Sizewell ‘A’ treated pond water leak

A report from Office for Nuclear Regulation (ONR) sets out the assessment of the treated pond water leak that occurred at the Magnox Limited Sizewell A site on 3 September 2011, the resultant enforcement decision, and related information.


On 3 September 2011, Magnox Limited (‘Magnox’) experienced a leak of approximately 13m3 of treated pond water within the Active Effluent Treatment Plant on its Sizewell A site. This water was of very low specific activity, represented 0.2% of the total volume of water in the pond, and did not compromise the integrity of fuel stored within the pond.

Whilst the direct consequences of the event itself are accepted to have been minor (representing negligible risk to the public and only a nominal risk to a small number of workers), ONR undertook a detailed review of the event to establish, amongst other matters, whether relevant legal duties had been met by the licensee, and whether there is underlying learning for the wider site or Magnox reactor fleet.

In light of this and other considerations, and in accordance with the principles of transparency and accountability, ONR decided to review the event formally by applying the Health and Safety Executive’s (HSE’s) Enforcement Management Model to determine the nature of proportionate enforcement action.

Consequently, this report sets out the:

  • nature of the event and immediate actions taken by Magnox;
  • significant findings of Magnox’s investigation of the event;
  • resulting intervention actions taken by ONR;
  • nature of the radiological significance of the event; and
  • considerations given to the event in applying HSE’s Enforcement Management
  • Model and final enforcement outcome.

Assessment and inspection work carried out by ONR in consideration of this request

ONR undertook detailed reviews of Magnox’s investigation of the event, of its findings, and of its proposed improvement action plan. 

ONR inspectors also made a number of plant inspection visits to develop confidence that specific important plant improvements had been effected prior to the restart of routine plant operations.

The ONR site inspector undertook a formal enforcement review of this event using the HSE’s Enforcement Management Model, to determine the appropriate nature of enforcement action to be delivered.

Whilst an enforcement decision will be made independently by ONR, it has appraised the Environment Agency of its intention to undertake, and progress in undertaking, an enforcement review of this event.

Matters arising from ONR’s work

There are no regulatory issues or concerns.


Whilst no-one was injured as a consequence of the event, and the risk gap (i.e. the gap between the actual and expected levels of risk control) is ‘nominal’, the ONR review has concluded that Magnox did not secure compliance in all respects with Licence Condition 34 (Leakage and escape of radioactive material and radioactive waste) and that work documentation (constituting an element of the safe system of work) was judged to be not adequate.

If this is considered in the context of HSE’s Enforcement Management model, and taking into account:

  • the nature, cause and relatively minor significance of the event;
  • the minor nature of the risk gap ;
  • the acknowledgement of compliance and administrative shortcomings by Magnox;
  • the proactive and responsible response from Magnox to the event; and  
  • a range of specific dutyholder and strategic factors.

This report concludes that the proportionate enforcement action in response to this event is that ONR write a formal letter to Magnox.


As a consequence of this enforcement assessment, it is recommended that ONR writes formally to Magnox setting out its conclusions as regards regulatory compliance, acknowledging improvements made to date, and to inform Magnox of ONR’s expectations that it:

  • completes its longer-term review of the vulnerabilities at Sizewell A’s Active Effluent Treatment Plant, and provide a copy of this review to ONR;
  • provides ONR with an appropriate description of improvement actions necessary to ensure that risk is minimised to levels that are as low as reasonably practicable; 
  • provides ONR with a suitably prioritised and timed plan; 
  • provides ONR with a commitment to communicate learning from this event to the wider magnox class fleet of reactors; and
  • provide ONR with progress updates at appropriate intervals.

You can read the report here.

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