Exactly 20 years ago this month, on 21 April 1986, workers at the Chernobyl nuclear plant in the former Soviet Union carried out an experiment at very low power with one of the facility’s two “RBMK” reactors. They were, however, unaware that their actions would make the reactor dangerously unstable. Its power rapidly increased, leading to the destruction of the core and a massive chemical explosion. The World Health Organisation estimates that between 40 and 50 staff and emergency workers died as a result of radiation released during the accident. It also resulted in widespread contamination and radiation exposure.

The Chernobyl disaster was a significant moment in the development of nuclear power, particularly in terms of the public’s attitude to this form of energy. It also highlighted how the Soviet nuclear industry was badly regulated, suffered from lax operation and training, and tolerated weak reactor designs. Surprisingly, one power station in Lithuania and three in Russia are still using RBMK reactors after design deficiencies related to Chernobyl were corrected, although the former is due to close in 2009 following appeals by the European Union on safety grounds.

Now, however, it appears that the tide is turning back in favour of nuclear power as countries contemplate the problems of climate change, rising energy prices and the fact that many nuclear plants are reaching the end of their lives. In the UK, for example, five of the nine “first-generation” Magnox power stations, most of which were built in the 1960s, have already closed after operating safely beyond their expected lives. The rest of the country’s 12 nuclear stations, which use mainly “second-generation” advanced gas-cooled reactors (AGRs), will all have closed by 2023. No new nuclear plants have opened since the UK’s single pressurized water reactor (PWR), Sizewell B, came online in 1990 (see “The physics of nuclear reactors”).

Given that the UK’s nuclear power stations together generate 20% of the country’s electricity’s needs, what will happen when these facilities shut? Can the UK reduce its emissions of carbon dioxide and have a diverse supply of electricity without nuclear power? Is it a problem that the country will be importing about 75% of its primary energy by 2020? To find answers to these questions, the UK government has recently launched a review of the country’s energy needs (see “Nuclear questions” Physics World January 2006 p14).

The review contains a consultation document entitled “Our energy challenge” that sets out four goals: to cut carbon-dioxide emissions; to ensure reliable supplies of energy; to achieve sustainable economic growth; and to ensure that every home is adequately and affordably heated. The review is also examining whether recent increases in energy prices have changed the assessment in the government’s energy White Paper of 2003, which was that new nuclear power stations might one day be needed to meet carbon-emission targets but that “new build” should be a future option only. The consultation ends on 14 April, with the review team reporting to Tony Blair in the summer.

New designs

While the UK hesitates, several other countries are already taking action. Finland has commissioned a new European pressurised water (EPR) reactor at Olkiluoto, which is currently being built by the French state-owned firm Areva and Siemens of Germany. It is set to open in 2009. Three companies in the US have said they intend to apply for permits to build two “advanced passive” AP-1000 power stations each, while France, Taiwan and China are either planning or building new stations.

One advantage of third-generation power stations like the AP-1000 and the EPR is that they include more “passive” safety features than first- and second-generation plants – that is, systems relying more on natural forces such as gravity, natural circulation and compressed nitrogen gas rather than relying on a multiplicity of pumps and valves. This makes these plants also far simpler than a conventional PWR.

In my view, the most likely candidates for new nuclear power stations in the UK – assuming they are approved – will be either Areva’s EPR or the AP-1000, which is designed by Westinghouse. Both are based on existing PWR technology, with which UK regulators are familiar. In terms of electrical output, the EPR generates 1.6 GW, while the AP-1000 generates 1.15 GW, which is close to the output of Sizewell B. Another option is the advanced CANDU reactor (ACR), which already operates in Canada and several other countries, but it is an unfamiliar system in Europe and is unlikely to be chosen.

As part of a study of the regulatory health and safety aspects of all future energy sources, the UK government has asked the Health and Safety Executive (HSE) – through the Nuclear Installations Inspectorate – to consider the concept of “pre-licensing” designs, as happens in the US. The HSE will report by June this year. If pre-licensing is approved, the views of the regulator might be available before an operator decides which type of plant to build and a public enquiry begins. This could make it more straightforward for a firm wanting to build a nuclear plant.

The next generation

Looking further into the future, the prospects are even more exciting. In 2000 the US Department of Energy launched an international initiative known as Generation IV, which seeks to carry out research into new nuclear power stations that could be ready to build by 2030. The initiative now includes nine countries – Argentina, Brazil, Canada, France, Japan, North Korea, South Korea, South Africa and Switzerland – plus the UK, which joined last year. (The European Union’s Euratom programme is also a member on behalf of other European countries.)

The long-term focus of Generation IV research is to build reactors that are economically competitive, safe and environmentally sound. The aim is also to make the reactors proliferation resistant, for example by allowing both plutonium and long-lived waste to be recycled together so that both can be destroyed through further exposure in the reactors. However, none of these targets are specific at this stage – indeed, particular reactor concepts may have distinct roles.

The Generation IV initiative has so far produced a “technology roadmap” that identifies six reactor concepts showing the most promise. Of these six, the UK government and industry has decided to focus on just three – the very high temperature reactor (VHTR) led by France; the gas-cooled fast reactor (GFR) led by the US; and the sodium-cooled fast reactor (SFR) led by Japan. In the UK, the Department of Trade and Industry will begin funding research on Generation IV concepts from this month to the tune of £5m a year over two years.

Each concept has its own advantages (see “Generation IV: the UK’s chosen designs”), but the most promising at this stage appears to be the various forms of high-temperature reactor cooled by helium and containing a graphite core. Indeed, demonstration plants based on this design are to be built in South Africa and the US, while experimental reactors already exist in Japan (HTTR) and China (HTR-10). Ironically, the prototype for all these reactors was the “Dragon” reactor experiment, which operated at Winfrith in the UK between 1964 and 1973.

Causes for concern

But does the UK have the industrial strength to build new nuclear power stations? It still runs a dozen nuclear stations and builds nuclear-powered submarines using PWRs, but it is more than 15 years since Sizewell B opened. New stations would require the country to use the expertise of overseas firms such as Areva or Westinghouse, which BNFL sold to Toshiba earlier this year. Some major components – such as the steel pressure vessel, the large steam turbine and the steam generators – would have to be built abroad.

Nevertheless, much of the construction would be civil engineering, at which the UK excels. Indeed, the Nuclear Industry Association (NIA) has estimated that its member companies could build at least 50% of any new nuclear power station. This proportion could rise to about 80% if a series of power stations is commissioned, because companies would then be more likely to invest in greater industrial capacity.

Maintaining the UK skills base is crucial, which is why the research councils’ Keeping the Nuclear Option Open (KNOO) programme is so essential. Led by Imperial College, with support from the universities of Bristol, Cardiff, Leeds, Manchester and Sheffield, it funds research into new reactors and provides training for postdocs and PhD students. The project includes exciting research into Generation IV systems, as well as research into advanced PWRs, materials and waste.

If new nuclear power stations are built, the UK government’s consultation document quite rightly highlights waste management as an issue that must be re-examined. The Nuclear Decommissioning Authority is currently responsible for dealing with existing radioactive waste from military and civil nuclear programmes, while in 2003 the government appointed the Committee on Radioactive Waste Management (CoRWM) to consult on what to do with nuclear waste over the very long term.

As the energy-review consultation document points out, the CoRWM has confirmed that waste from new nuclear reactors could be accommodated by the options for waste repositories being considered. I agree with the Royal Society, which earlier this year called on the CoRWM to work more closely with scientists before it makes a final recommendation to government in July on what form of waste repositories should be built.

I hope that the UK’s energy review will call for new nuclear stations to be built. They are, I believe, essential if we want a safe, secure and environmentally friendly mix of electricity supply. But nothing will happen unless the public supports new stations and unless business can raise the capital sums in what is a privatized and deregulated energy market. Turning words into action will be far from easy.

• See “The nuclear alternative” on pp42-43; print version only

The physics of nuclear reactors

Most existing nuclear power plants are pressurized water reactors, in which water is used both to carry heat away from the reactor core and as the “moderator” to allow the chain reaction to take place. The water is prevented from boiling by a pressurizer that maintains the pressure somewhat above saturation so that the water remains liquid. The core, enclosed in a steel pressure vessel, consists of low-enrichment uranium-oxide pellets made up into rods clad in zirconium alloy, which in turn are grouped into fuel assemblies.

Connected to the vessel are several “loops”, each of which takes the primary hot water to a generator, in which steam is produced by boiling secondary water. The loop then returns the primary water to the pressure vessel. A reactor building, known as the “nuclear island”, encloses the vessel and its surrounding pipe work and safety systems. Equipment outside the island, such as steam turbine-generators, is largely the same as for any fossil-fuelled station.

The above description is of a “thermal” reactor, so-called because the moderator allows neutrons to slow to thermal energies to cause fission. However, in “fast” reactors, like the gas-cooled fast reactor (GFR) and the sodium-cooled fast reactor (SFR), the neutrons are not slowed and so could destroy long-lived waste mixed in fuel through the process of transmutation. Fast reactors also generate energy from a larger proportion of the uranium than thermal reactors.

Generation IV: the UK’s chosen designs

Generation IV is an international research programme into new forms of nuclear reactor that might come on-line by 2030-2040. The UK is currently involved in three of the six main designs that are being studied, with the aim being to retain the country’s skills in nuclear-reactor design.

• Led by France, the very high temperature thermal reactor (VHTR) will be very safe and could produce both electricity and high-temperature-process heat to make hydrogen. The benefits for the UK are that it already has extensive experience of the operation, technology and licensing of gas-cooled graphite-moderated systems.
• The gas-cooled fast reactor (GFR) is being led by the US. The advantages of this design are that it can recycle actinide waste and could provide a long-term energy supply through extending the use of uranium reserves. The UK already has extensive design and development experience, including participation in European research programmes in the field.
• The sodium-cooled fast reactor (SFR), led by Japan, has three main benefits: the technical feasibility of one variant has already been proved; the reactor could recycle actinide waste; and it has potential as a long-term energy supply. The UK has considerable experience in this concept, through the prototype fast reactor programme at Dounreay and the European fast reactor programme.