Abstract
Until recently, criticality safety assessment codes had a minimum temperature at which calculations can be performed. Where criticality assessment has been required for lower temperatures, indirect methods, including reasoned argument or extrapolation, have been required to assess reactivity changes associated with these temperatures. The ANSWERS Software Service MONK® version 10B Monte Carlo criticality code, is capable of performing criticality calculations at any temperature, within the temperature limits of the underlying nuclear data in the BINGO continuous energy library. The temperature range of the nuclear data has been extended below the traditional lower limit of 293.6 K to 193 K in a prototype BINGO library, primarily based on JEFF-3.1.2 data. The temperature range of the thermal bound scattering data of the key moderator materials was extended by reprocessing the NJOY LEAPR inputs used to produce bound data for JEFF-3.1.2 and ENDF/B-VIII.0. To give confidence in the low temperature nuclear data, a series of MONK and MCBEND calculations have been performed and results compared against external data sources. MCBEND is a Monte Carlo code for shielding and dosimetry and shares commonalities to its sister code MONK including the BINGO nuclear data library. Good agreement has been achieved between calculated and experimental cross sections for ice, k-effective results for low temperature criticality benchmarks and calculated and experimentally determined eigenvalues for thermal neutron diffusion in ice. To quantify the differences between ice and water bound scattering data a number of MONK criticality calculations were performed for nuclear fuel transport flask configurations. The results obtained demonstrate good agreement with extrapolation methods. There is a discernible difference in the use of ice and water data.
Highlights
Criticality safety assessment codes had a minimum temperature at which calculations could be performed
Assessments for lower temperatures used indirect methods, such as reasoned argument or extrapolation, to assess reactivity changes associated with these temperatures
The temperature range of the BINGO nuclear data library was extended via modification of the existing BINGO library generation route, which is described in further detail in [2]
Summary
Criticality safety assessment codes had a minimum temperature at which calculations could be performed. Driven by the International Atomic Energy Agency’s transport regulation requirements to demonstrate criticality safety of a nuclear material transport package down to -40◦C, direct methods of criticality assessment have been developed. The ANSWERS Monte Carlo code MONK [1] uses the BINGO continuous energy nuclear data libarary and associated collision processor. Since MONK version 10B, the temperature of calculations is only limited by the temperatures tabulated on the BINGO library. With BINGO libraries, MONK includes a temperature interpolation capability for free gas nuclides based on runtime Doppler broadening. MONK version 11A (in development) includes stochastic interpolation of the S(α, β) data thereby providing full flexibility in the temperature dependence
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