Tritium 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. 

Helium-3 [14762-55-1], He, has been known as a stable isotope since the middle 1930s and it was suspected that its properties were markedly different from the common isotope, helium-4. The development of nuclear fusion devices in the 1950s yielded workable quantities of pure helium-3 as a decay product from the large tritium inventory implicit in maintaining an arsenal of fusion weapons (see Deuterium AND TRITIUM) Helium-3 is one of the very few stable materials where the only practical source is nuclear transmutation. The chronology of the isolation of the other stable isotopes of the hehum-group gases has been summarized (4). 

Table I. Integrated Tritium Inventory at a Single Crater Lip Site...

Perhaps the two most important implications of tritium inventory buildup in the torus are the locking up of the fuel in the PFMs (reducing the available fuel in the machine), and the need to keep the in-vessel T inventory within a licensed limit due to safety considerations. Based on postulated accident scenarios for ITER, the administrative in-vessel limit of mobilizable tritium has been set to be 1,000 g [2]. When the accumulated T inventory in the torus reaches this level, operation will have to be discontinued and dedicated T-removal procedures must be applied. 

G. Federici, R.A. Anderl, J.N. Brooks et al., Tritium inventory in the ITER PFCs predictions, uncertainties, R D status and priority needs, Fusion Eng. Design 39-40 (1998) 445-464... 

F.L. Tabares, D. Tafalla, I. Tanarro, V.J. Herrero, A. Islyaikin, C. Maffiotte Suppresion of hydrogenated carbon film deposition by scavenger techniques and their application to the tritium inventory control in fusion devices. Plas. Phys. Contr. Fusion 44, L37 (2002)... 

Tritium Inventory in the Materials of the ITER Plasma-Facing Components... 

Tritium is one of the main source terms in accident scenarios for ITER and to meet some of the key requirements for limitation of worker and public dose in accidents, limits need to be placed on the tritium inventory. Due to the ease of mobilization of tritium retained in co-deposited layers (co-deposited films in tokamaks start to decompose, releasing tritium, when exposed to air at temperatures > 520 K, [65-69]), a limit of 350 g is currently set for the in-vessel co-deposition inventory (and 120 g in the cryo-pumps). These limits are set to allow the full release of this inventory, under hypothetical accident conditions, without the need for public evacuation under the worst weather conditions. 

Erosion and deposition constitute one of the key topics in preparation of ITER operation, as they influence the in-vessel tritium inventory and the lifetime of divertor tiles and other PFCs. Assessment of erosion is discussed elsewhere [39,41]. 

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