Office for Nuclear Regulation

Radiological decoupling criteria and code validation for Design Basis Analysis (SCD4 and SCD6)

Executive summary

Purpose of Intervention

Post-accident radiological consequences analysis relies on the use of computational tools to provide quantitative understanding of off-site radiological consequences. This intervention reviewed NNB Generation Company (HPC) Limited’s (NNB GenCo) arrangements for providing radiological consequences analysis for design basis accidents in the Hinkley Point C (HPC) safety case. The key aims of this intervention were to assess the adequacy of the arrangements for validation of computer codes used for radiological consequences, and the arrangements for applying radiological decoupling criteria in Design Basis scenarios.

This intervention was carried out at a time when revised site-specific calculations are being produced in this topic area for inclusion in the PCmSR.

Interventions Carried Out by ONR

This intervention was performed in two parts, and assessed the following arrangements against License Condition (LC) 14:

Intervention 1: the implementation of “radiological decoupling criteria” within NNB’s arrangements for producing the radiological consequences analysis for design basis faults

Intervention 2: NNB’s processes for validating computer codes used for radiological consequences analysis in the Design Basis Analysis (DBA)

LC14 requires licensees to make and implement adequate arrangements for the production and assessment of safety cases consisting of documentation to justify safety during the design, construction, manufacture, commissioning, operation, and decommissioning phases of the installation.

The following guidance was used in this intervention:

Explanation of Judgement if Safety System Not Judged to be Adequate

Not relevant to this intervention.

Key Findings, Inspector's Opinions and Reasons for Judgements Made

The interventions highlighted that NNB GenCo is developing its Intelligent Customer role in the radiological consequences topic area, with several examples of good practice. Nevertheless, they identified a number of minor areas for improvement.

Intervention 1: application of radiological decoupling criteria

With regard to use of radiological decoupling criteria, the bulk of faults listed in the Fault and Protection Schedule (F&PS) appear to be bounded by a fault in the list of bounding faults. The process for identifying bounding faults appears to be an effective means of assigning bounding faults. Nonetheless, the intervention noted the following:

Intervention 2: validation of computer codes

With regard to NNB GenCo’s processes for validating computer codes, the intervention found that NNB GenCo is expected to provide technical leadership on methods and computational tools to ensure consideration of the UK context. At present NNB GenCo’s code validation procedures and requirements rely heavily on the processes of its suppliers.

In order to improve its control and visibility of code validation, NNB GenCo should give consideration to strengthening its acceptance process for code validation.

The intervention noted the recent development of guidance on code validation for the main supplier in France, Direction Technical, and that there is now a legal requirement to provide validation for codes used in relation to the first barrier. It is unclear, however, what material impact this will have in the radiological consequences area.

Conclusion of Intervention

While a number of findings have arisen from this intervention, these are not significant and can be addressed through the regular radiological consequences technical interactions. It is therefore considered overall that inspection ratings of Green are appropriate for LC14.