Office for Nuclear Regulation

Sizewell B – Inspection ID: 51965

Executive summary

  • Inspection date(s): 31/08/2022

Purpose of inspection

The aim of this inspection was to gain confidence in the use of the Risk Monitor (namely, RiskWatcher) for Risk Informed Decision Making at Sizewell B. This was a planned intervention and part of the 2022/2023 ONR integrated intervention strategy for operating reactors.

Subject(s) of Inspection

The following actives were the subject of this inspection:

  • LC 23 – Operating rules (RAG rating: Green)
  • LC 24 – Operating instructions (RAG rating: Green)

Key findings


A risk monitor uses the station Probabilistic Safety Analysis (PSA) risk model to provide real-time risk information on current and planned plant states. This information allows for risk informed operational and work planning decisions to be made. NGL currently use risk monitors at HYB, TOR and SZB.

In accordance with the Regulatory Strategy for HYB/TOR Risk Monitor Capability to End of Generation (CM9 2022/21893), ONR is undertaking Risk Monitor inspections at HYB and TOR to gain confidence in the use of their Risk Monitor (ESOP) for risk informed decision making at these stations. Although SZB uses a different risk monitor software (namely RiskWatcher), it was considered a good opportunity to introduce efficiencies by focusing on a single topic across multiple stations, and to ensure that learning is being promulgated within NGL.


NGL demonstrated how RiskWatcher (RW) is used for managing emerging plant unavailability, maintenance planning and outage planning. I sampled a number of RW entries, operations and maintenance records as well as had a number of demonstrations of RW in use. From the software demonstrations, discussions with users and sampling of records I found that:

  1. Adequate arrangements are available to integrate RW into station operations. RW is not required for Technical Specification compliance at SZB, however the Core Damage Frequency (CDF) risk indicator it provides is noted in the Shift Manager’s Log and taken into consideration in daily planning, work planning and outage planning. As such, there is an expectation on station personnel to utilise RW for station operations. I found that the RW users at SZB (MCR operators, WEC Engineers and NSG) are experienced and confident in using RW.
  2. Across all user groups of RW (operations, work planning, NSG) there is a focused effort to avoid entry into the higher risk bands in RW through appropriate work planning, and also to minimise time spent in higher risk bands, if entered due to emergent plant unavailability.
  3. RW risk calculation is only occasionally used to extend Action Completion Times (ACTs) at SZB. This is because the Technical Specification ACTs were risk informed by the PSA, therefore it is not required to extend the ACTs on a regular basis.

Judgements made

The inspection demonstrated how RW is used to manage emerging plant unavailability, calculation of extended action completion times, work planning and release of plant for maintenance and outage planning. In my opinion, adequate arrangements are available to integrate the use of RW into station operations. I judge that the users of RW at station are Suitably Qualified and Experienced in using RW to comply with the arrangements. Therefore, in my opinion, an inspection rating of GREEN is merited against LC23 (Operating Rules) and LC24 (Operating Instructions).

Whilst RW is not required for compliance with Technical Specifications at SZB, I found that it is well embedded into station operations. I found that the RW risk indicator is routinely used to inform daily operations, and for planning of maintenance and outage. This is a good use of the SZB LPSA.

I identified a number of points of good practice but also made a number of observations, which were conveyed to the licensee in the post-inspection de-briefing meeting.