Actinide-Only Burnup Credit for Pressurized Water Reactor Spent Nuclear Fuel–III: Bounding Treatment of Spatial Burnup Distributions

Review and Prioritization of Technical Issues Related to Burnup Credit for LWR Fuel

“…Thus, to be conservative, one must identify and assume an axial-burnup profile that is realistic but is limiting in terms of the value of k eff associated with the axially varying SNF nuclide compositions. To date, attempts to bound PWR profiles 21,28,29 have been based on the selection of limiting profiles from a set of calculated burnup-dependent profiles obtained from reactor operational data. 35,37 No attempt has been made to define a bounding profile for BWR fuel assemblies due to the lack of a similar database of burnup profiles.…”

Review of Axial Burnup Distribtion Considerations for Burnup Credit Calculations

“…[3][4][5][6][7][8][9][10][14][15][16] The transition between conservatism and nonconservatism depends on several factors, including the initial enrichment of the fuel, the cooling time considered, and the nuclides that are included in the criticality model, and appears to be strongly affected by fission product inventories. 9,15 When assuming an axially uniform distribution of isotopics, the most reactive region of a fuel assembly is at the axial mid-plane, because leakage increases as one moves away from the center.…”

“…8,15 The buildup of stable actinide and fission product absorbers, due to the decay of actinides and fission products, tends to reduce reactivity. However, because the concentration of actinides and fission products is much greater in the fuel mid-region than in the fuel ends, this tends to increase the relative reactivity of the fuel ends compared with the fuel mid-region and results in an increase in the end effect.…”

“…In ref. 8, analyses are presented for various axial reflector assumptions, including pure water, 50/50 water and stainless steel combination, and a few specific cask designs. The behavior and magnitude of the end effect is shown to be dependent upon the axial materials and the cask size; maximums are observed for pure-water reflection and minimum cask size (i.e., a single assembly).…”

Assessment of Reactivity Margins and Loading Curves for PWR Burnup Credit Cask Designs

“…8, 38) in this area have employed a relatively straightforward approachperform criticality calculations for each burnup shape to determine the profile that produces the greatest end effect. 39 to determine bounding profiles over coarser burnup ranges. The bounding profiles for three burnup ranges were determined (based on the 12 bounding profiles listed in Table 5) and suggested for use with a proposed actinide-only burnup-credit methodology.…”