Fusion Reactor Safety

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Fusion power generators are inherently safe. The magnetic confinement of the plasma must be carefully controlled and balanced to sustain the nuclear reaction. Any disturbance of the operating conditions will result in termination of the reaction. No combination of system failure, operator error, natural disaster or sabotage can cause the fusion reaction to run away. A nuclear explosion, melt down or similar catastrophic accident is not possible. A violent event, one of sufficient magnitude to disrupt the total reactor, could cause a chemical or electrical fire similar to any industrial fire.

Fission reactors produce of kilograms quantities of highly radioactive cesium 137, iodine 131, and strontium 90. Tons of uranium and plutonium are also present in a fission reactor. If released in an accident these radioactive isotopes can be absorbed by the body and retained for some length of time. The fusion reactions do not use or produce any of these toxic elements.

Tritium is the only radioactive material used in the fusion reaction. Tritium has a half-life of about 12.5 years and gives off relatively low energy electrons. It is one of the least hazardous radioactive elements. If released, the tritium would combine with oxygen to form water. Ingested tritium would tend to be eliminated within a few days. Consumption of large quantities of tritium free water would be a major element in the treatment for tritium exposure.

152 Rafelski, Johann and Jones, Steven E., "Cold Nuclear Fusion", Scientific American, Vol. 257, No. 1, July 1987, Page 84

153 Crura, Lawrence A., and Matula, Thomas J., "Shocking Revelations", Science, Vol. 276 30 May 1997, Page 1348

Contamination from a fission reaction can last for thousands of years because of the long half-life of the fission-produced radioactive isotopes. If, by an unusual set of circumstances a plot of ground becomes contaminated with a hazardous amount of tritium the area would be safe within a few decades because of the short half-life of the tritium.

Using the design postulated above the hazard of a fusion reactor can be estimated. Fusion reactors will present only a tiny fraction of the radiation hazard potential of a uranium fission reactor. There is only a small quantity of radioactive tritium present in the reactor. Based on the tritium usage in research reactors it would seem likely a power reactor would have less than a few kilograms of tritium at the power plant at any given time. This would probably be distributed 10% to 20% in the plasma, 20% to 40% ready for future insertion in the plasma and the un-recovered tritium present in the lithium coolant.

A violent event, such as the breaking of the vacuum chamber, would result in exposure of the plasma to the surrounding air. The tritium and deuterium from the plasma would react with the atmosphere to form water. The helium would mix with the atmosphere without reaction. The reaction products would be tritium and deuterium containing water. The deuterium containing water and the helium are both entirely non-toxic and could be ignored. Water containing weakly radioactive tritium is a mild hazard. The total amount of water resulting from the reaction would be in the range of 10 grams to 5 kilograms depending on the size of the reactor and its operating conditions. This small amount could easily be adsorbed by an atmosphere drier protection system. If the atmosphere drier system were to fail at the same time as the reactor, or be damaged by the same event that broke the vacuum chamber, little hazard would result. On escape from the system, the tritium would be diluted by the water in the atmosphere. It would present only a slight hazard to the environment. This would dissipate in a few years.

When the reactor is operating normally, the tritium is removed from the lithium continuously and the amount present at any given time is small. If an event breached the lithium cooling system, a lithium fire would be possible. In a lithium fire, any residual tritium remaining would be burned to water and released. The violence of the lithium fire will increase the mechanical difficulty of trapping the water containing the tritium. Fortunately, when lithium burns at high temperature the product is lithium oxide. As lithium oxide cools it becomes an excellent absorber of water. The oxide reacts strongly with water to form lithium hydroxide, a solid. To whatever degree this reaction occurred, the tritium would be tied up as a solid within the confines of the reactor complex. The amount of tritium released would be highly dependent on the details of the accident, condition of the reactor, weather and other similar variables. In general, should a lithium fire occur, the probability of some tritium escaping the reactor would be increased.

During its lifetime, a fusion reactor presents little radiation hazard. The internal structure, particularly the vacuum containment vessel and the heat exchanger, will be subject to intense neutron bombardment. The neutrons will convert some of the elements of the structure into long-lived radioactive isotopes. Selecting construction materials that do not easily become activated can minimize radioisotope production. No material is entirely resistant to neutron activation, thus the decommissioning of a fusion reactor will require the handling and disposal of potentially hazardous radioactive isotopes. Because of the lack of uranium, plutonium, and fission products, the total radiation exposure hazard from the decommissioned fusion reactor is 10,000 to 1,000,000 less than from a decommissioned fission reactor.

If the initial development effort is successful at implementing a CBFR using the boron-proton reaction, virtually all the hazard arising from radioactivity is eliminated. Table 3.1 shows that the boron proton reaction produces only helium and energy.

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