Does it really make sense to use fission gas plenums in a lead-bismuth-cooled fast reactor?

Abstract

In this paper the utility of using fission gas plenums in a lead-bismuth-cooled fast reactor (LBR) is called into question. It is shown that the primary coolant radiological activity levels due to the generation of 210Po from the neutronic activation of bismuth clearly overshadow the total radiological impact from the fission gas products even if only a tiny fraction of available unbound elementary polonium is considered, and then it does not make any difference if fission gases are collected in a gas plenum or if they are vented directly into the coolant. Moreover, the use of fission gas plenums in a LBR is not only futile from a radiological viewpoint but can neutralize the passive safety mechanism of capture and retention of the majority of polonium captured as lead-polonide (PbPo): the outrush of highly pressurized fission gases during a cladding rupture can act as a spray-like mechanism, and then could vaporize and disperse the retained PbPo in the form of a superfine dust or aerosol. The elimination of fission gas plenums will also result in direct mechanical benefits such as a reduction in pumping power requirements, potentially a smaller reactor vessel and related vessel internal components, deep burnups, a reduction in cladding failures, as well as an important reduction in the amount of high-level waste generated. All these benefits will translate into better reactor management and both social and economic benefits.