9.8 Summary
183
9.8 Summary
Accounting or quantitative estimation/determination of T inventory in a reactor
is one of the critical issues to establish fuel self-sufficiency and to avoid radiohazardousness. At present, however, the estimation of T inventory in a reactor
includes large error owing to the lack of data of H retention in PFM under the
reactor relevant conditions, in particular dependence of the H retention on fluence
and temperature for both PFM candidates of C and W remain as urgent tasks.
The mechanism of fuel retention is different between C-wall and W-wall. T corporation with C deposited layers on plasma shadowed area dominates the T retention
in the former, while T retention in bulk (by diffusive penetration and trapping) dominates in the latter. For both cases, the total T inventory after the long discharges seems
to be comparable level.
To attain the fuel self-sufficiency, the allowable fuel retention rate in PFM should
be less than around 0.1% and removal of T from PFM seems dispensable. To keep
T safety the maximum T inventory in the reactor is set to be around 1 kg. Therefore,
the reduction of the fuel retention rate and the fuel recovery become necessary tasks.
Although it seems quite hard to attain the fuel retention rate of 0.1%, it could
be attained after long time discharge or at the steady-state discharge in a reactor.
Because after long time plasma exposure, the rather thick surface layers of PFM
must be saturated with the fuel and only tiny part of injected fuel would diffuse into
deep. Higher temperature operation in a reactor compared to present tokamaks should
significantly reduce the saturation concentration. Thus, both the C-wall and the Wwall could attain the fuel retention rate below 0.1% at the steady-state operation.
Nevertheless, the total fuel retention in PFM must continuously increase owing to
the increase of deposited layers and deep penetration and could come across the site
limit given by T regulation low. H trapping in defects produced in PFM would add
the total fuel retention, which is still very uncertain. Hence, the removal or recovery
of T from PFM is mandatory not only for the fuel self-sufficiency but also for T
safety and the development of T removal technique from PFM is urgent. As for the
selection of PFM of a reactor, easy fuel removal is an important criterion different
from ITER for which it was the less fuel retention.
References
1. T. Tanabe, Tritium: Fuel for Fusion Reactors, Springer Japan, 2017 ISBN: 978-4-431-56458-4
2. J. Roth, E. Tsitrone, T. Loarer et al., Tritium inventory in ITER plasma-facing materials and
tritium removal procedures. Plasma Phys. Control. Fusion 50, 103001 (2008)
3. J. Roth, K. Schmid, Hydrogen in tungsten as plasma-facing material. Phys. Scr. T145, 014031
(2011)
4. A. Kirschner, D. Borodin et al., Modelling of tritium retention and target lifetime of the ITER
divertor using the ERO code. J. Nucl. Mater. 363–365, 91 (2007)
5. Y. Gotoh, T. Tanabe, Y. Ishimoto et al., Long-term erosion and re-deposition of carbon in the
divertor region of JT-60U. J. Nucl. Mater. 357, 138–146 (2006)
183
9.8 Summary
Accounting or quantitative estimation/determination of T inventory in a reactor
is one of the critical issues to establish fuel self-sufficiency and to avoid radiohazardousness. At present, however, the estimation of T inventory in a reactor
includes large error owing to the lack of data of H retention in PFM under the
reactor relevant conditions, in particular dependence of the H retention on fluence
and temperature for both PFM candidates of C and W remain as urgent tasks.
The mechanism of fuel retention is different between C-wall and W-wall. T corporation with C deposited layers on plasma shadowed area dominates the T retention
in the former, while T retention in bulk (by diffusive penetration and trapping) dominates in the latter. For both cases, the total T inventory after the long discharges seems
to be comparable level.
To attain the fuel self-sufficiency, the allowable fuel retention rate in PFM should
be less than around 0.1% and removal of T from PFM seems dispensable. To keep
T safety the maximum T inventory in the reactor is set to be around 1 kg. Therefore,
the reduction of the fuel retention rate and the fuel recovery become necessary tasks.
Although it seems quite hard to attain the fuel retention rate of 0.1%, it could
be attained after long time discharge or at the steady-state discharge in a reactor.
Because after long time plasma exposure, the rather thick surface layers of PFM
must be saturated with the fuel and only tiny part of injected fuel would diffuse into
deep. Higher temperature operation in a reactor compared to present tokamaks should
significantly reduce the saturation concentration. Thus, both the C-wall and the Wwall could attain the fuel retention rate below 0.1% at the steady-state operation.
Nevertheless, the total fuel retention in PFM must continuously increase owing to
the increase of deposited layers and deep penetration and could come across the site
limit given by T regulation low. H trapping in defects produced in PFM would add
the total fuel retention, which is still very uncertain. Hence, the removal or recovery
of T from PFM is mandatory not only for the fuel self-sufficiency but also for T
safety and the development of T removal technique from PFM is urgent. As for the
selection of PFM of a reactor, easy fuel removal is an important criterion different
from ITER for which it was the less fuel retention.
References
1. T. Tanabe, Tritium: Fuel for Fusion Reactors, Springer Japan, 2017 ISBN: 978-4-431-56458-4
2. J. Roth, E. Tsitrone, T. Loarer et al., Tritium inventory in ITER plasma-facing materials and
tritium removal procedures. Plasma Phys. Control. Fusion 50, 103001 (2008)
3. J. Roth, K. Schmid, Hydrogen in tungsten as plasma-facing material. Phys. Scr. T145, 014031
(2011)
4. A. Kirschner, D. Borodin et al., Modelling of tritium retention and target lifetime of the ITER
divertor using the ERO code. J. Nucl. Mater. 363–365, 91 (2007)
5. Y. Gotoh, T. Tanabe, Y. Ishimoto et al., Long-term erosion and re-deposition of carbon in the
divertor region of JT-60U. J. Nucl. Mater. 357, 138–146 (2006)