SMRs are uniquely suited to tightly coupled, integrated energy applications. SMRs are distinguished by their relatively small power production (10s to 100s of MW — electric) and design for inherent, passive safety. Plants could incorporate multiple SMR units at these production levels, such that they can more easily be sized to meet the specific end-user demand for the output streams (e. g. electricity, thermal input to a process application) or to maximize plant thermal efficiency. The smaller per-unit size offers increased flexibility for investors (lower initial capital outlay), reduces costs associated with load-balancing, eases siting and integration challenges, and ensures increased operational flexibility. The inherent, passive safety designed into SMR concepts supports the NHES goals of system safety, resiliency and environmental stewardship by minimizing the potential for negative consequences (e. g. radiological release) of a design basis or beyond design basis event.

Potential hybrid systems could utilize proven light-water reactor (LWR) technology or proposed advanced reactor technologies that would operate at higher temperature and, hence, provide higher temperature heat for non-electrical applications. Most currently operating LWRs produce on the order of gigawatts (GW) of electricity. Retrofit of existing LWRs to incorporate a non-electric output stream is considered among the potential applications of NHES technology. This option could offer an opportunity for extending the life of operating nuclear power plants that are currently experiencing the effects of competition from low-cost natural gas (potentially resulting in plant shutdown before license expiration) and increased grid penetration by subsidized renewable energy production sources. However, retrofitting an existing reactor facility could introduce significant challenges and hurdles in the relicensing process and may not be a worthwhile investment given the limited remaining plant life.

Reactor designs currently considered for NHES primarily fall into the category of SMRs (< 300 MW electric power), as these plant sizes couple well with many of the process applications considered and are an excellent fit for small-scale regional grids or isolated industrial applications that demand both thermal and electrical resources. Most currently operating electrical generation plants are less than 500 MWe in capacity, so an integrated energy system on this scale could be envisioned to replace aging plants, particularly aging coal plants that have significant CO2 emissions. Significantly smaller systems (~50 MWe) could be a good fit for NHES coupled with wind generation, as this is the approximate capacity of most individual wind farms. Because individual wind farms are affected by regional storm systems, the collective change in generation at the grid level can be on the order of GW in several tens of minutes. This magnitude of fluctuation has challenged the ability of natural gas-fired co-generation plants to accommodate [2].

Many of the SMR plant concepts would ultimately incorporate multiple units. For such an implementation, additional capacity can be added incrementally, with individual units built in phases as necessary to meet growth in market demand. These units could be operated independently or in concert as a group depending on the overarching control strategy. Modular build-out improves the financial investment profile of the overall project, where the plant owner could choose to first construct the basic plant units (e. g. nuclear generation, energy conversion system, and electrical power generation) to establish a revenue stream while the remainder of the plant is completed, building in the necessary interconnection points and control system structure to allow later addition of additional generation sources (e. g. renewable energy system or additional nuclear units) and thermal energy applications.

Small-scale, modular reactors incorporate significantly smaller components than large-scale plants, such that they can be factory-built. Large system components for traditional, large-scale baseload nuclear plants are often built on-site and are reliant on foreign suppliers. SMR component factories could utilize a domestic supply chain and could be sited very near to the intended plant site, or components could be easily transported to the intended plant location. One might even envision a future hybrid system implementation powering a domestic SMR component factory.

Modular construction also allows alternate operational scenarios and integrated system control strategies than would be possible for a hybrid system that incorporates a single large-scale nuclear plant. In a multi-unit plant in which each of those units provides a modest amount of thermal energy input, some of the input units could be dedicated to a particular output application. Other units could then be designated as ‘swing plants’ that switch output between applications as necessary based on customer demand, economic factors, required maintenance or refueling activities, etc.

The siting of an SMR plant having one or more nuclear units is significantly more flexible than traditional large-scale plants. The possibility to locate SMRs in densely populated regions (due to reduced exclusion zone) introduces the opportunity to site the plant closer to the final customer. The estimated US land availability (suitability) for small-scale versus traditional large-scale nuclear plants is discussed in Section 13.4.3. In a hybrid implementation, siting flexibility translates to siting of the industrial heat-use application near those population centers as well. By producing non-electricity products (heat, chemicals, etc.) near the point of use, the economical attractiveness of the planned facility is increased and the market size is enlarged (particularly as aging coal plants require replacement).

Smart grids could enable the implementation of smaller input sources, such as SMRs, by balancing the load dynamics at a local scale rather than at the large scale required by traditional, large-scale nuclear plants. In this case, SMR plants could be located based on other (non-grid) subsystem requirements. Siting could be in the vicinity of the process feedstock resource (e. g. coal, natural gas, biomass), near the end-user (e. g. local community or commercial industry), or near the coupled renewable input source. Such siting would reduce transport distances for both electricity and thermal energy, thereby minimizing transmission losses. Hence, SMRs offer operational flexibility by introducing a broad range of production opportunities and simplified coupling to renewable sources and to more process applications than large-scale nuclear implementations.

Multiple deployment opportunities can be envisioned for nuclear hybrid energy systems, particularly those utilizing small modular reactors. Early implementations might provide electricity and thermal energy for independent industrial implementations without an intention to connect to the main electrical grid, allowing the system to be optimized based only on internal energy demands that are likely more predictable than external demand from the grid. Alternately, an early hybrid energy park could provide electricity and heat to small, remote communities that currently rely on diesel power that must be trucked in to the region. Later implementations might integrate the hybrid system directly to the large-scale grid, while internally managing the thermal and electrical energy resources to meet the grid demand and maximize economic return. Considerations for potential hybrid system architectures are discussed throughout this chapter. The specific needs (desired commodities) of a potential customer and resources located at the intended site will aid in the design of an optimal energy system.