Analysis of Applicability of RELAP5 Mathematical Model for Cold Shutdown Conditions

Eksperiment ISP38 proveden u integralnom eksperimentalnom postrojenju BETHSY proračunat je upotrebom termohidrauličkog računalnog programa RELAP5/MOD3.2.2gamma. Eksperiment simulira gubitak sustava za odvođenje ostatne topline kad se elektrana nalazi u stanju hladne obustave, a otvoreni su otvori za održavanje tlačnika i parogeneratora. Proračun je napravljen s dva modela (nodalizacije) od kojih je prvi razvijen za stariju verziju RELAP5/MOD2 i prerađen za novu verziju, a drugi, znatno detaljniji, je razvijen za verziju RELAP5/MOD3.2.2. Provedena je analiza dobivenih rezultata te je nakon toga dana kvalitativna i kvantitativna ocjena točnosti rezultata. Identificirani su ključni dijelovi modela koji imaju znatan utjecaj na rezultate te se ispitao utjecaj načina modeliranja tih dijelova na rezultate. Na kraju je ocijenjena primjenjivost modela na odgovarajuće analize sigurnosti NE Krško.The experiment ISP38 performed in the BETHSY integral test facility was calculated using the RELAP5/MOD3.2.2gamma code. The experiment simulates the loss of the residual heat removal system (RHR) with the plant in cold shutdown conditions and with the pressurizer and steam generator manways open. Two calculations were performed, the first using an older nodalization developed for the RELAP/MOD2 version of the code, which was modified for use with the newer version, the second using a much more detailed nodalization developed for the RELAP5/MOD3.2.2 version. An analysis of the results was performed, after which it was possible to give a qualitative and quantitative accuracy evaluation of the results. Several parts of the facility were identified as important in determining the results, and several sensitivity study runs were made to evaluate the impact of changing the modeling approach of the selected parts. At the end, the applicability of the model for performing safety analysis calculations for NPP Krško was evaluated