Nuclear Energy Agency Online Bulletin

September 2000

Nuclear Safety Information

The first meeting of the CSNI Programme Review Group was held at the NEA on 3-4 July. The main points of the meeting are briefly summarised below:

Results of the work of the CSNI/PWG2 Task Force on Fuel Safety Criteria have recently been published in the CSNI report NEA/CSNI/R(99)25 entitled Fuel Safety Criteria Technical Review.

Most current safety criteria were established during the 1960s and early 1970s, and verified against experiments with fuel that were available at that time, mostly with unirradiated specimens. In order to optimise fuel cycle cost, the nuclear industry began work in the mid-1980s on new fuel and core designs with the aim of increasing fuel burnup, for example, for extending the cycle length or upgrading the power level. This again led to a number of basic design changes, e.g. new cladding materials. New demands on fuel and plant performance, however, have reduced the available margins. In addition, optimising fuel utilisation and core performance show a trend towards conditions where less operational and experimental experience exists.

The OECD/CSNI/PWG2 Task Force on Fuel Safety Criteria (TFFSC) was therefore given the mandate to technically review the existing fuel safety criteria, focusing on the "new" design elements (new fuel and core design, cladding materials, manufacturing processes, high burnup, MOX, etc.) introduced by the industry. It should also identify if additional efforts may be required (experimental, analytical) to ensure that the basis for fuel safety criteria is adequate to address the relevant safety issues.

Three separate meetings of TFFSC were held in June 1997, October 1997 and September 1998. Based on some individual members' contributions a draft of the report entitled "Fuel Safety Criteria Technical Review" was prepared for PWG2's consideration and submission to the CSNI for approval. The draft report was approved by the CSNI in December 1999. In the report, 20 different fuel-related criteria are discussed without attempting to categorise them according to event type or risk significance. For each of them, a brief description of the criterion as it is used in several applications along with the rationale for having such a criterion is presented. "New" design elements can affect fuel-related margins and, in some cases, the criteria themselves. Some of the more important effects are mentioned in order to indicate whether the criteria need to be re-evaluated. The discussion may not cover all possible effects, but should be sufficient to identify those criteria that need to be addressed. A summary of these discussions is given in Section 7 of the report.

As part of the assessment of the safety criteria, the Task Force looked at various issues, as they relate to one or more criteria, that have become of special interest. These topics included high burnup, core management, MOX, mixed cores, incomplete control rod insertion, and axial offset anomaly.

As a summary, the Task Force considered the current framework of fuel safety criteria generally applicable, being largely unaffected by the "new" or modern design elements; the levels (numbers) in the individual safety criteria may, however, change in accordance with the particular fuel and core design features. Some of these levels have already been - or are continuously being - adjusted; level adjustments of several other criteria (RIA, LOCA) also appear to be needed, on the basis of experimental data and the analysis thereof.

The report will also be available as an OECD/NEA publication in English and French versions.