Supercritical water reactor

Supercritical water reactor scheme.

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.^{[citation needed]}

Design

Moderator-coolant

The SCWR uses supercritical water 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 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 transmuted 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.

Control

SCWRs would likely have control rods inserted through the top, as is done in PWRs.

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-60, a strong gamma emitter, so cobalt-containing alloys are unsuitable for reactors. R&D focuses on:

• The chemistry of supercritical water under radiation (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 and challenges

Advantages

• The higher temperatures and use of a supercritical Rankine cycle 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 reactors)
• A fast SCWR can be a breeder reactor, and also burn the long-lived actinide 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 coefficient

References

• INL SCWR page
• INL presentation
• INL Progress Report for the FY-03 Generation-IV R&D Activities for the Development of the SCWR in the U.S.
• Generation IV International Forum SCWR website.
• INL SCWR workshop summary

See also

• Generation IV reactor.
• Breeder reactor
• Reduced moderation water reactor, 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 reactor:
  • Advanced Boiling Water Reactor (ABWR).
  • Economic Simplified Boiling Water Reactor (ESBWR) (generation III+).
• nuclear reactor.
• nuclear fuel.

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)