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Thorium

Page history last edited by Andrew Alder 5 months ago

A page of energy issues

 

Thorium is the other nuclear fuel. That is, like Uranium, Thorium is available on Earth in significant quantities and can be used to fuel nuclear power reactors.

 

And these are the only such materials with current technology. Some hope that nuclear fusion will enable us to use other fuels, quite a few of them in fact. But none have yet been shown to be practical. (Although that may not be what you've been told.)

 

But there are a few hurdles to be tackled before Thorium can make a significant contribution, and a few myths being promoted, for various reasons.

 

In particular, there are three hurdles that the proponents of Thorium tend to overlook.

 


 

 

The Hurdles

 

The Thorium fuel cycle depends on Uranium

 

Some background

The only fissile material available on Earth is Uranium-235, which is about 0.7% of natural Uranium... yes, less than 1%. And separating it from the other 99.3% is possible but difficult and expensive.

 

The rest of the Uranium is Uranium-238. Uranium-238 and Thorium (which is for practical purposes pure Thorium-232) can both be used as nuclear fuels but only indirectly. They are fertile, which means that a reactor can convert them to fissile material. Uranium-238 becomes Plutonium-239, and Thorium-232 becomes Uranium-233.

 

In fact all Uranium-fuelled reactors do this to some extent. We'll get to that later.

 

Natural Uranium can be used as nuclear fuel, and the first artificial nuclear reactors all did. But at first it's only the U-235 that is being fissioned sufficiently often to cause the chain reaction.

 

No matter how much natural Uranium you pile in one place, the chain reaction won't build up... an event known as criticality. But there are two ways of making Uranium go critical

 

One is to increase the amount of U-235 in the Uranium, a process known as enrichment. It's difficult and expensive but possible. This increases the probability that a neutron will react with U-235 rather than U-238. While U-238 fissions sometimes, this is not common enough to make U-238 fissile. So enrichment makes it easier to make the pile go critical.

 

The other is, the neutrons that U-235 emits when it fissions aren't very good at producing further fissions, because they are going too fast. If they can be slowed down, the probability of fission increases. A material that does this in a reactor is called a moderator.  Unfortunately the best moderators are also expensive.

 

Creating fissile materials (U-233 and Pu-239 in particular) from fertile materials (Th-232 and U-238 respectively) is called breeding. And in practice all reactors do a bit of it, but they don't generally do enough of it to fuel themselves. They consume more fissile material than they breed. 

 

But there is a special class of reactor called a breeder reactor which produces more fissile material than it consumes. And breeder reactors are an essential part of any Thorium-based nuclear power program. 

 

It 's important to understand that reactors that are not breeders can still contribute to nuclear programs that require fissile material. The Manhattan Project plutonium production reactors were not breeder reactors. They were useful because they were operated so that the Plutonium they produced was almost pure Pu-239, and separating it from the rest of the fuel then required only chemical processing. Bombs require very pure fissile material. Power reactors don't.    

 

The consequences

So while you can theoretically fuel a reactor with U-233 created by irradiating Th-232 in a reactor, there's a catch. Two of them in fact.

 

One is, you need a Uranium-235 and/or Plutonium-239 fuelled reactor to start the process, to create your first batches of Uranium-233. The Thorium/Uranium fuel cycle initially depends on input of fissile material from the Uranium/Plutonium fuel cycle, or it can't get started. 

 

The other is, you then need to design further reactors that are breeder reactors, meaning that they breed more U-233 than they consume. And this is possible but it is not a trivial task. And until you do this, the program will continue to depend on input from the Uranium/Plutonium fuel cycle. 

 

And it's interesting to note that some and perhaps all of the Thorium-based reactors (SMRs and others) currently being proposed are not breeder reactors. They depend on Plutonium as part of the initial fuel load, and that is admitted. But what is not said is, will they continue to require more Plutonium at every refuelling? The promoters generally don't say, for obvious reasons. Possibly they do not even know!

 

The Uranium/Plutonium fuel cycle is well developed

The first nuclear reactors and all their supporting infrastructure were paid for by the military.

 

The Manhattan Project used U/Pu just because they were in a hurry. See above. And the development of the PWR was based on this infrastructure.

 

Th/U has a lot of catching up to do. 

 

Thorium is nasty stuff

In some ways it's nastier than Uranium, in other ways less so. Both have their dangers.

 

From a recent post on a discussion group:

 

Thorium has a half life of about 14 billion years. The radioactivity of thorium is so low as to be insignificant. It is only when bombardment with neutrons transforms it to protactinium and subsequently onto U233 that it becomes highly radioactive. You could leve a hundred kilos of Thorium under your bed for years without any detriment .

 

All true enough but highly misleading. You'd want to seal the Thorium up very well. Its daughter products are very nasty indeed.

 

You could leave a hundred kilos of bare natural Uranium bars under your bed for years and probably nobody would suffer ill effects, but if you were to leave a single bare bar of Thorium under your bed for just a few years, you would be highly contaminated with its decay products and a danger to everyone you met.

 

The advantages of Thorium

Thorium has several advantages. But they are mostly not what you hear in the spin that is all too common.

 

Thorium is more common than Uranium. India and Australia in particular have lots of it, and world resources are greater too. But we're not short of Uranium, and some even predict that Uranium will not run out until the Sun expands enough to make the Earth uninhabitable anyway. So while nobody really doubts that the technology can be developed, there is no great hurry. Uranium is fine for now.  

 

Only India has real reasons to have real plans to exploit Thorium, and they do.  See India's_three-stage_nuclear_power_programme.

 

The main advantage of Thorium is political. The technology is not available to criticise. So nobody can prove that it will produce dangerous waste, or involve high-grade Plutonium in the "starter" fuel load, or cost a lot, or ask lots of other questions, when the reactor and fuel cycle design are both speculative anyway. Nobody will get upset that one will be built near them anytime soon for the same reasons.

 

See also

 

 

 

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