Reactor Physics Characterization of Transmutation Targeting Options in a Sodium Fast Reactor Page: 2 of 13
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Joint International Topical Meeting on Mathematics & Computation and Supercomputing in Nuclear Applications (M&C + SNA 2007)
Monterey, California, April 15-19, 2007, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2007)
Reactor Physics Characterization of
Transmutation Targeting Options in a Sodium Fast Reactor
Samuel E. Bays
Idaho National Laboratory
P.O. Box 1625, MS 3117, Idaho Falls, ID 83403
In sodium fast reactor designs, the fuel related inherent negative reactivity feedback is
accomplished mainly through parasitic capture in U-238. However for an efficient minor actinide
burning system, it is desirable to reduce or eliminate U-238 entirely to suppress further transuranic
actinide generation. Consequently, reactivity feedback is accomplished by enhancing axial
neutron streaming during a loss of coolant void situation. This is done by flattening "pancake" the
active core geometry. Flattening the reactor also increases axial leakage which removes neutrons
that could otherwise be used to destroy minor actinides. Therefore, it is important to tailor the
neutron spectrum in the core for optimized feedback and minor actinide destruction
simultaneously by using minor actinide and fission product targets.
The radiotoxicity and heat load attributed to long lived fission product and minor actinide Spent
Nuclear Fuel (SNF) isotopes are the limiting factors for repository capacity. Repository space is
maximized when these isotopes are removed from the nuclear fuel cycle's waste stream through
a process called transmutation. Long lived fission product isotope transmutation can be
accomplished by adding neutrons to the long lived atomic nucleus until a less stable nucleus is
created with a disproportionate neutron to proton ratio, which translates into a shorter half-life.
Similarly, minor actinides (MA) may be transmuted to shorter half-life isotopes using "thermal
spectrum" neutrons. Thermal spectrum reactors have a high MA neutron capture rate. Even so,
neutron capture efficiently removes the primary SNF isotopes Np-237 and Am-241. However,
the transmuted product actinides also have long half-lives. Thermal transmutation concentrates
higher mass actinides. Many of these isotopes (Cm-245 and heavier), produce high gamma and
neutron radiation fields when concentrated, presenting a fuel handling issue. The higher mass
actinide accumulation is less with "fast spectrum" neutrons because the relative fission-to-
absorption cross section ratios are higher than in the thermal spectrum. This suggests Sodium
Fast Reactors (SFR) as the means for MA destruction.
The two most basic sodium reactor control mechanisms inherent to the fuel are the Doppler
feedback provided by fertile uranium and the increase in axial neutron streaming that occurs
during coolant voiding. Generally for SFR with high MA loaded fuels, there is a tradeoff
between the optimal Doppler and void coefficients and the attainable actinide destruction
efficiency. This tradeoff stems from the fact that the mechanisms commonly used to remove
neutrons from the reactor during transients also remove neutrons in steady state operation. For
example, enhancing axial streaming through core geometry alterations can make the void
coefficient more negative but the increased overall leakage removes neutrons that could have
lead to MA fission. Alternatively, using fertile U-238 to make the Doppler coefficient more
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Bays, Samuel E. Reactor Physics Characterization of Transmutation Targeting Options in a Sodium Fast Reactor, article, April 1, 2007; [Idaho Falls, Idaho]. (digital.library.unt.edu/ark:/67531/metadc890309/m1/2/: accessed January 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.