ENERGY: Why a protactinium-233 threat to Green Nuclear won’t stop Thorium LFTR’s from becoming popular

LessThunk comment: Since the fifties, Thorium was considered to be of no real use to the weapons industry; that was the real reason the USA had little interest in it at that time. Today, it is clear that Thorium energy processing can yield a weapons grade isotope, namely protactinium-233.


Protactinium-233 is formed upon neutron capture by 232Th. It further either decays to uranium-233 or captures another neutron and converts into the non-fissile uranium-234 The 233Pa has a relatively long half-life of 27 days and high cross section for neutron capture (the so-called “neutron poison”). Thus instead of rapidly decaying to the useful 233U, a significant fraction of 233Pa converts to non-fissile isotopes and consumes neutrons, degrading the reactor efficiency. To avoid this, 233Pa is extracted from the active zone of thorium molten salt reactors, during their operation, so that it only decays to 233U.

The saving grace is this, if the 233U was pure there would be a potential problem, but in an LFTR (liquid-fluoride thorium reactor) design the process inevitably yields 232U contamination which yields thallium-208 and it is the gamma-signature of this element which saves the day.

The signature of Thallium-208 is easily traced and tracked, does not occur naturally and would present a heavy logistics challenge to any would-be proliferator.

Hooray for Thallium 208.




Categorised as: energy | interesting

Posted by: Phaedrus

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