Saturday, February 19, 2011

Why I am a Fan of Small Modular Light Water Reactors

The diagram above is from the Wikipedia article on Light Water Reactors.

I am one of those nuts you see in the grocery store reading labels. Manufacturers seem to be catching on to guys like me and are making the print smaller and smaller. For a few years I worked at a U.S. Navy Submarine Base being built for boomer submarines. I had to read a lot of stuff back then in the construction specifications to do my job as a civilian contractor. Because I was curious, I read a little more about the nuclear power plants that powered the subs and the warheads on the missiles they carried. I don't care much about the warheads, but it is nice to know how they work and why we should never need to work them. The power plants though were interesting.

The average age of a nuclear submarine's crew is about 24. So crew training and equipment complexity have to meet somewhere in the middle. The Navy reactors are about the simplest design there is, which makes it safe and simple to operate. Light Water Reactors are the Navy's choice.

Reactors need moderators to control the rate of the reaction. Regular water is both the moderator, coolant and energy transfer medium in a light water reactor. Because of the type of fuel used in the reactor, a moderator is needed for there to be a reaction. If there is a leak in the reactor vessel, the reaction slows as the water level drops. With no water there is no reaction so this is a simple safety shutoff system. If the reactor should get too hot, the water boils off which again slows the reaction down. This makes the reactor design have a negative temperature coefficient. In other words, it won't meltdown. Correction Boils off may be a poor choice of wording. If the water expands, it becomes less dense which slows the reaction. How the water expands depends upon the reactor design.

The drawing above shows the basic diagram of a pressurized water, light water reactor. The primary water loop is from reactor to heat exchanger. The water is heated by the nuclear reaction then moves to the secondary loop where the water is converted into steam which drives the turbine. The third water loop, used to cool the discharge from the turbine, is shown using water from a cooling tower, river or ocean for cooling. In most new power designs, that loop is replaced with another heat exchanger so the waste heat can be used for a useful purpose. That waste steam can also drive a low pressure steam turbine.

Turbines are designed for steam quality. High quality steam does not condense into water in the turbine, so the turbine can operate more efficiently. Secondary turbines can be designed to operate on low quality steam where the turbine discharge is a mixture of hot water and steam. Then very little cooling is needed before the discharge is just hot water ready to be pumped back through the secondary loop.

Navy reactors are what would be called Small Modular Reactors. The reactor vessel, heat exchanger and containment vessels are small in size and manufactured in secure facilities. This greatly reduces the cost of construction and improves security during construction. Complete reactors can be shipped by rail or barge to their installation site very securely because of their size and design.

While not cheap by any means, the small modular reactors have a long life and can produce electricity for about 5 to 10 cents per kilowatt hour. That is very competitive with other power plant designs.

Light water reactors designed for the U.S. Navy have an excellent safety record and proven performance. While I am all for new technology, there is something to be said for proven technology. This is why I am a fan of Light Water Reactors.

No design is perfect so I have to touch on things that are imperfect and how they are or need to be dealt with.

I am going to add one issue with LWR that I personally think is a non-issue. The LWR design is inefficient in its use of fuel. If there were no options, this inefficiency would lead to more nuclear waste that would need to be disposed of properly. Because of disposal issue, often irrational, spent fuel rod are stored on site. The spent fuel rods are not just useless, dangerous waste. Integral Fast Reactors (IFR), a type of breeder reactor, can use these spent fuel rods. Most of the interesting IFR designs are Generation IV. These Generation IV reactors would produce the highest thermal efficiency with the least nuclear waste. Civilian use breeder reactors and reactors that use the breeder reactor concept without being classified as breeder reactors are in use around the world. Canada's CANDU reactor design is an example of a more efficient reactor design where the principle of a breeder reactor is used but bred fuel is used making energy instead of making Plutonium. While CANDU has an exemplary safety record, the US has not embraced the design. The CANDU design has operated safely long enough that the US stance could and probably should change.

The CANDU and Generation IV designs can make use of spent LWR fuel rods reducing the amount of potential nuclear waste. Our current energy secretary has highlighted the need for a blend of Generation III and Generation IV technology. Though I am not sure this is a direct quote from the secretary, it is a good one: "Breeder reactors can “burn” some nuclear waste components (actinides: reactor-grade plutonium and minor actinides), which could turn a liability into an asset. Another major waste component, fission products, would stabilize at a lower level of radioactivity in a few centuries, rather than tens of thousands of years. The fact that 4th generation reactors are being designed to use the waste from 3rd generation plants could change the nuclear story fundamentally—potentially making the combination of 3rd and 4th generation plants a more attractive energy option than 3rd generation by itself would have been, both from the perspective of waste management and energy security." (From Wikipedia).

The advantages of Generation IV designs are obvious, but I am going to withhold recommending one over another until I do more research. The CANDU design though has a proven track record and offers many of the advantages of Generation IV.

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