The breeder reactor uses, not uranium, but plutonium as fuel. This decays or splits in a similar way to uranium-235, producing fast neutrons. However whereas in the conventional uranium-235 reactor the fast neutrons must be slowed with a moderator to enable further nuclear fusion reactions to take place, the fast reactor utilises the fast neutrons so no moderator is required.
The unique feature of the breeder reactor is that the reactor core contains, in addition to plutonium, some uranium-238. Uranium-238, the more common isotope of uranium, can capture the fast neutrons, becoming converted into plutonium in the process.
By careful design it is possible to make a breeder reactor actually produce (or breed) more plutonium than it burns, hence the name. Breeder reactors use liquid sodium as the coolant because the material does not slow down fast neutrons. However the use of this coolant can create severe technical problems.
Several countries have developed breeder reactors but none has entered full commercial service. The coolant, liquid sodium, has proved the Achilles heel in at least two projects, one in France and one in Japan. It is not clear that the nuclear fast reactor has any future for power generation.
There are a number of advanced reactor designs being developed across the world. These include the HTGR reactors discussed above as well as development of the various water-cooled designs. The latter are mainly aimed at improving safety and reducing the cost of construction. Passive safety features which operate in a failsafe fashion if any part of the reactor system fails are being pursued in many designs and modular construction is seen as a key to reducing overall construction cost and time.
The development of nuclear fusion has a history stretching back more than 50 years yet a commercial power plant based on the technology could still be 50 years away. The fusion reaction requires a temperature of 100 million°C. At this temperature all matter exist in a state called a plasma. The plasma must be controlled and contained by a magnetic field. There are no materials capable of resisting 100 million°C without becoming plasmas themselves.
Research into nuclear fusion has focussed on a torroidal magnetic containment for the fusion reaction, the most successful of which has been a design called a Tokamak. Tokamak's have been tested in experimental fusion reactors but no fusion reactor has yet been able to generate more energy than has been supplied to it. That is, the aim of an international project.
The next stage in fusion research and development is a project called International Thermonuclear Experimental Reactor (ITER), a project involving a large group of nations including the USA, Russia, Japan, the EU and China. The aim of ITER is to build a 500 MW fusion reactor to prove the concept. This is likely to cost around €4.5 billion but should be finished by the end of the first decade of the twenty-first century. If it is successful it could pave the way for the first generation of fusion power stations towards the middle of the twenty-first century.
The use of nuclear power raises important environmental questions. It is an apparent failure to tackle these satisfactorily that has led to much of the popular disapprobation that the nuclear industry attracts. There are two adjuncts to nuclear generation that cause the greatest concern, nuclear weapons and nuclear waste.
While the nuclear industry would claim that the civilian use of nuclear power is a separate issue to that of atomic weapons, the situation is not that clear cut. Nuclear reactors are the source of the plutonium which is a primary constituent of modern nuclear weapons. Plutonium creation depends on the reactor design; a breeder reactor can produce large quantities while a PWR produces very little. Nevertheless all reactors produce waste that contains dangerous fissile material. This is a subject of international concern.
The danger is widely recognised. Part of the role of the International Atomic Energy Agency is to monitor nuclear reactors and track their inventories of nuclear material to ensure than none is being sidetracked into nuclear weapons construction. Unfortunately, this system can never be foolproof. It seems that only if all nations can be persuaded to abandon nuclear weapons can this danger, or at least the popular fear of it, be removed. At the beginning of the twenty-first century such an agreement looks highly improbable.
The problem is political in nature. Nevertheless it carries a stigma from which the industry can never escape. The prospect of a nuclear war terrifies most people. Unfortunately for the nuclear power industry, some of the after effects of nuclear explosion can also be produced by a major civilian nuclear accident.
The contents of a nuclear reactor core includes significant quantities of extremely radioactive nuclei. If these were released during a nuclear accident they would almost inevitably find their way into humans and animals via the atmosphere or through the food chain.
Large doses of radioactivity or exposure to large quantities of radioactive material kills relatively swiftly. Smaller quantities of radioactive material are lethal too, but over longer time scales. The most insidious effect is the genesis of a wide variety of cancers, many of which may not become apparent for 20 years or more. Other effects include genetic mutation which can lead to birth defects.
The prospect of an accident leading to a major release of radionucleides has created a great deal of apprehension about nuclear power. The industry has gone to extreme lengths to tackle this apprehension by building ever more sophisticated safety features into their power plants. Unfortunately the accidents at Three Mile Island in the USA and Chernobyl in the Ukraine remain potent symbols of the danger.
This danger has been magnified by the rise of international terrorism. The threat now exists that a terrorist organisation might create a nuclear power plant accident, or by exploiting contraband radioactive waste or fissile material, cause widespread nuclear contamination.
So far a nuclear incident of catastrophic proportions has been avoided. Smaller incidents have not, and low-level releases of radioactive material have taken place. The effects of low levels of radioactivity have proved difficult to quantify. Safe exposure levels are used by industry and regulators but these have been widely disputed. Only the elimination of radioactive releases from civilian power stations is likely to satisfy a large sector of the public.
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