9.7 Fuel Removal/Recovery
181
The most reliable method is thermal desorption at higher temperatures. However,
all components in a reactor are difficult to take out to outside and require on-site
heating (main pumping system with T processing systems are always available). The
temperature required for full recovery is quite high and make all components to be
such high temperature is not realistic. Probably, baking the whole tokamak system
for conditioning to get good plasma assists T removal, but it would not be enough.
Hence, various techniques are proposed and studied [32].
Methods are divided into three categories, simple out gassing or heating, mechanical removal together with materials retaining T including oxidation, and isotopic
exchange.
For example, careful attention to optimizing the tritium fueling efficiency (minimizing the tritium introduced), tailoring the isotope ratio [33], and ending every
shot in a deuterium-only phase with sweeping of the divertor strike points [34] have
all been proposed. Seeding the divertor plasma with nitrogen may also help reduce
retention rates, either by increasing the chemical erosion of tritium-containing hydrocarbon co-deposits [35] or by reducing the availability of hydrocarbon radicals,
which suppresses deposit formation [36, 37]. This so-called nitrogen scavenging
effect may even reduce the formation of the deposits in shadowed regions of the
divertor far from the plasma [38], although issues such as nitrogen leakage out of
the divertor and possible collateral damage effects have yet to be fully assessed for
ITER conditions. Wall conditioning techniques, such as low-pressure RF discharges
in deuterium, could further contribute to reducing the rate of tritium accumulation. Although the efficiency of this type of technique would appear not to be very
high [34], it has no deleterious impact on the vessel conditions and can therefore be
employed frequently (i.e. inter-shot). During the maintenance phase, it may therefore
be necessary to employ other, more invasive recovery approaches. Surface heating
using scanning lasers [39, 40] or flash-lamps [41] under remote handling may be
an important option during this phase, for example, to remove tritium accumulated
near surface of CFC which needs higher temperatures than are easily accessible with
baking, up to ~750 K [42]. Since characteristics of fuel retention is different between
C and W, fuel removal or recovery is introduced in the following separately.
9.7.1 Removal/Recovery of T Retained in Carbon Materials
Since very high temperature (energy) is required to desorb or remove T retained
in carbon materials, removal or recovery of T retained in plasma-facing carbon
tiles is concerned both for T safety and fuel recovery to attain fuel self-sufficiency.
Although various methods have been examined in laboratory and tokamaks, no
methods have been established as efficient or reliable [43]. Because most of T is
retained near plasma-facing surface or deposited layers making C-H bonds, following
three methods have been applied (1) thermal desorption; (2) physically removal of
near-surface layers such as mechanical methods, sputtering by inert gas ions, or laser
181
The most reliable method is thermal desorption at higher temperatures. However,
all components in a reactor are difficult to take out to outside and require on-site
heating (main pumping system with T processing systems are always available). The
temperature required for full recovery is quite high and make all components to be
such high temperature is not realistic. Probably, baking the whole tokamak system
for conditioning to get good plasma assists T removal, but it would not be enough.
Hence, various techniques are proposed and studied [32].
Methods are divided into three categories, simple out gassing or heating, mechanical removal together with materials retaining T including oxidation, and isotopic
exchange.
For example, careful attention to optimizing the tritium fueling efficiency (minimizing the tritium introduced), tailoring the isotope ratio [33], and ending every
shot in a deuterium-only phase with sweeping of the divertor strike points [34] have
all been proposed. Seeding the divertor plasma with nitrogen may also help reduce
retention rates, either by increasing the chemical erosion of tritium-containing hydrocarbon co-deposits [35] or by reducing the availability of hydrocarbon radicals,
which suppresses deposit formation [36, 37]. This so-called nitrogen scavenging
effect may even reduce the formation of the deposits in shadowed regions of the
divertor far from the plasma [38], although issues such as nitrogen leakage out of
the divertor and possible collateral damage effects have yet to be fully assessed for
ITER conditions. Wall conditioning techniques, such as low-pressure RF discharges
in deuterium, could further contribute to reducing the rate of tritium accumulation. Although the efficiency of this type of technique would appear not to be very
high [34], it has no deleterious impact on the vessel conditions and can therefore be
employed frequently (i.e. inter-shot). During the maintenance phase, it may therefore
be necessary to employ other, more invasive recovery approaches. Surface heating
using scanning lasers [39, 40] or flash-lamps [41] under remote handling may be
an important option during this phase, for example, to remove tritium accumulated
near surface of CFC which needs higher temperatures than are easily accessible with
baking, up to ~750 K [42]. Since characteristics of fuel retention is different between
C and W, fuel removal or recovery is introduced in the following separately.
9.7.1 Removal/Recovery of T Retained in Carbon Materials
Since very high temperature (energy) is required to desorb or remove T retained
in carbon materials, removal or recovery of T retained in plasma-facing carbon
tiles is concerned both for T safety and fuel recovery to attain fuel self-sufficiency.
Although various methods have been examined in laboratory and tokamaks, no
methods have been established as efficient or reliable [43]. Because most of T is
retained near plasma-facing surface or deposited layers making C-H bonds, following
three methods have been applied (1) thermal desorption; (2) physically removal of
near-surface layers such as mechanical methods, sputtering by inert gas ions, or laser