Assessment of uncertainty identification for RELAP5/MOD2 and TRAC-BD1/MOD1 1 codes under core uncovery and reflooding conditions

Assessment calculations for the thermal hydraulic transient computer codes RELAP5/MOD2 (frozen version 36.02) and TRAC-BD1/MOD1 (frozen version 22) were performed, at Swiss Federal Institute for Reactor Research (EIR), under both core uncovery (boil-off) and reflooding conditions. The aim of the work being to assess the predicting capabilities of the frozen versions of the best estimate computer codes. Some of the reflooding and boil-off experimental data obtained from NEPTUN test facility at EIR are used for the assessment work. Model optimization calculations on nodalization and effect of available options (e.g. heat slab sizes) are performed with a selected base case and the same model is applied to the rest of the other experimental cases covering wide range of parameters. In this paper, the authors report the results of these assessment calculations and identify and point out the existing uncertainty areas in boil-off and reflooding phenomena. On the basis of the further calculations by changing some of the models such as bubbly/slug flow interfacial friction and film boiling heat transfer correlations in the frozen versions of the codes, improvements which eliminate some of the problems will be recommended