Monday, January 4, 2016

Addendum on IFR Scalability

This note is a follow up to my past post on IFR Breeding
How long would it take to produce enough Plutonium to Power up an LMFBR such as the IFR.  Well first that would depend on the quality of Plutonium you used.  If you got to use fuel a bunch of Plutonium based nuclear weapons, it might take 10 tons of 97% Pu-239 for a 1 GWe reactor..  If you were to use RGP the LMFBR might require 18 tons of RGP for a 1 GWe IFR.  That is because forms of Plutonium present in RGP do not fission after absorbing a neutron while present in the core of a reactor.

RGP contains two isotopes of Plutonium that do not fission after absorbing neutrons, the other isotopes of Plutonium frequently fission after absorbing neutrons, and the faster the neutrons, the more likely Pu-239 and Pu-241 are to fission.  Plutonium produces a lot of Neutronswhen it fissions, but bad things happen to well over a third of them.   Serious efforts to produce Sodium cooled fast breeder reactors has usually lead to a breeding ratio of 1.2 fissionable atoms of Plutonium to every 1 plutonium atom burned by nuclear fission.

There is a simple formula that burning 1 ton of fissionable atoms in the core of a reactor over a years period of time can produce 1 Billion watts of electricity for a year.  Since LMFBRs operate at somewhat higher temperatures, this might formula might suggest a little higher efficiency, but would actually not improve the breeding ratio.  Let us assume that our IFR produces 1.2 fissionable plutonium atoms per Plutonium atom burned.  How long would it take to produce enough plutonium to start a plutonium breeder?  Well we are producing 2400 lbs of plutonium a year in our breeder.  We require 2000 pounds of that to keep our initial breeder going at an optimal rate.  The other 400 pounds can go into reserve for the start up of a second breeder.  How long would it take to collect enough fissionable Pu to start a second breeder.

We are calculating that 10 tons of Weapons grade plutonium would start up our breeder.  Given an accumulation of 400 pounds a year, the answer is an astonishing 25 years.     That will never do is we wanted a scalable reactor system to rescue the planet from AGW.  Is it possible to scale up Plutonium production?

IFR advocates have claimed to me that breeding ratios as high as 1 to 1.7 our possible with the IFR.  This seems improbable, but if possible, might well require considerable R&D.  But consider how long it would take to accumulate a start chsarge for our second IFR, given a 1 to 1.7 conversion ration.  A 1.7 conversion ratio gives us 3400 pounds of fissionable plutonium.  That gives us about 7 years to accumulate a start up charge.  This is much better than the 25 years, but hardly leads to a planet saving rate of reactor startup.  Furthermore we do not even know if this conversion rate is possible, with appropriate nuclear safety.

So lets talk about the indian approach.  First the Indian government assumes that the domestic Uranium supply is large enough to develop a uranium industry.  Thus in order to accumulate enough nuclear waste to get a large number of LMFBRs going, the indian government is required to import a large number of reactors, together with their uranium based fuels.  Unfortunately Indian law discourages foreign reactor manufacturers from selling  new reactors to India.  At any rate Indian fast reactors are designed to produce both plutonium and U-233.  The first indian fast reactor produces Plutoniam at a 1 to 1.04 ratio.  It transforms Thorium 232 into  fissionable U-233 at a 1 to .14 ratio.
The Nice thing about U-233 is that it can be used to start up thermal thorium breeders with very good neutron efficiency.  One ton of U-233 will generate billion watts of electricity for a year.  And it can potentially produce 1,07 pounds of U-233 for every pound burned, although a 1 to 1 conversion ration would be sufficient.  It takes 7 years to save enough U-233 produced by an indian fast breeder, to start an Indian 1 GWe thermal breeder.    This is as good as the supposed super breeding fast breeder, with added safety measures.

However we do not need to go the fast breeder route.  We could start up thorium fuel cycle thermal breeders using U-235 or even RGP as its fuel.  Once we get the breeding cycle going, we would rely for thousands of years on the global supply of thorium, which is quite common, and virtually unused. There is a lot more energy avaliable in thorium, than in the global petroleum supply, but the oil companies are too stupid to recognize the potential binanza under their feet.

Unlike Uranium Plutonium cycle fast breeders Thorium cycle thermal breeders do not require huge ammounts of fissionable uranium and/or plutonium to start their brrding cycle.  Thus the fuel required to start one fast breeder can start 10 thermal thorium cycle breeders.  Given this disparity, if we want to save the planet with breeder reactors, LFTRs or heavy water thermal breeders are the routev to follow, and the LFTR is expected to be the lower cost design of the two.

1 comment:

Engineer-Poet said...

The situation for FBR fuel isn't as bad as you paint it.  Both Pu-240 and Pu-242 have fast-neutron fission cross-sections larger than U-235 at energies above 1 MeV (see tables 5 and 6 on page 13).  For Pu-240, this ratio goes as high as 1.4.  In short, it takes less reactor-grade plutonium to fuel a fast-spectrum machine than U-235.

Starting an FBR with a charge of MEU seems quite feasible.  Fermi 1 used a core enriched to 26.5%, and it was on the small side at 200 MW(t); the S-PRISM would be around 650 MW(t) and probably get by with substantially lower enrichment.  It would require on the order of 5-6x as much natural uranium to make a ton of FBR fuel vs. LWR fuel, but the beauty is that this would only be required once; after the reactor is started it generates more fissiles than it consumes.  Since a LWR changes out about 40% of its fuel every 18 months (45-month turnover), my guesstimate is that the lifetime requirement of enriched uranium for an FBR would require natural uranium equivalent to about 20-25 years of demand for a LWR of similar power.  This suggests that a 25-year buildout would have demand similar to an operating LWR fleet of the final size, after which uranium demand would be zero for many years due to the large inventory of depleted uranium created during the buildout and the previous operation of LWRs.

A growing count of FBRs would also provide a destination for reprocessed Pu from the existing LWR fleet, and get rid of all of the dry casks now dotting current and past plant sites.


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