VVER-1000/V320 decay heat analysis involving TVS-M and TVSA fuel assemblies

Abstract

MELCOR 1.8.4 is an integral computer code, developed for severe accident calculations. It is used primarily for the simulation of PWR and BWR types of reactors as there exists an internal database, suitable for modeling of their core inventory. Despite similarity between VVER-1000/V320 and PWR, accounting of specificities of Russian reactor designs is still required. Part of it is the simulation of core decay heat rate after the shutdown. MELCOR 1.8.4 distinguishes fifteen classes. Each of them contains chemical elements with similar properties. Twelve are involved in radioactive products decay. In current paper the authors present two boundary reactor core loadings, designed with corresponding fuel assemblies: TVS-M and TVSA. They have calculated decay heat after reactor shutdown from 100% and 104% of nominal power by SCALE 4.4a package. The amount of generated nuclides had also been estimated. Irradiation history had been accounted as proposed in Kolobashkin et al. (p. 141) [Kolobashkin, V.M., Rubtsov, P.M., Rujanskiy, P.A., Sidorenko, V.D., 1983. Radionuclide Inventory Estimation Handbook (on Russian). Energoatomizdat, Moscow, pp. 138–188]. Newly developed Core Inventory Estimation Tool (CIET), described in this paper, written and tested previously, has been used for the evaluation of core decay heat fractions, distributed over chemical classes. Twelve curves were generated by following the same numerical procedure implemented in MELCOR for representation of decay in W/kg. Comparison of chemical element decay rates to the defaults for PWR shows deviations from the expectations to maximal values of 37% in Uranium for TVSA fuel assemblies. The total number of radionuclides, separated in chemical classes, given in Gauntt et al. [Gauntt, R.O., Cole, R.K., Rodrigez, S.B., Sanders, R.L., Smith, R.C., Stuard, D.S., Summers, R.M., Young, M.F., 1997. MELCOR Computer Code Manuals. NUREG/CR-6119 Report, Vol. 1 and Vol. 2, SAND97-2398] was compared to the ones involved in default MELCOR decay heat calculation. The results have confirmed that neglected chemical elements give 0.9% and 1.1% of total core mass for TVS-M and TVSA and correspondingly 0.5% and 0.6% of total core decay heat. The authors estimated decay heat generated from reactor core, designed with TVS fuel assemblies according to Kolobashkin et al. (p. 141) [Kolobashkin, V.M., Rubtsov, P.M., Rujanskiy, P.A., Sidorenko, V.D., 1983. Radionuclide Inventory Estimation Handbook (on Russian). Energoatomizdat, Moscow, pp. 138–188] and juxtaposed it to decay heat calculated by SCALE 4.4a. Conclusions are drawn after the analysis of four computed core decay heat curves: TVSA on 100%, 104% of nominal power and TVS-M correspondingly on 100% and 104% of nominal power. The obtained data are suitable to be used as input in safety analyses. Comparisons were also made to decay rates obtained from default MELCOR computer code output.