Agreements on the design of new nuclear reactors for the UK drew a step closer when the Office of Nuclear Regulation and the Environment Agency awarded interim design acceptance certificates (IDACs) in December.
The two government organisations have been carrying out a generic design assessment (GDA) for two types of reactors in the UK: EDF and AREVA’s UK EPR and Westinghouse’s AP1000. But they have also identified a number of issues that need to be sorted out.
Both agencies say they are “satisfied” with how the designers of both reactors plan to resolve the remaining issues which, once completed, will generate the award of a full DAC. The Department of Energy and Climate Change (DECC) plans to finalise this process by the end of 2012. This would mean the blueprint for the reactors was approved.
A range of different concerns, including reactor chemistry, structural integrity and human factors are included among the dozens listed by the regulatory authorities.
Reports produced by government organisations state, for example, that there is “inadequate substantiation of human based safety claims” relating to EDF and AREVA’s EPR while they also take issue with the “completeness of the human factors safety case, specifically in the areas of human error mechanisms, operator misdiagnosis potential and violation potential” in the case of Westinghouse’s AP1000.
More design investigations, more improvements
The completion of the generic design assessment, however, will trigger another set of design investigations relating to the individual sites of the planned new reactors. These would mean adjusting the reactor designs to suit individual characteristics of the various sites.
Flooding and seismic behaviour are examples of differing features of the sites under consideration. Potential flooding could, for instance, determine the height of the key buildings above sea level and the extent of the sea defences and wall, which would vary according to site.
At the same time, the precise seismic behaviour of a site (described as hard or soft sites) depends on the geology. Only once the specific sites are characterised locally can the seismic methodology be applied to the generic design to finalise the site-specific design. To adapt to geological conditions, the design can be altered by, for instance, changing the separation distance between buildings. On a soft site, structures may need to be larger.
The installation of new reactors decades after the first ones were built presents enormous potential for improvements, given innovations in materials science, engineering and other fields that have been commercialised in the last 40 years. Forecasts of a changing climate suggest reactors could be operating in somewhat different, perhaps more extreme, conditions in future.
Information provided by the ONR indicates individual reactor makers have produced innovative ideas to combat potential problems. For instance, Westinghouse has proposed a new type of structure for steel/composite walls and floors in the AP1000.
This involves a ‘sandwich’ of concrete within steel plates – a different approach from previous buildings and numbers among the issues that need to be resolved before the full DAC can be awarded. “Westinghouse has to justify to us that this design is satisfactory and better,” states Kevin Allars, director of nuclear new build at the Office for Nuclear Regulation.
Extreme conditions, extreme measures
To combat potential extreme weather conditions, reactor makers are urged to consider every potential improvement under risk assessment principles that ensure the residual risk “as low as is reasonably practical” (ALARP). “We say to them: is there more you can do to improve safety and push the safety envelope to the edge? One area for that is external hazards. We’ve said all extreme hazards should be considered,” he says.
Other regulators have been consulted across the world, particularly in France, the USA, Finland and China, to broaden the knowledge base and maximise design improvement. “We work very closely with other regulatory authorities,” says Allars.
Issues that have come up include changes to instrumentation and control in the EPR. “All three regulators said we had the same issue that needed to be addressed and it sent a strong signal round the world that we were looking at this harmonised design,” states Allars.
An unexpected hazard, such as Fukushima in 2011, emerging after the final DAC had been awarded, would prompt a review of reactor design.
Jonathan Levy, media relations manager for EDF Nuclear New Build, explains: “EDF Energy reviews all significant operating experience, and assessments are carried out in order to determine as to whether the plant should continue to be constructed without a design change, or continue operating without any changes to mitigate the event.
This occurs whether there is a DAC or not and any changes that may affect the DAC or the station’s safety case, are communicated to the Office of Nuclear Regulation.
“The results of these evaluations are open to, and often are, reviewed by our regulator, the Office of Nuclear Regulation, who ultimately has the power to stop construction or shut down the plant.”
Source: NuclearEnergy Insider