Core decision

Britain’s fleet of nuclear reactors is close to sailing into the sunset. Of the 19 nuclear-powered power stations dotted around the coastline, 11 are no longer in operation and one more will cease generating in 2010. All are based on outmoded technology except Sizewell B in Suffolk — the only one that will continue operating beyond 2020. With the recent Energy White Paper coming down on the side of nuclear power, it seems certain the fleet will be replaced.

But what with? Current nuclear technology is far ahead even of Sizewell B and new reactors are being developed along strikingly different paths. The parties that will decide what sort of reactors will help keep the UK’s lights on — regulatory bodies, power generators, plant contractors, politicians, NGOs and communities — have some tricky choices to make.
Nuclear reactor technologies are described in generations. The earliest, Generation I (GI), are the Magnox reactors, such as the UK’s Calder Hall, the world’s first power-generating reactor, dating back to the 1950s.

Generation II reactors make up most of the world’s reactor fleet and include the advanced gas-cooled reactors (AGRs) used in the UK and the pressurised water reactors (PWRs) in the US. These have more standardised designs than the GI reactors, which were often experimental, bespoke designs. They are also much smaller.

A typical Magnox reactor is 9m high and 17m in diameter, while an AGR is about half the diameter and the PWR, derived from the power plant of a nuclear submarine, is 4m high and 4m in diameter. Sizewell B, the UK’s most modern plant, is a GII PWR, the only one of its type in the country.

The GI and GII plants operate in the same way — the heat generated from nuclear fission reactions in the fuel, which is based on uranium and contained inside the reactor, is carried away by a coolant, which is carbon dioxide (in Magnox and AGRs) or water (in PWRs). This heat is then used to generate steam from water, which is kept apart from any radioactivity. The steam spins turbines to generate electricity just as a steam turbine in a fossil-fuel powered generator.

Refinements in the design of PWRs has led to the development of Generation III reactors. Many of the refinements improve the safety of the plant, including multiple-redundancy systems to keep coolant circulating around the plant (coolant failure led to the accident at Three Mile Island in 1979 and the catastrophe at Chernobyl in 1986). Others improve the thermal efficiency of the plant in converting heat into electricity.

Further design improvements have given rise to a new generation of plants — the Generation III+. Still at the development stage, these have the same layout as a PWR but incorporate safety features that work differently to the multiple-redundant GIII systems.

The main thing that distinguishes GIII from GIII+ is a feature known as passive safety. In all reactor designs until now, the operation of the plant centres around controlling the nuclear fission reaction in the core and keeping the reactor safe: if the plant is left alone, a chain reaction will occur, which will go out of control and heat the core to the point where the fuel melts. Constant intervention is needed to stop this happening.

The concept of passive safety is to reverse this: to require constant intervention to keep the reaction going. If the plant is left alone, the reaction will stop and the core temperature will build to a peak level, below the melting point of the fuel, then begin to fall.

GIII+ designs are also simpler than GIII, require less material to build and have fewer systems.

Even more advanced are the conceptual designs known as Generation IV, which adopt different approaches to fuelling, controlling and cooling the reactor.
The UK’s new reactors are likely to be GIIIs, with some features that are claimed to liken them to the GIII+. Unlike the GI and GII reactors, they will not be designed in this country.

‘We had the capabilities and we proved them, but we lost them,’ said Robin Grimes, professor of materials physics at Imperial College, who is at the forefront of training a new generation of nuclear engineers.