NUCLEAR

Generation four: fission reinvented

The nuclear energy sector – which produces 35% of the European Union’s electricity – offers real potential for reducing hydrocarbon use. Although nuclear energy is neutral in terms of its greenhouse effect and is capable of generating a large amount of power, it also burns a limited resource, produces bulky waste and poses an enormous risk. How can these failings be mitigated and the nuclear industry’s economic competitiveness increased? This is the challenge which the fourth generation of reactors intends to take up.

Image virtuelle, en 3D, du réacteur EPR de 3ème génération. Le premier est actuellement en construction en Finlande. © Image & Process, AREVA NP
3D virtual image of the third-generation EPR reactor. The first one is under construction in Finland. © Image & Process, AREVA NP
Image virtuelle du circuit primaire du réacteur EPR. Il est constitué principalement de la cuve, des générateurs de vapeur, du pressuriseur et des pompes primaires. © Image & Process, AREVA NP
Virtual image of the primary circuit in the EPR reactor. It comprises mainly the reactor vessel, steam generators, pressuriser and reactor coolant pumps. © Image & Process, AREVA NP

Nuclear energy has come back into favour after two difficult decades: in late 2007, the AREVA industrial group met with the French and Chinese presidents to sign a contract to supply China with two European pressurised water reactors (EPR). Another order from the worldnuclear energy leader has led to the construction of the first pressurised reactor in Finland, which is expected to come into service in 2011 after a two-year delay and a further injection of funding.

EPR is the third generation of nuclear reactors. Although it has been improved, it burns uranium 235 and is incapable of mass-recycling the uranium from its spent fuel, meaning that it does not resolve the problem of limited fuel resources. “We know for certain that if nuclear energy continues to be developed using the current pressurised water reactors, which burn uranium 235 (representing less than 1% of the total content of natural uranium), three-quarters of known resources will have been committed by around the middle of the 21st century,” says Frank Carré, Deputy Director of the French Atomic Energy Commission’s Nuclear Deve - lopment and Innovation Division.

Burning all the uranium

The current technology uses water both as a moderator to reduce the velocity of nuclear reactions in the core of the reactor and as a heat-transfer fluid, transferring heat to the exchangers that generate the steam to drive a turbogenerator. This technology does not allow the 99% of uranium 238 in natural uranium to be burned for fuel. In the case of pressurised water reactors, which predominate at present, the uranium needs to be enriched with 3–5% of uranium 235 to make it suitable for burning.

However, the sustainable development challenge demands more: it calls for a fourth generation of fast-neutron nuclear reactors capable of burning all the uranium by converting it into plutonium. A number of initiatives testify to renewed interest in this technology, which has led to the creation of several prototypes since the 1960s. Apart from national projects in India, China and Russia, there is the Generation IV International Forum of leading nuclear technology nations, the International Atomic Energy Agency (IAEA) International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO), the United States Global Nuclear Energy Partnership (GNEP), and the European Union Sustainable Nuclear Energy Platform.

France, the United States and Japan are planning to build prototypes of sodium-cooled reactors in around 2025. Sodium is another heat-transfer medium that has been subject to intensive research. Sodium does not moderate the neutrons, allowing their velocity to convert natural uranium into plutonium (itself a nuclear fuel) and even to regenerate it efficiently, allowing the plutonium to be recycled roughly 10 times. This saves on uranium and reduces the amount of waste. A further advantage is that the large stocks of depleted uranium from existing nuclear power stations (220 000 tonnes in France) can be used as a fuel reserve.

Cooperation not competition

However, for the time being the excessive cost of this hypothetical technology limits its commercial viability, unless pressure on the uranium market makes it competitive soon. “Russia, India, Japan and China continued to develop these reactors after the United States stopped in the late 1970s and Europe stopped in 1998 when it closed down its Superphenix fast breeder reactor in France. In 2010, India is set to launch a prototype to generate 500 MW of electricity” (1). China is also in the world fast-neutron race with an experimental reactor planned for 2010.

At the moment, the technological challenges are so great that the rivals of tomorrow are joining forces. This has led to the Generation IV international forum, which includes the United States, France, Japan, South Korea, South Africa, Brazil, Argentina, the United Kingdom, Canada, Switzerland, the European nuclear power nations (EURATOM) as well as, most recently, China and Russia. Between 2000 and 2002, experts worked to select six potentially important systems for the 21st century. Grouped under the title “Generation IV nuclear energy systems”, they have not all reached the same degree of maturity.

Six potential technologies

Apart from sodium-cooled fast reactors, there is also renewed interested in very-hightemperature reactors. “The international effort in very-high-temperature technology has been revived by projects for the Pebble Bed Modular Reactor (PBMR) in South Africa (planned for 2014) and the Next Generation Nuclear Plant (NGNP) in the United States (planned for around 2020).” This technology could extend nuclear power applications to include industrialheat generation, in particular for the production of hydrogen, synthetic fuels and seawater desalination to produce drinking water, all of which are key resources for the future.

Two other innovative technologies involve developing new heat-transfer mediums for fast-neutron, lead and helium gas systems. “As gas is not such a good heat-transfer medium as sodium, gas-cooled fast reactors call for the development of refractory fuels that are able to withstand cooling accidents. On the other hand, compared to liquid-metal-cooled reactors (sodium and lead), they offer the advantage of a monophasic, chemically inert heat-transfer medium and easier access for in-service inspection and repairs.”

The last two technologies, molten-salt reactors and supercritical-water-cooled reactors, are seen as longer-term solutions for which the prototype-building phase is still a long way off.

The risks

All these systems will need to satisfy safety requirements, which have been stepped up markedly since the Chernobyl accident in 1986 and the attack on the World Trade Center in 2001. The systems will rely on robust containment envelopes and an accident-management system that minimises the need for human intervention – in particular for evacuating the reactor’s shut-down power. The risk of proliferation would be managed by a combination of IAEA controls, recycling methods deterring the diversion of nuclear material and regional cycle centres offering their services to countries operating reactors (supplying fuel and taking back spent fuel). Although this would spare such countries from equipping themselves with potentially hazardous technologies, it would also create an imbalance with nations that control the entire technology chain, in itself posing a geopolitical risk.

The advent of generation IV nuclear energy systems, which are set to play a key role in the world energy balance, appears to be heavily dependent on technology development prospects, environmental concerns and economic strategies. Their future exploration will also need to take into account rival nonnuclear energies and society’s acceptance of such energies.

Axel Meunier

  1. All quotes are from Frank Carré. 


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What about fusion?

Just like today’s reactors, fourth generation systems are still based on the fission principle, where a very heavy nucleus (uranium or plutonium) is struck by a neutron to split it into two lighter nuclei. The result is energy and the neutrons that allow the reaction to continue. Conversely, nuclear fusion, for which the ITER facility currently under construction at Cadarache in France represents an important demonstration phase, relies on the fusion of two light nuclei (deuterium and tritium in a first phase) into a heavier nucleus (helium).

The result is energy and a neutron that plays an essential role in regenerating tritium.

However, the fusion reaction can only occur in plasma at a temperature of some 100 million degrees, and ITER must demonstrate that it can be controlled in the presence of nuclear reactions. The expected next phase, in around 2040, is the construction of a demonstration rea ctor (DEMO) that will generate electricity and regenerate the tritium consumed. By that date, fourth-generation fission systems are expected to already be in commercial operation.



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