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 Springfields Fuels Ltd's (SFL) arrangements for the use of the MONK code to ensure that correct results are obtained. Similar interventions will be carried out at five other nuclear site licensees who make extensive use of this code.
The intervention was carried out at SFL's premises, near Preston, 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 code has developed, become more sophisticated and complicated to use. Secondly, we have seen recent examples of questionable practice within the UK in this area.
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.
From our sample, SFL 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 training and experience. The computer calculations and associated criticality safety documentation are tightly controlled and subject to rigorous "Quality Assurance" QA. Assurance is sought that contractors are "suitably qualified and experienced" SQEP for the tasks allocated to them.
The findings of this intervention provide confidence in SFL'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.