The U.S. Nuclear Regulatory Commission (NRC) issued a draft regulatory guide (DG) which would pave the way for nuclear power plants to be deployed closer to population zones.
If implemented, the new guidance would support the deployment of advanced reactors with novel safety features while allowing the NRC to avoid delays traditionally associated with new rulemaking processes.
The DG would also enable large light-water reactors (LWR) equipped with filtered containment venting systems (FCVS) or other advanced safety features to avoid being subject to population density-related siting limitations outlined in 10 CFR Part 100.

On October 18, 2023, the U.S. Nuclear Regulatory Commission (NRC) issued draft regulatory guide (DG), DG – 4034, for public comment. DG – 4034 includes a proposed Revision 4 for Regulatory Guide (RG) 4.7, “General Site Suitability Criteria for Nuclear Power Stations.” RG 4.7 provides guidance on the major health, safety and environmental site characteristics the NRC staff considers in determining the suitability of proposed sites for nuclear power plants. Essentially, it lays out the methods for determining whether a reactor may be located at a particular potential location.

If finalized, RG 4.7 Revision 4 would add alternative population-related criteria for consideration in determining site suitability. This would provide reactor operators with alternative means to demonstrate compliance with population-density rules, allowing them to be sited based on safety features and accident scenarios rather than on strict adherence to fixed population density requirements.

Existing NRC guidance compels nuclear reactor owners to incorporate specific population considerations into reactor siting proposals, which sometimes prove difficult to execute on the ground. For example, RG 4.7 currently requires that reactors be located at a site where the population density “… averaged over any radial distance out to 20 miles … is at most 500 persons per square mile (‘ppsm’).” This guidance reflects the NRC’s longstanding policy of recommending that nuclear reactors be sited in areas with low population densities located at considerable distances away from population centers. And the underlying policy stems from the NRC’s extensive large light-water reactor (LWR) siting experience, which takes into account the potential for releases of water coolant, zirconium alloy fuel cladding and radionuclide inventories in the event of an accident.

Advantages of this siting policy include that it: (1) facilitates emergency preparedness; (2) reduces the potential doses to large numbers of people in the event of a severe accident; and (3) reduces property damage in the event of a severe accident. However, by directing reactor siting based primarily on the “500 ppsm over 20 miles” benchmark, the policy runs the risk of bias against site consideration areas which “may have superior seismic characteristics, better rail or highway access, or shorter transmission line requirements, or less environmental impact on undeveloped areas, wetlands, or endangered species” (DG-4043 p. 19). Moreover, the benchmark creates a hurdle to siting reactors at retiring fossil fuel plants as many of the latter are located closer to population centers than the 500 ppsm/20-mile benchmark would allow.

In addition, this approach may not be appropriate for advanced reactor designs characterized by smaller sizes, novel safety features and enhanced fuel designs. Considered together, these aspects substantially reduce both the accident likelihood and the pace at which radionuclides would be released from such reactors.

On May 8, 2020, the NRC issued to the Commission policy memo SECY-20-0045, which evaluated the NRC’s population density and distance-based policy for siting advanced reactors. The SECY recommended the adoption of alternative siting criteria that directly relate to potential radiological consequences of accidents for specific types of advanced reactors, based on those reactors’ features. Under this approach, advanced safety features and accident scenarios could be considered when determining the appropriate distance from population zones and nearby population densities for sites at which such reactors may be located. On July 13, 2022, the Commission agreed and voted for the staff to proceed with implementing their proposed recommendation.

Notable Changes Introduced by RG 4.7 Revision 4
If finalized, Revision 4 to RG 4.7 would introduce new, technology-inclusive, risk-informed and performance-based criteria for complying with the commercial nuclear power siting suitability guidelines outlined in 10 CFR 100.21(h). Existing guidelines require applicants to site reactors in areas where the population density is within 500 persons per square mile. The guidelines cover areas located up to 20 miles from potential reactor sites. Revision 4 would update those same population density limitations to apply to areas located up to twice the distance at which a hypothetical individual could receive a total effective dose equivalent (TEDE) of 1 rem during the one-month period following the release of radionuclides during postulated accidents. Thus, assuming the nearest 500 ppsm area was located just one mile from a reactor, should it be possible for an individual in that area to be exposed to 1 rem TEDE following an accident, the guidelines would require the reactor to be sited at least two miles from that area. As such, Revision 4 effectively allows for the safety zone around a nuclear power site to be determined by potential radiation exposure instead of by a fixed distance.

Integral to this revised approach are changes to Appendix A of RG 4.7. The amended Appendix A would still require adherence to 10 CFR 100 which compels licensees to establish (1) an exclusion zone, (2) a low population zone (LPZ), and (3) a minimum distance of separation from the nearest densely populated area of 25,000 or more residents. However, the updated Appendix would also allow non-LWRs and light-water small modular reactors (SMR) equipped with novel safety features to be licensed in areas closer to population centers.

Specifically, the new guidelines would allow advanced reactor owners to show compliance with 100.21(h) based on the potential for a radiological source term (which depends, inter alia, on the type of fuel employed and coolant activity and concentration) and containment-type barriers for limiting the release of radionuclides that are integral to underlying reactor designs. Therefore, the LPZ and the minimum distance to densely populated areas could, under this method, be determined based on reactor design rather than specific, set distances. This is expected to create a more performance-based approach to the policy of siting reactors away from population centers in order to prevent or minimize societal risks. Moreover, as discussed below, these proposed changes could open the door to commercial nuclear power station siting in areas that have traditionally been off limits to conventional large LWRs.

In the past, the NRC had recommended that advanced reactor siting be addressed via updates to 100.21(h) include safety, fuel and other innovations. Implementing such recommendations has proven challenging or impossible given the diversity among emerging reactor designs and the slow pace at which the rulemaking process typically unfolds. By adopting the newly proposed Appendix A, the NRC will create a much more flexible framework for advanced reactor siting that will enable the NRC to adapt to reactor innovations without necessarily being subject to the challenges inherent in the rulemaking process.

Applicability to LWRs
The Revision 4 text does not provide any explicit guidance or prohibitions on whether large LWRs may avail themselves of the Appendix A siting methodology. However, statements by NRC staff in a recent public meeting indicate that large LWRs may be able to utilize containment and other technologies to limit the anticipated release of fission products in a postulated accident. This could allow such reactors to be sited under the Appendix A process, be located closer to population centers and facilitate the siting of new large LWRs. One main potential technology, acknowledged by staff during the meeting, is filtered containment venting systems (FCVS). In the event of an accident, an FCVS vents contaminated gas from reactor containment through dry or wet filters that remove the majority of radioactive particles prior to releasing the gas back into the environment. Designed to mitigate the consequences of core meltdowns, loss of coolant and other negative impacts of severe nuclear accidents, FCVSs may provide operators a means to site large LWRs under the Appendix A criteria. Although the United States does not require the use of FCVSs, many other countries have adopted them, and they are a well-established technology. The language of Revision 4 thus provides FCVS and other containment technology adopters with a means to satisfy existing reactor siting rules while potentially expanding the scope and diversity of the consumer populations they serve.

Nuclear site selection is a complex and challenging pursuit, and construction of nuclear reactors (particularly to replace other generation assets) has been limited by the NRC’s strict siting criteria. The NRC’s new proposed draft guidance may provide a means to address these challenges and allow for new technologies to expand the list of areas where reactors may be located. Beyond regulatory considerations, potential operators must consider the type of technology employed (e.g., LWR, non-LWR), plant security, availability of cooling water and materials transportation, access to the electricity grid, and environmental and socioeconomic impacts. Moving forward, technologies that enable plant operators to address these concerns (whether with overall advanced reactor designs or safety features like FCVS that can be added to existing LWR designs) while adhering to controlling NRC guidance stand to impact the deployment and geographic locations of both large LWRs and advanced reactors.

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