Friday, January 14, 2011

APS Physics | FPS | Liquid Fuel Nuclear Reactors

Robert Hargraves and Ralph Moir introduce iquid fuel reactors:
APS Physics | FPS | Liquid Fuel Nuclear Reactors
The 2009 update of MIT’s Future of Nuclear Power shows that the capital cost of new coal plants is $2.30/watt, compared to LWRs at $4/watt. The median of five cost studies of large molten salt reactors from 1962 to 2002 is $1.98/watt, in 2009 dollars. Costs for scaled-down 100 MW reactors can be similarly low for a number of reasons, six of which we summarize briefly:
Pressure. The LFTR operates at atmospheric pressure, obviating the need for a large containment dome. At atmospheric pressure there is no danger of an explosion.

Safety. Rather than creating safety with multiple defense-in-depth systems, LFTR’s intrinsic safety keeps such costs low. A molten salt reactor cannot melt down because the normal operating state of the core is already molten. The salts are solid at room temperature, so if a reactor vessel, pump, or pipe ruptured they would spill out and solidify. If the temperature rises, stability is intrinsic due to salt expansion. In an emergency an actively cooled solid plug of salt in a drain pipe melts and the fuel flows to a critically safe dump tank. The Oak Ridge MSRE researchers turned the reactor off this way on weekends.
Heat. The high heat capacity of molten salt exceeds that of the water in PWRs or liquid sodium in fast reactors, allowing compact geometries and heat transfer loops utilizing high-nickel metals.

Energy conversion efficiency. High temperatures enable 45% efficient thermal/electrical power conversion using a closed-cycle turbine, compared to 33% typical of existing power plants using traditional Rankine steam cycles. Cooling requirements are nearly halved, reducing costs and making air-cooled LFTRs practical where water is scarce.

Mass production. Commercialization of technology lowers costs as the number of units produced increases due to improvements in labor efficiency, materials, manufacturing technology, and quality. Doubling the number of units produced reduces cost by a percentage termed the learning ratio, which is often about 20%. In The Economic Future of Nuclear Power, University of Chicago economists estimate it at 10% for nuclear power reactors. Reactors of 100 MW size could be factory-produced daily in the way that Boeing Aircraft produces one airplane per day. At a learning ratio of 10%, costs drop 65% in three years.


LGM said...

What is the smallest size a LFTR could be built at? Could it be as little as in the tens or hundreds of kilowatts range?

What is the minimum amount of shielding that is required? In similar sense to the LFTR reactor's physical embodiment making it intrinsic safe, is it possible to design it so that its structure intrinsically inhibits/prevents the emission of radiation without the need for heavy and expensive containment structures?

Charles Barton said...

LGM, reactors produce lots of radioactive isotopes. People will need to be protected from radiation produced by radioactive isotopes in any reactor, But shielding need not be massive structures. One reactor built in Oak Ridge - the tower shielding facility - basically used air for shielding. Basically air shielding requires a lot of air between people and the source of radiation. Underground reactor placement also provides radiation shielding.

LGM said...


I was aware of air shielding. Some of NERVA test units were that way. Sort of OK when you are a long way from anywhere, although back-reflection is going to be an unwelcome issue to deal with.

What I had in mind when I asked my questions was whether a small reactor could be built wherein the structure itself provided much of the shielding. LFTR is such an elegant concept (intrinsic safe, controllable, non-proliforation, common fuel, compact etc. all already demonstrated) that perhaps there are a few other attributes inherent within it waiting to be detected and exploited. An economy in shielding would be welcome I thought (making the reactor smaller, easier to erect and easier to transport from site to site).



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