Office for Nuclear Regulation

Sizewell B – Inspection ID: 51238

Executive summary

Date(s) of inspection:

  • January 2023

Aim of inspection

The Sizewell B (SZB) category 1 heating, ventilation and air-conditioning (HVAC) chillers have been found to have an in-service availability history which is worse than that assumed by the station safety case.  These chillers are part of the HVAC systems which maintain acceptable temperatures for safety critical equipment in the following areas:

  • Main Control Room (MCR)
  • Safety Classified Electrical Equipment (SCEE)
  • Auxiliary Building

The SCEE rooms are of particular focus because of the potentially significant consequences of the primary protection system (PPS) overheating above the temperature it is qualified to.  A programme of work is therefore underway at EDF Energy Nuclear generation Ltd (EDF NGL) to justify replacement of the safety category 1 chiller units at SZB [1].

In advance of chiller replacement, a justification for continued operation (JCO) is in place [2].  This includes temporary measures at SZB to manage the situation should a cat 1 chiller fail.  The aim of this inspection is therefore to consider the implementation of the JCO at the station ahead of the warmer summer ambient air temperatures.  This inspection supports ONR’s regulatory engagement on the existing Level 3 regulatory issue 5497, “availability of nuclear significant HVAC chillers”

Subject(s) of inspection

  • LC10 – Training – Rating: Green
  • LC12 – Duly authorised and other suitably qualified and experienced persons – Not rated
  • LC23 – Operating rules – Rating: Green
  • LC24 – Operating instructions – Rating: Green
  • LC27 – Safety mechanisms, devices and circuits – Rating: Not rated
  • LC28 – Examination, inspection, maintenance and testing – Rating: Green
  • LC34 – Leakage and escape of radioactive material and radioactive waste – Not rated

Key findings, inspector’s opinions and reasons for judgement made

LC10 – Training

We held discussions with operations duly authorised persons (DAPs) and observed their ability to locate and interpret the relevant tech spec action condition logic and alarm sheets.  We questioned their decision making processes in the case of a room overtemperature and were satisfied that appropriate action would be taken.  We therefore considered formal training to be adequate with regards to the HVAC JCO.

We did consider that operations would benefit from addition context regarding the chiller issues and fault sequences and raised an action to call for directed reading to be issued on this subject (action 1 of RI-11184).

We assigned an integrated intervention strategy (IIS) rating of Green for LC 10.

LC23 – Operating rules and LC24 – Operating Instructions

We reviewed tech specs and tech spec basis associated with safety classified equipment room temperature and found that these reflect the operating rules derived from the PPS temperature qualification limits.  We reviewed associated alarm sheets and found that these have been updated in accordance with the JCO and contain appropriate guidance with regards to deploying temporary chillers and isolating non-essential equipment.

We considered that the tech specs would benefit from additional guidance on the conditions when a pre-emptive shutdown of the reactor should be considered; SZB informed us that work on this tech spec update is already underway.  We raised an action to track progress and encourage timely completion (action 2 of RI-11184).

We assigned an IIS rating of Green for LC23 and LC24.


We sampled recently completed maintenance records in order to ensure that the equipment which is claimed in the JCO is being adequately maintained.  Based on our sample, we considered that appropriate maintenance activities are being undertaken and satisfactorily recorded.  We noted that the temporary chiller maintenance could be better controlled to ensure that all chillers allocated to SCEE rooms are consistently captured in the routines and raised; we were pleased to observe that this improvement had already been put in place before the completion of the inspection.

We assigned an IIS rating of Green for LC28.


From the evidence sampled during this SBI against LCs 10, 23, 24, and 28, we judged that the implementation of the measures identified in the HVAC JCO is adequate and that the category 1 chiller issues are being adequately managed locally at the station.

There are no findings from this intervention that could significantly undermine nuclear safety at SZB. We raised three actions relating to issuing directed reading, improvements to tech specs, and control of temporary chillers; these will be managed by a new Level 4 regulatory issue, RI-11184.  The ongoing management of the chiller issue will be monitored as part of regulatory business and tracked via Level 3 regulatory issue 5497, Availability of nuclear significant HVAC chillers.

We therefore judged the overall IIS rating for the SBI to be Green.