Determination of the activity inventory and associated uncertainty quantification for the CROCUS zero power research reactor

Abstract

The paper describes the source term estimation of CROCUS, the zero power research reactor of EPFL, to be used for dispersion analysis under accidental conditions. To fulfil regulatory requirements, the source term of the CROCUS fuel is estimated through Monte Carlo simulations supplemented by uncertainty quantification, both obtained from the Monte Carlo code MCS developed at UNIST. Even though the depletion capabilities of MCS were pre-existing to this work, no verification has been documented so far. A comparison of MCS and SERPENT results for the determination of the activity inventory for the CROCUS fuel is presented; both codes agree within the 1% for the major isotopes contributing to both inhaled and ingested doses. The source term of CROCUS is calculated under a postulated accident. Eight isotopes, 90Sr, 91Y, 131I, 137Cs, 140Ba, 140La, 144Ce, and 239Pu produce the largest contributions to the effective dose to the public. For uncertainty quantification, nuclear data uncertainties, specifically cross sections and fission yields are considered. Three stochastic sampling methods are implemented in the MCS code namely, TMC, fast-TMC and fast-GRS methods. The performances of the two fast methods have been analyzed when applied to the CROCUS reactor. All three methods produce consistent uncertainty estimates and the two fast methods showed a reduced computational cost compared to the original TMC method. The fission yield uncertainty is the leading factor for the determination of uncertainty of the activity for 90Sr, 91Y, 131I, 137Cs, 140Ba, 140La and 144Ce isotopes. On the other hand, the cross section uncertainty is the leading factor for the uncertainty of 239Pu activity. Finally, the modeling of the irradiation history for CROCUS is simplified to reduce the computational cost. It is demonstrated that the activities for the short lived isotopes (131I and 144Ce) are very sensitive to the irradiation history specifications. Nonetheless, the irradiation history used for the determination of the nominal fuel inventory activity is conservative as it overestimates the activity of the short lived isotopes.