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Why have thorium nuclear reactors not been ore fully developed? Earth has more thorium than uranium. Is it technical difficulties or the fact that uranium technology was developed first for military reasons? Are there other reasons?

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The Oak Ridge National Laboratory had an experimental thorium reactor in the 1960s, but the program was terminated in the 1970s.

This can be partially be attributed to the needs of both the military and businesses, at the time. Unlike $^{235}{U}$, thorium is not naturally fissile, it needs to be bombarded with slow neutrons to ultimately produce $^{233}{U}$, which is fissile and can be used as a nuclear fuel.

One of the issues for the military was that the products of the thorium fuel cycle cannot easily be used for nuclear weapons production, which was an aim of the nuclear powers at the time. $^{233}{U}$ can be used in nuclear weapons but it is contaminated by $^{232}{U}$, which has a half life of 68.9 years, but is an intense gamma radiation emitter, which makes it hazardous to handle. This also makes it a problem in thorium based nuclear power systems.

Another reason is business had invested heavily in uranium reactors and developed that technology and it wanted a return on its investment. Thorium reactors were perceived as a competing technology.

Research into thorium reactors is being done by Norway, China, USA, India.

Some more information about thorium and thorium reactors:

Liquid Fluoride Thorium Reactors

Pros & Cons of Thorium

Thorium - World Nuclear Association

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As I wrote over on a similar question on our sister Physics site, the German THTR-300 Thorium High-Temperature Reactor operated for about 16,000 hours and the IAEA has data on it. So, a thorium-fuelled nuclear reactor has been developed and operated. So it's possible.

High-grade uranium ore hasn't been in short supply, and uranium ore is a tiny part of total costs of a nuclear reactor, so the fact that thorium is more common is irrelevant.

Commercial-scale electricity from nuclear fission may be many decades old, but there are only a few hundred generating reactors, across a large number of different designs. And that means, despite it being a multi-trillion euro/pound/dollar industry (at current prices), when it comes down to it the nuclear industry is still a series of prototypes.

Bear in mind that Thorium on its own is useless in a reactor - it just doesn't produce enough neutrons per fission. So you need uranium in there too.

So not only do you need everything that a uranium reactor needs, you need all the processing for the thorium fuel and materials to handle its chemistry too.

There are enough complications as it is, without starting to play with a whole new fuel cycle. We've got a lot more experience of managing the uranium fuel cycle than the uranium/thorium fuel cycle.

So we'd be taking something that's already incredibly complicated, and complicating it further, for no obvious economic advantage. Bad plan.

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