In-vessel source term calculation using chemical equilibrium approach for a medium sized sodium cooled fast reactor

Abstract

Quantifying the severe accident in-vessel source term for a fast reactor is a computationally challenging task owing to the complex phenomena and wide range of accident paths to be studied. A chemical equilibrium approach offers a relatively accident path independent way to quantify the source term. Here we follow a thermochemical equilibrium approach to the determination of in-vessel source term for a medium-size oxide-fuel sodium-cooled fast reactor. Assuming a uniformly mixed molten corium at a given temperature and volume, the quantity of radionuclides released to the cover gas volume in gaseous form is determined. The development of a modular code in Python for the calculation of equilibrium species in different phases from specified initial inventory at given thermodynamic conditions and its validation are discussed. From the calculations, it is observed that strontium, europium and other lanthanides form stable oxide species in the sodium as opposed to the elemental form in metal fuel reactors. As is the case with noble gases, almost complete inventory of alkali metals and antimony could be released to the cover gas in elemental form (Cs, Sb). The differences in release fractions between metal fuelled and oxide fuelled reactors are brought out in the study.