The different "generations" of nuclear technology
Nuclear reactor technology has been under continuous development since the first commercial exploitation of civil nuclear power in the 1950s. This technological development is presented as a number of broad categories, or 'generations', each representing a significant technical advance (either in terms of performance, costs, or safety) compared with the previous generation. They range from Generation-I nuclear systems, such as the first commercialised power plants of various designs (gas-cooled / graphite moderated, or prototype water cooled & moderated), through Generation-II designs, which are the standard light-water pressurised and boiling water reactors in operation today, to the Generation-III designs that are now in construction in several countries. The Generation-III designs are an evolution of current light-water reactor (LWR) technology with improved performance and extended design lifetimes, and also more favourable characteristics in the event of extreme events such as those associated with core damage. A typical example is the EPR - the European Pressurised-water Reactor.
From basic research on new reactor designs to actual commercial exploitation takes many years, if not decades. Already, the nuclear research community is looking at a range of advanced - so-called Generation-IV - reactor designs that could be commercially deployed from 2040. The nuclear reactors in use today, such as LWRs, require the energetic neutrons released in fission first to be slowed down to thermal energies (1 eV, or electron Volt) before they can initiative further fission events and therefore maintain the chain reaction. For this reason, these reactors are said to be thermal reactors. However, some Generation-IV reactors are able to 'burn' fuel without first slowing down the neutrons, and are therefore termed fast neutron reactors. Fast reactors are not new - they have existed for decades though have never been widely exploited commercially. They have the advantage that they can 'breed' large amounts of fissile material (Pu-239) from fertile material (U-238) and can therefore extract at least 50 times more energy than current reactors from a given quantity of uranium. This makes them particularly attractive as a means of increasing the sustainability of nuclear power in a scenario of dwindling uranium reserves.
However, fast neutron reactors (FNRs) also present specific technical challenges, in particular relating to safety, physical protection & non-proliferation, and economics, and all these aspects are currently being investigated as part of the global research effort into Generation-IV systems coordinated by the Generation-IV International Forum (GIF). In fact, six types of Generation-IV systems are currently being investigated. Four are fast neutron reactor designs, one is a thermal neutron reactor (very high temperature reactor, VHTR) and one is a supercritical water reactor (SCWR), which could be operated as either a thermal or fast reactor.
Thermal vs. fast reactors
Most nuclear reactors operating today are thermal reactors, so named because the fast-moving neutrons released in the fission process must be slowed down to thermal energies (much less that 1 electron volt) before they can sustain the chain reaction. This slowing down takes place in the moderator, and the most common moderator is ordinary water, as used in LWRs.
In fast reactors, the reactor core and fuel composition is designed so that a chain reaction can be sustained with more energetic, or fast, neutrons. This means a moderator is no longer needed and the reactor core is more compact. In addition, the coolant does not also have to serve as a moderator, as it does in LWRs, and alternative materials such as liquid metals can be used, which are more efficient at removing heat.
This in turn means that fast reactors can have a higher power density and are more compact than thermal reactors, thereby saving on construction materials. Crucially, because fast reactors operate in a more energetic neutron spectrum, they can more readily transform the U-238 in the fuel into plutonium through the capture of fast moving neutrons. When the fuel is discharged from the reactor, it can be reprocessed to extract the plutonium, which is then recycled in fresh fuel. Fast reactors can even be designed to produce, or breed, more plutonium from U-238 than they consume in the chain reaction. In such configurations they are called Fast Breeder Reactors. They therefore offer the possibility to develop a true 'closed fuel cycle', involving repeated recycling though reprocessing of discharged spent fuel in order to remove fissile material. This can be compared with many of today's thermal reactors operating using an 'open fuel cycle', in which the nuclear fuel is kept in the reactor for one cycle and once discharged is considered as waste.
The main advantage of a closed fuel cycle using Fast Breeder Reactors is that it enables much more energy to be extracted from the original uranium, of which the isotope U-238 makes up 99.3%. By also exploiting the U-238, instead of just the U-235, these reactors can produce more than 50 times more energy from the same quantity of natural uranium. Though a small amount of U-238 is converted to plutonium in thermal reactors, and can be recycled in fresh fuel (call MOX), only fast reactors offer the possibility of exploiting the full energetic potential of our uranium resources. This has major implications for the long-term sustainability of nuclear energy and the production of carbon-free energy.
Generation-IV: a global effort
Pre-commercial research on Generation-IV concepts is being coordinated at the global level by the Generation-IV International Forum (GIF). The fully ratified GIF member nations are Canada, China, France, Japan, Korea, Russia, South Africa, Switzerland, and USA, with Euratom also a member. Though commercial deployment of Generation-IV systems is not expected before 2040, the R&D effort must start now. GIF offers a forum for cooperation in this research on the basis of in-kind contributions to the common development of a number of more promising nuclear reactor concepts. These are the Sodium Fast Reactor (SFR), the Very High Temperature Reactor (VHTR), the Lead Fast Reactor (LFR), the Gas Fast Reactor (GFR), the Super-critical Water Reactor (SCWR), and the Molten Salt Reactor (MSR). These six concepts were chosen through expert solicitation and down-selection from a wide range of possible designs, and are considered to exhibit the greatest potential to show the desired Generation-IV characteristics of increased sustainability, competitive economics, high level of safety, increased proliferation resistance and, for some designs, the ability to cogenerate high grade heat for use in industrial processes (chemical industry, production of hydrogen or synthetic fuels, etc.)
Generation-IV research covers a broad range of disciplines and areas, and includes work on the fuel cycle as well as the reactor components. For example, the aim it to develop fast reactors that can also burn the minor actinides recycled from spent fuel. At the moment, when minor actinides are separated from the spent fuel, they end up in the waste, where they are responsible for much of the heat and radiation produced by the waste in the long term. By recycling the minor actinides back into the reactor, and by careful design of the fuel and operation of the reactor, they can be burnt in the core (transmuted) into less radiotoxic and shorter-lived radionuclides. This is not only an effective way of reducing waste quantities, but the recycling of the minor actinides along with the plutonium also greatly reduces the risk of proliferation because pure bomb-grade plutonium is at no point separated from the other components of the spent fuel. This would make the fuel cycle an extremely unattractive source of nuclear material for an illicit atomic weapons programme.