A primary motive for nuclear-renewable hybrid energy systems is the efficient alternative use of the generated heat when it is not needed for electric power production due to low net demand conditions. The proposed load-dynamic behavior of a system that incorporates variable renewable power generation introduces system complexities as a result of timing (when the heat is available), timescales (required response rate), and the large amount of excess heat that must be diverted to industrial applications. Industrial processes can potentially be designed to absorb the heat at timescales aligned with heat availability. Operating temperatures for each of the ‘classes’ of proposed SMRs result in different options for coupling to the selected process applications [18]. Table 13.2 provides a brief overview of possible process applications and the corresponding operating temperatures.

13.5.1 General considerations

A number of integration issues must be considered when coupling process heat applications to nuclear reactors. Key considerations include:

• reactor outlet temperature;

• reactor inlet temperature;

• fluid composition;

• pressure of primary coolant, heat transfer loop, and process heat application;

• primary coolant heat capacity;

• tritium migration.

The reactor outlet temperature defines the temperature and heat that can be provided to the process heat application. Low-temperature reactors can still provide process heat to most applications but will require temperature amplification via heat recuperation or high-temperature topping heat from fossil fuels and/or electric resistance heating to couple with higher-temperature process heat applications. Greenhouse gas emissions

can still be reduced relative to traditional generation sources in these cases. Process heat application studies under the NGNP program found that a reactor outlet temperature of 825-850 °C would provide sufficient heat to handle most applications. However, coupling a high-temperature reactor to a low-temperature process heat application can reduce the overall process efficiency due to inefficient use of the high-temperature heat. Optimal coupling of subsystems based on temperature outlet/inlet requirements may result in the most efficient use of the reactor thermal output, but regional needs may sway the selection of a system configuration based on economic factors.

Each generalized reactor type has a specified temperature difference (rise) and pressure difference (drop) across the core. For instance, a helium-cooled HTGR has a nominal temperature rise of 350-400 °C, while the core temperature rise for a fluoride salt-cooled high-temperature reactor is of the order of 100-150 °C. The temperature of the heat transfer fluid returned from the process heat application must be at or below the reactor inlet temperature. If it is at a higher temperature, heat must either be rejected to the environment or used in an additional process (i. e. power production using a bottoming cycle). A smaller core temperature rise corresponds to a higher average temperature process heat than designs having a larger core temperature rise, generally leading to a reduction in the number of reactors needed to provide the high-temperature heat.

The coolant and secondary heat transfer loop fluid composition can affect which process heat applications may be integrated. The melting temperature of molten salts and liquid metals (e. g. sodium) must be considered when applied to process heat applications. Some applications would return the working fluid at temperatures below the coolant solidification temperatures, which could plug heat exchangers and piping, without careful design of the heat transfer interfaces and potentially boosting the temperature of the return fluid. Coolants such as water, steam, carbon dioxide and helium do not have such issues, although condensation must be considered.

Process heat applications require heat not only at specific temperatures, but also specific pressures. Low-pressure coolants, such as liquid metals and molten salts, provide ideal pressure conditions at the core, but large pressure differences may occur at the process heat exchangers where high temperatures and pressure differences must be considered in the heat-exchanger design. High-pressure coolants, such as helium and pressurized water, may have the same effect if used for low-pressure process heat applications.

Process heat applications generally require a secondary heat transfer loop to isolate the primary core coolant loop from the process heat application. A primary purpose of this secondary loop is to reduce the migration of tritium from the reactor core to the process application. Additional heat transfer loops may be added to reduce tritium migration, but each loop reduces the process heat temperature based on the temperature difference required to transfer heat across the additional heat exchangers.

Some process heat applications use the heat rejected from the power conversion cycle of the reactor. Low-temperature applications, such as seawater desalination and district heating, may be able to use the heat rejected from the condenser of a Rankine cycle. The temperatures needed for these applications are higher than the usual rejection heat temperature such that some temperature amplification may be required. Heat may alternately be extracted from the low-pressure turbines at a higher temperature; this reduces the electric power generated, but the overall system thermal efficiency is increased due to the additional heat utilization.