Wednesday, January 30, 2008

Reactor uprates

Brian Wong has reported that MIT and Westinghouse have been developing new technologies that would, with minor modifications increase PWR electrical output by 50%. These developments include new "doughnuts" shaped design for fuel pellets. Other power technologies including the use of "spiked" or nanofluid water -- water interspersed with tiny particles of oxides and metals only billionths of a meter in diameter - to increase fluid heat transfer are being investigated by MIT. Not only would the new fuel design allow for increased reactor power output, but it would also increase reactor safety, by lowering operating temperatures. The "doughnuts" shaped fuel has by itself the potential to add 50% power output to existing and planned reactors.

Well developed approaches to uprating old reactors by more than 20% of designed output already exist. (See here and here) Major uprates are usually conducted in conjunction with major repair designed to extend reactor life. If reactor owners are going to replace parts anyway, it costs little extra to install new parts that push the system to towards maximum power safety will allow.

There are 3 types of current uprates:
• Stretch Power Uprate (SPU) – which yields up to 7% thermal power uprate through additional analysis of existing safety margins in the plant, usually with no hardware changes required.
• Measurement Uncertainty Recapture (MUR) power uprate that usually nets a 1.5% thermal power uprate through improved feedwater flow measurements.
• Extended Power Uprate (EPU) - which can result in 20% or more thermal power uprate through analysis of safety margins at higher powers and implementation of necessary plant modifications.

Increasing coolant flow rates is the key to uprating since increase of the reactor thermal power would be restricted by the maximum fuel temperature limit. Thus if coolant flowed through the fuel, thermal power could be increased without increasing fuel temperature. The modification to the reactor would cost far less per MW than building a new reactor.

Reactor uprating to date has had virtually no "green" opposition.

EPU uprates make significant improvements in reactor output. In 2006 the NRC approved a 20% EPU uprate for the Vermont Yankee reactor, increasing its generating capacity to 640 MWe. This uprate would be part of a refurbishing program that the NRC would expect in order to license the extension of the life of the 36 year old reactor for another 20 years. The reactor started operation in 1972 and an application for a 20 year life extension has been given to the NRC for its approval. Nuclear plants begin to face an ever increasing likelihood of major systems wearing out after the 40 year mark. Thus life extension licensing by the NRC is dependent on replacement of parts likely to ware out.

Both Boiling Water Reactors (BWRs) and Pressurized Water Reactors (PWRs) can be uptated, but PWRs yield better uprate results. MIT is, however, also working to improve BWR performance and safety. MIT has developed a new fuel design for BWRs that promises a 30% power density uprate while lowering coolant pressure by 30%, thus enhancing electrical output and reactor safety.

The suggested 50% uprate made possible by the MIT/Westinghouse annular fuel, and the 30% power increase for BWR's could be installed as a part of the NRC mandated 40 year refurbishment and would not impose exceptional expenses beyond the basic cost of the or repair and replacement.

Westinghouse has already developed uprates to the AP-1000 that increase its generating capacity to 1250 GWs. The Chinese have bought the uprate technology. A 50% uprate to the AP-1000 would being its power generation into the 1800 MW range. If this is possible, it would be very attractive to Westinghouse and its customers, because the modification would add little to AP-1000 construction costs. A 30% uprates would be also be attractive for new GE BWRs.

Uprating older American reactors with the new MIT technology would increase the output of the American reactor fleet by nearly half, with small materials input. Theoretically, even without new reactor construction, the nuclear contribution to the national electricity supply could be increased to nearly 30%, thus cutting national greenhouse gas emissions by 4%.

The fuel design modifications for LWR's have in my estimation a high probability of success. The "spiked" water approach has more problems in my estimation. It is less likely to come to fruition in the short term, and may never realize the potential MIT scientists envision for it. According to MIT scientist Jacopo Buongiorno, nanoparticles could agglomerate and settle quickly if chemical and thermal conditions are not carefully maintained within the reactor. This suggest that the presence of nanoparticles could pose significant safety hazards in the event of a nuclear accident. A temperature buildup might force the precipitation of nanoparticles, which in turn might cause coolant water to flash into steam. The result could be a potentially disastrous reactor explosion. The NRC, I think, will look long and hard before it licenses the use of nanoparticle coolant technology in reactors.

The MIT modification of LWR fuel shape does not, however, solve the problems of the nuclear industry. No matter how many modifications are built into LWR's to enhance there efficiencies, some fundamental disadvantages of LWRs can never be removed. Advances in LWR technology can keep LWR's competitive in cost with renewables, while decreasing materials input per MW of output. Long term radical design approach changes are needed to increase generating thermal efficiency, lower fuel and construction costs, streach the reactor fuel supply, enhance reactor safety and eliminate the problem of nuclear waste. The Molten Salt Reactor can achieve all of these goals, and represents the best alternative to the LWR approach.

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