Last Wednesday, Iain Clenahan gave a presentation on an incident that happened in February 1997. This post is my attempt at summarising the talk.
Hunterston B is an Advanced Gas-cooled Reactor (AGR) nuclear power plant commissioned in the late 1970s. To cool the reactors, carbon dioxide (CO2) is circulated at 40 barg. This gas a relatively high level of radioactivity, caused by receiving nuclear radiation while near the core of the reactor. The radioactivity of this gas decays to background levels within a couple of days. This circuit is mostly a closed loop however there is some consumption and purging of the gas required and so two to eight tonnes of make up gas are required each day.
The make up gas is supplied by a commercial supplier who also serves the food and drink industry including AG Barr who make Irn Bru. The gas is supplied by road tanker and can be delivered to one of two sets of tanks, the ‘strategic’ tanks which contain a backup supply and the ‘operational’ tanks which are used day to day and are closer to the process. These systems operate at a lower pressure of 20 barg.
On the 20th of February, during a routine survey, elevated radiation levels were noticed on the low pressure pipework. The operational tanks were confirmed to have been contaminated, so a delivery of CO2 was diverted to the strategic tanks to avoid any contamination while the issue was resolved. Initially it was thought that the contamination had come from some auxiliary equipment, the Burst Can Detection (BCD) and so this equipment was disconnected and the operational tanks were purged to remove the contamination. After this a sample was taken to confirm that the tanks were clear.
The following day the sample came back and proved that they were NOT clear. It wasn’t the BCD that had caused the contamination. The decision was made to divert all future CO2 deliveries to the strategic tanks. It took another four days until the 25th and 26th of February before the real reason was discovered. Three separate isolation valves were found to be passing, allowing gas to reverse flow through lines used to supply purge gas to help release equipment. After this was discovered, the purging proved effective and the radiation levels decreased.
On the 27th of February it was realised that a delivery had actually been made to the operational tanks on the second day of the incident, after the sample that was supposed to prove that everything was clear had been taken, but before the results were known. This had the potential to allow contaminated gas to get into the delivery truck and subsequently get to the bulk storage tank used to supply all the other CO2 users.
Due to the delay between the delivery being made and the realisation and checking, it was not possible to confirm if the bulk tank had been contaminated with reactor CO2, because the radiation would have decayed back to background levels by the time it was checked. As a result, there was no evidence that contamination had occurred, but this was not evidence that contamination had not occured. In light of this, a worst case assessment was carried out by assuming the headspaces were 100% reactor gas. This assessment indicated a potential dose of 0.05 μSv/l carbonated drink which wen compared with the public dose limit is 1000 μSv/year showed that the risk to public health was negligible.
The key points of the event that I took away was don’t be too quick to jump to conclusions. If they had kept an open mind early on in the investigation, they may have found the correct cause sooner rather than assuming it was the BCD and not waiting for the results.
There is more information and references available in the slide pack for the talk.Go Top