## 3 DEMO HCPB Single Pin Simulation Setup
### Diffusion-Limited Cases
#### 4.1.2 HT System, Diffusion-Limited, 300 Pa H\({}_{2}\) in Purge Gas  
Figure 7 shows diffusion-limited case results with 300 Pa H\({}_{2}\) in the Purge gas together with the previous low H\({}_{2}\) content results. In the prerun before the start of tritium release, H-permeation from the purge gas to the (undoped) coolant already approached a steady state. The initial HT and T\({}_{2}\) partial pressure levels (i.e., the prerun results) are very different. Quantitative permeation results are best read from the patch interface mass flows--see Figure 8. With 300 Pa H\({}_{2}\) in the purge gas, the permeation steady-state level is significantly lower and is approached faster than for 1 \(\times\) 10\({}^{-3}\) Pa H\({}_{2}\). H co-permeation seems to reduce tritium permeation by close to an order of magnitude (to about 0.6% of the T-generation rate) in this case. The tritium inventories in the regions of a single pin approach steady-state levels of about 6.4 \(\times\) 10\({}^{-10}\) g in the coolant, 0.17 \(\upmu\)g in the steel wall, and 0.1 mg in the ceramic breeder region. In summary about 10 g of tritium are expected to be present inside the entire DEMO reactor breeder zone (disregarding the neutron multiplier inventory). Figure 6: Transient tritium flow through patch interfaces breeder to and from steel and steel to and from coolant for single- vs. two-species model with no/\(1\times 10^{-3}\) Pa H\({}_{2}\) in the purge gas. Figure 7: Transient T\({}_{2}\) and HT partial pressure at coolant outlet for \(1\times 10^{-3}\) Pa vs. 300 Pa H\({}_{2}\) in the purge gas. 4.1.3. HT System, Diffusion-Limited, 300 Pa H\({}_{2}\) in Purge Gas, HT vs. T\({}_{2}\) as the Released Species  
In the diffusion-limited case, the purge gas outlet composition is mostly HT with very low T\({}_{2}\), regardless of whether HT or T\({}_{2}\) is released in the breeder zone in the presence of 300 Pa H\({}_{2}\). However, Figure 9 shows that the local T\({}_{2}\) concentrations in the breeder zone considerably depend on the released tritium species. Obviously, the high numeric values of rate constants imply an isotopic exchange downstream in the purge outlet channel where no pebbles are present and, therefore, no release takes place. In summary, permeation and wall inventories are not visibly affected from the local chemical form of tritium of HT vs. T\({}_{2}\). 1.4 HT System, Diffusion-Limited, 300 Pa H\({}_{2}\) in Purge, New \"Low\" Level: 1 \(\times\) 10\({}^{-4}\) Pa  
The perrun results for the initial (t = 0) coolant outlet partial pressures in Figure 7 differ considerably, while the subsequent increase during the transient with tritium release switched on is comparatively low. Due to the known high sensitivity of permeation to the counter pressure and the visible sign change from a small steady-state initial flow in opposite directions in the bottom curve in Figure 8, the simulation of Section 4.1.2 is repeated with a new initial \"low\" concentration of 1 \(\times\) 10\({}^{-4}\) Pa instead of 1 \(\times\) 10\({}^{-3}\) Pa for all H\({}_{2}\), HT, and T\({}_{2}\) species except the 300 Pa H\({}_{2}\) at the purge gas inlet. Figure 10 shows that now (compared to Figure 7) all initial tritium-related partial (counter) pressures are well-below 1 \(\times\) 10\({}^{-3}\) Pa. Figure 11 shows a visible counter pressure impact from the initial \"low\" level that makes permeation increase by about 20%. Nevertheless, this effect is still small compared to the hydrogen co-permeation effect from adding 300 Pa H\({}_{2}\) to the purge gas indicated in Figure 8. The steel wall tritium inventory steady-state level reduces from about 0.17 ug to about 0.15 ug for lower counter-pressures. Figure 8: Transient tritium flow through patch interfaces breeder to and from steel, and steel to and from coolant for 1 \(\times\) 10\({}^{-3}\) Pa vs. 300 Pa H\({}_{2}\) in the purge gas. Figure 9: T\({}_{2}\) in breeder purge gas at 20,000 s problem time for T\({}_{2}\) vs. HT release from pebbles. 1.5.