The influence of Xe-135m on steady-state xenon worth in thermal molten salt reactors

Abstract There has been a resurgence of interest in fuel-in-salt Molten Salt Reactors (MSR); a number of governments and private companies are currently undertaking efforts to develop and commercialize MSR technology. Recent nuclear models used in the TENDL nuclear data library have estimated the cross section of the metastable state of 135 Xe, 135m Xe, to have a much larger cross section than the ground state of 135 Xe. Thermal MSRs with continual online noble gas stripping of the fuel salt can operate in a regime where 135m Xe makes up a notable fraction of the xenon worth, necessitating the implementation of these new cross-sections in the neutronic analysis of these advanced reactor types. To estimate the effect of 135m Xe on reactor operation, a simplified mathematical model was produced with one neutron energy group and 135m Xe cross section data from the TENDL-2015 nuclear data library. 235 U and 233 U systems were investigated. It was found that the steady-state xenon reactivity worth was considerably higher for some modes of operation when 135m Xe was included in the xenon worth calculations. Based on available literature, it was found that proposed MSR concepts may operate in the modes of operation where 135m Xe has a notable impact on steady-state xenon worth. This work highlights the need to include 135m Xe in MSR models and the importance of acquiring evaluated cross-sections for 135m Xe.