Calculation of decay heat rate in VVER-1000 spent fuel pool including uncertainties

Abstract

This paper presents a decay heat rate analysis of the VVER-1000 (unit 6 in “Kozloduy NPP” Plc.) spent fuel pool (SFP) using the method outlined by the United States (US) Nuclear Regulatory Commission (NRC) Regulatory Guide (RG) 3.54 (Revision 2, 2018), titled “Spent Fuel Heat Generation in an Independent Spent Fuel Storage Installation.” The usage of RG 3.54 for SFP calculations is extremely attractive because it enables fast determination of the decay status of a large number of stored fuel assemblies (FAs) in an SFP and further evaluation of their uncertainties.Two batches of fuel FAs in an SFP were analyzed in the study: The first batch consisted of 402 FAs that were stored for 321 days of reactor power operation from 2014 to 2015. The second batch consisted of 355 FAs that were stored for 326 days of reactor power operation from 2018 to 2019. The total decay heat rate and the corresponding decay heat rates of the FA that was placed in each of three VVER-1000 SFP sections were evaluated. Additionally, the propagation of the input uncertainties introduced by the initial fuel mass, burnup, and operation time on full power effective days was estimated.The applied method was derived from the uncertainty method, which was previously introduced by Gesellschaft für Anlagen-und Reaktorsicherheit (GRS), for free from deliberate pessimism safety analyses (named best estimate). The variation in the selected input parameters produced decay heat generation uncertainty as follows: ±73.2 kW (2014)/±59.1 kW (2018) at the beginning of each batch storage, and correspondingly, ±12.0 kW (2015)/±10.7 kW (2019) at the end of each batch storage. The results indicate that the method is sufficiently sensitive to the initial mass variation and irradiation uncertainties.The degree of conservatism (via a parameter referred to as a ‘safety factor’), introduced in RG 3.54 to address the numerical and nuclear data uncertainties in pressurized water reactor (PWR) decay heat calculations, was calculated for the stored VVER-1000 FA. An original approach for calculation of the scaling coefficient kcorr is suggested in the paper for estimation of the level of conservatism using the safety factor in RG 3.54 and other sources. The obtained results are reported.The recommendations by RG 3.54 were implemented in an input script for the MAXIMA Computer Algebra System (CAS). The MAXIMA CAS calculations of the VVER-1000 SFP decay heat rate were verified with the corresponding Oak Ridge Isotope Generation (ORIGEN) (SCALE 6.1) data at the beginning and end of the analyzed time periods. The authors determined that the obtained values of kcorr = 0.91–0.93 are useful in the re-evaluation of the RG 3.54 degree of conservatism. Furthermore, the scaled safety factor decay heat generation output was replaced with the calculated upper uncertainty bound. The upper boundary values in the decay heat rate uncertainty range showed flexibility and reduced conservatism in comparison with the upper 10% of the conservative bound of the nominal values.