OECD CSNI ISP 15: pre-test analysis of a split break transient in FIX-II loop performed at Pisa University by RELAP4/MOD6 code

The document deals with the description of results obtained by the Relap4 code in the pretest-blind 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 is the well-known computer code developed at Idaho National Laboratory in US: the code is in use at UNIPI since more than a decade. 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 pre-test calculation submitted (by UNIPI) to Studsvik before the execution of the test. This is called blind-pre-test analysis: the comparison of about 40 calculated time trends with measured data (once the experiment is performed: this is part of the ISP documentation issued as OECD report) 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).