8.3 Dust
145
show amorphous like structure at lower temperature area. These deposited layers are
easily exfoliated resulting in dusts due to the different expansion coefficients between
the layers and the substrate when large power is loaded.
Owing to less erosion rate, the deposited amount for W-wall is much less than that
for C-wall. Consequently, observed dust level of JET-ILW is orders of magnitude less
compared with that of the JET-CW. However, when the power load is high enough to
give surface melting of the metallic wall, released droplets from the melting surface
become dust. Furthermore, re-solidified layers are easily cracked to give particle
ejection from the surface under high heat load, which is a major concern for utilization
of W as PFM.
In a reactor, there is an additional concern. The resuspension of dust could be a
consequence of lss of coolant accidents (LOCA) and lss of vacuum accidents (LOVA)
and it can be dangerous because of dust radioactivity, toxicity, and capable of causing
firing and/or explosion [32].
It should be mentioned that there is a lack of experimental data on thermochemical properties and shape of dust particles, dust production rates are unknown,
and there are uncertainties in the dust–plasma and dust–wall interactions. Hence,
much more studies in dust-related areas are required including improvement of dust
measurement, dust cleaning, and the reduction of dust production.
8.4 Recycling and Retention of Fuels
Since the burning efficiency of fuel in a fusion reactor is poor, large amount of fuels
are put through and most of them are exhausted. Moreover, fueling efficiency, which
means the fraction of fuels going into plasma to the total fuel throughput is also
rather poor. Therefore, any loss of the fuels in a reactor including consumed ones by
burning, i.e. the difference between the throughput and the exhaust, influences fuel
self-sufficiency. In addition to the fuel loss in a reactor or in-vessel T retention, some
loss cannot be avoided in all T processing systems for fueling, exhaust, breeding,
recovery, refinement, isotope separation, and storage. Among all, the in-vessel T
retention (mainly in PFM and deposited layers) is the largest. Without establishment
of the fuel self-sufficiency by T breeding in each rector, enough number of fusion
reactors as energy sources cannot be built. Thus, the in-vessel T retention should
be determined precisely not only for the fuel self-sufficiency but also T safety. In
these respects, T breeding and recovery are also quite important engineering tasks.
However, they are out of scope of this book and not discussed. Please refer textbooks
such as [32]. The following sections concentrate on the in-vessel fuel retention.
145
show amorphous like structure at lower temperature area. These deposited layers are
easily exfoliated resulting in dusts due to the different expansion coefficients between
the layers and the substrate when large power is loaded.
Owing to less erosion rate, the deposited amount for W-wall is much less than that
for C-wall. Consequently, observed dust level of JET-ILW is orders of magnitude less
compared with that of the JET-CW. However, when the power load is high enough to
give surface melting of the metallic wall, released droplets from the melting surface
become dust. Furthermore, re-solidified layers are easily cracked to give particle
ejection from the surface under high heat load, which is a major concern for utilization
of W as PFM.
In a reactor, there is an additional concern. The resuspension of dust could be a
consequence of lss of coolant accidents (LOCA) and lss of vacuum accidents (LOVA)
and it can be dangerous because of dust radioactivity, toxicity, and capable of causing
firing and/or explosion [32].
It should be mentioned that there is a lack of experimental data on thermochemical properties and shape of dust particles, dust production rates are unknown,
and there are uncertainties in the dust–plasma and dust–wall interactions. Hence,
much more studies in dust-related areas are required including improvement of dust
measurement, dust cleaning, and the reduction of dust production.
8.4 Recycling and Retention of Fuels
Since the burning efficiency of fuel in a fusion reactor is poor, large amount of fuels
are put through and most of them are exhausted. Moreover, fueling efficiency, which
means the fraction of fuels going into plasma to the total fuel throughput is also
rather poor. Therefore, any loss of the fuels in a reactor including consumed ones by
burning, i.e. the difference between the throughput and the exhaust, influences fuel
self-sufficiency. In addition to the fuel loss in a reactor or in-vessel T retention, some
loss cannot be avoided in all T processing systems for fueling, exhaust, breeding,
recovery, refinement, isotope separation, and storage. Among all, the in-vessel T
retention (mainly in PFM and deposited layers) is the largest. Without establishment
of the fuel self-sufficiency by T breeding in each rector, enough number of fusion
reactors as energy sources cannot be built. Thus, the in-vessel T retention should
be determined precisely not only for the fuel self-sufficiency but also T safety. In
these respects, T breeding and recovery are also quite important engineering tasks.
However, they are out of scope of this book and not discussed. Please refer textbooks
such as [32]. The following sections concentrate on the in-vessel fuel retention.