Office for Nuclear Regulation

Hunterston B planned Intervention

Executive summary

Purpose of intervention

The purpose of this intervention was to undertake Licence Condition [LC] compliance inspections at EDF Energy Nuclear Generation Ltd’s [NGL’s] Hunterston B power station, in line with the planned inspection programme contained in the Hunterston B Integrated Intervention Strategy [IIS] for 2015/16.

Interventions Carried Out by ONR

This intervention included compliance inspections against the following Licence Conditions:

System based inspection 19 – Heating and ventilation; Licence Conditions 10, 23, 24, 27 and 28 were assessed as part of this intervention

In addition I attended the quarterly site stakeholder group for Hunterston and undertook a monthly meeting with the independent nuclear assurance station evaluators.

Explanation of Judgement if Safety System Not Judged to be Adequate

I judged that the heating and ventilations systems inspected did not adequately fulfil the requirements of the underlying safety case. This judgement was made in light of there being inadequate and insufficient maintenance of a safety mechanism that is claimed by the safety case as being necessary for a tenable and habitable central control room [CCR] should an accidental release of carbon dioxide coolant gas or fire occur.

Notwithstanding this judgement, I secured appropriate assurance that, should the emergency CCR ventilation system be unavailable during an onsite incident or offsite nuclear emergency, there is sufficient provision to sustain control through the alternative indication centre. 

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

The system based inspection was undertaken with support from two NGL independent nuclear safety assurance [INA] evaluators. During the inspection I made the following observations and judgements:

From an LC23 and LC24 perspective, I examined the safety case claims associated with the station’s contaminated extract ventilation system and the station’s CCR ventilation system. These claims relate to the control of airborne contamination levels on the station, and the minimisation of spread and concentration of contamination to the environment.

I observed that the conditions of operation for availability of contaminated extract system fans, dampers and filtration systems are consistent within the safety case and environmental technical specifications. I observed that the availability requirements for CCR ventilation extract fans, dampers and filtration systems, as defined within the safety case, have inadequate visibility within station operating instructions. As a result, the station is not required to restore CCR ventilation extract capability according to a defined completion time, commensurate with the requirements for availability of other ventilation systems.

From a sample of plant item operating instructions associated with availability of contaminated extract ventilation systems, I observed that instructions do not adequately comply with the station’s quality management arrangements. In light of these observations, I have allocated IIS ratings of 4 – Below Standard for LC23 and LC24. A candidate regulatory issue is identified through which to monitor for timely and adequate response by the licensee.

From an LC27 perspective, I observed generally satisfactory management of system defects; however the inspection revealed two incidents of previously unrevealed and apparently unrelated failure of instrumentation associated with interlocks that ensure emergency recirculation of CCR air, necessary for ongoing tenability during an accidental coolant gas release. I also observed in one area of the contaminated extract ventilation system long-standing and extensive corrosion of ductwork and HEPA [high efficiency particulate air] filter housings within a high humidity environment.

From an LC28 perspective I sampled a range of maintenance records to verify timely completion of maintenance inspections and tests [MITs] as required by the station plant maintenance schedule. In each case the MIT had been completed on time. I observed evidence of a backlog in scanning completed maintenance check-sheets; this is a repeat observation from previous inspections. The station provided satisfactory assurance that the overall backlog on the station has reduced. This will be followed up in accordance with an existing regulatory issue.

I observed that the safety case functional requirements for CCR ventilation extract dampers, to automatically close upon detection of elevated CO2 coolant gas, are not subject to adequate examination inspection maintenance and testing. In light of these observations, I have allocated IIS ratings of 4 – Below Standard for LC27 and LC28.

From an LC10 perspective, I examined the qualification manual requirements for operator technicians and duly authorised plant engineers; I confirmed that staff are provided with appropriate system specific training associated with CCR and contaminated extract ventilation systems. I allocated an IIS rating of 3 – Adequate for LC10.  

The intervention was performed in line with ONR’s guidance requirements (as described in our technical inspection guides) in the areas inspected.

Conclusion of Intervention

In light of the below standard ratings for LC23, 24, 27 and 28, three candidate regulatory issue are identified through which to monitor for timely and adequate response by the licensee.

There are no findings from this inspection that could significantly undermine nuclear safety.