Tag Archives: nuclear

Infrared imagery of the transport of nuclear waste

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As much as I loathe Greenpeace, they’ve released some fantastic infrared imagery of nuclear waste being transported by train. The intent seems to be to try and get people to think that the casks are emitting something dangerous, but I think they do quite the opposite.

The waste is being transported in CASTOR containers from Germany to France for reprocessing and then back to Germany for reuse.

I haven’t been able to find much information from Greenpeace about the images, but I love how they’re described by National Geographic (who should know better) as Red-Hot Nuclear-Waste Train Glows in Infrared despite the fact that they’re not even close to red hot: at somewhere around 35°C they’re actually colder than the people watching them.

Images copyright Greenpeace

How does the damage caused by exposure to radiation vary as the dose of radiation increases?

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Most people assume that if you double the amount of radiation you double the damage caused, and that there is no threshold below which no damage is done. This is called the Linear No Threshold (LNT) model and is represented by the graph below:

The LNT model has been the subject of some disagreement in recent years. The American Nuclear Society said in a 2001 Position Statement* that:

“There is substantial and convincing scientific evidence for health risks at high dose. Below 10 rem (which includes occupational and environmental exposures) risks of health effects are either too small to be observed or are non-existent.”

This linear threshold model holds that there is a limit below which no damage is caused, but that damage then increases linearly beyond that limit.

There are other possible models. Damage may increase in an exponential way, with very low damage at low doses but increasing amounts of damage at higher doses.

In a logarithmic model the damage would be very large at first, but taper off as the dose increases.

So which model is correct?

The LNT model remains the most commonly used by regulatory bodies, but there is growing interest in threshold models and in the idea of radiation hormesis, the idea that a small dose of radiation is actually good for the body by somehow stimulating the body’s repair systems. I think the most likely candidate is a J-shaped curve with a significant threshold but then a fairly rapid linear increase in damage caused.

* American Nuclear Society, Health Effects of Low-Level Radiation, Position Statement 2001.

Finding hidden nuclear reactors with neutrinos

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French physicists from the École Polytechnique and the Commissariat à l’Energie Atomique et aux Energies Alternatives have published a paper that looks at the possibility of finding clandestine or rogue nuclear reactors by using mobile neutrino detectors transported by supertankers.

The fission of uranium and plutonium produces fission fragments that have too many neutrons. In order for these radioactive isotopes to become stable they must lose their excess neutrons and they do this by the process of beta decay in which a neutron becomes a proton, with the emission of an electron and an electron antineutrino.

Nuclear reactors are therefore prodigious producers of neutrinos; for every gigawatt of thermal energy generated by a reactor about a thousand million million million electron antineutrinos are produced.

For example: the fission of uranium-235 can produce xenon-140 and strontium-94 fission fragments (along with two neutrons that go on to cause further fissions, thereby continuing the chain reaction):

Both xenon-140 and strontium-94 are neutron rich and must undergo a number of beta decays before they become stable and each of these beta decays results in the emission of an electron antineutrino.

Neutrinos are tiny, almost massless particles that pass through matter without interacting with it (about fifty million solar neutrinos pass through your body every second). Because they don’t interact it is impossible to prevent them from being leaving the reactor and this is what makes their detection an interesting possibility for identifying hidden reactors: burying your reactor a mile underground in a mountain won’t work, a mile of rock is nothing to a neutrino.

Neutrinos’ weakly interacting nature is a curse as well as a blessing. Neutrino detectors have to be very large so that they have a reasonable chance of capturing a neutrino “event” in a reasonable timeframe. SuperKamiokande, a neutrino detector in Japan containing fifty million kilograms of ultra-pure water in a cylinder 39m across and 41m tall, detects only about fifteen events per day.

Part of the SuperKamiokande detector, with Japanese physicists and rubber dingy for scale.

The French physicists’ paper suggest a cylindrical detector 46m across and 95m long which would be transported to its location and submerged two kilometres underwater. The detector would be filled with a hydrocarbon called linear alkylbenzene doped with gadolinium (to increase the detection rate) and surrounded by thousands of photomultiplier tubes that pick up the flashes of UV light caused when a proton in the detector “absorbs” the electron antineutrino. They suggest that they could easily locate a three hundred megawatt research reactor producing fuel for a nuclear weapon to within “a few tens of kilometres” from three hundred kilometres away after only sixth months’ observation.

via The Physics arXiv Blog

Long half-life ≠ dangerous

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Nuclear waste is often quoted as having a “half-life of millions of years” as if this is a bad thing in and of itself.* But there’s another way of looking at it.

Radioactive decay occurs when an unstable atom emits either a helium nucleus, a high-speed electron, an electromagnetic wave called a gamma ray or more rarely one of a number of other possibilities. Being in the way of these emitted particles and waves is generally considered to be a Very Bad Idea.

Radioactive decay occurs at random, with each atom having a chance of decaying at any given moment. The more likely it is that atoms decay, the quicker they decay, and the shorter their half-life.

Imagine the radioactive atoms are ammunition cartridges; when they decay the cartridge “goes off” and a bullet is released. Now imagine you’re standing next to two piles of cartridges representing some nuclear waste: one pile with a short half-life and one pile with a long half-life

The bullets in the short half-life pile will go off over a short period of time, and the bullets in the long half-life pile will go off over a longer period of time. Which pile would be safer to stand next to?

Caesium-135 and caesium-137 are both common isotopes found in nuclear waste: Cs-135 is formed when xenon-135 produced as a fission fragment decays by beta emission; and Cs-137 is formed as a fission fragment itself (a uranium nucleus splits to form one caesium-137 and one rubidium-98 nucleus).

Cs-135 has a half-life of 2.3 million years and emits beta particles with an energy of 267 keV. Cs-137 has a half-life of 30 years and emits beta particles with an energy of 605000 keV. On a graph of 100 years the change in caesium-135 is invisible; only at a scale of a million years does the change become visible:

If you stood next to a million atoms of Cs-137 for a year 22840 atoms would decay, for a total energy release of 2.2 nanojoules. Standing next to a million atoms of Cs-135 for a year less than one atom (0.301) would decay and the total energy released would be 13 femtojoules, less than 150 thousandth of the energy released by the caesium-137.

So you have a tradeoff: caesium-135 is less dangerous than caesium-137 but becomes less dangerous more quickly. Both Cs-135 and Cs-137 decay to form stable (non-radioactive) barium so if you can turn a profit selling barium then you’re better off buying a truckload of Cs-137; you’ll be able to sell it as barium sooner.

* It’s worth bearing in mind that nuclear waste eventually becomes safe. Chemical waste from the production of solar cells like silicon tetrafluoride and cadmium telluride remain toxic forever.

Uranium-233 and the thorium future

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When people think of nuclear fuel they tend to think of uranium and plutnonium, or more specifically their fissile isotopes: uranium-235, plutonium-239 and plutonium-241. But there is another fissile isotope that doesn’t get the attention it deserves: uranium-233.

A fissile isotope is one that can sustain a nuclear chain reaction. There is only one naturally-occurring fissile isotope: U-235 which makes up 0.7% of mined uranium (the other 99.3% being non-fissile U-238). Plutonium-239 and -241 are both “bred”, created artificially in a reactor: Pu-239 from the inert U-238 and Pu-241 from Pu-240 which is itself bred from Pu-239.

Making plutonium-239:

In the first stage U-238 is bombarded with neutrons (n) to create U-239. This U-239 then undergoes beta decay* to form neptunium-239:

This neptunium then undergoes a second beta decay to form Pu-239:

Making plutonium-241:

To create plutonium-241 the plutonium-239 from the previous step is bombarded with neutrons to form first Pu-240 and then Pu-241.

Breeding uranium-233 from thorium:

Uranium-233 is produced by bombarding thorium-232 with neutrons to create Th-233 which then undergoes two beta decays to form U-233. This can all be done inside the reactor itself.

Using thorium as a nuclear fuel has a number of significant advantages: it is made up of only one isotope which means that no costly (in both financial and energy terms) enrichment processes are necessary and thorium is at least four to five times more abundant in Earth’s crust than uranium.

Thorium can be used in a molten salt reactor, where it is dissolved into uranium fluoride to form a fluid that is both fuel and coolant (the full name of this reactor is the liquid fluoride thorium reactor). The advantage of using molten thorium as both fuel and coolant is that the reactor then has passive (“fail-safe”) safety: if the fuel begins to overheat then the reaction rate decreases, making a meltdown impossible.

The reaction takes place at a pressure of one atmosphere, meaning no pressure containment vessels are needed. Thorium reactors produce far less waste than uranium reactors do, and the waste produced is far safer: after 10 years 83% of the waste can be sold to recyclers and reused.

More information:

* Completists will notice that I’ve missed out the electron antineutrinos produced in beta decay; I’ve removed them for simplicity since they don’t really play a role here.