Energy From Thorium Discussion Forum

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PostPosted: Mar 05, 2011 10:17 am 
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But is doppler sufficient control if you don't have the advantage of a softer spectrum where criticality is maintained by a strong moderating coolant, such as water? If there is sodium coolant voiding, heavy earthquake induced LOCA, or maybe just sodium coolant expansion, the spectrum hardens and your doppler is reduced. Isn't that a problem in some beyond design basis accidents? Wouldn't you rather use a fuel that has less doppler in the first place and rely on fuel dilatation for control? Severe heatup of metallic fuel will start to plasticise and melt it at quite low temperatures, inducing strong negative reactivity and providing an initial heat sink. With oxide fuel the cladding will fail first, right?

What happens if you have an oxide fuel and there is a loss of heat sink? The stronger doppler will not allow you to reduce power quickly so your sodium could boil, voiding the coolant and inserting large positive reactivity, risking overpower transients. You'd need a Chernobyl proof containment or have a huge pool of sodium handy. Probably the latter is more attractive, but it will make your reactor bulky.

I am no nuclear engineer, so correct me if I'm wrong, but oxide fuelled fast reactors seem very tricky to me. Experience with the Monju breeder was at best not good.

The EBR prototypes led to an interesting technique to accomodate swelling, based on injection molding and sodium filler slugs. Perhaps this is much harder though, with thorium, given its higher melting point. One other technique that might work is drip melt casting of little thorium-plutonium pebbles. If the fuel rods are filled with pebbles, perhaps a few mm in diameter, almost as small as TRISO kernels, but metallic, that will allow easier outgassing of fission product gasses and accomodate swelling much better.

It seems to me that metallic fuel is easier on the cladding, so would be a natural candidate for higher burnup fuels.


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PostPosted: Jun 05, 2011 12:28 pm 
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If TerraPower wants to build a small test reactor how small can a TWR be? Would a fuel cylinder 20cm diameter by 60cm length be critical?


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PostPosted: Jun 05, 2011 5:06 pm 
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edpell wrote:
If TerraPower wants to build a small test reactor how small can a TWR be? Would a fuel cylinder 20cm diameter by 60cm length be critical?

Certainly, with a high enough fuel fissile enrichment and reasonably low power, to allow heat removal in such a small reactor.

Just as an example, look at the 300 MWth (94 MWe) Enrico Fermi Reactor built at Lagoona Beach, on Lake Erie, about 30 miles southwest of Detroit.
It had an active core 30.5 inches in diameter by 31.2 inches high, with 26.7% enriched U fuel in a U-Mo alloy (10% U) with Zr cladding, and a thick DU breeding blanket surrounding the active core (Note that fast reactor cores tend to be designed as short, squat cyliders, typically quite a bit wider than high, for heat exchange reasons).
It was started up in 1966 and decommissioned in 1972, following re-start in late 1970, after a partial core meltdown on Oct.5 1966.

Unfortunately, regulatory policy does not allow civilian nuclear reactors to use fuel with highly-enriched uranium (anything over 19.9%).
So TerraPower’s test reactor core will likely not be as small as you suggest.

Also, it is not clear (or at least not publicised by TerraPower) what sort of initial core load it takes to get that “travelling wave” going properly – is it small & high-enriched, or larger & low-enriched ?
Without seeing their detailed analysis or burn simulations, it would be a wild guess.


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PostPosted: Jun 09, 2011 12:54 am 
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jaro wrote:
edpell wrote:
If TerraPower wants to build a small test reactor how small can a TWR be? Would a fuel cylinder 20cm diameter by 60cm length be critical?

Certainly, with a high enough fuel fissile enrichment and reasonably low power, to allow heat removal in such a small reactor.

Just as an example, look at the 300 MWth (94 MWe) Enrico Fermi Reactor built at Lagoona Beach, on Lake Erie, about 30 miles southwest of Detroit.
It had an active core 30.5 inches in diameter by 31.2 inches high, with 26.7% enriched U fuel in a U-Mo alloy (10% U) with Zr cladding, and a thick DU breeding blanket surrounding the active core (Note that fast reactor cores tend to be designed as short, squat cyliders, typically quite a bit wider than high, for heat exchange reasons).
It was started up in 1966 and decommissioned in 1972, following re-start in late 1970, after a partial core meltdown on Oct.5 1966.

Unfortunately, regulatory policy does not allow civilian nuclear reactors to use fuel with highly-enriched uranium (anything over 19.9%).
So TerraPower’s test reactor core will likely not be as small as you suggest.

Also, it is not clear (or at least not publicised by TerraPower) what sort of initial core load it takes to get that “travelling wave” going properly – is it small & high-enriched, or larger & low-enriched ?
Without seeing their detailed analysis or burn simulations, it would be a wild guess.

How about 19.9% LEU metal containing additional Reactor Grade Plutonium oxide for increased enrichment?


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PostPosted: Nov 10, 2011 8:59 pm 
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Went to a seminar/lecture today by one of the engineers from TWR, here's what I learned-

The TWR engineers plan on using Ferritic-Martensitic Steel (HT9) whose complex microstructure allows it to withstand stresses from neutron irradiation more effectively than typical Austenitic Steel. They are obtaining this steel from a Japanese steel maker, Kobe Steel. It experiences fewer vacancy-interstitial pairs (aka "Frenkel Defects", I need to thank my Materials professor). The TWR core environment generates radiation in the 500-600 dpa range. So far the HT9 has been tested to a maximum 250-300 dpa range and extrapolating from the current data into the 500-600 dpa range would be irresponsible even given the 1024 Xeon processors Terrapower has at their disposal. Instead they're having to spend millions of dollars to have it tested. The quoted 60 year "lifespan" of the reactor is more like 40 years, if that, depending on whether a materials solution can be found. The reactor also experiences problems with irradiation creep, swelling, and ductile to brittle transitions...I'm a little ignorant in this area being only a sophomore having not hit my upper division yet, but these sound like problems common to all solid-fueled reactors, especially for those in the fast spectrum (high neutron fluxes).

The company says "no reprocessing", but the engineer who spoke today admitted that the utility that buys the reactor is likely the one who will be making the decision or not to go in and recover the plutonium/fission products. He conceded that the "burn and bury" option isn't viable and that the disposal of the waste in a geologic repository would be a better decision. Also, He said Terrapower had not been spending much of its time thinking about the backend of the reactor's lifetime as it's been concentrating primarily on the frontend. The first prototype reactor will be 500MW.

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PostPosted: Nov 10, 2011 9:22 pm 
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Jonathan Wyers wrote:
The company says "no reprocessing",


Major mistake.

Jonathan Wyers wrote:
Also, he said Terrapower had not been spending much of its time thinking about the backend of the reactor's lifetime as it's been concentrating primarily on the frontend.


Another major mistake. I'm really amazed that people who've had this much money and this much attention haven't given thought to something I started thinking about five minutes after I heard of fluoride reactor technology in 2000.

Welcome Jonathan!


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PostPosted: Nov 11, 2011 12:04 am 
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Does nuclear physics show irradiate and burn to be possible? Is it possible to make the neutron flow linear?


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PostPosted: Nov 11, 2011 3:09 am 
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Thank you Mr. Sorensen! I'm happy to be here, and I am sorry that I wasn't sooner.

I agree. The TWR is at best a temporary solution and will only push our "waste problem" out another 60, well, 40 years. I appreciate what they're trying to do: eliminate the need to remove, reload, or readjust fuel elements as in LWRs (though TWR's require reshuffling, I still don't know how they physically go about this, he showed us some computer models on some optimal shuffling schemes, but nothing about how that shuffling occurs, and he also said the reactor is supposed to be sealed?), eliminate current stockpiles of nuclear waste, and utilize the more abundant U-238. But this reactor's incapacity for net breeding, easy reprocessing, operation at high temperatures, and its materials problems (which are numerous), and confusion regarding disposal dim its potential. In the end, the TWR only detracts attention (investors) from other reactor options such as LFTR, risks accidents due to the Na coolant, and uses up a portion of our finite, but extremely valuable inventory of fissile material.

@jagdish: This guy was a materials engineer so I tried not to ask him many question regarding the nuclear aspect of the reactor. Our time was also short. When I'm more educated than I am now and can solve the Neutron Transport/Boltzmann equation (fun fact, Boltzmann killed himself because he couldn't according to one of my professors, though I'm doubtful that's why), I might be able to tell you. :-)


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PostPosted: Nov 12, 2011 4:33 am 
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Welcome Jonathan, interesting stuff, I didn't know that there are materials that can tolerate that sort of neutron exposure. It's also interesting to note that the TWR still sounds very underdone technically. It will be interesting to see how it develops.


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PostPosted: Dec 07, 2011 8:37 am 
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'Computing billionaire Bill Gates has confirmed he is discussing developing a new and safer kind of nuclear reactor with China.
"The idea is to be very low cost, very safe and generate very little waste," said the Microsoft co-founder in Peking.
Mr Gates has largely funded a US company, TerraPower, that is developing a reactor that can run on depleted uranium.
"TerraPower is having very good discussions with various people in the Chinese government," he said, cautioning that they were at an early stage.
He said as much as a billion dollars will be put into research and development over the next five years.'

http://www.independent.co.uk/news/world ... 73422.html


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PostPosted: Dec 07, 2011 3:02 pm 
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Thanks Jonathan for your informative post on the TWR. I lived in Gainesville for a year in 1997. Really nice place.


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PostPosted: Dec 07, 2011 3:47 pm 
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http://www.foxnews.com/scitech/2011/12/ ... ith-china/

Quote:
BEIJING – Microsoft co-founder Bill Gates confirmed Wednesday he is in discussions with China to jointly develop a new and safer kind of nuclear reactor.

"The idea is to be very low cost, very safe and generate very little waste," said the billionaire during a talk at China's Ministry of Science and Technology.

Gates has largely funded a Washington state-based company, TerraPower, that is developing a Generation IV nuclear reactor that can run on depleted uranium.



Will this be a thorium version of the TWR?

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PostPosted: Dec 07, 2011 4:26 pm 
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Yeah, the presenter was talking about how Terrapower was looking for overseas support/cooperation. Someone asked who, and he said he wasn't at liberty to say who. I told the person who asked it was China, which drew a smile from the presenter. Glad to hear its official now though.

A TWR variant could use Thorium, since Th-232 transmutes to U-233 in a similar fashion as U-238 transmutes to Pu-239 (though U-238 requires a fast spectrum!). However, decisions would have to be made on deciding upon a moderator and coolant. Right now, Terrapower is very happy with the fact that the coolant it has selected, Sodium (Na), is compatible with its fuel and fuel cladding. Additionally, if it used Thorium, it would very likely be in a solid form, which would disallow any simplistic reprocessing techniques. Terrapower's "doctrine" states there should be no reprocessing. Understand, the driving idea behind the TWR is a reactor that burns up depleted uranium (U-238) and doesn't require you to pop open the reactor and pull out and rearrange the fuel assemblies (aka in-core fuel management) throughout the lifetime of the reactor. This is part of what made the presenter's job so hard. What material exists (or can be fabricated) that can withstand high neutron fluxes and thermal stresses for 40+ years without significant deformation? In this case, significant isn't a whole lot, the margin for error is tiny.

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PostPosted: Dec 07, 2011 4:33 pm 
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@ Lindsay, thanks, I'm happy to be here :-)

@clumma, You're welcome! and yeah, I like it, I used to live here when I was a kid, so coming back here for school, it's a little bit like coming home.

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PostPosted: Dec 07, 2011 4:54 pm 
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Someone else picked up on this too. There's a topic devoted to TWRs here-
viewtopic.php?f=57&t=2144

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