Working Group on Fuel Safety Margins (WGFSM)

This working group's objective was to advance the current understanding and to address safety issues related to fuel safety margins, in particular to systematically assess the technical basis for current safety criteria and their applicability to high burn-up and to the new fuel designs and materials being introduced in nuclear power plants. The needs and priorities for future research programmes in the area of fuel safety behaviour were also examined, with the aim to understand and adequately model key phenomena and to quantify safety margins. This group was originally called the Special Expert Group on Fuel Safety Management (SEGFSM), later evolving into the WGFSM before becoming the Working Group on Fuel Safety (WGFS).

Publications and reports
WGFSM work

The seventh Plenary Meeting of the Special Expert Group on Fuel Safety Margins (SEGFSM, now WGFS) was held on 26 June 2006, at OECD headquarters.

As a follow up to the joint CSNI-CNRA session in December 2004, a discussion on high burn-up fuel issues took place during the first part of the SEGFSM plenary meeting. The objective was to update and review the database gathered through testing in RIA and LOCA conditions and to assess the status of development of new criteria taking into account the effect of burn-up. 

The current state of code predictions for LOCA and RIA safety analysis at increased burn-up.

  • JAEA (Japan) experimental basis on RIA and LOCA;
  • USNRC-sponsored fuel research on LOCA and RIA;
  • evaluation of mechanical strength and ductility of high burn-up nuclear fuel cladding in KAERI (Korea);
  • influence of Hydrogen uptake on the ductile-brittle transition of Zr1Nb cladding (KFKI AEKI, Hungary);
  • new RIA safety criteria in Switzerland (HSK, Switzerland);
  • an overview of existing high burn-up databases and criteria for RIA and LOCA (NEA Secretariat).

As a follow up to the CSNI discussion on high burn-up issues the NEA Secretariat prepared a document that set out a Review of High Burn-up RIA and LOCA Database and Criteria. This document presents an up-to-date summary of existing databases from various laboratories in a consistent manner, in order to facilitate the comparison of data from different sources.

The plenary meeting then reviewed the status of ongoing activities and discussed future activities, in particular:

Review of the adequacy of existing codes for the simulation of high burn-up fuel behaviour in accident conditions

The SEGFSM Task Force (TF) on the review of the adequacy of existing codes for the simulation of high burn-up fuel behaviour in accident conditions initiated at the end of 2004 was a benchmark for the Halden-irradiated LOCA test (Test No. 650.3). In total, seven blind calculations were presented during the SEGFSM plenary meeting. Thes were from the following organisations: VTT Finland, IRSN France. GRS Germany (2x), TUV NORD EnSys Hannover (Germany), JNES (Japan) and PSI (Switzerland). A special workshop had been proposed during the SEGFSM plenary meeting with the aim of comparing pre-test and post-test calculations.

The workshop was organised on 13-14 September 2005 in Halden, Norway. There was a big spread of results from both pre-test and post-test calculations. This was partly due to the complexity of the test modelling and partly due to non-unified initial and boundary conditions. The benchmark participants recommended to repeat the post-test calculations with unified boundary conditions based on the ATHLET thermal-hydraulic model of the test facility. Such calculations would enable comparison of "fuel models" in the codes and help in better understanding the different results. The CSNI endorsed continuation of the second phase of the benchmark at its December 2005 meeting.

Revision of the 1986 CSNI state-of-the-art report (SOAR) No. 129 on "PWR Fuel Behaviour in Design Basis Accident Conditions"

The group agreed that the work will focus first on LOCA and RIA. A revised table of contents was proposed and work on the report continues. It is expected that the report will be completed during 2007.

Recommendations on methodologies for experimental work to revise or produce new LOCA fuel safety criteria

Progress has been made by the SEGFSM Task Force on producing new recommendations on the experimental methodologies used to revise or produce new LOCA fuel safety criteria. The TF met for the third time on 13-14 April 2005 in Prague, Czechia. The TF discussed in detail current LOCA test methodologies, in particular ring compression tests and thermal shock (quench) tests, including the oxidation process and the ways in which test results are presented. It reviewed and updated the tables containing the principal test characteristics as performed in different laboratories, along with "recommended" ones. The tables were distributed to the SGFSM members with a request to provide final comments. The SEGFSM agreed to close the task and publish the tables as a SEGFSM Technical Note.