Tritium retention in next step devices and the requirements for mitigation and removal techniques

Abstract

Mechanisms underlying the retention of fuel species in tokamaks with carbon plasma-facing components are presented, together with estimates for the corresponding retention of tritium in ITER. The consequential requirement for new and improved schemes to reduce the tritium inventory is highlighted and the results of ongoing studies into a range of techniques are presented, together with estimates of the tritium removal rate in ITER in each case. Finally, an approach involving the integration of many tritium removal techniques into the ITER operational schedule is proposed as a means to extend the period of operations before major intervention is required.

Dynamics of erosion and deposition in tokamaks

“…In contrast, mitigated disruptions [40] would lead to tolerable wall loads and can even have a positive affect on the release of trapped fuel from the wall [41]. Therefore, mitigated disruptions are discussed as a routine discharge termination procedure in ITER [2].…”

“…Of special concern are co-deposited layers growing in remote locations, areas shadowed from direct contact with plasma. These remote locations are hard for currently available cleaning techniques to access [2] and can offer a reservoir for 2 tritium accumulation. Gaps of castellated PFCs are a special type of remote areas specific for ITER [3].…”

Redeposition of amorphous hydrogenated carbon films during thermal decomposition

“…A number of investigations conducted in fusion plasma devices as well as in laboratory experiments have been devoted to study the deposition of such redeposited layers [3][4][5][6][7][8][9][10][11]. Redeposited layers growing in remote areas without direct plasma contact are soft hydrocarbon films with high hydrogen content and it seems that hydrocarbon species with a relatively high sticking coefficient contribute dominantly to deposition [12].…”

“…Redeposited layers growing in remote areas without direct plasma contact are soft hydrocarbon films with high hydrogen content and it seems that hydrocarbon species with a relatively high sticking coefficient contribute dominantly to deposition [12]. A major concern for future fusion devices such as ITER is the large amount of hydrogen isotopes trapped in these redeposited films because then this trapped hydrogen will partly be tritium [1,11] from fusion devices were recently reviewed by Counsell et al [11]. Among others, heating of redeposited films in vacuum either with a rapidly scanning laser beam [13,14] or with a flash lamp [15,16] was proposed as a possible method to thermally desorb tritium.…”

Temperature dependence of the chemical sputtering of amorphous hydrogenated carbon films by hydrogen

“…In the present design for ITER it is also foreseen to build parts of the divertor-the strike zone-from CFC (carbon fiber composites) material [1,2]. With this choice, erosion and redeposition of carbon accompanied by co-deposition of hydrogen isotopes is expected to be one of the dominant tritium retention mechanisms [1,3,4].…”