The Functional Containment Licensing Argument
- sarahgibboney
- Apr 14
- 5 min read
Updated: Apr 15
Regulatory Guide (RG) 1.232, titled “Guidance for Developing Principal Design Criteria for Non-Light Water Reactors,” provides the Nuclear Regulatory Commission's (NRC) guidance on adapting the general design criteria in 10 CFR Part 50, Appendix A, “General Design Criteria for Nuclear Power Plants,” for non-LWR designs.
This guide is essential for non-LWR reactor designers, applicants, and licensees to develop principal design criteria as required by NRC regulations. One of the key policy issues discussed in the RG is the concept of functional containment performance requirements, which is closely associated with the use of specified acceptable system radionuclide release design limits (SARRDLs) in place of specified acceptable fuel design limits (SAFDLs). This association is particularly relevant for advanced reactor developers as they navigate the regulatory landscape to license non-LWR technologies.
Understanding SAFDLs
From General Design Criterion (GDC) 10 in Appendix A to 10 CFR 50:
Criterion 10—Reactor design. The reactor core and associated coolant, control, and protection systems shall be designed with appropriate margin to assure that specified acceptable fuel design limits (SAFDLs) are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences (AOOs).
The SAFDL-based approach outlined in GDC 10 in Appendix A to 10 CFR 50 has traditionally been used by light-water reactor (LWR) developers. This approach sets design limits for parameters such as departure from nucleate boiling ratio (DNBR) and peak fuel temperature to prevent any damage to fuels during normal operations and anticipated operational occurrences. For example, the safety basis for the AP1000, a well-known LWR, uses SAFDLs with acceptance criteria based on the relevant requirements of GDC 10.
Examples of phenomena requiring SAFDLs in LWRs include:
Departure from Nucleate Boiling
Fuel Temperature Design Basis
Core Flow Design Basis
Hydrodynamic Stability
These phenomena may not be applicable to advanced reactor technologies like high-temperature gas reactors (HTGRs), sodium-cooled fast reactors (SFRs), or integrated molten salt reactors (IMSRs). Therefore, the methodology defining SAFDLs, phenomena, and parameters for these advanced reactors would need to be approved by the regulator.
The term “specified acceptable fuel design limits (SAFDLs)” for LWRs refers to design limits set to ensure that the fuel rods do not fail, the fuel system’s dimensions remain within operational tolerances, and their core cooling capabilities are not reduced below those assumed in the safety analysis. These objectives address GDC 10, and the design limits that accomplish these objectives are called SAFDLs. In the event of a fuel rod failure, the fuel rod leaks, breaching the first fission product barrier (i.e., the fuel cladding). The applicant must account for fuel rod failure in its dose analysis for postulated accidents, as required by 10 CFR 100. The radiological dose consequences criteria given in 10 CFR 50.34(a)(1) are referenced in 10 CFR 100.21 “Non Seismic Siting Criteria.”
Either SAFDLs or SARRDLs will need to be established to ensure a plant design complies with dose limits in 10 CFR 20 during normal operations and AOOs. Since the GDC in 10 CFR 50 Appendix A are not regulatory requirements for non-LWR designs but provide guidance in establishing the Principal Design Criteria (PDC) for non-LWR designs, non-LWR applicants would not need to request an exemption from the GDC in 10 CFR Part 50 when proposing PDC for a specific design.
ARDC 26, derived from GDC 26 to be applicable to advanced reactors:
(2) A means which is independent and diverse from the other(s), shall be capable of controlling the rate of reactivity changes resulting from planned, normal power changes to assure that the design limits for the fission product barriers are not exceeded.
In GDC 26 for ARDC 26, the term “specified acceptable fuel design limits” is replaced with “design limits for fission product barriers” to be consistent with the AOO acceptance criteria while also addressing liquid fueled reactors which may not have SAFDLs and instead SARRDLs. Using this alternative terminology in an application will necessitate regulator interaction to request acceptance but may not require an exemption.
Understanding SARRDLs and the Concept of Functional Containment
The term “specified acceptable system radionuclide release design limits” (SARRDLs) is frequently mentioned in Appendix C of RG 1.232, titled "Modular High-Temperature Gas-Cooled Reactor Design Criteria." This concept is particularly relevant for advanced reactor designs, where the traditional SAFDL-based approach may not be applicable.
In advanced reactor designs, the concept of functional containment is closely associated with the SARRDL-based approach. Functional containment refers to the ability of the reactor system to limit the release of radionuclides under normal and anticipated operational occurrences (AOOs). Unlike the SAFDL-based approach, which focuses on preventing fuel damage, the SARRDL-based approach aims to limit the amount of radionuclide inventory released by the system.
Design features within the reactor system must ensure that the SARRDLs are not exceeded during normal operations and AOOs. This approach allows for some small increase in circulating radionuclide inventory during an AOO, provided that the normal operation radionuclide inventory limit is also established to ensure appropriate margin. The radionuclide activity circulating within the reactor system is continuously monitored to ensure that both the normal operation limits and SARRDLs are not exceeded.
The SARRDLs are established so that the most limiting license-basis event does not exceed the siting regulatory dose limits criteria at the exclusion area boundary (EAB) and low-population zone (LPZ). Additionally, the 10 CFR 20.1301 annualized dose limits to the public must not be exceeded at the EAB for normal operation and AOOs.
The SARRDL concept is more tailored for certain advanced reactor technologies, such as those using TRISO particle fuel in Modular High-Temperature Gas-Cooled Reactors (MHTGRs), than for metallic fuel with cladding used in Sodium-Cooled Fast Reactors (SFRs). The NRC has not yet reviewed or approved the concept of replacing SAFDLs with SARRDLs, but the SARRDL-based approach offers a promising alternative for ensuring the safety and regulatory compliance of advanced reactor designs.
In Appendix B to RG 1.232 titled 'Sodium-Cooled Fast Reactor Design Criteria,' the term “specified acceptable fuel design limits” (SAFDLs) is replaced with “design limits for fission product barriers” to be consistent with the anticipated operational occurrences (AOO) acceptance criteria while also addressing liquid fueled reactors which may not have SAFDLs.
Pursuing a SAFDL-based Approach versus a SARRDL-based Approach
When deciding between a SAFDL-based approach and a SARRDL-based approach, advanced reactor developers must recognize that engagement with the regulator is essential. The existing regulations are primarily tailored for light-water reactors (LWRs), and any deviation from these established frameworks requires thorough interaction with the Nuclear Regulatory Commission (NRC).
The SAFDL-based approach, traditionally used for LWRs, focuses on preventing fuel damage by setting design limits for parameters such as departure from nucleate boiling ratio (DNBR) and fuel peak temperature. This approach ensures that the fuel rods do not fail, the fuel system’s dimensions remain within operational tolerances, and their functional capabilities are not reduced below those assumed in the safety analysis.
On the other hand, the SARRDL-based approach is more suited for non-LWR reactor designs, where the emphasis is on limiting the release of radionuclides under normal and anticipated operational occurrences (AOOs). This approach aligns with the concept of functional containment, ensuring that the radionuclide inventory released by the system remains within acceptable limits.
Regardless of the approach pursued, advanced reactor developers must engage with the NRC to seek acceptance and approval. The SAFDL-based approach may require modifications to address the unique characteristics of non-LWR advanced reactors, while the SARRDL-based approach, being relatively new, necessitates detailed discussions to establish its validity and applicability.
In both cases, the goal is to ensure that the plant design complies with regulatory dose limits and safety criteria. By working closely with the regulator, advanced reactor developers can navigate the complexities of the regulatory landscape and achieve a design that meets both safety and regulatory requirements.
