Hunterston B power station (HNB) Reactor 4 is currently undergoing its periodic shut down for examination, maintenance, inspection and testing under Licence Condition 28 (LC 28) and will need Consent from ONR before it is restarted. This report presents the findings of the second mechanical engineering intervention in support of the Hunterston B Reactor 4 2014 periodic shut down.
I undertook on-site meetings and plant inspections between 17th and 18th September 2014 with relevant staff to determine the adequacy of work being undertaken during the reactor 4 periodic shut down, and arrangements associated with the requirements of LC 28. The activities examined were selected due to their significance to nuclear safety, with due consideration being given to any fleet wide interventions and including:
The intervention was performed in line with ONR’s guidance requirements (as described in our technical inspection guides) in the areas inspected.
I inspected the safety protection systems associated with the gas circulator lubrication system. As part of my outage inspection, I considered the adequacy of protection arrangements around the GC lube oil systems prior to the reactor going back to power. The protection systems associated with the Gas Circulator lubricating oil system in my judgement have significant shortfalls in terms of plant protection and the absence of an engineered interlock to prevent mal-operation of the lubrication oil system of an in-service circulator whilst the reactor is shutdown. The short term improvement identified by EDF through their internal investigation shall, in my judgement, enhance the plant protection system by introducing more robust administrative controls such as colour coded, quadrant specific padlocks as well as updated instructions which place greater emphasis on the use of human performance tools, and introduce human factor considerations (albeit at an early stage with more work to be conducted around integration of human factors).
I am satisfied that, based on the evidence sampled, the gas circulator event is not a realistic fault sequence whilst the reactor is at power. The plant operations which led to the failure are not undertaken whilst operating at power and are limited to shut down periods and therefore have a lesser impact on nuclear safety.
Based on the evidence and discussions held with the Gas Circulator System Engineer and Component Engineer around maintenance tasks, I am satisfied that arrangements for undertaking and recording of maintenance in line with Licence Condition 28 are adequate. I further welcome the decision that a fleet wide initiative is now underway to harmonise the way in which Gas Circulator operational histories are recorded and judge that this will lead to improvements across the fleet. I judge therefore from a mechanical engineering perspective an IIS Rating of 3 (adequate) to be appropriate for Licence Condition 28.
Extensions to existing safety cases known as Justifications for Continued Operation (JCO’s) relating to Gas Circulator re-baring work, motor winding life and Nylicon coupling replacement are required due to the changes in the Gas Circulator replacement plan. The completion of the work associated with the JCO is still expected to be completed within the original commitment dates and continue to be bounded in my judgement by the extant safety case. I am also satisfied, based on the discussions held, that the increased risks associated with the 1 year extension to Gas Circulator 4A1 maintenance schedule are reasonable. In my judgement the extensions continue to demonstrate the risk is shown to be as low as reasonably practicable and do not pose a significant challenge to nuclear safety.
Overall, I am satisfied that arrangements for operations and maintenance carried out at Hunterston B are adequate and in line with my expectations. Whilst there is longer term work which can be completed to further improve safety, I am satisfied the risks have been reduced so far as is reasonably practicable at present.
From a mechanical engineering perspective, and subject to satisfactory completion of the re-start administrative enhancements, I have no objections to the return to service of Hunterston B Reactor 4.