The Office for Nuclear Regulation has given EDF Energy the all-clear to bring reactor 4 at Hunterston B back into operation – but the operation covers only a limited amount of operation likely to give it four months on line, until a further safety case is made.

The reason for the shutdown at Hunterston B –  reactor 3 and reactor 4, which have both been out of operation since last year – is that the graphite blocks in the core have been cracking. This is a result of the fact that they are irradiated all the time the reactor is operating, and it has been a major point of contention between those arguing that it will and should mean the reactors have very limited remaining life, and owner EDF Energy, which up to now has presented safety cases for contunued operation that have convinced the regulator. The condition of the graphite blocks has been monitored for a number of years by the regulator and EDF.

Last year the reactors were shut because cracking at reactor 3 was found to have approached operating limits set by the regulator (although not safety limts – the operating limits are set so as not to approach this level). Reactor 4 was also shut, although the level of cracking was somewhat lower. ONR now says reactor 4 can operate for up to around four months, because that will not take it beyond the level of cracking  in reactor 3.

It remains to be seen whether the ONR will allow the two reactors to restart for a longer term. ONR said, “Whilst we are content to allow operation for a limited period, the longer term condition of the reactor remains uncertain and the licensee will need to justify safe operation beyond this period.” EDF Energy submitted a ‘safety case’  for a longer term return to operation for reactor 4, which if agreed by the regulator would also lay the groundwork for longer term operation at reactor 3. The new safety case was submitted in June and will likely take several months to be examined by the ONR.

Read the ONR report