Office for Nuclear Regulation

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Inspection of Sellafield Ltd's arrangements for the use of the MONK code

Executive summary

Purpose of intervention

Modern criticality safety cases produced in the UK are often based on Monte Carlo computer calculations carried out using the MONK code. This intervention consisted of an inspection of Sellafield Ltd's arrangements for the use of the MONK code to ensure that correct results are obtained. Similar interventions are being carried out for at least five other nuclear site licensees who make extensive use of this code.

Sellafield Ltd intends to replace MONK with a new in-house developed code called METEOR in the near future. This new code is not covered by this intervention. However, given the significance of this change, it is currently intended to carry out an intervention specifically on METEOR.

Purpose of Intervention

The intervention was carried out at Sellafield Ltd's Albion Square premises, where their criticality safety specialists are based. There are two reasons for carrying out this cross-sites intervention. Firstly, such an intervention has not been carried out for some time, whereas the MONK code has moved on. Secondly, we have seen recent examples of questionable practice within the UK in this area.

Interventions Carried Out by ONR

This intervention was based on a standard question set. This was sent to the licensee before the intervention. The responses to the questions formed the basis of the intervention. During the inspection we sought further detail on the licensee's arrangements for managing the quality of their safety documentation and MONK calculations.

Explanation of Judgement if Safety System Not Judged to be Adequate


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

From our sample, Sellafield Ltd appears to have a robust system in place to enable them to carry out MONK calculations correctly. The competence of the staff is assured through a formal SQEP training process and practical experience. The computer calculations and associated criticality safety documentation are tightly controlled and subject to rigorous Quality Assurance procedures. Assurance is sought that contractors are SQEP for the tasks allocated to them.

In particular, the ONR inspectors were impressed by the:

Conclusion of Intervention

The findings of this intervention provide confidence in Sellafield Ltd's arrangements for managing their criticality safety documentation and carrying out MONK calculations. Based on the responses to our question set, our questions on the day, and our observation of their computer system, we assign an IIS rating of Green. This requires no formal action from the licensee.