Heysham 1 power station in Lancashire was recently served with an Improvement Notice from the Office for Nuclear Regulation (ONR), after 30 tonnes of carbon dioxide (CO2) accidentally leaked from a corroded pipe on 16 March 2015. The operator of the site, EDF Energy Nuclear Generation Ltd (NGL), has been given until the end of September to demonstrate improvements in their arrangements for examination, inspection, maintenance and testing of facilities.
The ONR inspectors judged that as a result of the CO2 leak, ‘NGL had failed to meet this condition of its nuclear site licence, therefore compromising the safety of its employees.’ Fortunately, however, they reported that, ‘There was no release of radioactive material, no persons were injured and the two reactors remained operational during the event.’ Whilst harmless in small quantities, CO2 can be extremely hazardous to health if its level increases.
The corroded pipe at Heysham 1 was part of a CO2 delivery system that distributes cooling gas to its two reactors. These reactors are known as the ‘AGR’ type (advanced gas-cooled reactor), developed from an older design known as ‘Magnox’. The UK currently has seven fully operational AGR nuclear power stations which all use carbon dioxide to cool the reactor core. Sizewell B is the UK’s only power station to use a pressurised water reactor (PWR) instead of gas.
In an AGR the CO2 cooling gas is stored on site and circulated through the core to transfer heat from the reactor to the boilers. The circulator assembly is totally enclosed to facilitate removal and replacement from its position in the bottom of the vessel. This also acts as a secondary containment system. In addition, nitrogen is also stored on site for emergency injection into the coolant gas circuit—this enables rapid shut-down of the nuclear reactor.
The storage and delivery infrastructure for CO2 cooling gas and nitrogen shut-down gas is on a different scale than many ordinary applications. Thirty tonnes of CO2 is a significant leak by any standards. Anyone in the vicinity of such a leak would be rapidly overcome, probably without warning, and rendered unconscious. It only takes an increase from 0.04% CO2 (ambient level) to 1.5% to cause drowsiness and headache; 3% causes dizziness; 10% is fatal.
Personnel working in areas of potential CO2 leaks would benefit from a reliable gas sensor—either a permanent system fixed to the building or a portable device for personal use. Areas where nitrogen is stored or used should be monitored for oxygen depletion. Analox has a number of instruments specifically for such applications—the dual Aspida O2/CO2 detector offers personal monitoring in confined spaces; the Ax60 is a permanently fitted multi-point CO2 system.
Without being able to monitor a gas leak the site was lucky to only receive an Improvement Notice, and that none of their employees were seriously hurt.
Author: Paul Smith, Technical Writer