The UK's Office for Nuclear Regulation (ONR) has published its assessment of the Periodic Safety Review (PSR) for Hunterston B in Ayrshire and Hinkley Point B in Somerset. Both are advanced gas-cooled reactors (AGRs) that started up in 1976 and are scheduled to close in 2023.
ONR also published an update of EDF Energy Nuclear Generation's (NGL) revised graphite core safety case. In a statement in February, ONR confirmed that EDF had carried out an adequate PSR for the two plants, adding that it had accepted EDF's revised graphite core safety case for both sites, but had included recommendations as part of this acceptance. To comply with a nuclear site licence, a periodic review - a comprehensive study of plant safety - is carried out every 10 years to justify continued safe operations.
The revised safety case provides new limits and conditions of operation in response to key-way root cracking (KWRC) of the graphite in the core, which is an expected part of the ageing process as reactors approach the end of their design life. Acceptance of the safety case depends on a revised inspection and monitoring strategy.
Chief Nuclear Inspector Richard Savage said: "Our assessments in a range of technical disciplines, along with our frequent specialist inspections, and discussions with external experts, led ONR to make a number of recommendations with the aim of improving future safety cases. This includes the requirement for NGL to prepare a safety case for the graphite core, defining the safety limits marking end-of-generation."
A total of 15 regulatory assessments were commissioned covering: structural integrity; mechanical engineering; civil engineering; electrical engineering; control and instrumentation; chemistry; graphite; fuel safety; internal hazards; external hazards; fault studies; human factors; leadership and management for safety; radioactive waste management and decommissioning; and radiological protection.
In November 2015, EDF Energy said it had found cracks in three of the graphite bricks in unit 3 of its Hunterston B plant. Similar cracks were found in October 2014 in two of the graphite bricks of unit 4. In both cases, the company said the cracks had no safety implications.
"Continued operation of HPB/HNB reactors is now supported by NGL's safety case NP/SC 7716 which sets an operational limit of 20% cracking in the core. The justified period of operation of each reactor at HPB/HNB is therefore dependent upon the findings from the inspections at each outage," ONR said.
A significant nuclear safety concern for operation beyond the onset of KWRC was the ability to safely shutdown the core during a seismic event. ONR said that, in addressing the concern, NGL identified and implemented a series of "reasonably practicable modifications" to the plant, such as establishing diverse shutdown capability of the core, in order to support plant life extension.
Inspection will "play a crucial role in supporting the period of safe operation of the reactor in late life," the regulator said, adding that certain improvements are necessary, such as the development of a capability to measure the condition of control rod channels. NGL should develop improved inspection and monitoring technology; in particular equipment capable of performing visual inspection and dimensional measurements of control rod channels, it said. NGL should determine end-of-life criteria for the reactors, which is likely to include measures of core distortion as well as numbers and morphology of cracks, ONR said.