Discussion

For nuclear heat sources, cycles with a reaction taking place at a temperature > 900\(^{\circ}\)C should not be considered because it would be higher than what Generation IV concepts are expected to provide. Also the range of 800 .. 900\(^{\circ}\)C is very questionable because of the exponential increase in cost of materials with increasing temperatures \cite{verfondern2007nuclear}.  
In HTE, only a small percentage of energy is needed as heat, which is around 90 kJ/mol for an electrolyzer operating at 950°C. This splits up into ~ 60 kJ/mol of H2 around 200°C to decompose steam and only 30/40 kJ/mol (maximal realistic value) at 950°C for the electrolyzer itself, if supposed that the heat of the products is recovered to heat up the reagents. Therefore the HTGR will only provide less than 10 % of useful heat energy to the electrolyzer, resulting in an increase of efficiency from around 40 to 42% \cite{verfondern2007nuclear}.

Conclusions

\cite{verfondern2007nuclear}: The question is whether it is really worth coupling an HTE system to an HTGR and achieving such a low theoretical gain of efficiency:

Acknowledgments

The project was funded by the National Science Center based on decision number DEC-2016/23/B/ST8/03056.