Energy From Thorium Discussion Forum

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PostPosted: May 18, 2017 5:21 pm 
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I'm not a nuclear engineer. I have a bare bone notion of what I'm talking about...
My best hope is perhaps I inspire others onto something. Maybe after radical changes. And most of all details which are clearly lacking, since I don't know enough to estimate the behavior of the initial idea to get into any real detail.

Would a large enough fast MSR have to be to sustain criticality on natural uranium ?
How huge would such a contraption have to be ?
Would it breed (maybe by making it even bigger) ?
After startup, could the make up fuel be pure Th232 ?
Once it has enough Th232 content, would it breed U233 ?

Perhaps such a huge contraption would be very sub critical when it starts with natural uranium and need to achieve higher fissile before it achieves criticality.
Maybe give it an ADS like neutron beam and focus it on a small area of the reactor, trying to generate a localized criticality, that would gradually help increase reactivity.
The goal would be that such a machine would normally operate with fissile to fertile ratios slightly higher than typical MSR burners, by virtue of having a much larger fissile/fertile mass, once it gets to its fissile ratio targets, a large chunk of the core is extracted and replenished with Th232, keeping this cycle.

Give this reactor a plain pyro reprocessing facility, that solely separates fission products from the rest.

In essence the idea is to have a reactor that primarily functions as completely untraditional blanket (obviously it will have fission products, heavier transuranics and other non ideal elements).

The driving issues that made me conjure this was simply the conviction that the traditional two fluid LFTR is pretty much D.O.A. for NRC certification.
This should not have any sort of Pa233/U233 extraction systems. The fuel has to be this mix of Th232/U238/U233/U235/Pu240-3 and so on.

If someone could come up with a real design, the most I hope is some attribution as inspiration only. Have all the fun and profit you can if this idea has any potential at all !

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PostPosted: May 18, 2017 5:24 pm 
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You'll never be able to attain criticality in a fast spectrum with uranium at natural levels of enrichment, no matter what the kind of reactor you propose.


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PostPosted: May 18, 2017 6:58 pm 
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Then how is it that any reactor can attain criticality on natural uranium ?
Isn't the problem of fast reactors not having enough distance for neutrons to find a target and avoid being lost ?
Once they do find a target, they produce more neutrons than a thermal reactor.
Or maybe such a reactor would need to be really truly impractical ? Like millions of tons of nuclear fuel.

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PostPosted: May 18, 2017 8:04 pm 
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macpacheco wrote:
Then how is it that any reactor can attain criticality on natural uranium ?


They use moderators such as heavy water or graphite to slow their neutrons to thermal equilibrium with their surroundings, while avoiding much absorption in the moderator. In this way they are able to achieve criticality with the low fissile content of natural uranium.


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PostPosted: May 19, 2017 7:26 pm 
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Kirk Sorensen wrote:
macpacheco wrote:
Then how is it that any reactor can attain criticality on natural uranium ?


They use moderators such as heavy water or graphite to slow their neutrons to thermal equilibrium with their surroundings, while avoiding much absorption in the moderator. In this way they are able to achieve criticality with the low fissile content of natural uranium.


Kirk, I understood 100% of what you said, in the level you explained.
But when I think about microscopic and macroscopic cross sections, it doesn't add up.

I watched through all of your videos, specially the whole protospace video at least 3x.

How about a thought experiment.
If we had a 1Km radius vat of natural uranium+salt coolant.
Neutron losses would be essentially irrelevant even for fast neutrons due to the ultra long distances neutrons would have to travel before they're lost.
Nearly every neutron would either find a fissile or fertile to hit.

If U235 fission produces 2.4 neutrons on average, there are enough neutrons to promote U238 to Pu239, fission more U235 and still some left.

Maybe the problem is too many neutrons would breed Pu239 and not enough U235 fission would happen, due to cross section differences between fast and thermal fission. So the reactor never starts up. But unless the proportion of breed/U235 fission is the same, then a thermal natural enrichment reactor would be impossible too. Get my point ?

That possibility was one of the reasons I suggested perhaps giving that design an ADS neutron source. Those are not fast neutrons.
Perhaps the energy budget for an ADS neutron source to produce enough Pu239 so fast fission can be sustained would be astronomical (say with a 1000 tons of natural uranium reactor).

Apologies for the persistence. And thanks.

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PostPosted: May 20, 2017 3:09 am 
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It is worth noting that in another few hundred thousand years, a natural uranium fueled reactor on Earth won't work at all any more.

When every mk of reactivity is nearly 120MWd/t of additional burnup (2% of total in a CANDU-6!), it changes your perspective.


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PostPosted: May 20, 2017 5:47 am 
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The difference between fast and thermal spectrum is the ratio of the fission cross section of 235U over absorption cross section of 238U. This ratio is around 10 for high energy neutrons and around 250 for thermal neutrons. In natural uranium you have around one 235U atom for 140 238U atoms. That is why a fast natural uranium reactor can not go critical no matter the size.

It is still possible to build a natural uranium reactor cooled by molten salt, we just have to use graphite or heavy water to thermalize the neutrons.


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PostPosted: May 20, 2017 1:51 pm 
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macpacheco wrote:
How about a thought experiment.
If we had a 1Km radius vat of natural uranium+salt coolant.
Neutron losses would be essentially irrelevant even for fast neutrons due to the ultra long distances neutrons would have to travel before they're lost.
Nearly every neutron would either find a fissile or fertile to hit.


Control might be a problem. Would it not go super critical and boom? Negative temperature coefficient may not work so well in large 3d spaces.


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PostPosted: May 20, 2017 1:53 pm 
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fab wrote:
The difference between fast and thermal spectrum is the ratio of the fission cross section of 235U over absorption cross section of 238U. This ratio is around 10 for high energy neutrons and around 250 for thermal neutrons. In natural uranium you have around one 235U atom for 140 238U atoms. That is why a fast natural uranium reactor can not go critical no matter the size.

It is still possible to build a natural uranium reactor cooled by molten salt, we just have to use graphite or heavy water to thermalize the neutrons.


Is it possible with graphite, or do you need a more space efficient moderator like hydrogen?


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PostPosted: May 20, 2017 2:33 pm 
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Quote:
Control might be a problem. Would it not go super critical and boom? Negative temperature coefficient may not work so well in large 3d spaces.


You can have a cube of natural uranium of an infinite size, you still can not go critical if the neutrons are not thermalized : most of the neutrons will be absorbed by 238U atoms, preventing the occurence of a chain reaction.

Quote:
Is it possible with graphite, or do you need a more space efficient moderator like hydrogen?


With natural uranium it is not possible to have a chain reaction if you use hydrogen as a moderator : hydrogen absorbs too much neutrons. It is possible with graphite and deuterium (heavy water) because they absorb less neutrons.

The United States used the reactors at the Hanford Site to create the plutonium for their weapons program. These reactors were fueled by natural uranium, moderated by graphite and cooled by water. There are several molten salts which absorb less neutron than water, so I think it is possible to create a natural uranium reactor, cooled by a molten salt (so we have the nice low pressure) and moderated by graphite or heavy water. It is technically possible but maybe not practical, using enriched uranium will permit the use of a simpler and more compact design.


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