Scale-4 Analysis of Pressurized Water Reactor Critical Configurations: Volume 3-Surry Unit 1 Cycle 2

Abstract

The requirements of ANSI/ANS 8.1 specify that calculational methods for away-from-reactor criticality safety analyses be validated against experimental measurements. If credit for the negative reactivity of the depleted (or spent) fuel isotopics is desired, it is necessary to benchmark computational methods against spent fuel critical configurations. This report summarizes a portion of the ongoing effort to benchmark away-from-reactor criticality analysis methods using selected critical configurations from commercial pressurized-water reactors. The analysis methodology selected for all the calculations in this report is based on the codes and data provided in the SCALE-4 code system. The isotopic densities for the spent fuel assemblies in the critical configurations were calculated using the SAS2H analytical sequence of the SCALE-4 system. The sources of data and the procedures for deriving SAS2H input parameters are described in detail. The SNIKR code module was used to extract the necessary isotopic densities from the SAS2H results and to provide the data in the format required by the SCALE criticality analysis modules. The CSASN analytical sequence in SCALE-4 was used to perform resonance processing of the cross sections. The KENO V.a module of SCALE-4 was used to calculate the effective multiplication factor (k eff) of each case. The SCALE-4 27-group burnup library containing ENDF/B-IV (actinides) and ENDF/B-V (fission products) data was used for all the calculations. This volume of the report documents the SCALE system analysis of two reactor critical configurations for Surry Unit 1 Cycle 2. This unit and cycle were chosen for a previous analysis using a different methodology because detailed isotopics from multidimensional reactor calculations were available from the Virginia Power Company. These data permitted a direct comparison of criticality calculations using the utility-calculated isotopics with those using the isotopics generated by the SCALE-4 SAS2H sequence. These reactor critical benchmarks have been reanalyzed using the methodology described above. The two benchmark critical calculations were the beginning-of-cycle (BOC) startup at hot, zero-power (HZP) and an end-of-cycle (EOC) critical at hot, full-power (HFP) critical conditions. These calculations were used to check for consistency in the calculated results for different burnup, downtime, temperature, xenon, and boron conditions. The k eff results were 1.0014 and 1.0113, respectively, with a standard deviation of 0.0005. viii

Scale-4 Analysis of Pressurized Water Reactor Critical Configurations: Volume 4-Three Mile Island Unit 1 Cycle 5

“…1 and 2 of this report, 8,26 similar reactor criticality calculations have been performed for Surry Unit 1 and Sequoyah Unit 2 PWRs. Even though each reactor model is unique in its own respect, several significant differences between TMI-1 BOC-5 and the two other criticals should be considered when comparing results.…”

“…Even though effective cross sections are felt to be insensitive to minor variations in burnup, it is necessary to set a maximum range of burnup for which an average burnup is an acceptable approximation in determining cross sections. As discussed earlier, a range of no more than 2 GWd/MTU has been found to be acceptable; 25,26 this value was used in subdividing assembly groups into similar-burnup subgroups. As shown in Fig.…”

“…No attempt was made to account for burnup asymmetries within an assembly, because this information was not readily available and would have little, if any, detectable effect on the computed solution. 8,26 Thus, only a single fuel pin description is necessary to describe all fuel pins in a given assembly. In addition, in this model axial power distributions are ignored, and assemblies are represented by a model that assumes a constant (average) power distribution along the active length of the assembly.…”

Sensitivity and Parametric Evaluations of Significant Aspects of Burnup Credit for PWR Spent Fuel Packages

“…However, collection and use of composition-specific cross sections for all 23 fuel isotopes for a KENO V.a model with multiple burnup zones has the potential to be an onerous and very time-consuming task when a large number of axial zones is desired. Experience has shown 29,30 that because fission-product nuclides represent only a small fraction of the total number density of the fuel isotopes, fission-product cross sections are relatively insensitive to changes in isotopic content, and therefore resonance-corrected cross sections calculated for any single burnup zone in a multizone fuel pin model are appropriate. This situation is also true of many fuel activation products and minor actinides; however, cross sections for seven actinides are known to have a more significant burnup dependence.…”

“…The effect of this approximation has been found to be small (<0.1% ∆k/k). 29,30 Composition-specific cross-section processing can be performed using the SCALE CSASN sequence. SAS2H calculations are performed to obtain burnup-dependent isotopics for each burnup zone in a multizone model.…”