Supercritical water reactor
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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.
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[edit] Design
[edit] Moderator
The SCWR uses water as a neutron moderator. Moderation comes primarily from the high density sub-critical 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.
The main advantage for having a fast neutron spectrum is the higher power rating available than using thermal neutron spectrum, because of high power density.
[edit] Fuel
The fuel is traditional LWR fuel. However, it is likely the SCWR will use cladding fuel element (for single fuel element, if there is a collection of fuel elements called fuel assembly) like the BWR to reduce the chance of hotspots causing local variations in core properties. Because of the SCWR operating at exceeding condition than the current experience with LWRs and LMFBRs, thus specific criteria for material especially for cladding must be developed for safe operation to maintain fuel rod integrity during abnormal transient, rated power operation as well as releasing the fission product 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, over pressure damage and creep failure. Hydrogen injection is for reducing oxidation corrosion.
[edit] 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.
[edit] Control
SCWRs would likely have control rods inserted through the top, as is done in PWRs.
[edit] 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 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.
[edit] Advantages and disadvantages
- Higher thermal efficiency. Supercritical water-cooled reactors (SCWRs) are promising advanced nuclear systems because of their high thermal efficiency, about 45% vs. about 33% efficiency for current light water reactors (LWR).
- Unknown chemistry.
- Materials constraints.
- The need of circulation pumps, pressurizer, steam generator, steam separator and dryer can be eliminated, so Balance of Plant is considerable simple than current nuclear power plant.
- Water coolant has high specific enthalpy because coolant under supercritical condition.
- Specific start-up procedures required to avoid instability.
[edit] 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).
[edit] 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.
- Generation III reactor:
- Advanced Boiling Water Reactor (ABWR).
- Economic Simplified Boiling Water Reactor (ESBWR) (generation III+).
[edit] External links
- 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)
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