[Simon Gunson]

I have heard proposals to create a Thorium reactor in New Zealand. This is said to be safer than a uranium reactor because (presumably) the reaction is not self sustaining ?

Please could anyone explain therefore how a Thorium reactor works if not self sustaining ?

Also the Thorium reactor is said to be able to rehabilitate nuclear wastes from uranium reactors by returning U235 to stable U238 etc.

Comments please ?

[AWK]

http://cavendishscience.org/bks/nuc/thrupdat.htm
http://www.princeton.edu/~globsec/publications/pdf/7_3kasten.pdf
http://www.hinduonnet.com/fline/fl2104/stories/20040227006011500.htm
http://www.uic.com.au/nip67.htm

[Elgon]

The main reason why people think that a thorium reactor might be less harmful is the fact that the radiotoxicity of the spent fuel from a thorium reactor is considered lower than the radiotoxicity of spent fuel from a conventional reactor.

The thorium reactor as described by Rubia is just a different take on the concept of an accelerator driven subcritical reactor.  This concept has been studied in several places in the world, mainly for the transmutation of long-lived radionuclides in spent fuel to shorter-lived nuclides.

[Simon Gunson]

Hey thanks guys... I was a former anti-nuclear protestor in the 70s, but that was then. I think that technology has stepped up several notches since then and that nuclear energy needs another chance. Especially given oil shortages and greenhouse gas emission concerns.

I will go read this stuff on Thorium reactors. I think in terms of waste vitrification of waste and deep burial means that waste need no longer be a polluition threat to groundwater.

Someone made a comment on TV in New Zealand last week that with a thorium reactor, if it overheats one pretty much just switches it off. I take it that refers to how reactions may not be self sustaining ?
Is that why they would say that ?

Any explanations what they might have ment by that comment please ?

[AndyHolland]

Thorium itself is not a fuel, its a material that is irradiated that becomes fuel - namely U233 instead of U235.

The advantage of U233 is it is not useful to make into nuclear weapons - anti proliferation. Additionally, thorium is plentiful and a thorium breeder reactor can be made using H2O moderator. U233 is very radioactive.

The downside from a nuclear safety standpoint is that, similar to Plutonium, U233 has fewer delayed neutrons, which means the control system has to respond faster to transients. The delayed neutron fraction is responsible for assuring that the reactor does not go prompt critical (a very bad thing).

This is a design problem that can be overcome with a good system, but in general it is probably less safe owing in theory to the activity of the U233 fuel and the control system considerations.

These can be overcome with a good design - for example, a reactor that uses online refueling with little excess reactivity is one that is inherently safer, even if fueled by U233.*

* Assuming one does not have a large power defect or set it up to expliode like Chernobyl

[pantone159]

The advantage of U233 is it is not useful to make into nuclear weapons - anti proliferation.

I have heard that the USA successfully tested a U-233 weapon at one point.  This route wasn't pursued for some reason, but is apparently possible.

[AndyHolland]

The term was "useful" not possible. It is not useful because it is radioactive, hard to construct, and easy to detect remotely.

I would not be suprised that the US government wasted our tax money building a U233 bomb - as inefficient from a predetonation standpoing as that would be. However, I have not heard of them doing so.

What they did do was make a Plutonium bomb from LEU core waste, to demonstrate the possibility of creating a nuclear bomb with large Pu240 contamination (causes predetonation). The bomb was very large physically, and the yields were terrible. It was totally impractical, however, it led Jimmy Carter to kill Plutonium recycle in the US.