This summarises the main categories of nuclear reactor that are currently in service, as well as the main reactor types that are being considered in various countries for future use.
The main drivers for reactor development are:
The pressurised water reactor (PWR) is the dominant form of nuclear power generation system. This is due to economic advantages drawn from its compactness and the relative simplicity of its primary circuit. It is widely used in large-scale power production and in naval marine propulsion. Ordinary, or ‘light’ water is used as the coolant, although the chemistry is modified to give enhanced corrosion resistance and to aid reactor reactivity control. Since light water is used, a nuclear chain reaction cannot be sustained in natural uranium since light water is a poor neutron moderator, so a PWR must use enriched uranium as its fuel. Relatively low levels of enrichment, typically 3 per cent, is used in power station reactors, although naval reactors require considerably higher levels of enrichment.
As it is at such a high pressure, the water in a PWR reactor does not boil, so a heat exchanger known as a steam generator must be used. This takes the thermal energy from the reactor and raises the steam required by the turbine. Modern power station PWR designs have 2, 3 or 4 steam generator loops, housed within a domed containment building. Steam temperatures and pressures produced by a PWR support a well proven steam turbine system that achieves a cycle efficiency between 32 and 36 per cent, giving dependable and economic electricity production. British Royal Navy submarines incorporate a Rolls-Royce designed and built PWR to raise steam, for propulsion and on board electricity generation. The Royal Navy’s latest submarines now use the second generation of a larger Rolls Royce PWR2 core that lasts four times longer than predecessors, eliminating the need for removal from service for mid-life refuelling.
We have manufactured components for civil PWRs and provided digital instrumentation and control systems for over 200 PWR reactors in 20 countries.
We continue to invest in capability to become a major global supplier of components in the next phase of PWR construction. Also, recognising the additional potential for lower power units in applications where a traditional PWR is too large, we have participated in integral reactor developments in which the primary circuit is contained within a single large pressure vessel.
The second most significant power reactor type is the Boiling Water Reactor (BWR). It also uses light water as a coolant and enriched uranium as a fuel. A BWR is simpler than a PWR in that steam for the turbines is produced directly inside the reactor vessel, rather than in a separate a steam generator.
For a given power the BWR is larger than a PWR, but it offers a slightly higher efficiency and has a simpler system. Some utilities have BWRs and PWRs, with the technology selection being based on economic considerations and projections prevailing at the time the plant was ordered.
A third type of power station reactor makes use of ‘heavy water’ instead of ‘light water’ as a coolant. Heavy water, also called deuterium, has the ability to slow neutrons and so sustain a nuclear chain reaction.
This means that the fuel does not need to be enriched, and is a more effective use of fuel - a heavy water moderated reactor uses less mined uranium than a PWR of equivalent power.
In Canadian Deuterium Uranium (CANDU) reactors, water is pumped through pressure tubes surrounding the fuel elements, enabling direct steam delivery to turbines – as in a BWR – or via steam generators as in a PWR. CANDU (for which Rolls-Royce Canada is an equipment supplier) is the only heavy water moderated reactor design that is currently offered for construction of new power stations.
In the early years of nuclear power there was considerable interest into gas cooled reactors. These reactors differ considerably to water-cooled reactors and, although they have a number of attractive features, have been surpassed by PWR and BWRs. No new gas-cooled power reactors have been built for over 20 years.
In the UK, gas-cooled reactors have enjoyed considerable success, but it is unlikely that this technology will be used again in the foreseeable future. The current UK fleet of Advanced Gas-Cooled Reactors (AGRs) use graphite as a moderator and carbon dioxide as a coolant and attain a thermal efficiency of over 40 per cent. Although new AGRs, or derivatives of them, will not be built, the existing stations still have many years of service ahead of them and Rolls-Royce continues to support AGR Plant Life Extension (PLEX) work.
In addition to the water-cooled and gas-cooled reactors there are also other reactor types that have been built for specific purposes.
Graphite-moderated helium-cooled reactors such as the Pebble Bed Modular Reactor (PBMR) and prismatic-fuel high-temperature gas reactor (HTGR) may represent the future of nuclear power. They have high efficiency, intrinsic safety and ability to utilise a variety of nuclear fuel types. Rolls-Royce has undertaken several concept design studies relating to HTGR, including a study of the gas turbine systems that would be required to achieve the high efficiency promised by these high-temperature reactors.
Fast reactors do not have a moderator. Their principal advantages are:
Fast reactor coolants may be liquid metal. For example; sodium, lead or lead-bismuth alloy, combined with a secondary circuit for electricity generation, which could achieve high thermal efficiencies.
Rolls-Royce are cooperating with France's Commissariat A L'Energie Atomique et aux Energies Alternatives (CEA) on development of ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration), a sodium-cooled Generation IV fast reactor concept.
Gas-cooled equivalents may operate at temperatures compatible with gas turbine power conversion, which finds commonality with the gas turbine studies undertaken for the graphite-moderated helium-cooled reactors. Rolls-Royce is participating in studies for the ‘GoFastR’ gas-cooled fast reactor concept.
Future reactors could potentially be fuelled with thorium, an abundant fertile nuclear fuel, providing even longer-term security of energy supply. There are also some safety benefits to the Thorium cycle.
Currently there is an international debate regarding the technical and commercial viability of various Thorium-fuelled reactor concepts. Thorium fuel could potentially be used in various reactor concepts, primarily Graphite-Moderated Helium-Cooled reactors, but also potentially Water-Cooled Reactors (of various types) or Fast Reactors. A Molten-Salt-Liquid-Fuel Thorium reactor concept has been demonstrated in the past.