Heysham Agreement to Essential Systems Outage Program (ESOP)

BE has requested that the regulator agrees to the implementation of the Essential Systems Outage Program (ESOP) risk monitor.

 This is as a replacement for the existing risk monitor (Essential Systems Status Monitor (ESSM)) at Heysham 2.


Plant operators are required to make decisions on the release of post-trip safety related plant for maintenance or repair.  There are two main elements to the current arrangements for controlling the amount of essential safety plant permitted to be out of service: (i) check of compliance against a number of ‘deterministic rules’; and (ii) ESSM calculation (probabilistic assessment) of the risk increase factor (relative to the level of risk when all plant is available).  The risk model of the plant integral to the ESSM has been improved with a more up-to-date model produced which is of greater scope and more representative of the plant.  British Energy did not consider that an update to utilise the revised risk model within the ESSM was practical so the ESOP was developed for this purpose, as a replacement for the ESSM.

The ESOP is an operator aid that can check compliance with the deterministic maintenance rules and carry out a probabilistic assessment.  The ESOP interfaces with the latest station probabilistic safety assessment (PSA) model to carry out the risk assessment.  British Energy claims that the overall arrangements for controlling the amount of essential safety plant permitted to be out of service will largely remain unchanged compared to those currently in place with the ESSM.

Assessment and inspection work carried out by Office for Nuclear Regulation (ONR) in consideration of this request

The proposal has been assessed by an ONR PSA specialist and found to be acceptable.  The PSA Specialist and the Site Inspector have also confirmed, through site inspection and subsequent follow up, the licensee’s readiness to implement the ESOP.

Matters arising from ONR’s work

There are no regulatory issues or concerns.


The proposal has undergone technical review within the licensee’s organisation and it has been subject to ONR assessment and inspection.  There is no technical or regulatory reason that prevents agreement being given for implementation of the ESOP at Heysham 2.


Agreement to implementation of the ESOP at Heysham 2 should be given by issuing Licence Instrument 566 to the licensee.

Full project assessment report.

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