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

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 135Xe, 135mXe, to have a much larger cross section than the ground state of 135Xe. Thermal MSRs with continual online noble gas stripping of the fuel salt can operate in a regime where 135mXe 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 135mXe on reactor operation, a simplified mathematical model was produced with one neutron energy group and 135mXe cross section data from the TENDL-2015 nuclear data library. 235U and 233U systems were investigated. It was found that the steady-state xenon reactivity worth was considerably higher for some modes of operation when 135mXe 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 135mXe has a notable impact on steady-state xenon worth. This work highlights the need to include 135mXe in MSR models and the importance of acquiring evaluated cross-sections for 135mXe.