Thorium reactor
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Understanding Thorium reactors
Thorium (Th) is element 90, that is, all its atoms have 90 protons and therefore 90 electrons. Almost all Thorium atoms in nature have 142 neutrons, so the atomic weight is 232 (90+142). Th232 is only very slightly radioactive, having a half life of 14.5 billion years, probably about the age of the universe. It would have been formed, along with all the very large atoms such as Uranium U) and Plutonium (Pu), in massive star explosions (super novae) in which there was enough energy to create such atoms.
The half life of the most of the longest lived large atoms is too short for any of them to be found in detectable quantities in nature, except U238 and small amounts of U235.
Unlike U235, Th232 is not by itself suitable for use in a fission reactor. If it is hit by a neutron, it absorbs it, and does not split, whereas U235 splits, yielding energy and enough additional neutrons to continue the chain reaction.
However, when Th232 is hit by a neutron, it becomes Th233 which soon decays into Pa233 (protactinium) which later (about one month half life) decays into U233.
U233 (an artificial isotope of uranium) is a good fission reactor fuel.
Thus in a suitable reactor, in which additional neutrons are provided from some source, Th232 can be turned into U233, which then powers the reactor by undergoing fission.
One method of supplying the extra neutrons is to use a proton accelerator to smash a beam of protons into a lead target, which causes large numbers of neutrons to fly out, hitting the nearby Th232.
Another method is to mix Th232 (probably as the oxide) with fissile fuels such as plutonium 239.
Facts:
- Australia has about 3x as much thorium (by product of mineral sands mining) as uranium
- Only about 0.5%~1% of the uranium (U235) ever gets used
- Uranium has to be enriched to raise the U235 to about 5% - a chemically nasty and energy intensive process
- Most of the thorium is potentially usable after neutron bombardment
- Australia has some of the largest thorium deposits in the world
- An accelerator bombarding nuclear waste tends to convert most of it to short lived isotops
Thus in principle, accelerator driven thorium reactors have some 250~500 times the potential of conventional uranium reactors.
Some major challenges include making an accelerator that is powerful enough and runs for long enough between servicing. Also, if the Pa233 remains in the neutron field, it tends, undesirably, to absorb a second neutron.
Evidence
I will write more later. For now, Google for accelerator and thorium to get 168,000 references!
