144
8 PMI in Large Tokamaks
DistribuƟon of surface mass density
Fig. 8.9 Collected dusts in divertor region of JT-60U a full carbon tokamak operated mainly at
around 473–573 K (reprinted with permission from [30])
Some are directly redeposited at the remote area with loose adhesion to the substrate
owing to lower temperature, some caused by exfoliation of deposited layers on PFS
and some made by disruption.
Although the amount of dust mobilized after disruptions is known to be proportional to the dynamic vessel forces, it is not possible to infer what fraction of the
dust (if any) might be created by disruptions. However, disruption mitigation which
is mandatory in a fusion reactor is found to reduce the amount of the dust seen after
moderate disruptions by a factor of 4 [31].
Dust has not yet been of a major concern in existing tokamaks mainly because
the quantity is small, and they are not nuclear facilities. Since dusts in a reactor
contain tritium and neutron activated elements, they will represent operational and
potential safety issues. From a safety point of view, they must be controlled based on a
defense in depth approach designed to provide reliable confinement systems, to avoid
potential hazard and failures, and to measure and minimize the dust inventory [31].
Dust formation critically depends on the kind of PFM. In general, higher erosion
results in larger deposition and consequently more dusts. In case of C-wall, owing
to its chemical erosion, significant amount of C is transported and deposited at areas
facing less plasma flux and shadowed from plasma. Depending on the temperature
of deposited area, the properties of the deposited layers, their fuel retention, and
their micro-structure are changed. At lower temperature area, deposited C exhibits
amorphous like structure with high fuel retention, while at higher temperature area, it
changes to layered structure but not fully graphitized. For the case of W, the deposited
layers include a lot of impurities in addition to fuel, such as oxygen, carbon and often
8 PMI in Large Tokamaks
DistribuƟon of surface mass density
Fig. 8.9 Collected dusts in divertor region of JT-60U a full carbon tokamak operated mainly at
around 473–573 K (reprinted with permission from [30])
Some are directly redeposited at the remote area with loose adhesion to the substrate
owing to lower temperature, some caused by exfoliation of deposited layers on PFS
and some made by disruption.
Although the amount of dust mobilized after disruptions is known to be proportional to the dynamic vessel forces, it is not possible to infer what fraction of the
dust (if any) might be created by disruptions. However, disruption mitigation which
is mandatory in a fusion reactor is found to reduce the amount of the dust seen after
moderate disruptions by a factor of 4 [31].
Dust has not yet been of a major concern in existing tokamaks mainly because
the quantity is small, and they are not nuclear facilities. Since dusts in a reactor
contain tritium and neutron activated elements, they will represent operational and
potential safety issues. From a safety point of view, they must be controlled based on a
defense in depth approach designed to provide reliable confinement systems, to avoid
potential hazard and failures, and to measure and minimize the dust inventory [31].
Dust formation critically depends on the kind of PFM. In general, higher erosion
results in larger deposition and consequently more dusts. In case of C-wall, owing
to its chemical erosion, significant amount of C is transported and deposited at areas
facing less plasma flux and shadowed from plasma. Depending on the temperature
of deposited area, the properties of the deposited layers, their fuel retention, and
their micro-structure are changed. At lower temperature area, deposited C exhibits
amorphous like structure with high fuel retention, while at higher temperature area, it
changes to layered structure but not fully graphitized. For the case of W, the deposited
layers include a lot of impurities in addition to fuel, such as oxygen, carbon and often