I think we have to build some sort of fast spectrum reactor to destroy existing transuranics but I do not know what the best technology is.

There is also the helium cooled very high temperature reactors but those strike me as a disaster waiting to happen given the relatively low heat carrying capacity of helium. But the high outlet temperatures translate into high efficiencies.

You had mentioned reactivity excursions with fast reactors. I can see two contributing factors to this problem.

One, you don’t have the time it takes to slow (thermalize) fast neutrons as you would in a thermal reactor so that delay is eliminated from the reactivity escalation.

Two, in a thermal reactor that is water moderated, steam is a less efficient moderator and so you have a negative void coefficient, that is when the reactivity level increases, more steam is produced and the effectiveness of the moderator is reduced and so the reaction is dampened. But in a fast reactor where moderation is not desired, if the cooling medium, say liquid sodium, acts as a moderator or just absorbs neutrons to any degree, if boiling is reached, the voids are going to slow/absorb neutrons less increasing the rate of reaction creating a positive void coefficient. That positive feedback only destablizes things further which would seem to hold a strong potential for rapidly increasing reactivity and destruction of the reactor core.

But a number of these reactors have been operated and the major problems with the liquid sodium cooled reactors so far seems to have been leaks in the plumbing resulting in sodium reacting with atmosphere or leaks in the secondary heat exchanger resulting in sodium reacting with water. These cases the major hazards have not been nuclear but chemical.

The corrosive nature of salts and the lower heat carrying capacity, the chemical reactivity of liquid sodium with atmosphere and water, both of these are engineering problems. I do not know which is more difficult.

With the very high temperature helium cooled reactors I would imagine loss of forced coolant would have a high probability of core destruction?

I agree with you that we should be building a reactor farm at Yucca Mountain instead of a waste repository. I also agree that you need fast-spectrum reactors to destroy transuranic waste. But rather than using the sodium-cooled fast reactor, which has a multitude of safety issues, I am very intrigued with the liquid-chloride reactor concept, which is another kind of molten-salt reactor. I believe the chloride reactor could be built in ways that would be far safer and simpler than the sodium-cooled solid-core reactor.

Whereas the liquid-fluoride reactor optimizes as a thermal-spectrum reactor that can run on thorium (with a conversion ratio of > 1.0), the liquid-chloride reactor is a very fast spectrum reactor designed to destroy transuranics. I think we should build a few chloride reactors to destroy all the transuranics we’ve produced in light-water reactors, breed U-233 from thorium using the excess neutrons these chloride reactors will produce, then use the U-233 to start literally thousands of fluoride reactors which will then run exclusively on thorium.

I would further suggest other reactors be sited underground at locations where water table contamination in the event of loss of containment would not occur.

Not to dismiss your suggestion but one of my primary concerns is that we find a way to use up the existing actinides that we’ve created in water cooled reactors before we intentionally create more.

I would also suggest that the pyrometallic / fast neutron fuel cycle can also incorporate Thorium.

But you are correct in as much as sodium cooled reactors have been plagued with sodium leaks, sodium-air fires, sodium-water fires, and that fast reactors are more prone to rapid reactivity excursions.

You might want to learn more about the liquid-fluoride reactor, which can operate on a thermal-neutron spectrum and with non-volatile core fluids (salts instead of metallic sodium). When operated with uranium-233 generated from thorium, the liquid-fluoride reactor produces essentially no transuranics. Furthermore, since it uses salts instead of metallic sodium, it avoids all the issues with sodium-water fires that bedevil the liquid-metal fast breeder concept.

Energy from Thorium

Thorium is abundant and can be completely converted to energy in the reactor.