Is Carbon a Realistic Choice for ITER's Divertor? Page: 4 of 18
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Serious plasma material interaction issues need to be addressed and resolved in order for
ITER to be able to study burning plasmas[1,2]. This article focuses on the choice of material for
the most challenging plasma facing region - the divertor. ITER's physics goals with fusion
power gain, Q=10, are projected to be met in high confinement regimes that entail a steep
pedestal region with a transport barrier that periodically relaxes in an edge localized mode
(ELM), sending heat and particle flux from the confined plasma primarily to a narrow region on
the divertor target plate. Here the heat flux from ELMs and disruptions could exceed 1 MJ/m2
and cause material damage. Tungsten is one candidate material, however this would melt
under high heat flux and in the worst case, loss of the melt layer could drastically shorten the
divertor lifetime. For this reason carbon-fiber-composite (CFC) is the currently favoured material
as it does not melt, (though it is subject to brittle destruction). The ITER physics base is
founded mostly on tokamaks with carbon plasma facing components (PFCs). However, a major
obstacle to the use of carbon in ITER is tritium retention. An excessive tritium inventory in the
torus would present a safety hazard in the form of a potential tritium release to the atmosphere in
case of a loss of vacuum accident. Due to the ease of mobilisation of tritium trapped in
codeposited layers (co-deposited films in tokamaks start to release tritium when exposed to air at
temperatures >520 K, ), a limit of -350 g is currently set for the in-vessel codeposition
inventory. This limit is set to avoid the need for public evacuation in event of the full
accidental release of the tritium inventory under the worst weather conditions. Independent of
safety considerations, tritium is expensive and the supply is limited so it is important to avoid
inventory build up in inaccessible locations. Though predictions are uncertain, the inventory
limit could be reached in as little as 100 pulses under the worst conditions. At this point ITER
will only be able to continue its planned burning plasma program if the tritium can be removed
from the vessel. An analysis of the tritium removal rate required for ITER to meet its physics
mission and a review of removal techniques is presented in . Public sensitivities to tritium
issues have previously led to closure of one fission reactor and it is especially surprising that a
proven means to remove tritium from ITER is nowhere is sight.
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Skinner, C.H. & Federici, G. Is Carbon a Realistic Choice for ITER's Divertor?, report, May 13, 2005; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc779415/m1/4/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.