During its operational lifetime, the graphite core of an advanced gas-cooled reactor (AGR) is subject to a number of ageing mechanisms and must be regularly monitored to ensure continuity.  The core’s geometry must be within acceptable limits, its structural integrity must be preserved and the optimum reactor performance must be maintained.  But how to monitor such activities?  One way is during a statutory reactor outage, which includes the removal of small graphite samples from the wall of a fuel channel which are then analysed at an off-site facility.  This activity is carried out using specialist Graphite Trepanning equipment which must be able to accurately, efficiently and safely retrieve the required number of samples without compromising the integrity of the core.

Risktec Solutions, the specialist risk management consulting firm, has recently assisted British Energy in undertaking a detailed safety analysis of their existing fleet of Graphite Trepanning equipment against modern assessment criteria.  By back-tracking to first principles in the assessment of the existing equipment, Risktec was able to clearly determine the exact features and functions of the equipment which provide a safety benefit and identify the degree of reliance which must be placed upon each function in the management of nuclear safety.  As a result, a detailed Safety Requirements Specification for the equipment was produced to identify whether the existing equipment satisfied its assessment criteria, and if any shortfalls were identified, provided a clear statement of requirements for remedial action.

Following on from this assessment, Risktec worked closely with the equipment’s design authority to develop and substantiate a range of essential modifications, covering electrical, mechanical and procedural aspects of the equipment.  The successfully modified equipment was delivered to site as scheduled for the trepanning campaign at Hunterston B nuclear power station where the modifications provided both dose reductions and availability improvements.  The equipment has also been used at Hinkley Point B nuclear power station and further trepanning campaigns at other British Energy stations are planned.

Gareth Ellor, Technical Officer at British Energy said “Prior to the deployment of the Graphite Trepanning equipment during the 2003 outage programme, we committed to modify the equipment to ensure it was fully compliant with current standards.  Risktec’s structured and disciplined approach to the safety assessment of our existing equipment whilst ensuring best value in terms of nuclear/personnel safety and equipment reliability and efficiency was fundamental to the list of modifications required to achieve this commitment.  By adopting a pro-active yet hands-on approach throughout the implementation of these modifications, Risktec supported British Energy to comply with current standards.  We have been extremely happy with the performance of the equipment over recent campaigns and Risktec has been instrumental to this success.”

Andy Reynolds, Principal Engineer at Risktec said “the development of a comprehensive safety case for the equipment which provides traceability from the hazards associated with a trepanning campaign through to the specific engineered and procedural safeguards, will also ensure that any future modifications required to the equipment can be more readily assessed.  This of course impacts on safety, thereby maintaining the long-term safety justification for trepanning operations.”