Principles of operation and comparison of both types of solid oxide electrolyzers are shown in Fig. \ref{437284}. The working principles of both types of solid oxide electrochemical cell are different, defining their influence on the steam cycle when integrated with a nuclear reactor. SOE with oxygen ion conduction generates hydrogen inside the steam flow by extracting oxygen, whereas conduction of hydrogen protons results in pure hydrogen and introducing oxygen into the steam at the same time. Both solutions have their pros and cons. Producing hydrogen inside a steam flow causes certain difficulties with obtaining this gas at adequate purity afterwards. On the other hand, putting oxygen inside the steam cycle may increase corrosion processes, but the amount of oxygen in the steam flow is two times smaller than the adequate amount of hydrogen.
As the operation of a solid oxide electrochemical cell can be reversed, it can be operated in both modes: electrolysis and fuel cells. Much work has been done on modelling fuel cell operation (Solid Oxide Fuel Cells: oxygen ion conductors---inter alia \cite{van_Biert_2019,Wu_2019,Nassef_2019,Lv_2019,Guo_2019,Shen_2019,Ghorbani_2019,Botta_2019,Chaudhary_2019,Mozdzierz_2019,Stoeckl_2019,Xia_2018,Prokop_2018,Lv_2018,Baldinelli_2018,Conti_2019}; and proton conductors---inter alia: \cite{Wachowski_2019,Kultz_Unti_2019,Dzierzgowski_2019,Hakimova_2019} . Modeling attempts relating to solid oxide electrolyzers can be found at, inter alia: \cite{Fogel_2019,Jiang_2019}. Thus we adapted the fuel cell models (both O=SOFC and H+SOFC) for purpose of modeling high temperature electrolysis.
Hydrogen Production in Solid Oxide Electrolyzers coupled with Nuclear Reactors
There are five main, mature nuclear power reactor technologies: gas-cooled (GCR), fast breeder (FBR), heavy-water (HWR), boiling-water (BWR), and pressurized-water (PWR). PWR is currently the most popular -- 2/3 of reactors in operation and almost all reactors under construction are of that type. This is because of their operating and safety features, and despite the relatively low live-steam parameters they provide (compared to GCR and FBR).
Westinghouse’s AP-1000 secondary circuit was selected for this investigation from amongst the several PWR designs of today generation III/III+ (featuring increased safety) . There were two main reasons: the flow diagram as a whole is simplified (fewer heat exchangers, pumps, valves, pipes and cables), which makes it cheaper and more reliable; and the safety systems are almost fully passive. Therefore, this technology should be perceived as the most promising.
PWRs will remain a leading solution in medium term, as successful designs of generation IV (featured by fully passive safety systems) are not slated to appear soon. There are serious problems with the fuel elements of high-temperature reactors (proofness), which result in the coolant temperature being lowered to 750°C or less. The other prospective technology---fast breeder reactor---is a strategic one. This means it is not for export and there is no exchange of experience or ideas, so its development is slower.
This analysis considers PWRs, the scheme being based on two cycles of water/steam with the turbine being powered by non-radioactive steam (see Fig. \ref{424195}). The steam is generated in a special steam generator producing saturated steam, rarely slightly overheated. The low parameters of steam require specific measures to be taken to avoid blade erosion. Operation of the turbine in the wet steam area is accompanied by additional energy losses that strongly reduce internal efficiency, compared to the efficiency achieved with super heated steam; wet steam erosion causes damage to the flow part, mainly rotating blades. Thus, the low steam parameters in the nuclear power plant turbines cooperating with water-cooled reactors mean there is a need to modify the thermal cycle of the turbine in relation to the standard steam turbine cycle. The impermissible moisture level of the steam in the turbine, impermissible due to the erosion of the flow part and the reduction in efficiency, is countered by introducing external water separators in the thermal system, usually including interstage steam superheated with steam.