British Energy Generation Limited, the licensee of Sizewell B nuclear power station, has requested agreement from HSE Nuclear Directorate to modifications to the reactor pressuriser. This request is in line with the licensee’s responsibility, as set out in its nuclear site licence (licence condition 22). British Energy Generation Limited has provided justification that the reactor is safe to operate in a written safety case.
The pressurised water reactor at Sizewell B consists of two main circuits, primary and secondary. The primary circuit and part of the secondary circuit are situated within a domed concrete and steel containment vessel. Amongst other things, this large structure is intended to contain the water resulting from any primary circuit leaks. The containment is sealed during operation
The primary circuit contains water that is circulated through the reactor and four steam generators, while the secondary circuit contains water that boils to form steam in the steam generators. The steam is used to drive turbines to generate electricity. The primary circuit water is heated in the reactor, but most of this water remains liquid. The exception is in a vessel connected to the rest of the circuit called the pressuriser. This is kept slightly hotter with the aid of electrical heaters. Inside the pressuriser there is both liquid water and steam.
On 16 March 2010, a small leak of less than 5 litres / minute was detected. The reactor was safely shut down and the causes of the leak investigated. It was subsequently traced to the bottom of the pressuriser. This region contains 78 electric heaters in tubes which are installed through the main body of the pressuriser. Due to a fault within a heater element, one of these tubes had developed cracking such that there was a water path into the containment building. There were no radiological consequences for either the public or workers as a result of this event
Within Sizewell B, the components are classified according to the consequences of their failure. The heater tubes do not have the highest classification, thus even if the entire heater tube had failed and had come away from the pressuriser, the consequence would be limited to one which could be managed by shutting down and using safety systems to inject water into the primary circuit. The actual leak that occurred was only marginally above a level at which operators are required, under the safety case, to shut down the reactor
Following the investigation of the leak, the licensee carried out repairs before requesting to restart the reactor
Assessment and inspection work carried out by ND in consideration of this request
HSE Nuclear Directorate has considered of the safety of the reactor, based on a number of activities. The most significant of these are
- Assessment of safety case documents that the licensee has prepared. The licensee has prepared extensive documentation covering the cause of the leak and all aspects of the repair. These have been considered by HSE Nuclear Directorate’s site inspector as well as by specialists in structural integrity, nuclear fuel, chemistry and electrical systems.
- Inspection of the reactor site to determine whether the repairs could be carried out adequately, i.e. to a sufficient standard and without compromising the safety of the workforce.
- Visiting a number of off-site facilities where the repair techniques were developed.
- Consideration of the surveillance work undertaken by the licensee’s internal regulator
Matters arising from ND’s work
Inspection and assessment work undertaken by HSE Nuclear Directorate has found that:
- The root cause of the failure has been adequately investigated and identified by the licensee.
- All electrically failed heaters have been removed from the pressuriser, thereby minimising the risk of future cracking.
- The repairs to failed heater tubes in the pressuriser undertaken by the licensee adequately ensure that the risk of a failure of the pressure boundary is acceptably low.
- The potential for debris and materials, arising from the failed heaters and repair programme, to enter the reactor coolant circuit and adversely affect safety is acceptably low; particularly regarding the potential for fuel failure.
- Although further failure of pressuriser heaters cannot be ruled out, adequate steps have been taken by the licensee to prevent a repeat event. In particular, commitments have been given to replace all remaining pressuriser heaters within an acceptable timescale, with a different type which is less susceptible to failure. Also, additional instrumentation for monitoring the repairs has been installed to provide early warning should further failures of existing heaters occur before they are replaced.
HSE Nuclear Directorate has sufficient confidence that the modifications to the pressuriser have been adequately conceived and executed and that the Sizewell B reactor is safe to operate
HSE Nuclear Directorate agreed to the request from the licensee and issued Licence Instrument 521 accordingly.