Supercritical water reactor
Encyclopedia
The supercritical water reactor (SCWR) is a Generation IV reactor
concept that uses supercritical water
(referring to the critical point
of water, not the critical mass of the nuclear fuel) as the working fluid
. SCWRs resemble light water reactors (LWRs) but operating at higher pressure and temperature, with a direct once-through cycle like a boiling water reactor
(BWR), and the water always in a single, fluid state like the pressurized water reactor
(PWR). The BWR, PWR and the supercritical boiler are all proven technologies. The SCWR is a promising advanced nuclear system because of its high thermal efficiency
(~45% vs. ~33% for current LWRs) and simpler design, and is being investigated by 32 organizations in 13 countries.
as a neutron moderator
and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (PWR
), or jet/recirculation pumps, steam separators and dryers (BWR). Also by avoiding boiling, SCWR does not generate chaotic voids (bubbles) with less density and moderating effect. In a LWR this can affect heat transfer and water flow, and the feedback can make the reactor power harder to predict and control. SCWR's simplification should reduce construction costs and improve reliability and safety.
The neutron spectrum will be only partly moderated
, perhaps to the point of being a fast neutron reactor
. This is because the supercritical water has a lower density and moderating effect than liquid water, but is better at heat transfer, so less is needed. In some designs with a faster neutron spectrum the water is a reflector outside the core, or else only part of the core is moderated.
A fast neutron spectrum has three main advantages:
, a strong gamma emitter, so cobalt-containing alloys are unsuitable for reactors. R&D focuses on:
Generation IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
concept that uses supercritical water
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...
(referring to the critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...
of water, not the critical mass of the nuclear fuel) as the working fluid
Working fluid
A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder...
. SCWRs resemble light water reactors (LWRs) but operating at higher pressure and temperature, with a direct once-through cycle like a boiling water reactor
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...
(BWR), and the water always in a single, fluid state like the pressurized water reactor
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
(PWR). The BWR, PWR and the supercritical boiler are all proven technologies. The SCWR is a promising advanced nuclear system because of its high thermal efficiency
Thermal efficiency
In thermodynamics, the thermal efficiency is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a boiler, a furnace, or a refrigerator for example.-Overview:...
(~45% vs. ~33% for current LWRs) and simpler design, and is being investigated by 32 organizations in 13 countries.
Moderator-coolant
The SCWR uses supercritical waterSupercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...
as a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (PWR
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
), or jet/recirculation pumps, steam separators and dryers (BWR). Also by avoiding boiling, SCWR does not generate chaotic voids (bubbles) with less density and moderating effect. In a LWR this can affect heat transfer and water flow, and the feedback can make the reactor power harder to predict and control. SCWR's simplification should reduce construction costs and improve reliability and safety.
The neutron spectrum will be only partly moderated
Reduced moderation water reactor
The Reduced-Moderation Water Reactor , also referred to as the Resource-renewable BWR, is a proposed type of light water moderated nuclear power reactor, featuring some characteristics of a fast neutron reactor, thereby combining the established and proven technology of light water reactors with...
, perhaps to the point of being a fast neutron reactor
Fast neutron reactor
A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons...
. This is because the supercritical water has a lower density and moderating effect than liquid water, but is better at heat transfer, so less is needed. In some designs with a faster neutron spectrum the water is a reflector outside the core, or else only part of the core is moderated.
A fast neutron spectrum has three main advantages:
- A higher power density, generating more power for the same size of reactor
- A conversion ratio of greater than 1, which makes breeder reactors possible. This allows for the efficient use of the greater than 99 % ( U238 ) of natural uranium.
- The fast neutrons split actinides, while long-lived fission products can be transmutedNuclear transmutationNuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'...
with excess neutrons
Fuel
The fuel will resemble traditional LWR fuel, likely with channelized fuel assemblies like the BWR to reduce the risk of hotspots caused by local pressure/temperature variations. The enrichment of the fuel will have to be higher to compensate for the neutron absorption by the cladding, which can't be made from the zirconium customary in LWRs, as zirconium would corrode rapidly. Stainless steel or nickel alloys may be used. The fuel rods must withstand the corrosive supercritical environment, as well as a power surge in case of an accident. There are four failure modes considered during an accident: brittle failure, buckling collapse, overpressure damage and creep failure. To reduce corrosion, hydrogen can be added to the water.Material
The conditions inside an SCWR are harsher than those in LWRs, LMFBRs and supercritical fossil fuel plants (with which much experience has been gained, though this does not include the combination of harsh environment and intense neutron radiation). SCWRs need a higher standard of core materials (especially fuel cladding) than either of these. In addition, some elements become very radioactive from absorbing neutrons, e.g. cobalt-59 captures neutrons to become cobalt-60Cobalt-60
Cobalt-60, , is a synthetic radioactive isotope of cobalt. Due to its half-life of 5.27 years, is not found in nature. It is produced artificially by neutron activation of . decays by beta decay to the stable isotope nickel-60...
, a strong gamma emitter, so cobalt-containing alloys are unsuitable for reactors. R&D focuses on:
- The chemistry of supercritical water under radiationRadiolysisRadiolysis is the dissociation of molecules by nuclear radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux...
(preventing stress corrosion cracking, and maintaining corrosion resistance under neutron radiation and high temperatures) - Dimensional and microstructural stability (preventing embrittlement, retaining strength and creep resistance also under radiation and high temperatures)
- Materials that both resist the harsh conditions and do not absorb too many neutrons, which affects fuel economy
Advantages
- The higher temperatures and use of a supercritical Rankine cycleRankine cycleThe Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is...
improves efficiency (~45% versus ~33% of current LWRs) - The higher efficiency means better fuel economy and a lighter fuel load, so residual (decay) heat would be less
- Supercritical water has excellent heat transfer, allowing high power density, a small core and a small containment structure
- An SCWR is a lot simpler, making it cheaper and more reliable
- Water is liquid at room temperature, cheap, non-toxic and transparent, simplifying inspection and repair (compared to liquid metal cooled reactorLiquid metal cooled reactorA liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use but have also been extensively studied for power generation...
s) - A fastFast neutron reactorA fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons...
SCWR can be a breeder reactorBreeder reactorA breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...
, and also burn the long-lived actinideActinideThe actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
isotopes - A heavy-water SCWR can breed fuel from thorium (4x more abundant than uranium), with increased proliferation resistance over plutonium breeders
Challenges
- Extensive material development and research on supercritical water chemistry under radiation
- Special start-up procedures needed to avoid instability before the water reaches supercritical conditions
- Supercritical water expands more than liquid water when heated, so an SCWR can be less stable than a PWR (but more than a BWR)
- A fast SCWR needs a relatively complex reactor core to have a negative void coefficientVoid coefficientIn nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant...
See also
- Generation IV reactorGeneration IV reactorGeneration IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
.
- Breeder reactorBreeder reactorA breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...
- Reduced moderation water reactorReduced moderation water reactorThe Reduced-Moderation Water Reactor , also referred to as the Resource-renewable BWR, is a proposed type of light water moderated nuclear power reactor, featuring some characteristics of a fast neutron reactor, thereby combining the established and proven technology of light water reactors with...
, a concept that is in some ways similar and in others overlapping to the SCWR concept, and is under development apart from the Generation IV program.
- Generation III reactorGeneration III reactorA generation III reactor is a development of any of the generation II nuclear reactor designs incorporating evolutionary improvements in design developed during the lifetime of the generation II reactor designs...
:- Advanced Boiling Water Reactor (ABWR).
- Economic Simplified Boiling Water ReactorEconomic Simplified Boiling Water ReactorThe type of nuclear reactor formally known as the Economic Simplified Boiling Water Reactor is a passively safe generation III+ reactor derived from the predecessor Simplified Boiling Water Reactor and the Advanced Boiling Water Reactor...
(ESBWR) (generation III+).
- nuclear reactorNuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
.
- nuclear fuelNuclear fuelNuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
.
External links
- Idaho National Laboratory Supercritical-Water-Cooled Reactor (SCWR) Fact Sheet
- UW presentation: SCWR Fuel Rod Design Requirements (Powerpoint presentation).
- ANL SCWR Stability Analysis (Powerpoint presentation).
- INL ADVANCED REACTOR, FUEL CYCLE,AND ENERGY PRODUCTS WORKSHOP FOR UNIVERSITIES (Portable Document Format|PDF).
- Natural circulation in water cooled nuclear power plants (IAEA-TECDOC-1474)