Improved prompt and delayed photon heating computational methods for fast reactors

Abstract

In order to accurately calculate the photon heating distribution of fast reactor, the photon cross section processing code NGAMMA has been developed. NGAMMA can process MATXS format library and generate multi-group forms of photo-atomic cross sections, prompt photon production cross sections, delayed photon production cross sections, and neutron-photon KERMA factors for neutron-photon coupled heating calculations. An improved 21-groups photon library containing both prompt and delayed photon data is generated using the NGAMMA code based on the 1968n × 94γ MATXS format database. The photon library was verified using the fast reactor benchmark problem ZPR-6/7 and MET-1000. The results of the calculations show that the use of the 21-groups photon library generated by energy group collapse using 94-groups photon fluxes results in a significant improvement in the accuracy of the prompt photon heating calculations, especially in the non-fuel region, resulting in a relative error of less than 1% between the photon heating calculations and the MCNP calculations, compared to an error of 7% using the previous photon library. For the ZPR-6/7 benchmark problem, consideration of the delayed photons results in a 30% improvement in photon heating in the fuel region. For the MET1000 benchmark problem, the total power results of the neutron-photon coupling calculations based on the improved photon library are in good agreement with the MCNP, with a maximum relative error of no more than 3% in the total power of the fuel assembly. For reflector assemblies where photon power is a major contribution to heat release, the maximum relative error in prompt photon power is only −5.96%. Considering delayed photon significantly affects the photon heating, and for the MET1000 benchmark problem, considering delayed photon results in an overall increase of more than 30% in the photon power of the fuel assembly.