Energy From Thorium Discussion Forum

A thorium cycle liquid salt reactor was designed for aircraft aplications, but that was in the '50s.

Realistically, how small could a safe thorium reactor be? The application I'm thinking about is marine for a small icebreaker like the Coast Guards 140 footers, possibly using the hight temperatures available to power heating elements or a steam bubbler system.

http://www.uscg.mil/datasheet/140wtgb.asp

These vessels need to be extremely powerful but small enough to manuver in rivers and small harbors. The practical draft would be limited to ~12 ft.

Any thoughts?

Why a steam bubbler if you can use compressed air? Slightly more energy efficient...

Everybody probably knows about the Russian nuclear icebreakers.

Wondering about how small you can build a thorium reactor has been one of my idle thoughts for many years.

First of all, you need to achieve criticality--every fission needs to lead to another fission for the reactor to run. Since your average absorption in U-233 or U-235 produces a little more than two neutrons, you need to make sure at least one of them causes another fission. In a reactor with rapid moderation (light-water or heavy water) and a low uranium enrichment (lots of absorptive U-238) you can do this with a pretty small reactor, since the mean free path of the neutron is fairly small, and it is thermalized fairly quickly.

If your fuel has greater levels of enrichment, you can get even smaller. Naval reactors have really high enrichment levels and are cooled and moderated by light water, so there's lots of targets (U-235), rather little absorptive material (U-238), and neutrons get slowed down quickly in light water.

In a fluoride reactor using graphite moderation, the mean free path of the neutron is greater, since graphite isn't nearly as good at slowing down the neutron as the hydrogen in water.

In a thorium-fueled reactor, you not only need to get the neutron to cause fission (for criticality) you need another neutron to find a thorium to convert to U-233 to replace the fissile nucleus you just used up.

A small reactor along the lines of what David has proposed might work, since the "core" of the reactor essentially leaks half of the neutrons to the shield/blanket of thorium, where they are absorbed and the uranium is removed. The minimum size of that design would depend on the thickness of graphite needed for decent moderation.

Using D2O instead of graphite in a fluoride reactor could lead to more rapid moderation in the D2O, but the D2O would have to be thermally insulated from the hot graphite salt, and the thickness of the insulation would be some level of "penalty" against the concept. I've been curious for a long time just how insulated D2O would fare against uninsulated graphite for moderation capability.

The 140's already use a bubbler system. Given the very high operating temperatures of the liquid metal reactor the idea of a steam bubbler was just to leverage that heat and go one better than cold air.

How small? How about the Indian CHTR design: 1m x 1m hermetically sealed unit (including axial reflector) weighing less than two tonnes, with a nominal power of few hundreds of kW (limited by heat removal capacity of passively safe system) and a 15 year life?

http://indian-nuclear-society.org.in/co ... Paper1.pdf

http://www.energyfromthorium.com/forum/ ... .php?t=297

Could maybe power a locomotive or a small ship. Could also be stacked to provide more power

Very interesting design.
But the paper states that the power is one hundred kW thermal.
Also, according to the table,


That seems like a low burnup for such high-enrichment fuel.
LWRs are approaching that kind of burnup, on fuel with less than 5% enrichment.
On top of that, TRISO fuel is far more difficult to manufacture than the UO2 fuel pellets used in LWRs -- and even more so than UF4/ThF4.

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Several hundred kW is quoted as it's capacity elsewhere, but the paper suggests that output is restricted by the amount of energy that can be extracted by passive circulation methods used to transfer heat from the reactor to the turbine, chemical plant or hydrogen generator.

I think you could get a lot more power out of it using a heat pipe or something similar, but you would have to ensure you don't get overheating/damage if the turbine etc. breaks down. I don't think this should be a problem for a supervised plant in a ship. The CHTR is designed to be left unattended, so I think that is the reason for the low power output - just like satellite power plants.

As a comparism with coal and oil power, the reactor weighs less than two tonnes and that includes fuel for 15 years. How much would the equivalent amount of oil or coal used to generate 100kW for 15 years weigh?

Good point.
But one should also look at how much uranium mining and fuel reprocessing was needed to produce the required 33.75% U233 in the core.
Also, after only 68000 MWd/t of burnup, there will still be lots of valuable U233 left in the spent fuel which, being in the TRISO form, is difficult to reprocess.
But the value of that U233 is so high, that I can't imagine it would just be sent to disposal as waste.

Another good comparison, besides coal, is other small power reactors.
There is a very nice summary of various concepts on the UIC web site at their Nuclear Issues Briefing Paper # 60 -- Small Nuclear Power Reactors.

Particualrly interesting is the comparison to Toshiba's 4S reactor which, with fuel enriched to just under 20% U235 (equivalent to 12% U233) gets a 30-year life, producing 10MW electric.
With the fuel being metallic, recovery of unused uranium is of course far easier than with TRISO fuel.
And while the basic design uses liquid sodium coolant, there is also the L-4S, which is a Pb-Bi cooled version of 4S.

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The proposed future Indian Thorium-U233 reactors all seem to be designed on the principle of breeding U233 in the thorium to compensate for the U233 burned in order to minimize control movement necessary. As U233 is a strategic material as far as the Indians are concerned, I am absolutely sure that the intention is to recover all the U233 remaining in the CHTR after 15 years, and breed a little more if possible.