Fusion reactors, radioactive inventory

A D—T fusion reactor is expected to have a tritium inventory of a few kilograms. Tritium is a relatively short-Hved (12.36 year half-life) and benign (beta emitter) radioactive material, and represents a radiological ha2ard many orders of magnitude less than does the fuel inventory in a fission reactor. Clearly, however, fusion reactors must be designed to preclude the accidental release of tritium or any other volatile radioactive material. There is no need to have fissile materials present in a fusion reactor, and relatively simple inspection techniques should suffice to prevent any clandestine breeding of fissile materials, eg, for potential weapons diversion. 

One of the key chemical problems associated with lithium in fusion reactors is extraction of the tritium that has been either generated in or trapped by lithium. A figure of merit for tritium extraction is the blanket tritium inventory. Very large inventories require excessive start-up inventory and are a potentially large radioactive effluent in the event of a catastrophic accident. 

Compared with fission reactors, operation of fusion reactors is more complicated because of the high ignition temperatures, the necessity to confine the plasma, and problems with the construction materials. On the other hand, the radioactive inventory of fusion reactors is appreciably smaller. Fission products are not formed and actinides are absent. The radioactivity in fission reactors is given by the tritium and the activation products produced in the construction materials. This simplifies the waste problems considerably. Development of thermonuclear reactors based on the D-D reaction would reduce the radioactive inventory even further, because T would not be needed. The fact that the energy produced by fusion of the D atoms contained in 1 litre of water corresponds to the energy obtained by burning 120 kg coal is very attractive. 

An important aspect for the safety of fusion reactors consists in the possibihty to decrease in future the decay heat and the radioactive products inventory. In fact, the use of materials with reduced or short-hved activation and with low tritium retention, together with a limited operation power density, would minimize the above mentioned safety problems, bringing the plant towards intrinsic safety conditions (for which no active systems are necessary). 

Another factor, unique to MSRs, down rates this reactor in comparison with other liquid-cooled reactors in terms of isolation. In an MSR, the fuel is dissolved in the molten salt. The fission process produces tritium, the radioactive form of H2. This places an additional requirement on the intermediate heat transfer loop to ensure that tritium does not reach the H2 production facility. Significant work has been conducted to develop methods to ensure tritium does not cross the heat exchanger. Most of this work is associated with development of fusion reactors that have very large tritium inventories. It is unclear how serious this issue is. 

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