UO2.
1.
2.
L. Pajo, Ph.D. thesis 2001, University of Helsinki,
http://urn.fi/URN:ISBN:952-10-0122-4.
Nuclear Engineering International, World Nuclear
Industry Handbook 2012.
DOCUMENT REVISION HISTORY
Document INFL-UOFP
Version
No.
Version
Date
1
2016-09-01
Description Changes made by
of Changes
Initial Draft
M. Wallenius
(author)
3. DATA INTERPRETATION
Some of the above mentioned signatures are selfexplanatory, i.e. the results do not need any comparative
data to give an implication. Such a signature is, for
instance, the age, because it can be calculated directly
from the measured parent/daughter ratio using the
common decay equation. However, the elemental
impurities are a good example for a comparative
signature, where one needs comparison data to elucidate
the results. For example, the quality control data from fuel
manufacturers could be utilised for this purpose.
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