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Wednesday, September 13, 2017

Is there any way to neutralize radioactive material?

Yes there is, and Windows Vista is somehow involved.

I 've always been amused by the scene in Terminator 2 where Robert Patrick gets frozen in liquid nitrogen. After Arnold shoots him, everyone has nothing to do, so they just sit around waiting for him to reassemble. If only they had half a brain, they could have sequestered the various drops of the evil terminator. But I guess if they did, he would have re-assembled into a little midget and the movie would've turned into a comedy.

T-1000 [in high-pitched squeaky voice]: Call to John. I know this hurts.

Sarah Connor: What? Oh, there you are. Aww, so cute! Yes, that does hurt. Ouch!

That movie is used by modern-day techno-doomsayers as a warning about artificial intelligence, but I think of it as more of a metaphor for radioactive material. These days we always try to sequester it, usually by vitrification, which is encapsulation in glass, which is what Sarah Connor ought to have done.

But who wants to have hundreds of these little guys running around potentially stabbing people in the foot? What we really need is some way to neutralize it. I'm not talking about chemically altering it to lower its bioavailability, but changing its physical properties to make it non-radioactive. Is there any way to do this?

Gamma-ray transmutation
Gamma-ray transmutation of radioactive 126Sn by a laser-accelerated electron beam. Tin-126 has a half-life of 23,000 years; Tin-124 and Tellurium-125 are stable. (redrawn from ref. [3])

Conspiracy theories

A Bing search on this topic plunges us into an ocean of crackpots, conspiracy theorists, and misinformation. For instance, one colorful scientist named Paul Brown once started a company called Nuclear Solutions Inc. Its goal was to commercialize a photo­disinte­gration (aka photo­deac­tivation) process whereby a gamma ray induces the expulsion of a neutron from an atom, rendering it non-radioactive or giving it a shorter half-life. Unfortunately, Brown, a race-car aficionado, died in a car crash and a conspiracy theory was born.


But photoneutrons are well known in physics. One team of researchers[1] beamed gamma rays into a target and measured the cross section of a variety of (γ,n) reactions, including 185Re(γ,n)184Re and 178Hf(γ,n)177Hf. (In this notation, the left is the starting material, the right is the product, and the part in parenthesis shows what goes in and what comes out.)

Photodisintegration uses up energy for nuclei lighter than iron and (usually) gives off energy for elements heavier than iron. It's important: in supernovas, photodisintegration is what converts much of the mass of the star to subatomic particles, including alpha particles. These particles are then reassembled to make all the larger elements.

Diagram of traveling wave reactor
Diagram of proposed traveling wave reactor from Terrapower's paper [4].

Gamma rays can also transmute radioactive elements, sometimes producing isotopes with shorter half-lives and sometimes not. Examples: the 242Am(γ,n)241Am reaction increases the half-life from 141 to 432 y, while the 242Pu(γ,n)241Pu reac­tion decreases the half life from 3×105 to 14 y.[2]

So there's no question that photodisintegration works. The current method is to use an electron beam generated by a 1 kW laser. The laser interacts with a near-critical-density plasma, producing electrons. The electrons pass through a target such as tantalum, which produces high-energy semi-collimated gamma rays when their path is deflected by an atomic nucleus, a phenomenon known as bremsstrahlung.

Wang et al[3] used this to transmute radioactive tin-126 into other elements. They obtained about one billion transmutated atoms per laser shot. That might sound like a lot, and it was a definite improvement over previous attempts, but it still translates to only 4×1012 atoms, or about a billionth of a gram, every hour. Remarkably, about one in 100 gamma rays was effective. But considering that about 2,000 metric tons of nuclear waste are produced every year, neutralizing it would take a very, very, very long time.

There are other major challenges that suggest that photodisintegration is the wrong way to go.

  1. It remains to be demonstrated that destroying a radioactive atom takes less energy than the parent atom would produce by disintegrating. If it takes too much, it would negate the benefits of nuclear power. Although destroying a high-Z atom releases some energy, capturing this energy would require carrying out the process in a power plant.
  2. A better technology exists: fast neutron reactors and breeder reactors are far more efficient at using up elements with large fission branches, such as uranium, and they produce a greater percentage of short-lived radioactive waste, mostly 137Cs (t1/2=30.1 years) than thermal reactors. The reason they're not in widespread use is that they require enriched uranium fuel, which is more expensive than ordinary fuel. However, a breeder reactor can be 100 times more efficient in using the fuel, which more than compensates for the greater fuel cost. However, these reactors could also produce plutonium, potentially adding to the risk of nuclear proliferation during the reprocessing stage, which is why President Carter's DoE secretary banned them in 1994.

Breeder reactors

Only one commercial breeder reactor is operating in the world at present, in Beloyarsk Nuclear Power Station, in Zarechny, Sverdlovsk Oblast, Russia. It's an antiquated liquid sodium design, but amazingly it has never blown up. Since 2012 it has operated as a burner, which means it destroys actinides rather than replenishing them.

It won't be alone for long. China, India, Japan, S. Korea, and Russia all believe that uranium prices will not remain low forever, and they're actively researching fast breeder reactors.

Blue screen of death
Bill Gates's first invention was not very popular

Considering the benefits that breeder reactors can provide in reducing nuclear waste and greatly extending the supply of uranium, this makes sense. And as luck would have it, there are newer technologies, such as the proposed traveling-wave reactor, that don't require fuel reprocessing and run on depleted uranium instead of enriched uranium.

Bill Gates, famous for inventing the Blue Screen of Death, is an investor and chairman of the board of one such company called Terrapower, which is heavily promoting their traveling-wave reactor, and he features prominently on their website. The company claims that their design improves fuel utilization by 40-fold over light water reactors. There's enough uranium available in seawater—about 3.3 μg/liter—to provide US-levels of energy for 10 billion people for 130,000 years. Since this uranium is constantly replenished, it could provide, as the company puts it in one of its brochures, virtually free energy ($0.00004 per kWh) “for as long as the sun shines and the rain falls.” They estimate that the amount of uranium on Earth represents $100 trillion worth of electricity.

Overhyped, maybe, but if the U.S. government can permit them, maybe someday we can say that watching all those blue screens of death was not for nothing . . . .

1. Olariu S, Olariu A. Gamma Resonances near Threshold for the Production of Thermal Photoneutrons. arXiv:1205.2199 Link

2. Hirlimann C (2016). Laser induced nuclear waste transmutation arXiv:1603.08737 (non-technical article).

3. Wang XL, Xu ZY, Luo W, Lu HY, Zhu ZC, Yan XQ (2016). Transmutation prospect of long-lived nuclear waste induced by high-charge electron beam from laser plasma accelerator. arXiv:1705.05770

4. Ellis T, Petroski R, Hejzlar P, Zimmerman G, McAlees D, Whitmer C, Touran N, Hejzlar J, Weaver K, Walter JC, McWhirter J, Ahlfeld C, Burke T, Odedra A, Hyde R, Gilleland J, Ishikawa Y, Wood L, Myhrvold N, Gates, WH III (2010). Traveling-Wave Reactors: A Truly Sustainable and Full-Scale Resource for Global Energy Needs. Proc. Int. Cong. Advances in Nuclear Power Plants (ICAPP), San Diego, California, June 13–17, No. 10189. Link

sep 13, 2017; last edited sep 13, 2017, 5:45 am

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