Old nuclear event in the open

Friday, 12 June 2009

An incident over two years ago at the shut-down Sizewell A nuclear power plant has become a focus of public attention due to timing of the release of information as well as the potential consequences of the event.

Following a pipe break on 7 January 2007, levels of water in a used nuclear fuel cooling pond began to drop. Alarm systems did not function, and even if they had operators would have been unaware. It is conceivable that the leak could have gone undetected for the ten hours it would have taken for the pool to drain almost completely, raising the possibility that the fuel could have been exposed and triggered a radioactive fire.

 

Operators on duty responded well to a warning given by staff that actually saw water that had leaked and the situation was stabilised safely within 45 minutes. The level in the pond dropped by around 33cm (compared to a total depth of several metres) but was returned to its normal level within 24 hours. Nobody was harmed by the incident and the water that leaked and was discharged to sea amounted to a radiation release of less than 1% of the site's annual limit.

 

Classified on the International Nuclear Event Scale at Level 1 (an anomaly), a summary of the event was included in the Nuclear Installation Inspectorate's (NII's) quarterly report, publications of the Nuclear Safety Advisory Council and meetings with local people. However, details of how the event came about and how serious it could have been only became clear after a request under the Freedom of Information Act by John Large, a consultant employed by the Shut Down Sizewell campaign. This route of discovery has added an atmosphere of high drama to the news and raised questions about the procedures the NII uses to report and act on events.

 

The water leak

 

The NII's preliminary investigation of the event notes a number of alarming things about the Sizewell A cooling pond which indicate that Magnox South broke several conditions of its licence.

 

An outdated but working control and instrumentation set-up for the system that maintained the water level in the cooling pond had been replaced. However, the contractor "replaced it with one that is a poor design, poorly installed and did not work on the day." The system was meant to send an alarm to the Sizewell A main control room if water levels were more than 15cm above or below normal. A second alarm was to be sent if water levels dropped more than 40cm below normal. Neither of these systems worked.

 

Even if the pond alarms had been sent, this would only have activated an alarm that had already been on for two days. Operators had been ignoring this 'standing alarm' for several shifts and none had logged it or raised it as a defect. "The tolerance of standing alarms in the main control room after the reactors were shut down raises issues associated with [licence conditions on] operator training and their suitability for the job," noted the NII.

 

The PVC recirculation pipe that broke did not meet design specifications and it is not clear that it ever did. Furthermore, "No evidence could be found of this pipework ever having been on the Sizewell A maintenance schedule." Sizewell A is a Magnox gas-cooled nuclear power plant built in the early 1960s.

 

Media reports looked to the worst-case scenario, which the NII report said could be the exposure of the used fuel in the pond, a subsequent fire and release to the environment.

 

The NII told World Nuclear News today that it did not now believe such a fire to be credible. Due to the location of the pipe break, around 60cm of water would always remain in the pond. Although this is not enough to cover the fuel or give radiation protection, it would greatly reduce the heat load of the fuel.

 

A spokesman told WNN that a 'pessimistic' calculation showed the maximum heat load of the skips containing used fuel to be around 5.2 kW, while heat loads below 7 kW would not cause a fire. Furthermore, even at 7 kW, the skips would not reach 600ºC until 36 hours had passed, the NII said.

 

This timing is important because scheduled checks on area were made every 12 hours on weekends, when this event occurred. The NII report said the pond could have drained in ten hours, had operators not quickly taken the correct action and shut down a recirculation pump.

 

NII response

 

Despite reporting this event in the usual way, the NII has now come under scrutiny. It said that it does not normally publish the full versions of all its reports, but that it does release them to anyone that requests.

 

After its investigation, the body was faced with three possible courses of action. It could have issued a 'Direction' to the licensee demanding to certain action, an 'Improvement Notice' or moved forward with prosecution.

"After much discussion," the NII decided to issue a Direction, which Magnox South quickly complied with "to the letter". After this, the NII treated the matter as settled, in part because of the good safety record of the company and the site up to that time.

 

The NII said that the course of action was "entirely in line with published policies" and its established decision-making process.

 

"Prosecution does not depend on staff resources. If we think one is appropriate, we will take that course. Issuing a Direction, which is a serious legal device, does not preclude a prosecution and if it had not been complied with or not satisfied, then a prosectution would remain an option," summarized the NII.