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 Post subject: Startup charges
PostPosted: Sep 25, 2010 4:10 am 
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We often hear on the forum that LFTRs can be started up on LEU 20%. However in the French TMSR modelling, even 33% U235 takes decades to get net breeding, and well over half a century in doubling time. The problem comes from the drain of U238 even in a fastish spectrum.

Interesting result from the TMSR modelling is that spent nuclear fuel Pu startup has much better breeding. A Pu/LEU 50-50 start is probably just as poor as HEU 33%, so it seems that for isobreeding LFTR we should be considering only Pu (and probably other TRU with it) as startup. Or do David Leblanc's idea of LEU fuel makeup, setting the U233 from the blanket aside. The latter option might mean fierce proliferation headaches, and does not solve the political waste problem.

So my question is: should we consider only TRU startup?


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 Post subject: Re: Startup charges
PostPosted: Sep 25, 2010 4:33 am 
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Sure but I hate to make deployment dependent on processing the LWR spent fuel. I've been thinking along the lines of a mix of LEU20 and SNF/Pu. Going pure SNF/Pu has a challenge as well - for a fast spectrum you need lots of fissile but plutonium solubility won't let you put in as much as you'd like. In a slower spectrum the plutonium isn't as attractive - until you get to a really slow spectrum.
Another thought is to start with LEU20+Pu and then add Pu as needed in the early years to keep the reactivity up until we replace some of the u235 with u233.


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 Post subject: Re: Startup charges
PostPosted: Sep 25, 2010 5:19 am 
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Lars wrote:
Sure but I hate to make deployment dependent on processing the LWR spent fuel. I've been thinking along the lines of a mix of LEU20 and SNF/Pu. Going pure SNF/Pu has a challenge as well - for a fast spectrum you need lots of fissile but plutonium solubility won't let you put in as much as you'd like. In a slower spectrum the plutonium isn't as attractive - until you get to a really slow spectrum.
Another thought is to start with LEU20+Pu and then add Pu as needed in the early years to keep the reactivity up until we replace some of the u235 with u233.


With faster spectra we can add som NaF to make FLiNaBeU (in the ballpark of 33:33:33:1) that has a much higher solubility for trifluorides at a given temperature. It is also lower melting and saves us 50% of the expensive 7LiF, also producing a bit less tritium and being less viscous as gravy (pardon the pun!).

We talked about Pu being expensive so you just want to use it as startup charge. Shipping Pu on a regular basis to plants seems like a bit of a disadvantage as well.

There is a conundrum with the LEU vs TRU startup. If we look at the French TMSR modelling LEU is very poor in breeding the U233 that you need to start up more reactors, so less good than you say it will be. TRU only in fastish spectrum (TMSR) is much better in breeding quickly & breeding more on average, but may be limited in availability in a truely grand transitional scheme. Like the end game paper you posted.

An interesting twist, for sure - we may not have enough TRU waste!!!

We should use that point as good PR in communications. For example it is a good justification to stop MOx fuel recycling, use the money to develop LFTR and pyroprocessing, and build more water reactors. Waste is Good.


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 Post subject: Re: Startup charges
PostPosted: Sep 25, 2010 5:29 am 
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Please see: 'thorium molten salt reactors: detailed calculations and concept evolutions in view of a large nuclear energy production'

http://docs.google.com/viewer?a=v&q=cac ... -eAjkbEE2Q

Figure 12.


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 Post subject: Re: Startup charges
PostPosted: Sep 25, 2010 1:04 pm 
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Cyril R wrote:

There is a conundrum with the LEU vs TRU startup. If we look at the French TMSR modelling LEU is very poor in breeding the U233 that you need to start up more reactors, so less good than you say it will be. TRU only in fastish spectrum (TMSR) is much better in breeding quickly & breeding more on average, but may be limited in availability in a truely grand transitional scheme. Like the end game paper you posted.

An interesting twist, for sure - we may not have enough TRU waste!!!

We should use that point as good PR in communications. For example it is a good justification to stop MOx fuel recycling, use the money to develop LFTR and pyroprocessing, and build more water reactors. Waste is Good.

The french reactor is pretty fast and uses a lot of fissile (5 tonnes / Gwe or so. I would prefer no graphite like the french (to minimize waste) and lower fissile inventory (to reduce costs, soften the spectrum, and increase wall life) and if possible be able to startup on a mix of LEU20 and SNF/Pu to enable deployment w/o building a breeder or shipping HEU around.

I do not know if we can accomplish all those things. I expect we will need to feed in additional SNF/Pu for a few years to get the reactor going. I haven't seen this considered in any papers and I don't have the appropriate s/w so I'm not sure how this will play out. It does look like we will have to have a means to extract rather fresh plutonium to make it work though.

BUT if you add graphite then the spectrum softens sufficiently that I'm sure you can start on LEU20 alone or mixed with SNF/Pu as suits the market. Just wish there was a way to avoid generating tons (literally) of graphite waste per year.

I agree that a very good PR pitch is we can replace all your coal power plants and consume all the plutonium you've got - burning it up in the end rather than burying it.


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 Post subject: Re: Startup charges
PostPosted: Sep 26, 2010 11:25 am 
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Cyril R wrote:
Please see: 'thorium molten salt reactors: detailed calculations and concept evolutions in view of a large nuclear energy production'


" The actinide loss rate is taken equal to 10^(-5) over these 10 days " is this figure actually reasonable/feasible ?


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 Post subject: Re: Startup charges
PostPosted: Sep 26, 2010 1:10 pm 
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Alex P wrote:
Cyril R wrote:
Please see: 'thorium molten salt reactors: detailed calculations and concept evolutions in view of a large nuclear energy production'


" The actinide loss rate is taken equal to 10^(-5) over these 10 days " is this figure actually reasonable/feasible ?


Probably not; the French researchers are mostly physicists, not chemists and engineers. Not to mention business minded people. That must also be one of the reasons for their choice of impractical reactor geometry...

10^-4 loss per cycle seems very agressive already, if we choose to avoid the fluorinator and have only distillation units. It doesn't make that big a difference on the breeding gain anyway (minus 0.0037 on the CR/BR). When loss rate gets to 10^-3 it gets serious and barely allows isobreeding. With 10^-2 breeding with thorium is out of sight permanently. A 10 day cycle is pretty rapid.


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 Post subject: Re: Startup charges
PostPosted: Sep 27, 2010 12:41 am 
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This is a relatively early paper in the French teams writings. The 10 day period was done to reduce losses to Pa capture. If we are using either just standard fluorination or vacuum distilling you won't be recovering any of the plutonium, curium, or americum. It will all flow with the fission products.

The 10^-5 number appears to be one taken for sake of calculations and so far as I can tell has no experimental data to back it up. It is based on the liquid metal extraction method which both ORNL and the French team bank on but I see little data to say that it would really work well and some experimental data from ORNL that does not look encouraging.

It is common to see recovery rates in the 90-99% for experimental data and 99% to 99.99% as hoped for in papers. But it seems to me that quoting a leak rate isn't the best metric since it will be specific to the initial concentration and economic tradeoffs.

ORNL did do studies using very aggressive fluorination (falling drop method ORNL 4224) that got the plutonium concentrations down to 0.01mg (Pu) / g (salt). If we really want to minimize plutonium in the final waste stream we could use this technique. Imagine a salt consisting of the fission products less the off-gases, noble metals, and salt seekers that come out with vacuum distillation (Zr and Cs). I would guess what remains of the fission products is significantly less than 400kg/GWe-yr. If we use this aggressive fluorination we end up with around 4 grams Pu escaping to the final waste stream per GWe-yr. At the time ORNL did the experiments they used HastalloyN and the hot (624-640C) fluorine gas was corrosive. ORNL talked about freezing salt on the walls of the fluorinator to protect them. I'm thinking we are better with graphite now and could build the fluorinator out of graphite. Since there is no neutron flux and no mechanical stresses the weaknesses of graphite are not a problem. I'm not sure it is worth the trouble and in any event doesn't address curium or americum.

I suspect that any technique and machinery to extract plutonium from fission products will have to be treated rather like enrichment capability for proliferation reasons.

But ORNL did think this was a reasonable approach to removing the plutonium from spent fuel.
"At the present time, it appears unlikely that molten-salt volatility methods will be used for processing reactor fuels that contain plutonium. If such processing is contemplated, it should be noted that the falling-drop method is the only approach that has shown promise for the rapid removal of plutonium with low reaction-vessel corrosion rates. For example, our experimental data indicate that fluorination in an 11 ft high tower will result in the removal of 99% of the plutonium from 100u diamter drops at 640C." Pg24.


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