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Supercritical water reactor scheme.

The Supercritical water reactor (SCWR) is a Generation IV reactor concept that uses supercritical water as the working fluid. SCWRs are basically LWRs operating at higher pressure and temperatures with a direct, once-through cycle. As most commonly envisioned, it would operate on a direct cycle, much like a BWR, but since it uses supercritical water (not to be confused with critical mass) as the working fluid, would have only one phase present, like the PWR. It could operate at much higher temperatures and pressure than both current PWRs and BWRs.

Supercritical water-cooled reactors (SCWRs) are promising advanced nuclear systems because of their high thermal efficiency (i.e., about 45% vs. about 33% efficiency for current light water reactors (LWR) and considerable plant simplification.

A key issue in natural circulation is constituted by the stability of the flow mainly when two phase conditions are concerned and when the feedback with neutron kinetics is possible.

The main mission of the SCWR is generation of low-cost electricity. It is built upon two proven technologies, LWRs, which are the most commonly deployed power generating reactors in the world, and supercritical fossil fuel fired boilers, a large number of which are also in use around the world. The SCWR concept is being investigated by 32 organizations in 13 countries.

The reactors are intended for use in nuclear power plants to produce nuclear power from nuclear fuel.

Contents

Design

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Moderator

The SCWR uses water as a neutron moderator. Moderation comes primarily from the high density supercritical water. This high-density water is either introduced from cooling tubes inserted into the core or as a reflector or moderated-part of the core.

The neutron spectrum will be only partly moderated, perhaps to the point that the SCWR will technically become a fast neutron reactor. There are three main advantages for having a fast neutron spectrum. Firstly fast neutron reactors have a higher power density and can therefore generate more power for the same size of reactor. Secondly, the fast neutrons are able to split the long lived actinides, destroying the most long lived nuclear waste through nuclear transmutation. Thirdly, since fission events induced by fast neutrons produce more neutrons per fission event, it becomes possible to design a breeder reactor, which could extract roughly 100 times as much energy from the same quantity of uranium as could a traditional reactor design.

Fuel

The proposed fuel will resemble traditional LWR fuel. However, it is likely the SCWR will use channelized fuel assemblies like the BWR in order to reduce the risk of hotspots caused by local variations in core thermal hydraulic properties. Because the SCWR will operate under conditions exceeding current experience with LWRs and LMFBRs, new criteria for core materials (especially for the fuel cladding) must be developed to ensure safe operation to maintain fuel rod integrity during abnormal transients, normal power operation as well as concerns for release of the fission products caused by oxidation corrosion of the cladding. There are four failure modes considered for the fuel rod's integrity during abnormal transient conditions: mechanical failure, buckling collapse, overpressure damage and creep failure. It is expected that hydrogen will need to be injected into the coolant to reduce oxidation corrosion of the sheath.

Coolant

The coolant will be supercritical water. Operating above the critical pressure ensures the coolant remains single-phase in the core. At a lower pressure it would boil, producing chaotic voids (bubbles) with less density and therefore less moderating effect, making the reactor power output hard to predict and control. At extreme pressure, above the critical point, steam and liquid can be considered to be the same density, and indistinguishable. The hope is that more of the heat produced from fission can be converted into electricity in reactors cooled and/or moderated with supercritical water. Additionally, the elements handling water's phase change from liquid to gas in conventional light water reactors are not needed. This simplification should reduce construction costs and improve reliability and safety. Current LWRs need recirculation and jet pumps, pressurizers, steam generators, and steam separators and dryers, all or most of which would not be required.

Control

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

Material

This section describes the R&D needs for SCWR material. The actual R&D needed to select and/or develop materials that be compatible and resist under condition above condition current LWR. The R&D proposed for SCWR designs focus on the following key areas:

  • Oxidation, corrosion, and stress corrosion cracking
  • Radiolysis and water chemistry
  • Strength, embrittlement, and creep resistance
  • Dimensional and microstructural stability

In addition to those performance factors, the cost of the material and its effect on fuel utilization must also be considered to meet the economic and sustainability requirements of Generation IV designs.

Advantages and disadvantages

Advantages

  • Supercritical water-cooled reactors promise to have thermal efficiencies of approximately 45% versus the current 33% of light water reactors.
  • The supercritical coolant has a high specific enthalpy.
  • The SCWR design is far simpler than current designs, eliminating circulation pumps, pressurizers, steam generators, steam separators and dryers.

Disadvantages

  • Many of the materials needed for the SCWR are either expensive, rare, or do not exist.
  • Specific start-up procedures required to avoid instability.
  • Unknown chemistry.

References

1: INL SCWR page'

2: INL presentation (Portable Document Format|PDF).

3: INL Progress Report for the FY-03 Generation-IV R&D Activities for the Development of the SCWR in the U.S. (Portable Document Format|PDF).

4: Generation IV International Forum SCWR website.

5: INL SCWR workshop summary (Portable Document Format|PDF).

See also

  • 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.

External links


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