Wednesday, May 27, 2009

Scalability and Breeder Start up

Scalability is a deal breaker in global warming technology. One of the nice things about the LFTR is that it is scalable. You can build them in factories ship them off to coal-fired generation facilities, dig a whole into the ground, plant them, hook um up to the Grid, and turn them on. And then stand back and let them work. Every now and then you might add some thorium and remove some U-233 that would be used to start a new reactor.

Basically you could build as many as you wanted too in the LFTR factory. You would need a start up charge of fissionable material - U-233, U-235, or Pu-239. The start up charge would initiate the chain reaction in the reactor, and begin the breeding process. Later fuel will be derived from breeding, so no further nuclear fuel from external sources would be required to keep the chain reaction going.

The number of start up charges, the material composition of start up charges, and the size of each charge would pose a potential limit on LFTR scalability. LMFBRs would also require start up charges.

Neutron speed would play an important role with faster neutron reactors requiring more fissionable materials to keep a chain reaction going. For example French researchers studying Molten Salt Reactors operating at various neutron speeds found that a Thermal TMSR requited a charge of 790 kgs of U-233 in order to maintain breeding in a 1 GWe reactor. An Epithermal TMSR required 2400 kgs to fulfill the same conditions. While a Fast TMSR required 5200 kgs of U-233. The French also reported that a standard fast neutron reactor - I assume a LMFBR -would require 12,25o kgs of plutonium.

An S PRISM related study "S-PRISM Fuel Cycle Study: Future Deployment Programs and Issues," suggested that as of the year 2000, four hundred tons of plutonium could be recovered from spent nuclear fuel. This in turn would provide enough plutonium to supply start up charges for twenty-two, 1520 MWe S-PRISM facilities with ab output of 33,440 MWe. That is about 12 tons per 1 GWe of reactor capacity.

Clearly then neutron speed has an adverse effect on reactor scalability.

On the other hand neutron speed also influences the fission rate per neutron absorption, this in turn influences neutron production. Pu-239 fissions 25% more often in a fast reactor than in a thermal reactor. On the other hand it still take more Pu-239 to maintain a chain reaction in a fast reactor than in a thermal reactor. Reactor physics tricks and fuel cycle also seem to influence start up charge size.

A recent discussion on the EfT form produced quite a lot of useful information. "Jagdish" reported that
Indian 500MW PFBR is designed to use only two tons of plutonium.

"Honzik" pointed to French research of epithermal/fast Thorium Molten Saalt Reactors. The French, modeling the use of transuranium materials from spent nuclear fuel, in a 1 GB reactor had calculated a need for 7.3 tons of fissile elements (87.5% of Pu (238Pu 2.7%, 239Pu 45.9% , 240Pu 21.5%, 241Pu 10.7%, and 242Pu 6.7%), 6.3% of Np, 5.3% of Am and 0.9% of Cm). Alternatively the reactior would require a start uo charge of 4.6 tons of U-2330.

Lars reported that
The minimum for unity breeding from the French group is 1.5 tonnes u233 / GWe.
Alex P noted:
the french design has an only radial, not axial, blanket, so for comparison I'd think that the fissile start-up in a LFTR with a fully encompassing blanket can be at least one tonn of u-233 per GWe, or even lower
David LeBlanc noted:
The French TMSR design running without graphite moderator needs upwards of 5 tonnes of U233 or 8 or more tonnes of fissile Pu. They could drop this somewhat if they just wanted to barely break even but not very much since they'll start losing too many neutrons that would migrate into the axial reflectors. In designs in which the blanket is nearly fully encompassing you can get by with much lower fissile concentrations. It is only speculation for now but based on early Oak Ridge studies using sphere within sphere designs I think we could probably get things down to 500 Kg of u233 or maybe even lower but 1000 kg is a fine for a conservative estimate. These designs with lower fissile concentration would also be fairly soft spectrums since the salt itself can do a modest job at moderating the neutrons.
The problem of plutonium in nuclear breeding should be noted. In thermal breeders plutonium suffers from poor neutron economy, while in fast neutron reactors plutonium neutron economy improves but does not compensate for the added requirement for fissile material. Radial and axial thorium blankets in a breeder appears to lower fissile demand by as much as 300%.

The S-PRISM design would appear far less scalable than Epithermal or thermal MSRs. David LeBlanc's estimates are based on the use of blankets with Epithermal MSRs. If we estimate that 2 kgs of reactor grade plutonium from spent nuclear fuel about 1 kg of U233, 500 kgs of U-233 would be a similar startup charge to a ton of RGP. Thus the same amount of RGP that will start 33 GWe worth of S-Prisim FBRs will also start 200 GWe worth of LFTRs. Clearly the LFTR offers scalability advantages over the IFR/S-PRISM.

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