Comment by InclinedPlane | Hacker News Reader

Comment by InclinedPlane | Hacker News Reader

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There are numerous different reasons. Thorium proponents often gloss over a lot of the difficulties of the system, leading to a false sense of how easy it would be to make such reactors. One of the biggest problems is that a Thorium reactor is actually a U-233 reactor, which is bred from the Thorium, and U-233 is not so easy to work with. It emits a huge amount of gamma radiation, which is highly penetrating and dangerous for human life.

To work with U-235 or Plutonium you only need a glove box, but to work with U-233 you cannot have humans physically close to the material, you need waldos and closed circuit cameras and so forth. This naturally increases the cost of working with the fuel. But wait, it gets worse. As I said, gamma radiation is highly penetrating and heavily ionizing, which means that it damages delicate materials quite easily. Especially seals, made out of rubber or silicone or what-have-you, and electronics. This makes fuel cycle handling hugely challenging and also makes reactor construction rather challenging as well.

Now, likely we could overcome these problems but they are nevertheless huge problems.

One of the big reasons why Uranium/Plutonium reactors have caught on is because you can use 1950s technology to build reactors and process fuel. That's not the case with Thorium/U-233.

This pretty much nails it, that is why things have to be very different with Thorium and the combination "very different" and "nuclear" leads to an over abundance of caution.

One of the reasons the travelling wave reactor is "interesting" is that starts and ends with 'low grade' radioactive material, and works very much like a 'brushfire' which burns fuel ahead of it and leaves behind fully utilized fuel. The downside is that it doesn't really "stop" in the sense that you start one of these candles burning and for the most part it goes 10 years and then sputters out, you can harvest the energy or not but you can't really turn it off. (at least not in the early designs)

So much of the engineering issues with Thorium are mastering the fuel cycle and that is something the US DoE hasn't spent a whole lot of time investigating. Its an interesting question what we could do with a 1950's attitude toward researching nuclear power uses and 2010's level of technology.

LFTRs are not all that difficult. It is a WHOLE lot simpler than a Traveling Wave reactor!

We have different definitions of "difficult" I suspect.

The Thorium fuel cycle produces U232, that stuff kills at a distance, through walls. What that means is that there are a number of scenarios, one of which Fukishima just went through, where the core gets uncovered and rather than leaking Cesium it shoots gamma rays everywhere killing anything trying to get near it. That is not the case with the U238 fuel cycle.

Not saying it can't be dealt with, just saying its different, and by being different it is dangerous in different ways.

"The Thorium fuel produces U232..." Yup, which is one of its nicest features. It means that the U233 is unlikely to be used for weapons. And since it never leaves the well shielded containment area in a LFTR, there is no hazard.

If the kettle is breached in the LFTR, the salt will probably just condense on any small break and seal it. If the break is large, the salt drains into a drain tank which is still in the shielded containment volume. No worries, mate! Oh, and since there is no significant pressure and no volatile chemicals like liquid sodium, there are no forces trying to disperse the materials. Inherently MUCH safer than any PWR or LMFR.

Has anyone considered using molten lead as a "seal"? I imagine you could reduce the number of places that need seals and then have it set up so that everything that needs to get into the sealed box passes through a small pool of lead that is kept molten. So long as everything around the lead has a melting operating temperature tolerance greater than 621 F it should be able to function through a molten lead seal.

It's not the U-233 itself that emits gamma, but the U-232 it tends to be contaminated with. This can be avoided by chemically separating protactinium during the decay chain from thorium, but the U-232 is often considered desirable as a anti-proliferation measure.

By using liquid fuel and transmuting in place, you never handle U-233, contaminated or not. After reactor startup (which requires a good neutron source), you just keep feeding more thorium to the reactor and removing fission products.

Thorium is kind of like diesel, it doesn't work well in a spark ignition engine. Thorium doesn't work very well in a solid fuel reactor. Liquid Fluoride Thorium Reactors (LFTRs, pronounced "lifters") on the other hand work very nicely. All the "reasons" you mention apply to solid fuel reactors, not LFTRs. The man who invented both preferred the LFTR.

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