Nuclear hydrogen chemical cycles

The thermochemical cycles (S-I > 850°C) or hybrid cycles (S-electrolysis > 850°C) still feature many uncertainties in terms of feasibility and performances. Uncertainties still exist in parts of the flow sheet and technologies needed to provide high temperature heat whether from solar or nuclear nature. Potential assets of thermochemical cycles lie in a theoretical potential for a global efficiency above 35% and a scaling law of the hydrogen plant after the volume of reactants instead of the total surface of electrolytic cells. In return, their practical feasibility and economic viability have to be entirely demonstrated. Especially, a global efficiency above 30% is to be demonstrated to compete with alkaline electrolysis. Moreover, the safety of co-located nuclear and chemical plants has to be demonstrated. 

Safely implementing a thermochemical nuclear hydrogen generation scheme requires a robust understanding of the interaction between the nuclear plant and the chemical plant. In turn, this requires robust models of the chemical plant, reactor thermal-hydraulics and reactor physics. Efforts have been conducted in both the transient modelling of the sulphur-iodine (S-I) and hybrid sulphur (HyS) thermochemical cycles, as well as coupling to models of the pebble bed modular reactor (PBMR-268) (Brown, 2009). 

Chemical plants themselves are subject to a variety of accident scenarios. These scenarios have not been modelled in detail for an S-I cycle system and are not fully understood. A thorough literature review indicates not one single paper regarding proposed chemical plant initiated accident scenarios in a coupled nuclear hydrogen generation system. However, it is evident that, in terms of safety, these scenarios could be very important. The chemical plant acts as the heat sink for the nuclear reactor, and any scenario which impedes the ability of this heat sink to function effectively is potentially serious. 

Chemical heat pump systems are important, if the utilization of low-quality heats ( 80 °C), e.g., solar or geofliermal or nuclear (LWR) heat, waste heat in factories, is considered for heat storage and temperature upgrading to improve the total efficiency of the system. Among the many chemical cycles that have been considered to store and transport heat energy in the form of chemical reaction enthalpy, the coupled processes of hydrogenation / dehydrogenation can be adapted here. 

Hydrogen can be obtained firom different sources as fossil fuels (natural gas reforming, and coal gasification), renewable fuels (biomass), algae, and vegetables or water (electrolysis and thermo-chemical cycles). Many different energy sources can be used in most of these processes heat from fossil fuel or nuclear reactors, electricity from several sources as solar energy. 

The sulphur-iodine (S-I) cycle is a three-step process to thermochemically split water i) a chemical reaction of sulphur-dioxide, iodine and water forms hydriodic acid (HI) and sulphuric acid (H2S04) ii) high-temperature (450°C) process heat from an advanced nuclear reactor decomposes the HI to release hydrogen and recover iodine iii) high-temperature (850°C) process heat from an advanced... 

A modelling and experimental effort has identified a new uranium thermochemical cycle (UTC) for the production of hydrogen from water. The peak temperature within the cycle is below 700°C - a temperature achievable with existing high temperature nuclear reactors and some solar systems using commercially auailable materials. This paper describes the new process and some of the experimental work. It is an early report of chemical feasibility. Much work will be required to determine engineering and economic viability. 

A recent screening of several hundred possible reactions (Besenbruch et al 2001) has identified two candidate thermochemical cycles for hydrogen production from water (i.e., cycles that enable chemical reactions to take place at high temperatures) with high potential for efficiency and practical applicability to nuclear heat sources. These are the sulfur-iodine (S-I) and calcium-bromine-iron (Ca-Br) cycles. Also, Argonne National Laboratory (ANL) has identified the copper-chlorine (Cu-Cl) thermochemical cycle for this purpose (Doctor et al 2002). A hybrid sulfur-based process that does not require iodine but has a single electrochemical... 

Doctor, R.D., D.C. Wade, and M.H. Mendelsohn. 2002. STAR-H2 A Calcium-Bromine Hydrogen Cycle Using Nuclear Heat. Paper presented at Spring 2003 National Meeting, American Institute of Chemical Engineers, New Orleans, La., March 30-April 3. 

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