Quote:
Originally Posted by John Bull
Engineers and scientists at the Oak Ridge plant ran a thorium generating plant for 20 yrs w/o incident and they found that it could not do a 3 mile island or Chernobyl because of it's design and the fact that the isotope it uses is incapable of that kind of runaway.
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OK, looks like somebody's been believing everything he reads in
Wired again. I don't want to turn this into a physics class, but despite the article's claims that thorium will save the human race this just isn't so. Most of what has been in the press about thorium lately has been a load of bunk sponsored by the nuclear power industry in India (for reasons I will detail below).
Myth #1 - Thorium reactors can't melt down or explode.
By design, the fuel cycle in a thorium reactor produces or "breeds" uranium-233. U-233 makes a quite lovely nuclear bomb if one so desires and you have be very careful with a thorium reactor or it could, literally, go "boom". All breeder-type reactors are inherently dangerous in this way. Despite the claims, thorium reactors are perfectly capable of going super-critical and melting down or even just exploding outright. You can make a thorium reactor that runs at sub-critical phase and would have little risk of a true "Chernobyl" type accident, but such a reactor would be so inefficient that its cost would far exceed its usable electrical output.
We no longer allow construction of plutonium breeder reactors for these same reasons. A "sub-critical" plutonium breeder actually came very close to nuclear detonation during an accident in 1966. The resulting explosion and radioactive contamination would have pretty much taken out Detroit, Toledo, and most of the surrounding area. Uranium reactors have (slightly) higher potential for "melt-down" type accidents but I'll take that small extra risk over a full-fledged nuclear detonation any day, thank you.
Myth #2 - Thorium reactors are "cleaner" than uranium.
This is true for thorium itself, but it ignores the problem of waste.
First off, the thorium fuel cycle also produces uranium-232. This is an EXTREMELY dangerous isotope - one of the nastiest that there is - because it produces significantly more radiation than other isotopes in its class. The presence of U-232 in the spent fuel from a thorium plant makes the waste
much more dangerous than that from a uranium fuel reactor. Some people consider U-232 to be equivalent to plutonium as a terrorist problem because small amounts can be used to make a very, very nasty "dirty bomb". We currently have no way to process spent fuel from thorium reactors to make it safe enough for transportation to permanent storage facilities because the U-232 is so dangerous.
Second, if you want to make an efficient thorium reactor you need to use what's called a "blanket" design. This requires the use of plutonium - another EXTREMELY dangerous isotope - as part of the fuel assembly. It's claimed that the Pu would be transmuted to other, safer isotopes in a thorium reactor and this is true to some extent. However, in order to eliminate enough Pu in the fuel assemblies to make them reasonably safe you'd have to run the reactor to what's called "burn-out" - meaning that you'd have to let the thing sit there making no electricity for 10-20 years to burn away the Pu before you can go in and reload it. In short, it ain't gonna work like people claim and the waste from your thorium reactor is gonna be some of the nastiest, most dangerous stuff man ever made.
Myth #3 - Thorium can be used in the reactors we already have.
Thorium reactors require very specialized cooling systems. You can't run them with the typical light water systems that you find in uranium reactors - they just don't work with that. Instead you have to use exotic designs like pressurized heavy water or molten salt as the coolant. This makes thorium reactors much, much more expensive to build and, if molten salt is used, significantly more dangerous to operate than uranium light water systems. (Molten salt coolants are explosive on contact with air so even a minor coolant leak becomes a damn serious problem in a real big hurry.)
It is possible to adapt some existing pressurized heavy water reactors (PHWR's) to use thorium in a blanket design. This is, however, extremely frowned upon from a safety viewpoint as PHWR's produce more waste than light water designs. Given how nasty thorium waste is this is the last thing you want to do.
In short, physicists and nuclear engineers pretty much know what they're doing. They don't use thorium for very good reasons outside of certain reactors for research and other specialized needs. It's not the wonder fuel that the magazine articles claim it is.
So why is the press so gung-ho about thorium? Because India is spending big bucks to promote thorium as a fuel.
India has very little uranium, but it has loads of thorium reserves and an extreme need for new electrical generation. The Indians want to both use and sell thorium fuel - something that the rest of the world isn't too thrilled about and has refused to help with. They are now finishing construction of a blanket-design thorium plant at Madras and have seeded thorium into the uranium fuel of some of their PHWR's in other parts of the country. In order to short-circuit the criticism the Indian nuclear power agencies have been papering the planet with news stories about their new miracle fuel.
I, for one, am particularly troubled that a country like India is building a risky reactor design using a fuel system that will produce copious amounts of waste that would be very useful for terrorism and that it is doing so without the cooperation from other countries with significantly more experience and expertise. I'm not knocking the Indians when it comes to brains. They've proven that they're as good as the next guy when it comes to high tech. I'm just not all that comfortable having this reactor go up in a country that routinely changes it's government with a pistol or a bomb instead of an election.
It's possible, with sufficient investment in research and another 20 to 30 of science, that we could someday build thorium reactors that are both safe and efficient enough to justify the cost. I wouldn't hold my breath. Uranium is a far superior and safer fuel given the technology we have now and it's still so dangerous that we're not building any more uranium plants either. And even if we do come up with a decent reactor design for thorium we still have the nasty waste to deal with. As with most "miracles" you read about in the popular press, this one is also too good to be true.
As for cold fusion, well, I'll believe it when I see it powering my water heater. Yes, there's something there. No, it ain't something that's happening any time soon. I'm hoping that my grand-kids see it happen before they die.
Cheers,
Mazo.
Nuclear fusion
