The purpose of the visit to Hunterston B was to discuss and review a sample of the statutory surveillances, inspections and tests undertaken by EDF-Nuclear Generation (NGL, the Licensee) on the pre-stressed concrete pressure vessel of Reactor 4. The ONR inspection was to support NGL's anticipated request for consent for return to service of Reactor 4 following completion of the 2014 periodic shutdown.
This was a technical discussion regarding the intended scope of examination, maintenance and testing being conducted on the pre-stressed concrete pressure vessel under site Licence Condition 28. Note some inspections and maintenance are conducted safely with the reactor at power.
I noted that NGL's Appointed Examiner (Apex) has planned the statutory surveillances, inspections and tests in accordance with their Branch Instructions
Settlement and tilt survey points, E and F, have been re-established and they allow the licensee to measure any tilt of the reactor. These points were last measured in 1985.
The work during this outage included a full tendon (262G) replacement that had been subject to historic intermittent wetting. Initial visual examination of the strands showed no signs of corrosion. However, the withdrawn samples were then stored such that further surface corrosion inadvertently occurred. I have placed an action on the Apex to investigate this.
NGL is considering implementing grease sampling in all their fleet, which will include, Hunterston B.
The Apex reported that one of the five tendon tensile test samples did not reach the minimum values on breaking load test and load at 1% extension test. I have asked the Apex to investigate why this sample did not attain the required values.
During the visit of to the upper stressing gallery, I noticed that tendon 327A only had one strand instead of 7 strands. The Apex was not aware of this one strand tendon. I have placed an action on the Apex to investigate this.
A Start-Up Statement, that summarises the work, will be submitted in support of the request for consent to return the reactor to service. This follows the expected process. No adverse judgements were made.
I conclude from the information presented at the discussion that these statutory surveillances, inspections and tests, along with the additional tendon samples are proceeding as normal. I believe there has been an improvement to the surveillance by re-establishing the tilt survey points.
I placed three actions on the Apex to:
The above actions have been recorded in the outage database under reference number R4-1. It is not expected that these actions will prevent return to service.
I have recommended comparing the results of the top cap deflection on both reactors to monitor the behaviour of any construction cracks.