OECD CSNI ISP 15: post-test analysis of a split break transient in FIX-II loop performed at Pisa University by RELAP4/MOD6 and RELAP5/MOD1 codes", University of Pisa Report, DCMN - RL 067(83), Pisa (I), Dec. 1983, OECD CSNI Workshop on ISP 15, Nykoping (S), Dec. 15-16, 1983

The document deals with the description of results obtained by the Relap4 code in the post test (open) simulation of a LOCA experiment performed in the Boiling Water Reactor (BWR) experimental simulator FIX-II installed at the Nykoping Studsvik Research Center in Sweden. The Relap4 (already adopted for the blind-pretest analysis) and the Relap5 are the well-known computer code developed at Idaho National Laboratory in US: the codes are in use at UNIPI since more than a decade (case of Relap4) and since about 4 years case of Relap5). The FIX-II loop is an Integral Test Facility (ITF) simulating with full height, full pressure, full linear power a Swedish type BWR. The concerned test was selected as International Standard Problem 15 (ISP 15) by OECD/NEA/CSNI. The document describes the results of the post-test calculations (i.e. calculations performed after the availability of experimental data) and are submitted (by UNIPI) to Studsvik for the issue of the final comparison report.. This is called open post-test analysis: the comparison of about 40 calculated time trends with measured data is discussed. Comments are given in relation to the comparison between blind-pre-test analysis results and experimental data (now available): all of this is part of the ISP 15 documentation issued as OECD report. The activity allows an objective evaluation of the capabilities of the computer code and of the code user team in predicting the scenario of an accident. This is relevant for demonstrating the capabilities in evaluating safety margins of existing NPP, with main reference to BWR (in this case).