Monte Carlo study of pebble bed reactor fuel reactivity and isotopics

Abstract

This thesis sets the groundwork for a numerical benchmark of the VSOP code suite, which has the potential of being an integral part of the design and licensing of future modular high temperature gas reactor designs. The goal of this thesis was to develop models, techniques, and data to be used in a future verification of the VSOP code. Pebble bed critical experiments carried out on the Swiss PROTEUS facility were chosen as a starting point for a pebble bed benchmark using the Monteburns code. Monteburns, a linking code that uses the ORIGEN2 code to burn MCNP inputs, was used to evaluate the neutronics of the PROTEUS LEUPRO-1 unit cell. Models were made at various levels of simplification for the actual spherical geometry as well as the equivalent cylindrical geometry, to test their ability to reproduce results described in several published reports. These models were then used to calculate reactivity, spectral indices, and bumup isotopics for a cold 100 MWD/kgU burnup scenario. The results of these calculations showed that the methods employed in this thesis are in good agreement with work previously published regarding benchmarking of codes on LEUPRO-1: k-inf differences are generally less than 0.5%. The Monteburns burnup isotopics were compared to results from a CASMO-4 burn of the cylindrical geometry equivalent of the LEUPRO-1 unit cell with excellent agreement, even better than that typically achieved for LWR unit cell benchmarks. The results provide a reliable basis for proceeding with the benchmarking of the VSOP code's burnup capabilities at MIT and elsewhere.