122
7 Fundamentals of Hydrogen Recycling
In addition, at higher temperature, reemitted H is not necessary to be molecules.
In chemical equilibrium in following dissociation of H 2 molecules, atomic hydrogen
is preferred at higher temperatures.
H 2 + G = H + H,
(7.4)
where ΔG is free energy change accompanied with the reaction with temperature
dependence of
G = H − T S,
(7.5)
where ΔH and ΔS are, respectively, enthalpy and entropy changes accompanied by
the reaction. ΔG decreases with increasing temperature and is represented by the
equilibrium constant as,
G = −RT /I n (P(H )
2
/P(H 2 )),
(7.6)
where P(H) and P(H 2 ) are partial pressures of H and H 2 , respectively. Accordingly, dissociation of hydrogen molecules become easier at higher temperatures as
evidenced in Fig. 4.15, in which atomic reemissions from metals (Mo, Ta, and W)
are appreciable above 1100 K under 3 keV D
3+ irradiation at steady state [10, 11].
The atomic reemission was observed for graphite as shown in Fig. 7.5 [12]. Thus,
irrespective of materials, the atomic reemission becomes appreciable above around
1000 K. The atomic reemission on the graphite limiter surface was confirmed in
TEXTOR plasma [13]. This means more than half of reemitted H should be atoms
on the divertor surface of ITER and a reactor of which temperature would be more
than 1200 K. Hence, H recycling in a reactor would be somewhat different from that
observed in present tokamak operated below 1000 K.
It should be also noted that the saturation concentration attained under energetic
H injection cannot be kept after stopping the injection and major part of H in surface
saturated layers must be subsequently released out. The releasing rate and total
release (or how much remains) significantly change with materials and temperature.
At lower fluence, there is one experiment in which release of deuterium (D) injected
in Ni was measured changing its temperature as shown in Fig. 7.6 [14]. In the figure
are compared the amount of D retained in Ni under the injection (referred to as
injection), D released during the decay period of 3 min (evolution), and desorbed
thermally (desorption) for 30 keV D
+ injection into Ni. The evolution corresponds
to dissolved D or dynamic retention, and the desorption corresponds to trapped D
or static retention. Above 800 K almost all D retained during the injection was
immediately (within a few seconds) released after stopping the injection. Although
both dissolved and trapped D decrease with increasing the temperature, the latter
decreases more significantly and disappears at elevated temperatures, i.e. the static
retention significantly decreases at elevated temperatures.
7 Fundamentals of Hydrogen Recycling
In addition, at higher temperature, reemitted H is not necessary to be molecules.
In chemical equilibrium in following dissociation of H 2 molecules, atomic hydrogen
is preferred at higher temperatures.
H 2 + G = H + H,
(7.4)
where ΔG is free energy change accompanied with the reaction with temperature
dependence of
G = H − T S,
(7.5)
where ΔH and ΔS are, respectively, enthalpy and entropy changes accompanied by
the reaction. ΔG decreases with increasing temperature and is represented by the
equilibrium constant as,
G = −RT /I n (P(H )
2
/P(H 2 )),
(7.6)
where P(H) and P(H 2 ) are partial pressures of H and H 2 , respectively. Accordingly, dissociation of hydrogen molecules become easier at higher temperatures as
evidenced in Fig. 4.15, in which atomic reemissions from metals (Mo, Ta, and W)
are appreciable above 1100 K under 3 keV D
3+ irradiation at steady state [10, 11].
The atomic reemission was observed for graphite as shown in Fig. 7.5 [12]. Thus,
irrespective of materials, the atomic reemission becomes appreciable above around
1000 K. The atomic reemission on the graphite limiter surface was confirmed in
TEXTOR plasma [13]. This means more than half of reemitted H should be atoms
on the divertor surface of ITER and a reactor of which temperature would be more
than 1200 K. Hence, H recycling in a reactor would be somewhat different from that
observed in present tokamak operated below 1000 K.
It should be also noted that the saturation concentration attained under energetic
H injection cannot be kept after stopping the injection and major part of H in surface
saturated layers must be subsequently released out. The releasing rate and total
release (or how much remains) significantly change with materials and temperature.
At lower fluence, there is one experiment in which release of deuterium (D) injected
in Ni was measured changing its temperature as shown in Fig. 7.6 [14]. In the figure
are compared the amount of D retained in Ni under the injection (referred to as
injection), D released during the decay period of 3 min (evolution), and desorbed
thermally (desorption) for 30 keV D
+ injection into Ni. The evolution corresponds
to dissolved D or dynamic retention, and the desorption corresponds to trapped D
or static retention. Above 800 K almost all D retained during the injection was
immediately (within a few seconds) released after stopping the injection. Although
both dissolved and trapped D decrease with increasing the temperature, the latter
decreases more significantly and disappears at elevated temperatures, i.e. the static
retention significantly decreases at elevated temperatures.