The advanced gas-cooled reactors (AGRs) in use in the UK generate heat as a result of the nuclear reaction which takes place in the reactor core. The reactor core is cooled by high-pressure gas, which is circulated through the core to the main boilers. The boilers generate steam which is used to drive turbines which, in turn, generate the electricity.
Powerful gas circulator pumps are used to circulate the gas and they are monitored during operation to ensure they are working properly. This safety system provides an automatic reactor trip (shutdown) function in the event that the gas circulator pumps operate below their required duty as this would indicate that there could be a problem affecting the gas circulators and hence the cooling of the reactor.
The equipment which performs this function is known as gas circulator speed trip units (GCSTUs). At Heysham 1 nuclear power station, this electronic equipment is now around 30 years old and whilst it is still working satisfactorily at the moment, it is obsolete and requires replacement.
British Energy, the licensee of Heysham 1 nuclear power station, requested permission from HSE Nuclear Directorate (ND) to replace the existing GCSTUs with modern equivalent equipment. This request is in line with the licensee’s responsibility, as set out in its nuclear site licence.
Assessment and inspection work carried out by ND in consideration of this request
- An ND specialist inspector in control and instrumentation engineering assessed the safety case, written by the licensee, to demonstrate that the proposal properly describes the relevant engineering aspects of the modification and verify that there were no safety issues.
- An ND site inspector checked that the processes used by the licensee to create and review the safety case had been followed properly.
Matters that arose from ND’s work and outcome of ND’s consideration
- That the correct process for producing and assessing the safety case for the replacement of the equipment had been followed by the licensee.
- The replacement units are designed along similar lines to the original equipment but with modern electronic components.
- The replacement units are similar to replacements that have already been installed on other UK power stations previously and without problem.
- That the safety case produced by the licensee demonstrated that the proposed replacement of the original equipment by the replacement GCSTU equipment has sufficient integrity and reliability to ensure appropriate levels of safety.
- That there is an additional benefit in replacing the GCSTU equipment since it will enable old instrumentation cables to be changed at the same time.
ND’s assessment of the licensee’s safety case for the replacement of the Heysham 1 GCSTUs, coupled with discussions and subsequent agreements with the licensee’s staff, provided confidence that the proposed modification is required to maintain suitable and satisfactory performance of this nuclear safety related protection system.
It is recommended that a licence instrument is issued to allow British Energy to replace the existing GCSTUs with new equipment.