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, alongside an update of EDF Energy Nuclear Generation's (NGL) revised graphite core safety case. Hunterston B and Hinkley Point B are Advanced Gas-cooled Reactors (AGRs) that both started up in 1976 and are scheduled to close in 2023.
In a statement on 24 February, ONR confirmed that EDF had carried out an adequate PSR for the two plants - HNB and HPB - and said 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 ten years to justify continued safe operations. This requirement means that the site licence company regularly reviews and reassesses safety at nuclear sites, making improvements where necessary.
The revised safety case provides new limits and conditions of operation in response to key-way root cracking of the graphite in the core, which is an expected part of the aging process as reactors get closer to their 'end of life'. Acceptance of the safety case is also reliant on a revised inspection and monitoring strategy.
Chief Nuclear Inspector Richard Savage said: "From our extensive review of the safety case processes and procedures, we confirm that EDF has justified future operations for the period through to end of generation (currently 2023) and defueling/decommissioning activities to 2027.
However, ONR said confirmation is subject to operations being supported by a "detailed understanding" of the condition of the core at each reactor.
"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 the following topic areas: 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.
Hunterston B is a nuclear licensed site operating two Advanced Gas-cooled Reactors - units 3 and 4. The graphite core of each of the reactors is made up of around 6000 graphite bricks - 3000 of these are the graphite bricks containing fuel channels - which are all connected together. Graphite ageing is one area used to determine the lifespan of an AGR nuclear power station. Greater understanding of the ageing process by sampling and modelling can lead to them operating safely for longer, giving the UK secure and reliable low-carbon electricity, the company has said.
The ONR said in its report the fundamental nuclear safety requirements of the graphite core are affected by the two principal ageing and degradation mechanisms affecting the HPB/HNB graphite bricks; graphite weight loss and irradiation induced dimensional change. The stresses within the graphite bricks later in life can result in cracks originating from the key-ways on the periphery of the bricks, known as key-way root cracking (KWRC). This is of nuclear safety concern because it potentially affects the functioning of the keying system of the core, which holds the bricks in alignment, the regular said. Along with the graphite weight loss, the progression of KWRC will probably determine the lifetime of the reactors, it added.
"The rate of KWRC was however uncertain, due to the differences in irradiation between bricks and the variability in the material properties of the bricks. NGL had attempted to derive the rate of cracking from statistical analyses. The PSR3 submission, based on mid-2014 data, predicted KWRC in 2019. However, the first KWRC was observed at HNB in 2015," the report said.
"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," it added.
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's damage tolerance assessment was focused on brick cracking but ONR noted that the assessment had not yet defined a limit on the graphite channel bore distortion, nor channel bow or tilt.
"Given that these parameters were measures of changes in the geometry of the graphite channel hence bear direct influence on the normal operation, ONR considered that it would ultimately be necessary for a safety limit, end-of-life criteria, on the change of the channel shape to be substantiated in NGL's safety case,” the regulator said.
NGL should determine end-of-life criteria for the reactors. This is likely to include measures of core distortion as well as numbers and morphology of cracks, it said.
For the "late life operation" of the graphite core, NGL needed to demonstrate the geometry of the control rod channels would be maintained to ensure "free and unimpeded" control rod insertion in normal operation, fault and seismic conditions. "Due to the significant uncertainties in graphite ageing with irradiation and weight loss, [we] considered that NGL should review the scope of its damage tolerance assessment, following each outage," ONR said.
Researched and written
by World Nuclear News