# Neutronic Activity for Development of the Promising Alternative Water-Cooled DEMO Concepts
## 2 WCLL-db Blanket Study
### Results of the Neutronic Simulations for the WCLL-db DEMO
#### 2.3.2 Safety Aspects of the PbBi Use in the WCLL-db Blanket  
The PbBi (\(\mathbf{T_{melt}}\) = 123.5 \({}^{\circ}\)C) eutectic implemented as a separating layer between water and PbLi in the double-bundle cooling tubes has an inherent safety risk for the environment because of \({}^{210}\)Po generation in Bi under neutron irradiation. \({}^{210}\)Po is a result of neutron capture reaction on \({}^{209}\)Bi (100% in natural Bi) with the production of \({}^{210}\)Bi (T\({}_{1/2}\) = 5.01 days) that then decays to \(\alpha\)-radioactive \({}^{210}\)Po (T\({}_{1/2}\) = 138.38 days). Polonium is highly volatile and readily evaporates from the PbBi eutectic inducing significant surface contamination of the primary cooling loop [15]. \({}^{210}\)Po is extremely toxic; this and other polonium isotopes are some of the most radiotoxic substances to humans. With one microgram being more than enough to kill the average adult, \({}^{210}\)Po is 250,000 times more toxic than hydrogen cyanide by weight. It is also thought that one gram of \({}^{210}\)Po is enough to kill 50 million people and sicken another 50 million [16]. Therefore, a special study was carried out within this work in order: (1) to demonstrate the potential environmental risk of the PbBi use in the WCLL-db blanket in the case of, nevertheless, acceptance of this material in its design and (2) to indicate alternative compounds that could replace PbBi without a loss of the blanket functionality. A number of additional calculations were done to demonstrate the alternatives to the use of PbBi eutectic in the WCLL-db blanket. The results of the TBR simulations with the replacement of PbBi in the WCLL-db blanket with various Pb-based compounds [17] are given in Table 2. The replacement of the PbBi with the compounds listed in Table 2 does not result in the reduction of the TBR compared to the layout case with a 220 mm radial thick manifold. All these compounds have a melting temperature much higher compared to the one of PbBi (123.5 \({}^{\circ}\)C), which could be a design safety issue in case of their prompt solidification. Therefore, these results obviously cannot be directly used for the design changes because technological aspects of the use of these compounds in the blanket must be investigated first. In spite of this, the main message of these results is to demonstrate the replicability of PbBi in the WCLL-db without loss of its tritium breeding performance. In case of the need, such analyses could be continued. In the case of the WCLL-db design will be accepted without the PbBi replacement, the accumulation of \({}^{210}\)Po in the blanket must be assessed and compared with some safety limits available so far for \({}^{210}\)Po contamination. For this purpose, an activation analysis of the irradiated WCLL-db blanket was performed using the automated procedure [18]. \begin{table}
\begin{tabular}{c c c} \hline
**Compound** & **Melting Point, \({}^{\circ}\)C** & **TBR** \\ \hline Pb\({}_{90}\)Mn\({}_{10}\) & 290 & 1.16 \\ Pb\({}_{95}\)Ba\({}_{5}\) & 290 & 1.16 \\ Pb\({}_{80}\)Sn\({}_{20}\) & 280 & 1.16 \\ \hline \end{tabular}
\end{table} 
Table 2: TBR results with alternative chemical compounds. Figure 6: Three-dimensional distribution of the tritium production rate. The activations calculations were performed only for the irradiated PbBi eutectic in the IB and OB blankets. Neutron flux intensities and spectra presented with CCFE 709-group structure for activation analysis [19] were computed with MCNP6.2 utilizing JEFF-3.3 transport data. A mapping of the neutron spectra was done in different geometry cells containing PbBi inside the blanket structure. The statistical uncertainty in all calculations of the total neutron flux does not exceed 1%. For some low energy bins, for instance, 1\(\cdot\)10\({}^{-7}\)-1\(\cdot\)10\({}^{-4}\) MeV, the relative error was found to be as high as \(\sim\)10%.