Minor Actinides Loading Optimization for Proliferation Resistant Fuel Design - BWR Page: 3 of 7
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Proceedings of Global 2009
Paris, France, September 6-11, 2009
Paper 9082from civilian nuclear power applications that could be used
in the production of nuclear weapons. Based on critical
mass considerations, the 235U enrichment limit for
proliferation resistance is 20 wt%. However, unlike
uranium, any isotopic mix of plutonium has a finite critical
mass, i.e., a potential explosive material. Hence, there is no
general isotopic concentration threshold for plutonium
isotopes from a critical mass point of view. Nevertheless,
the suitability for weapons usage varies significantly for
the different plutonium isotopic compositions. Ref. 2 lists
the important proliferation resistance characteristics of
plutonium isotopes and summarized in Table 1 of Ref. 1.
238Pu 240Pu, and 242Pu have high spontaneous neutron
generation, which reduces the bomb yield significantly. In
addition, 238Pu has high decay heat, which further
complicates the design of explosive devices. Because 238Pu
is highly proliferation resistant, G. Kessler3 pointed out that
for 238Pu/Pu ratios above 6%, its proliferation resistance
can be considered as effective as 235U < 20% or 233U <
12%.
Burning MA of 237Np and/or 241Am mixed in the LWR
high burnup fuel enhances the transmutation of which
decay to 238Pu. To protect plutonium even in the low
burnup fuel, per Ref. 1, the author loaded Np rather
heavily (0.5 wt%), such that the mixing MA provided a
high fraction of 238Pu at very low burnup while providing
adequate proliferation resistance. The disadvantage of the
heavy loading of Np/Am is that more plutonium is
generated in the discharged fuel. The Pu/238U ratio at the
discharged burnup (51 GWd/t) for U02 only and U02
mixed with 237Np 0.5 wt% are 0.97 and 1.54,1 respectively,
which indicates the undesired plutonium generation is
much higher in the mixed 237Np and U02 fuel. In this
study, we mix only 0.125 wt% 237Np and/or 241Am in the
MA Loading Optimization (MALO) approach to achieve
the goal of proliferation resistance as well as minimize the
plutonium production in the spent fuel.
For future advanced nuclear systems, the MAs are
considered more as a resource to be recycled, or
transmuted to less hazardous and possibly more useful
forms, rather than simply as a waste stream to be disposed
of in an expensive repository facility. As a result, MAs
will play a much larger role in the design of advanced
systems and fuel cycles, not only as additional sources of
useful energy, but also as direct contributors to the
safeguard of the generated Pu in the spent fuel. 237Np and
241Am can be transmuted and decayed to the highly
proliferation resistant isotope 238.
In the following study, a typical Boiling Water Reactor
(BWR) fuel unit lattice cell model with U02 fuel pins will
be used to investigate the effectiveness of MALO approachfor enhancing proliferation resistance for future advanced
nuclear energy systems.
III. BWR UNIT LATTICE CELL MODEL AND MALO
APPROACH
A typical BWR (10x10) unit lattice cell, as shown in Fig. 1
of Ref. 1, has been chosen as the basis for the fuel
neutronics analysis of U02, NpO2, and AmO2 with 95% of
theoretical density. The fuel rods have a radius of
0.409 cm and are clad with 0.063 cm of Zr. The fuel pins
are arranged in a square fuel lattice. The detailed lattice
cell parameters are tabulated in Table II of Ref. 1. The
unique feature of a BWR is that the moderator water
density decreases from bottom to top of the core. We
divided the water coolant channel and fuel pin into 24
axial nodes as shown in Fig. 2 of Ref. 1. The validation of
the relative fission power local to average ratio (L2AR)
profiles along the 24 fuel nodes at the beginning of life
(BOL) and at the end of life (EOL) for a discharged
burnup of 50 GWd/t are discussed in Ref. 1.
Increasing the fuel discharge burnup can improve the
proliferation resistance and reduce the spent fuel storage
volume. In this work, U02 with 235U enrichment of 4.95
wt% was used. For the high burnup fuel with 235U
enrichments of 4.95 wt%, three mixed oxide (MOX) MA
cases for U02+NpO2, U02+AmO2, and U02+NpO2+AmO2
were established. The 235U enrichment, NpO2, and AmO2
composition of the 4 proposed study cases are summarized
in Table I. Case-4 represents the MA optimized loaded
fuel, which minimizes plutonium production in the
discharged high burnup fuel, while meeting the
proliferation resistance goal of 238Pu/Pu ratio greater than
6% and maintaining a lower, yet still quite effective MA
reduction rate.
TABLE I
U02 - 235U enrichment, NpO2, and AmO2 composition of
the 4 study cases.
U02 - 235U NpO 2 AmO2
ID enrichment (wt%) (wt %) (wt%)
Case-1 4.95 -- --
Case-2 4.95 0.125 --
Case-3 4.95 -- 0.125
Case-4 4.95 .025 0.105
IV. MONTE CARLO BURNUP METHOD - MCWO
The physics analyses were performed using the
computer code MCWO4 which couples MCNP5
calculations with the ORIGEN2.26'7 fuel burnup
calculations. MCWO has been verified at the Idaho
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Chang, G. S. & Zhang, Hongbin. Minor Actinides Loading Optimization for Proliferation Resistant Fuel Design - BWR, article, September 1, 2009; [Idaho]. (https://digital.library.unt.edu/ark:/67531/metadc933269/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.