The Hype About Thorium Reactors
by Gordon Edwards, Canadian Coalition for Nuclear Responsibility, December 26 2021.
www.ccnr.org/thorium_hype_2021.pdf
There has recently been an upsurge of uninformed babble about thorium as if it were a new discovery with astounding potentiality. Some describe it as a nearly miraculous material that can provide unlimited amounts of problem-free energy. Such hype is grossly exaggerated.
Thorium and Nuclear Weapons
One of the most irresponsible statements is that thorium has no connection with nuclear weapons. On the contrary, the initial motivation for using thorium in nuclear reactors was precisely for the purposes of nuclear weaponry.
It was known from the earliest days of nuclear fission that naturally-occurring thorium can be converted into a powerful nuclear explosive – not found in nature – called uranium-233, in much the same way that naturally-occurring uranium can be converted into plutonium.
Working at a secret laboratory in Montreal during World War II, nuclear scientists from France and Britain collaborated with Canadians and others to study the best way to obtain human-made nuclear explosives for bombs. That objective can be met by converting natural uranium into human-made plutonium-239, or by converting natural thorium into human-made uranium-233. These conversions can only be made inside a nuclear reactor.
The Montreal team designed the famous and very powerful NRX research reactor for that military purpose as well as other non-military objectives. The war-time decision to allow the building of the NRX reactor was made in Washington DC by a six-person committee (3 Americans, 2 Brits and 1 Canadian) in the spring of 1944.
The NRX reactor began operation in 1947 at Chalk River, Ontario, on the Ottawa River, 200 kilometres northwest of the nation’s capital. The American military insisted that thorium rods as well as uranium rods be inserted into the reactor core. Two chemical “reprocessing” plants were built and operated at Chalk River, one to extract plutonium-239 from irradiated uranium rods, and a second to extract uranium-233 from irradiated thorium rods. This dangerous operation required dissolving intensely radioactive rods in boiling nitric acid and chemically separating out the small quantity of nuclear explosive material contained in those rods. Both plants were shut down in the 1950s after three men were killed in an explosion.
The USA detonated a nuclear weapon made from a mix of uranium-233 and plutonium-239 in 1955. In that same year the Soviet Union detonated its first H-bomb (a thermonuclear weapon, using nuclear fusion as well as nuclear fission) with a fissile core of natural uranium-235 and human-made uranium-233.
In 1998, India tested a nuclear weapon using uranium-233 as part of its series of nuclear test explosions in that year. A few years earlier, In 1994, the US government declassified a 1966 memo that states that uranium-233 has been demonstrated to be highly satisfactory as a weapons material.
Uranium Reactors are really U-235 reactors
Uranium is the only naturally-occurring material that can be used to make an atomic bomb or to fuel a nuclear reactor. In either case, the energy release is due to the fissioning of uranium-235 atoms in a self-sustaining “chain reaction”. But uranium-235 is rather scarce. When uranium is found in nature, usually as a metallic ore in a rocky formation, it is about 99.3 percent uranium-238 and only 0.7 percent uranium-235. That's just seven atoms out of a thousand!
Uranium-238, the heavier and more abundant isotope of uranium, cannot be used to make an A-Bomb or to fuel a reactor. It is only the lighter isotope, uranium-235, that can sustain a nuclear chain reaction. If the chain reaction is uncontrolled, you have a nuclear explosion; if it is controlled, as it is in a nuclear reactor, you have a steady supply of energy.
But you cannot make a nuclear explosion with uranium unless the concentration of uranium-238 is greatly reduced and the concentration of uranium-235 is drastically increased. This procedure is called "uranium enrichment", and the enrichment must be to a high degree – preferably more than 90 percent U-235, or at the very least 20 percent U-235 – to get a nuclear explosion. For this reason, the ordinary uranium fuel used in commercial power reactors is not weapons-usable; the concentration of U-235 is typically less than five percent.
However, as these uranium-235 atoms are split inside a nuclear reactor, the broken fragments form new smaller atoms called “fission products”. There are hundreds of varieties of fission products, and they are collectively millions of times more radioactive than the uranium fuel itself. They are the main constituents of “high-level radioactive waste” (or “irradiated nuclear fuel”) that must be kept out of the environment of living things for millions of years.
In addition, stray neutrons from the fissioning U-235 atoms convert many of the uranium-238 atoms into atoms if plutonium-239. Reactor-produced plutonium is always weapons-usable, regardless of the mixture of different isotopes; no enrichment is needed! But that plutonium can only be extracted from the used nuclear fuel by "reprocessing" the used fuel. That requires separating the plutonium from the fiercely radioactive fission products that will otherwise give a lethal dose of radiation to workers in a short time.
Thorium Reactors are really U-233 reactors
Unlike uranium, thorium cannot sustain a nuclear chain reaction under any circumstances. Thorium can therefore not be used to make an atomic bomb or to fuel a nuclear reactor. However, if thorium is inserted into an operating nuclear reactor (fuelled by uranium or plutonium), some of the thorium atoms are converted to uranium-233 atoms by absorbing stray neutrons. That newly created material, uranium-233, is even better than uranium-235 at sustaining a chain reaction. That’s why uranium-233 can be used as a powerful nuclear explosive or as an exemplary reactor fuel.
But thorium cannot be used directly as a nuclear fuel. It has to be converted into uranium-233 and then the human-made isotope uranium-233 becomes the reactor fuel. And to perform that conversion, some other nuclear fuel must be used – either enriched uranium or plutonium
Of course, when uranium-233 atoms are split, hundreds of fission products are created from the broken fragments, and they are collectively far more radioactive than the uranium-233 itself – or the thorium from which it was created. So once again, we see that high-level radioactive waste is being produced even in a thorium reactor (as in a normal present-day uranium reactor).
In summary, a so-called “thorium reactor” is in reality a uranium-233 reactor. Some other nuclear fuel (enriched uranium-235 or plutonium) must be used to convert thorium atoms into uranium-233 atoms. Some form of reprocessing must then be used to extract uranium-233 from the irradiated thorium. The fissioning of uranium-233, like the fissioning of uranium-235 or plutonium, creates hundreds of new fission products that make up the bulk of the high-level radioactive waste from any nuclear reactor. And, as previously remarked, uranium-233 is also a powerful nuclear explosive, posing serious weapons proliferation risks. Moreover, uranium-233 – unlike the uranium fuel that is currently used in commercial power reactors around the world – is immediately usable as a nuclear explosive. The moment uranium-233 is created it is very close to 100 percent enriched – perfect for use in any nuclear weapon of suitable design.
Uranium-232 — A Fly in the Ointment
There is a complication that arises in the form of another human-made uranium isotope, uranium-232. In a thorium reactor, the uranium-233 that is created is accompanied by a very small quantity of uranium-232. As it happens, U-232 (along with its decay products) gives off very powerful gamma radiation that makes it difficult to fabricate an atomic bomb, given the danger to the workers and the heat generated by the intense radioactivity of U-232 and its decay products. But these difficulties can be overcome, or even avoided altogether, by making suitable adjustments to the reactor operation.
Without going into too much detail, when a thorium-232 atom absorbs a neutron, it is transformed into an atom of protactinium-233, which in turn is spontaneously transformed into an atom of uranium-233. But if either of those two non-thorium atoms absorbs an additional neutron, before the conversion is complete, atoms of uranium-232 can be created – which act as unwanted pollutants. However, if the protactinium atoms are removed from the reactor core, addition neutron collisions are avoided and an uncontaminated supply of almost 100 percent pure uranium 233 can be obtained by just waiting for the spontaneous conversion to be completed.
- Continued below
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