Office for Nuclear Regulation

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Dungeness B Power Station to witness pre-stressed concrete pressure vessel tendon surveillance activities on reactor 21

Executive summary

Purpose of intervention

This is a report of a planned compliance inspection undertaken at EDF Energy Nuclear Generation Ltd's (NGL's) Dungeness B (DNB) power station, to support a planned intervention for the Civil Nuclear Reactor Programme (CNRP) of the Office for Nuclear Regulation (ONR).

The purpose of this intervention is to witness tendon stressing and inspection activities being carried out on the Reactor 21 pressure vessel, and to check the Licensee's compliance with its maintenance arrangements during this work. The opportunity was taken to look at the bearing arrangement between the pre-stressed concrete pressure vessel (PCPV) and its foundation and supports.

Interventions carried out by ONR

This intervention will form a part of ONR's assessment of the Licensee's claims on the adequacy of the reactor pressure vessel after its current statutory maintenance outage, and will inform ONR's judgement on its consent to return Reactor 21 back into operation. ONR's assessment will take account of several specialist assessments and interventions covering a range of topic areas.

Explanation of judgement if safety system not judged to be adequate

Not applicable.

Key findings, Inspector's opinions and reasons for judgements made

The reactor pressure vessels at this site are pre-stressed concrete pressure vessels (PCPVs) comprising of a thick-walled concrete cylinder that is pre-stressed by many multi-strand tendons. The tendons run in ducts in the sidewalls and top and bottom caps of the vessel. The tendon loads place the concrete into a compressive state that negates the tension from the internal reactor gas pressure. Water cooling circuits within the concrete maintain the materials at suitable temperatures, and various reactor coolant and steam pipes penetrate the vessel walls.

I inspected the Licensee's documents and procedures for the tendon maintenance and inspection activities and the tendon stressing equipment calibration and anchorage inspections.

Licence Condition 28 requires the Licensee to make and implement adequate arrangements for the regular and systematic examination, inspection, maintenance and testing of all plant which may affect nuclear safety. This was adequately demonstrated for the tendon surveillance activities seen during the site visit by my observation of the procedures being followed and the records being kept. The IIS rating of 3 ('Adequate') is in my opinion appropriate for this intervention.

There are no findings from this inspection that could significantly undermine nuclear safety.

The licence conditions were inspected against ONR's published guidance requirements (as described in our technical inspection guides).