Thursday, 17 December 2015

A politician asks: can thorium be profitable for Norway?

It will be profitable when done with the right reactor design. Today's PWR, BWR, designs are too expensive. E.g. Hinkley C in Britain is horrendously expensive. ThorCon (a startup company aiming to make molten salt reactors (MSR), using thorium), reckon they can make a safer reactor, within 6 years, for 20% of Hinkley C prices. ThorCon are not unique. There are a host of other startups and molten salt reactor designs: Transatomic Power, Terrestrial Energy, Seaborg Technologies, Flibe Energy, Moltex SSR, Chinese designs, the Russian MOSART, the EVOL MSFR in France, Japanese, and Czech reactors. Bill Gates TerraPower are said to be working on a molten salt reactor too.

Note:

  1. There are 3 practical nuclear fuels (fissile materials): plutonium-239, uranium-235, and uranium-233 (Pu-239, U-235, U-233).
  2. Uranium consists of a natural mixture of two isotopes: U-238 : U-235, in a ratio of 1000 : 7. Only U-235 is fissile. It must generally be concentrated 5 times (or more) to be used as fuel. In a reactor some of the U-238 "breeds" to Pu-239. U-235 is the only naturally occurring fissile material.
  3. There is about 3½ times more thorium available in the world than uranium. Thorium can be 'bred' to make a kind of uranium not found in nature : U-233. (much like Pu-239 is bred from U-238) U-233 performs better than any other fissile material in a thermal nuclear reactor (and 99%+ of reactors are thermal). This is the reason nuclear engineers love it.
  4. A molten salt reactor is a very good natural fit for thorium. Molten salt reactors are also:
    • safer - they do not use water compressed at 80 atmospheres. So the possibility of catastrophes like Chernobyl or Fukushima is eliminated.
    • more efficient - because they operate at much higher temperatures (almost 400ºC above a PWR temperature), they convert more heat to electricity
    • potentially less expensive - the intrinsic safety of a MSR allows many expensive safety boondoggles to be dispensed with.
    • for more see: Advantages of molten salt reactors

The magic of thorium is really uranium-233. Poor uranium-233 had all its limelight stolen by the thorium upstart. U-233 has excellent neutronic performance in the thermal neutron spectrum : 90.3% of neutrons hitting a U-233 atom cause fission and release the atom's energy. Only 7.7% of neutrons are wasted (captured and absorbed). The fission : capture ratio (table below) shows that U-233 is 3.1 times better than Pu-239. I'm measuring this performance in terms of fissionable atoms (and neutrons) wasted. The more that are wasted the more difficult it is for nuclear power to be sustainable. To be sustainable we must be able to breed more fissile material from thorium or uranium-238 than we use up in make energy. The couple: thorium and uranium-233 are sustainable in both the thermal and fast neutron spectra.

In practice, uranium-233 can only be made from thorium. That's why thorium is talked about. But it's really about uranium-233.

The relative performance of 3 fissile materials U-233, U235, Pu-239 under thermal neutron bombardment (aka - in a reactor) :

Thermal cross-section (barn)
capturefissiontotal% fissionfission : capture ratio
U-2334553157692.2%11.80
U-2359969279187.5%6.99
Pu-2392691025129479.2%3.81

Now it's not so much that plutonium-239 and uranium-238 don't make a sustainable couple too. They do. But it's U-238 + Pu-239 for the fast neutron spectrum only, but thorium-232 + U-233 for either the fast or thermal neutron spectra.

Wednesday, 9 December 2015

"Hey, these guys had a pretty good idea. Let's go back to it."

Alvin Weinberg MSR Questions (2004). Youtube link

Dr Laurence Miller: One of the things that I considered was the choice between the liquid metal fast reactors and ...

Alvin Weinberg: [Interrupting] Excuse me. How do you increase the volume in here?

Dr Laurence Miller: You mentioned where did we go wrong. And one of your aims was the molten salt reactor and instead of that we went for the liquid metal reactor. Do you think that was a serious mistake?

Alvin Weinberg: Yes I think it was a mistake but, I guess, what I'm talking about goes much beyond the kind of reactor that we're going to build, or that will take over. Because no matter what kind of reactor you build, apprehensive members of the public, or Ralph Nader for that matter, will be able to use this confusion between phantom risk and real risk to scare people out of nuclear energy. So ... I think that that issue overrides the question of whether molten salt is better than liquid metal. I happen to think that molten salt is better than [liquid] metal.

Moderator: I have a follow-on question from cyberspace that we got by email right before today's colloquium and its related to Dr. Miller's question. This question comes from Kirk Sorensen who is with the NASA Marshall Space Flight Center and it has to do with the molten salt breeder reactor. He would like for you to comment on the inherent safety of the molten salt reactor and how we might be able to restart a molten salt reactor programme.

Alvin Weinberg: The molten salt people, who included most famous figures nuclear energy, in particular Eugene Wigner, are all dying off and once they're dead then I suppose you can reinitiate a program on molten salt. Are molten salts inherently safer than liquid metal fuel pin reactors? I think they are, as much as anything, because you don't have supercritical amounts of uranium involved in the system. You add uranium, bit by bit, as you need it because the material is molten. But I'm much impressed with the fact that, despite molten salt reactors having in a sense been a failure in that we don't have people building molten salt reactors now, the molten salt reactor experiment, which produced seven and a half megawatts of heat was one of the most important, and I must say brilliant achievements of the Oak Ridge National Laboratory. And I hope that, after I'm gone, people will look at the dusty books that were written on molten salts and will say "Hey, these guys had a pretty good idea. Let's go back to it."

Monday, 7 December 2015

Radon is not nearly as scary as the authorities make it out to be.

Under normal circumstances, radon is not a threat to your health. No matter what the EPA tell you. There's no positive correlation between lung cancer rates and the distribution of background radiation. Radon-222 is a decay produce of radium-226, which is a decay product of uranium-238. Radon is a heavy gas which accumulates in homes and offices. According to the EPA it is a huge threat to life. In reality it is not such a big threat. It can easily be demonstrated that the effect of other background radiation on the lungs (e.g. from carbon-14 and potassium-40 decay) greatly exceeds the effect of radon-222 by about 100 to 1. So according to EPA logic there should be 100 times as much lung cancer due to potassium-40 and carbon-14 (compared to their projected radon cancers). There's not. There is no good evidence that, at the low doses found, radon-222 causes any measurable lung cancer. The EPA's worry about radon is based upon a projection of a bad mathematical model.

If anything, a map of the USA shows a negative correlation between radon concentration and lung cancer!

Perhaps better described as a positive correlation between radon concentration and no lung cancer!

For an average 70kg person:

  • Your lungs weigh approximately 1.3 kg; 1.86% of your body.[1]
  • We experience about 8300 Bq of radiation (mostly K-40 and C-14).[2]
  • It follows, our lungs experience about 158 Bq (mostly K-40 and C-14).
  • When doing mild activity, we breath about 14 litre/minute = 2.3 × 10-4
  • US average radon air concentration is 37 Bq m-3 (EPA).[4]
  • It follows that a US citizen's lungs are exposed to 37 × 2.3 × 10-4 = 0.00863 Bq radon

When accounting for the effect of that radon on the body we should assume it follows the main decay chain. That's because the other branches of the decay chain have very low probabilities. The main radon decay chain is shown below. It has 4 alphas in it [5]:

energy (MeV)½-life
Rn-222 → Po-218(alpha)5.593.8235 d
Po-218 → Pb-214(alpha)6.0023.1 min
Pb-214 → Bi-214(beta, gamma)1.02426.8 min
Bi-214 → Po-214(beta, gamma)3.27219.9 min
Po-214 → Pb-210(alpha)7.687164.3 µs
Pb-210 → Bi-210(beta)0.06422.2 y
Bi-210 → Po-210(beta)1.1635.012 d
Po-210 → Pb-206(alpha)5.305138.4 d

To convert to millisievert we need to weigh the decays. I won't convert to millisievert but we will weight radon far more heavily than other natural radiation experienced in our lungs. Comparing the effect of radon with other radionuclides we experience: mostly potassium-40 and carbon-14. When converting from grays to millisievert the weighting is normally:

alphabetagamma
2011

Because the EPA are so worried about radon, I'll assume they must know something I don't. So I will weight it twice and also assume every decay in the main decay chain counts. The main radon decay chain is worth 86 according to normal weighting. 4 alphas, 4 betas, and 2 gammas = 20 x 4 + 4 × 1 + 2 × 1. I'll double that to 172. Let's compare the lung radiation contribution of radon with K-40 and C-14:

radon= 172 × 0.00863= 1.5
K-40 and C-14= 158
The effect of K-40 and C-14 combined is about a hundred times that of radon. [even after I weighed radon twice as much as I should have!]

The following table shows the radioactivity found in a typical adult human body of 70,000 grams (about 154 pounds)[2]:

NuclideTotal MassTotal Activity (Bq)
Uranium90 µg1.1
Thorium30 µg0.11
Potassium-4017 mg4,400
Radium31 pg1.1
Tritium0.06 pg23
Polonium0.2 pg37
Carbon-1420 pg3,840
Total:8,302
Source: Radionuclides in the Ocean

References

  1. Lungs (Wikipedia)
  2. Radionuclides in the Ocean
  3. Radon decay chain
  4. Radon (Wikipedia)
  5. Radioactive series of radium-226
  6. EPA: The National Radon Action Plan
  7. Mohan Doss comment on radon at 'The Conversation'

Postscript

I said "perhaps better described as a positive correlation between radon concentration and no lung cancer!". Here it is: