The Effect of U-234 Content on the Neutronic Behavior of Uranium Systems
- Univ. of New Mexico, Albuquerque, NM (United States)
- Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
When analyzing uranium systems, the usual rule of thumb is to ignore the U-234 by assuming that it behaves neutronically like U-238. Thus for uranium systems, the uranium is evaluated as U-235 with everything else being U-238. The absorption cross section of U-234 is indeed qualitatively very similar to that of U-238. However, thermal absorption cross section of U-234 is about 100 times that of U-238. At low U-235 enrichments, the amount of U-234 is quite small so the impact of assuming it is U-238 is minimal. However, at high enrichments, the relative ratio of U-234 to U-238 is quite large (maybe as much as 1 to 5). Thus, one would expect that some effect of using the rule of thumb might be seen in higher enriched systems. Analyses were performed on three uranium systems from the set of Benchmarks. Although the benchmarks are adequately characterized as to the U-234 content, often, materials used in processing are not as well characterized. This issue may become more important with the advent of laser enrichment processes, which have little or no effect on the U-234 content. Analytical results based on the relationship of U-234 activity to that of U-235 have shown good predictive capability but with large variability in the uncertainties. Rucker and Johnson noted that the actual isotopics vary with enrichment, design of the enrichment cascade, composition of the feed material, and on blending of enrichments so there is considerable uncertainty in the use of models to determine isotopics. Thus, it is important for criticality personnel to understand the effects of variation of U-234 content in fissile systems and the impact of different modeling assumptions in handling the U-234. Analyses were done on LEU, IEU and HEU benchmarks from the International Handbook. These indicate that the effect of ignoring U-234 in HEU metal systems is non-conservative while it seems to be conservative for HEU solution systems. The magnitude of change in k-effective was as high as 0.4%, which has implications on selection of administrative margins and the determination of the upper subcriticality limit.
- Research Organization:
- Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Univ. of New Mexico, Albuquerque, NM (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
- DOE Contract Number:
- AC05-00OR22725
- OSTI ID:
- 1025849
- Resource Relation:
- Conference: 9. International Conference on Nuclear Criticality Safety (ICNC 2011), Edinburgh (United Kingdom), 19-22 Sep 2011
- Country of Publication:
- United States
- Language:
- English
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73 NUCLEAR PHYSICS AND RADIATION PHYSICS
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ABSORPTION
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CROSS SECTIONS
DESIGN
LASERS
PERSONNEL
PROCESSING
SAFETY
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Nuclear Criticality Safety Program (NCSP)
Uranium Systems Analyzation
U-234
U-235
U-238
Benchmarks
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