## 3 DEMO HCPB Single Pin Simulation Setup
### Surface-Limited Cases
#### 4.2.2 HT System, Surface-Limited, 300 Pa H\({}_{2}\) in Purge Gas  
In the next simulation, the purge gas contains 300 Pa H\({}_{2}\). In the prerun before the start of tritium release, this H-permeation from the purge gas to the (undoped) coolant had already reached a steady state. Figure 18 compares the results with 1 \(\times\) 10\({}^{-3}\) Pa H\({}_{2}\) and 300 Pa H\({}_{2}\) in the purge gas. Now the dominant species in the coolant is HT while the T\({}_{2}\) partial pressure rises only very slightly. HT obviously contributes lower than twice the T\({}_{2}\) contribution in both the single-species and the 1 \(\times\) 10\({}^{-3}\) Pa H\({}_{2}\) two-species cases.  
Again, the quantitative result may be read better from the patch mass flows, see Figure 19. Compared to the low hydrogen purge gas case, now (1) the steady state tritium permeation level is about 25% lower and (2) is achieved significantly faster.  
The transient tritium inventory in the steel wall that separates breeder and coolant approaches a steady-state value of 0.19 \(\upmu\)g/pin. All observations might be compiled together with the following description: the mixed-molecule species (which is negligible in the low H\({}_{2}\) case) opens an additional dissociation + recombination path in parallel that basically should already accelerate surface exchange processes. Moreover, the H\({}_{2}\), HT, and H concentrations enter the rate equations of this path. In the 300 Pa H\({}_{2}\) purge gas scenario,  
Figure 19: Transient tritium flow through patch interfaces breeder to and from steel, and steel to and fromcoolant for surface- limited conditions for 1 \(\times\) 10\({}^{-3}\) Pa vs. 300 Pa H\({}_{2}\) in purge gas.  
Figure 18: Transient T\({}_{2}\) and HT partial pressures at coolant outlet for 300 Pa H\({}_{2}\) in purge gas compared to T\({}_{2}\) in a single-species system (without H).  
these concentrations are significantly higher than the T\({}_{2}\) and T concentrations. Therefore, from the rate equations, significantly lower tritium levels are expected in the solid wall (asymptotic level now is 0.19 \(\upmu\)g/pin while it was 1.52 \(\upmu\)g/pin with 1 \(\times\) 10\({}^{-3}\) Pa H\({}_{2}\)) that overcompensate for the possibly higher recombination rate on the coolant side of the wall. In summary, the key finding is that co-permeation of a major species (H) reduces wall inventory as well as permeation flow of the minor species (T).  
2.3 HT System, Surface-Limited, 300 Pa H\({}_{2}\) in Purge Gas, HT vs. T\({}_{2}\) as the Released Species  
So far, the release of tritium from the pebble bed had been done to T\({}_{2}\) exclusively, even in the 300 Pa H\({}_{2}\) case. In an additional simulation, the same amount of tritium was released to the purge gas with 300 Pa H\({}_{2}\) exclusively as HT. Unlike in the corresponding diffusion-limited Section 4.1.3 where high rate-constants had already equalized the purge gas outlet composition, the released species in this simulation at the purge gas outlet had twice the partial pressure of T\({}_{2}\) as HT, but close to zero T\({}_{2}\) were found. The transient behavior is not visibly affected--see Figure 20. Steel inventories and coolant gas results can hardly be distinguished from the corresponding above results in Section 4.2.2 (and therefore they are not shown here). Probably this is a consequence of all species' rate constants being taken as equal.  
4.2.4 HT System, Surface-Limited, 300 Pa H\({}_{2}\) in Purge Gas, K\({}_{\text{r.HT}}\)\(\times\) 2  
For completeness, the enhanced recombination constant scenario (Section 4.1.5) is also simulated for the surface-limited case. Figure 21 shows that permeation is reduced by about 15% with the enhanced HT recombination rate constant. The inventory in the steel wall between the breeder zone and the coolant channel reduces by a factor of about two from about 0.19 \(\upmu\)g/pin to about 0.095 \(\upmu\)g/pin.