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Thorium | |||
Thorium is a naturally occurring, slightly radioactive metal. When pure, thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium oxide (ThO2), also called thoria, has one of the highest boiling points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.
Thorium, as well as uranium, can be used as fuel in a nuclear reactor. Although not fissile itself, thorium-232 (Th-232) will absorb slow neutrons to produce uranium-233 (U-233), which is fissile. Hence, like uranium-238 (U-238), it is fertile.
In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (U-235 or Pu-239), a breeding cycle similar to but more efficient than that with U-238 and plutonium (in slow-neutron reactors) can be set up. The Th-232 absorbs a neutron to become Th-233 which normally decays to protactinium-233 and then U-233. The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle.
Problems include the high cost of fuel fabrication due partly to the high radioactivity of U-233 which is always contaminated with traces of U-232; the similar problems in recycling thorium due to highly radioactive Th-228, some weapons proliferation risk of U-233; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available.
Nevertheless, the thorium fuel cycle[?], with its potential for breeding fuel without the need for fast neutron[?] reactors, holds considerable potential long-term. Thorium is significantly more abundant than uranium, so it is a key factor in the sustainability of nuclear energy.
India has particularly large reserves of thorium, and so have planned their nuclear power program to eventually use it exclusively, phasing out uranium as an input material. This ambitious plan uses both fast and thermal breeder reactors.
The isotopes of thorium range in atomic weight from 212 amu (212-Th) to 236 amu (236-Th).
Powdered thorium metal is often pyrophoric and should be handled carefully. Thorium disintegrates with the eventual production of "thoron", an isotope of radon (220-Rn). Radon gas is a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential.
Exposure to thorium in the air can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases.
surface of the sawdust, the heat of summer had been drying out
long time. Perhaps for yards and yards around, the interior of
plunging back.html">back to the stalled wagon, suddenly his foot slumped
out with surprise or pain. Nan, horrified, saw the flames and
For the moment the girl felt as if her heart had stopped beating.
"Oh, Tom! Oh, Tom!" she shrieked, and sprang toward him.
Tom was struggling to get up. His right.html">right leg had gone into the
not get it out. A long yellow flame shot out of the hole.html">hole and
side of his head.
But Nan did not scream again. She needed her breath, all that
her back feebly, and tried to tell her to avoid the fire.
Nan rushed in, got behind him, and seized her cousin under the
tried.
Chapter XXVII
chains, the shrieking of the wind, the reverberating cracks of
scarcely hear what the imperiled Tom shouted to her. Finally she
left. Nan tugged that way. Tom pulled, too, heaving his great
fingers spread like claws. His right leg came out of the hole,
beaten down. Tom got to his feet, shaking and panting. He
for the smoking timber cart. The terrified horses tried again
nor did the mired wheel.html">wheel budge.
"Oh, Tom! Oh, Tom!" begged Nan. "Let us make the poor horses
almost beat Nan to the ground; but Tom Sherwood worked furiously.
He placed the butt of the lever.html">lever he had cut under the hub of the
fulcrum. Tom threw himself upon the end of the lever. Nan ran
began to rise slowly.
The sawdust was not clinging, it was not like real mire. There
broken in and the wheel had encountered an impediment of a sound
.
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