The parameters obtained are collected in Table \ref{tab:comparison}. Nuclear power plant load can be reduced by 20% by utilizing this energy for hydrogen generation. As reactor thermal power is changed by only about 10% due to steam consumption in electrolysis processes, the resulting efficiency is 7% lower, but this does not take into account the hydrogen generated.
\[\kappa\equiv\mu\]
At this point, it is difficult to compare O=SOE vs H+SOE, from the very first viewpoint. H+SOE has the advantage of generating more hydrogen with higher purity and releasing at the same time as oxygen to the steam cycle, from which oxygen is quite easily separated by the deaerator. On the other hand, H+SOE generates hydrogen at a very high temperature. This results in problems with transport and storage of the gas, and the gas has to be cooled using available steam cycle equipment and monitored by sensors \cite{Li_2018}. The energy cost of hydrogen generated in the proposed nuclear reactor integrated with solid oxide electrolyzers is 38.83 and 37.55 kWh/kg H2 in O=SOE and H+SOE, respectively. This calculation is limited to the sizing of the electrolyzers under the given assumptions, relating to the integration of SOE with the AP1000 nuclear cycle. These numbers can be compared to conventional, PEM and alkaline-based electrolysis, which offer a hydrogen production cost of 50 .. 65 kWh per kilogram of hydrogen.
The advantage of O=SOE is that it generates hydrogen at low temperature and high pressure, albeit requiring additional devices to purify the hydrogen, which is obtained in a mixture with steam. Also, hydrogen is not fully separated from water by the deaerator, thus the steam is contaminated by hydrogen. Consequently, a steam turbine generates slightly higher power (see Table \ref{tab:comparison} vs Fig. \ref{283651}).
This study shows that, for nuclear heat sources, cycles with a reaction taking place at a high temperature can be considered even for classic steam turbine cycles (not only Generation IV concepts). Other issues arise relating to cost, as in the range of 800 .. 900\(^{\circ}\)C there is an exponential increase in the cost of heat-resistant materials \cite{verfondern2007nuclear}.
Conclusions
In this study, two types of high temperature electrolyzers (O=SOE and H+SOE) were investigated for the purpose of hydrogen generation integrated with a nuclear power plant. The results obtained were compared against exclusively a Westinghouse AP1000 nuclear power plant steam turbine cycle. The main parameters of the electrolysis were tailored to match an operational temperature of the electrolyzers, and the water utilization factor for both technologies were set at the same value. The application of high temperature electrolysis on the deaerator steam feed delivers a few advantages: first, there is almost no modification of the nuclear steam turbine cycle; second there is a 20% increase in the flexibility of the nuclear power plant with almost constant thermal load of the nuclear reactor and, third, high pressure hydrogen is obtained for commercial purposes.
The analyzed systems were not optimized, due to the non-evident objective function of the optimizing process. While the sizing of the electrolyzers of two different types for integration with the AP1000 plant was considered, detailed thermal integration was not covered by this study. It should be noted that the temperature of hydrogen exiting O=SOE and H+SOE varies substantially, leaving space for further optimization of the SOE-AP1000 combination. As cycle efficiency cannot be upgraded by this solution, there are other parameters which can be improved, including: (i) modulation of the nuclear power plant (measured by minimum load); (ii) hydrogen generation for commercial purposes, and (iii) utilization of by-product oxygen outside of the nuclear station. This analysis will be subject to future study alongside other technical issues such as hydrogen purification \cite{Ye_2019}.
Acknowledgments
The project was funded by the National Science Center based on decision number DEC-2016/23/B/ST8/03056 , and the Ministry of Science and Higher Education of the Republic of Poland through the statutory grant at the Institute of Power Engineering CPE/040/STAT/19.