The purpose of the inspection at Heysham 1 was to review EDF’s investigation into the cause of the crack, examine the cracks on the steel members, assess the potential consequences if the structural support was to fail and investigate if any other systems (pipework) are affected.
ONR has carried out an intervention against Licence Condition 28. This requires the Licensee, EdF-NGL, to implement adequate arrangements for regular and systematic examination, inspection, maintenance and testing of all plant which may affect safety
ONR inspected the DC Heater trains at Heysham 1 Reactor 1 & Reactor 2 and examined the cracks developed in the support beam of DC Heater 2.
ONR met with the Event Recovery Manager to investigate the cause of the cracks on the support beams of DC Heater 2 and gather information of the proposed connection arrangements to transmit the loading safely to the primary beam.
The ONR site inspection of the DC Heater 2 support structure at Heysham 1 Reactor 1 & Reactor 2 confirmed that cracks have developed in one of the secondary support beams. One of the support beams in R1 had a crack at each end. One of them extends the full width and length of the web, and the other only extends half of the web’s length. The cracks in R2 are also located at each end of one of the support beams but are less severe than the cracks in R1. The full length and extension of the cracks will be confirmed by subsequent non-destructive testing.
The site technician confirmed that the DC Heater train experienced higher level of vibrations when the power on the reactor was reduced. It was visible during the site inspection that the excessive vibrations had caused damaged to pipework lagging. The Station did not provide any evidence that condition reports reporting excessive vibrations in the DC Heater train have been raised.
The Civil Design Group (CDG) has substantiated the structure, in its current condition, for static loading. Station was not able to provide information regarding the substantiation of the structure under the dynamic loading in its current condition. The CDG is in the process of developing a short term solution to ensure that the loadings on the cracked beams are transmitted safely and a longer term solution. Station was therefore not yet able to provide information on the type of connection arrangement or the long term solutions.
Design Authority and CDG are identifying if DC Heater 2 is seismically qualified. I stated to the Event Recovery Manager that despite the seismic loading being the bounding case for DC Heater 3, it could not extend to DC Heater 2 due to the different support arrangements. The fatigue effects caused by excessive vibration should be considered when assessing structures subject to vibrations.
The Station has commenced an investigation and is developing both short term and long term repair strategies. For Heysham 1 Reactor 1 the DC Heaters have been taken out of service, so I am satisfied that the Licensee has taken suitable steps to address the immediate safety concern. In terms of Heysham 1 Reactor 2, I was satisfied that the damage was less severe and a short term solution was to be implemented.
It appears that the cracks on the beams were caused by fatigue due to excessive vibration, as well as a poor engineering connection design. The inspection revealed that the site technicians were aware of an increase in vibrations in the system since the reactors were at reduced power. There was also obvious damage to the pipework lagging. The Station did not provide any evidence reporting the increment on vibrations or the damage that this may be causing on the pipework in this area. Therefore, there was a failure on the Station to recognise this problem and to follow their procedures. I have judged that the inspection should be rated as Below Standard (4) with respect to the Integrated Intervention Strategy (IIS) Guide.
I have created an ONR issue to follow up with the Civil Design Group and the Station the assessment of the cracked beams in the DC Heater 2 and to ensure that the consequences of high level vibration on the plant are adequately addressed.