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Core values

Today's reactors are designed to head off Fukushima-style disasters, explains Andrew Sherry. Better blueprints are key to industry safety.

As the world watches the Fukushima disaster unfold in Japan, questions are being raised about the safety and reliability of nuclear power stations under compounded conditions, where a number of severe accidents happen simultaneously.

Unlike the accident at Chernobyl a quarter of a century ago, the crisis at Fukushima was triggered by natural forces rather than human error. Yet the management of the crippled reactors and spent-fuel storage ponds has been a human story, with operators, the fire service and army working amid local devastation to inject cooling water by any means possible into the reactors and spent-fuel ponds. We now, abundantly, know that to be safe, nuclear plants must be able to withstand both extremes of nature and the human factor.

Such robustness is hugely down to reactor design — to some extent, the 'hidden player' in this disaster. The Fukushima blueprint is decades old, and reactor design has evolved significantly in the interim. So much so, that many of the battles faced by the Fukushima team could never have arisen in a new plant.

What does this mean for the British nuclear industry? Part of the answer could arrive this autumn. Chris Huhne, Britain's Secretary of State for Energy and Climate Change, recently commissioned a report by chief nuclear regulator Mike Weightman on the lessons to be learned from Fukushima regarding Britain's nuclear programme. The report will be made public, allowing lessons to be implemented effectively and in an appropriate manner. And its timing — issued as a draft in May and in full in September — will ensure that the United Kingdom, rather than responding to Fukushima in knee-jerk fashion, will look at the facts fully as they emerge and translate these into key actions to improve homegrown nuclear-plant safety.

Fukushima's boiling-water reactors (BWRs), dating from the 1960s and 1970s, will undoubtedly feature in that assessment. Design-wise, today's nuclear reactors and plants have moved on, as accumulated operational experience, developments in instrumentation and control systems, and better materials and manufacturing technologies have combined in a process of continual improvement. A modern car, with airbags, crumple zones, anti-locking brakes and safety belts, is a significantly safer mode of transport than a 1960s car. In much the same way, the modern nuclear reactor has already addressed many of the factors that contributed to the incident at Fukushima.

Currently, around 60 new nuclear reactors are being built around the world, and they include a number of design features that would significantly enhance the performance of the reactor under the kind of challenging conditions the Fukushima team has had to tackle.

Reactor designs now incorporate a multi-layered 'defence in depth' approach to mitigating safety risks, whatever the trigger. The two designs discussed below have been reviewed within Britain's Generic Design Assessment by the Office for Nuclear Regulation (ONR).

The EPR reactor, an advanced pressurized-water reactor designed by French public multinational AREVA, and the Westinghouse AP1000 each include three protective barrier layers preventing the release of radioactivity into the environment. These barriers are a zirconium alloy cladding encasing the fuel (uranium dioxide pellets); the reactor pressure vessel, a 200-millimetre-thick ferritic steel forging; and a double-layered reinforced-concrete containment building able to withstand severe accident loading such as an earthquake or aircraft impact.

The EPR includes two additional diesel generators, a supplementary back-up system to the four main back-up diesel generators. They are located separately from the main generators to eliminate, as far as possible, common-cause failures between the two systems.

For normal cooling, the AP1000 pressurized-water reactor design also has back-up diesel generators, although its passive system of core cooling requires neither pumps nor human intervention and so safeguards against a total loss of power. The passive safety system uses natural forces such as gravity and convection, as well as valves that open on a loss of power and compressed gas, which combine to eliminate the need for safety-related electrical power. This system also provides operators with a three-day grace period following a serious accident: with safe management of the decay heat within the reactor core during this period, operators can consider longer-term management issues.

The Fukushima incident involved dramatic explosions in units 1 and 3, caused by hydrogen, probably originating from oxidization of the Zircaloy fuel cladding, accumulating in the building and igniting. Both the EPR and the AP1000 include passive recombiners that catalytically combine hydrogen with oxygen to form water, thereby minimizing the risk of hydrogen detonation.

The worst-case scenario at Fukushima, highlighted by UK chief scientist John Beddington in a 15 March talk to the UK Embassy in Tokyo, related to a full core meltdown. The EPR and the AP1000 designs have features to manage and cool the molten core in such an event. The AP1000 floods the reactor cavity to cool the outside of the reactor's pressure vessel, preventing failure and the spilling of molten core debris into the containment. In the EPR, a molten core would flow into a large concrete cavity underneath the reactor pressure vessel, where it can spread over a large surface area and cool safely.

Crisis scenarios aside, we need to remember that the vast majority of Japan's nuclear power stations shut down safely after the earthquake and tsunami hit. Hundreds of people actually took refuge in the Onagawa nuclear plant, more than 100 kilometres away from Fukushima.

Ramping up safety in older reactors such as these is a distinct possibility, too, for instance through retrofits such as installing passive recombiners where not present already, or by installing additional backup diesel generators.

It needs to be said, however, that better engineering is not an all-in-one solution. Human input, as Fukushima has shown, is key to the competent handling of nuclear-industry crises. But built-in safety is the basis. Before any new UK nuclear power stations are built, the ONR will have to be satisfied that their safety features are robust. Public information on these issues must improve dramatically too. Engineers, scientists and the media have a window of opportunity here, to inform and educate on energy needs, climate change, nuclear safety and the realities of radiation.

Andrew Sherry
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Andrew Sherry is director of the Dalton Nuclear Institute at the University of Manchester, UK. e-mail: andrew.sherry@manchester.ac.uk

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John Beddington's talk

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Sherry, A. Core values. Nature (2011). https://doi.org/10.1038/news.2011.209

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