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 Post subject: Net breeding
PostPosted: Dec 05, 2010 10:36 am 
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As far as i have understand it, it take different times for the numerous LFTR design to reach a CR > 1 or higher. Several decades for the french TMSR reactor. So how long would it take it for David LeBlancs tube in a tube/shell design?

Thx in advance. :D


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 Post subject: Re: Net breeding
PostPosted: Dec 05, 2010 4:23 pm 
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You may be slightly confusing breeding ratio with doubling time - by definition, a ratio is unitless.

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 Post subject: Re: Net breeding
PostPosted: Dec 05, 2010 4:42 pm 
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Mh so the good news is i am only slightly confused :lol:

Want i want to know is how long will it take for a LFTR design to produce enough U233 to be self sufficient?


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 Post subject: Re: Net breeding
PostPosted: Dec 05, 2010 5:35 pm 
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Except for the DMSR, all the molten salt designs usually discussed here are intended to be self-sufficient in fissile (CR>=1.0, iso-breeder or better) from when they are started, provided that the correct startup fuel is available - U233 or a mix of actinides extracted from the spent fuel of existing reactors. If startup is on low-enriched uranium (<20% U235, >80% U238) it may be necessary to continue feeding uranium for some years. How much and for how long depends on the reactor design, and in general it is easier to start thermal (lots of moderator) designs than fast designs this way.


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 Post subject: Re: Net breeding
PostPosted: Dec 05, 2010 5:45 pm 
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In my opinion, it will be less expensive to produce the start up charge using enriched 235U than to breed your own fissile. This will also remove the expansion constraint of doubling time. That is we can expand the fleet as fast as we like limited by the enrichment capacity. As a crude estimate we have around 400 GWe world wide nuclear power so there should be something like enough enrichment for 400 tonnes fissile / yr. We need something like 2 tonnes fissile as a start up charge. So, if we can start up from LEU we can start over 100 reactors per year using the current enrichment facilities.


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 Post subject: Re: Net breeding
PostPosted: Dec 05, 2010 9:28 pm 
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Right. Marcus, I'm going to write a description which is probably a bit simple for you, because I'm in that kind of mood right now. If folks like it I'll stick in on my blog.

A lot of the early work on nuclear reactors talked about breeding ratio. People thought about breeding ratio a lot. Back in the 1950s and 1960s, they thought that Uranium was a fairly rare metal, and it was obvious that you'd want to power the whole work off nuclear reactors. So they didn't think there would be enough uranium for a once-through fuel process, which got most of it's energy from fissioning the 0.7% of natural uranium that is U-235. Instead, they thought they'd have to get most of their energy from U-238, which as you know is fertile but not fissionable.

Natural uranium with 0.7% U-235 and 99.3% U-238 doesn't sustain a chain reaction in light water reactors and especially not in fast breeder reactors. It has to be enriched to have a higher fraction of U-235. So, they figured they would make fuel elements from Uranium enriched to 3.5% or 5% U-235. After a few years of operating those fuel elements in a reactor with a breeding ratio greater than 1.0, when those fuel elements had accumulated enough fission products that they had too little reactivity, they would reprocess them, separating out the fission products and mixing back in the depleted Uranium left over from making the initial fuel elements. Note that because the breeding ratio was greater than 1.0, the fuel elements would have a little more fissile (Plutonium 239 and 241 now rather than Uranium-235) than they started with. In this way, a reactor could keep going for hundreds of years with no more mined uranium that what was used to start it initially.

But the problem was that you'd have to mine all the uranium needed to run the reactors for hundreds of years right at the beginning. So the next idea was to start reactors, not with newly mined and enriched Uranium, but with the extra Plutonium that the presumed breeder reactors would be making. If your breeder reactors yield 106% of their initial fissile load after 3 years of operation, it's going to take 50 years for one of those reactors to make enough extra fissile to start another reactor. That's a long time. You can speed it up a bit by having two reactors make enough fissile to start another after just 25 years. Then you have three reactors make fissile for 17 years to start the next one, and so on. The relevant metric for a exponentially expanding scheme like this is "doubling time", which is, how long does it take to double the number of reactors. Remember that the benefit of this scheme is that you can defer mining uranium.

And then they discovered a lot of uranium in the ground. Uranium prices crashed. The initial benefit of breeder reactors, which was that you could defer all the expense of mining uranium, turned out to be a small expense deferred. In fact, it turned out to be much cheaper to mine uranium for start charges than to reprocess the used fuel elements to take out the fission products. So that's what we do now.

The thing I realized which caused me to write all this is that the benefit of a breeding ratio greater than 1.0 was just the deferral of the cost of mining. Getting from 0.4 every 4.5 years, which is where LWRs are now, to 1.0 has the advantage of reducing the amount of uranium mined by a factor of 100, because once you get to a breeding ratio of 1.0 you can remove fission products and mix in depleted uranium. But beyond 1.0, there is no improvement in the amount mined.

So, I'm astonished that the price of mined uranium was ever high enough to motivate the development of commercial reactors with BR>1.0. I haven't been able to find anything which gives uranium prices in the 1950s yet.


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 Post subject: Re: Net breeding
PostPosted: Dec 06, 2010 12:42 am 
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Addendum: because the fast breeder reactors needed a much larger load of U-235 to get started than a light water reactor, they had to mine more uranium to get it going. If you compare a fast breeder to MOX reprocessing in an LWR, it's going to take at least two decades for the MOX/LWR to consume more natural uranium as input than the fast breeder. I guess that's not a completely fair comparison because the fast breeder with metallic fuel was going to be easier to reprocess than the LWR with oxide fuel.

Iain


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 Post subject: Re: Net breeding
PostPosted: Dec 06, 2010 12:48 am 
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From http://en.wikipedia.org/wiki/Uranium_market Uranium prices reached an all-time low in 2001, costing US$7/lb.. IIRC, the US government used to buy Uranium at $8 which was above free market price to encourage a dependable supply. Don't remember if that was for metal or U3O8.

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 Post subject: Re: Net breeding
PostPosted: Dec 06, 2010 4:19 am 
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Uranium price history, 1948-2007. Expensive in the late 70's.

Image

There are other factors to consider, such as weapons programmes. Fast breeders could help get more Pu239 weapons. That was worth a lot of money to some people, evidently.


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 Post subject: Re: Net breeding
PostPosted: Dec 06, 2010 8:05 am 
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iain wrote:
.......The thing I realized which caused me to write all this is that the benefit of a breeding ratio greater than 1.0 was just the deferral of the cost of mining....
Not quite, there is also the cost of enrichment. That has always been more expensive than mining, and with energy-hungry diffusion enrichment plants, the EROEI for the system takes a big hit. The relatively low cost and small energy requirement of centrifuge enrichment has helped to further tilt the argument away from breeders compred with the situation in the 60's. Today, there's no point in CR>1.01, unless you want to use some spare neutrons for another purpose, or are concerned about security of supply while expanding the fleet.


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 Post subject: Re: Net breeding
PostPosted: Dec 09, 2010 1:50 pm 
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Marcus wrote:
As far as i have understand it, it take different times for the numerous LFTR design to reach a CR > 1 or higher. Several decades for the french TMSR reactor. So how long would it take it for David LeBlancs tube in a tube/shell design?

Thx in advance. :D



Do you maybe mean how long it takes for the conversion ratio to drop down to 1.0 after startup and no fission product removal? The French TMSR (now MSFR) with a high loading of 5.5 tonnes of U233 per GWe can have as high as 1.13 for a breeding ratio (but that's still a 40 year doubling time because of the high starting load). If you run the TMSR without removing fission products I think it takes about 20 years before the conversion ratio drops below 1.0

For my tube within tube design I have a lot more flexibility on the startup fissile load (likely as low as 500 kg per GWe) and still break even because with a completely encompassing blanket salt I can go to low fissile concentration and let those neutrons wander quite awhile before they find a fissile. In the TMSR with axial reflectors, they can't go too low in fissile or the reflectors eat up too many neutrons. This 500 kg limit would just barely break even so we'd have to remove fission products maybe every 3 to 6 months. If we load more fissile at the start, the breeding ratio goes up and we can start to go years or even decades when the spectrum starts to get hard like the TMSR.

Like others have mentioned, as long as you have a source for startup (and you can use Low Enriched Uranium for Two Fluid designs) then there is virtually no reason to aim for high breeding ratios. Thus it mainly becomes an equation balancing the cost of more startup fissile versus the cost of faster fission product removal.

David LeBlanc


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 Post subject: Re: Net breeding
PostPosted: Dec 09, 2010 1:55 pm 
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Iain,

I think your post makes overall sense but it is very hard to tell when you are talking about fast breeders and when about PWRs. You seem to mention 3.5% to 5% enrichment for a breeder which doesn't make sense. If you meant a fast breeder you need at least 15% U235 to start these up (and a massive amount, likely 20 tonnes of U235 per GWe since you typically need about a third more than fissile Pu).

David L.


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 Post subject: Re: Net breeding
PostPosted: Dec 09, 2010 5:08 pm 
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Thanks guys, i had it totally wrong i thought it would need time to reach a cr of 1 or better not that the cr drops over time :oops:


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 Post subject: Re: Net breeding
PostPosted: Dec 09, 2010 7:07 pm 
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David wrote:
If you meant a fast breeder you need at least 15% U235 to start these up

In the four Design Studies of a 1000-MW(e) Fast Reactor, published in the spring 1965 issue of Power Reactor Technology, the core average fuel enrichment ranged from 13% for the Combustion Engineering design, to 20.8% for the one by Allis-Chalmers. Westinghouse came in at 15.7%, and GE at 18%.
Today, there would be much more emphasis on reducing fuel enrichment than back in 1965, for political reasons.
An SFR with 12% enrichment is certainly doable.


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 Post subject: Re: Net breeding
PostPosted: Dec 10, 2010 2:53 am 
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If you have the maximum politically acceptable enrichment of 20% with o.2% U235 tailings, 97.5% 0f uranium will be DU tailings. The fast reactor is best used as a trigger or kindling for the remaining DU. The idea gravitates to a travelling wave reactor or some variation of it. Or you could have a fast spectrum MSR. As matters stand, the world is awash with DU and used LWR fuel, and the latter is used to scare people away from nuclear power.
As far as breeder fuel is concerned, uranium is best for old nuclear power users like the US, France and Canada and thorium for India, China or Japan. Russians may do whatever fancies them.


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