Around the world, approximately a quarter of the energy demand is attributed to the transportation sector.  In order to address problems such as the fossil fuel resource depletion and a country's dependence on imported petroleum, the idea of hydrogen serving as a transportation fuel has become popular over the years due to its efficiency and versatility of use.  However, the majority of hydrogen is currently generated from natural gas, leading to significant carbon dioxide emission in the chemical process.  As a result, researches have been devoted to identify other potential energy sources for hydrogen production to mitigate climate change. One of the existing solutions is hydrogen production from nuclear reactors. In comparison with other hydrogen schemes, the nuclear reactor emits zero greenhouse gas, and is capable of supporting large-scale hydrogen production consistently. 
|Fig. 1: Nuclear reactor system, involving thermochemical process as the hydrogen production method. |
The fundamental concept behind nuclear-hydrogen system is to use the heat of the reactors to generate hydrogen via thermochemical or electrochemical processes. In other words, a hydrogen production plant is combined with an advanced nuclear power plant, which serves as a heat source in driving the chemical reactions. Fig.1 illustrates the flow from nuclear reactor to hydrogen production via a thermochemical process to power generation. 
In coupling a specific nuclear reactor to a relevant hydrogen scheme, the hydrogen production process properties play an essential role. For instance, in order to have a good match of hydrogen plant and nuclear reactor, it is imperative for the reactor coolant to have a minimal temperature reduction when transferred to the chemical plant. The commonly employed nuclear reactors are: 1. modular helium reactor (MHR), 2. advanced high-temperature reactor (AHTR), 3. advanced gas reactor (AGR), 4. secure transportable autonomous reactor (STAR-H2) and 5. sodium-cooled fast reactor (SFR). Each individual reactor operates at a specific pressure where different coolants are utilized, exiting the reactor at a particular temperature. Fig. 2 shows the potential technology option for various hydrogen production processes. 
|Fig. 2: Proposed nuclear reactor types for specific hydrogen production methods. |
In general, the thermal energy generated by the nuclear reactors serves as a primary heat source to produce hydrogen through thermochemical processes such as steam-methane reforming (SMR) as well as thermochemical water splitting, and/or electrochemical processes such as water electrolysis as well as high-temperature steam electrolysis as shown in Fig.3.  For direct electrolysis of water, a minimum temperature of 2500°C is required for hydrogen production. Through thermochemical processes like the water-splitting cycle which involves a series of chemical reactions, a lower temperature is required to accomplish the same overall results. 
The overall performance of the nuclear-hydrogen system will depend on the operating conditions, coolant choice, conversion efficiency, and reactor type as these variables will affect the economical and technological feasibility. Although the nuclear technology has achieved certain degree of maturity, this particular power generating scheme remains largely theoretical. No prototype of a hydrogen-producing reactor has been built, which means that the cost effectiveness and efficiency of the process remain to be measured and evaluated. In particular, thermal to electricity conversion efficiency is the main technical challenge that has a direct impact on the total cost of the system. 
|Fig. 3: Proposed hydrogen production methods by nuclear energy. |
© I-Tso Chen. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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