ML20148M158
| ML20148M158 | |
| Person / Time | |
|---|---|
| Site: | Nuclear Energy Institute |
| Issue date: | 05/13/2020 |
| From: | Mccullum R Nuclear Energy Institute |
| To: | Andrea Kock Division of Fuel Management |
| Aragaus C | |
| Shared Package | |
| ML20148M157 | List: |
| References | |
| Download: ML20148M158 (4) | |
Text
RODNEY MCCULLUM Senior Director Decommissioning & Used Fuel 1201 F Street, NW, Suite 1100 Washington, DC 20004 P: 202.739.8082 rxm@nei.org nei.org May 13, 2020 Ms. Andrea Kock Division of Fuel Management Office of Nuclear Material Safety and Safeguards U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Subject:
Defining Spent Fuel Performance Margins: An experience-based approach to protecting against gross rupture of cladding in dry storage Project Number: 689
Dear Ms. Kock:
The Nuclear Energy Institute (NEI)1, on behalf of its members, appreciates NRCs willingness to engage in a dialogue in response to industrys November 8, 2019 white paper entitled Defining Spent Fuel Performance Margins. NRCs December 2019 response to this white paper set the tone for this dialogue with the following statement:
The Nuclear Regulatory Commission (NRC) embraces the concept of applying risk principles and operating experience to identify areas of safety margin and to focus the NRCs reviews of spent fuel dry storage and transportation systems on the most safety significant issues. In this regard, the NRC agrees with the philosophy outlined in the white paper. The NRC also agrees that changes and improvements can be made without the need to pursue rulemaking. Additional discussion is needed before the staff can offer a perspective on the specific recommendations outlined in the Nuclear Energy Institute (NEI) white paper. The NRC supports holding a series of workshops in 2020 to expand on the individual recommendations outlined in the white paper. The staff envisions these workshops will provide a forum to discuss each recommendation, explore the benefits and viability, discuss anticipated schedule and resource needs, prioritize planned activities, and align on next steps.
To date, the first three in the series of workshops envisioned by staff in NRCs response have been completed 1
The Nuclear Energy Institute (NEI) is the organization responsible for establishing unified industry policy on matters affecting the nuclear energy industry, including the regulatory aspects of generic operational and technical issues. NEI's members include entities licensed to operate commercial nuclear power plants in the United States, nuclear plant designers, major architect/engineering firms, fuel cycle facilities, nuclear materials licensees, and other organizations and entities involved in the nuclear energy industry.
Ms. Andrea Kock May 13, 2020 Page 2 and these have resulted in a number of steps being taken to implement the recommendations of the white paper. In the third of these workshops Electric Power Research Institute (EPRI) presented recommendations for future work related to gross rupture of cladding in dry storage (e.g., determining the safety objective and clarifying the interpretation). In response to these recommendations NRC and EPRI are planning to form a Phenomena Identification and Ranking Table (PIRT) team to begin with a clear safety objective and develop an actionable definition for gross rupture.
Addressing the gross rupture issue will be an important next step in achieving the dry storage licensing process improvements that both industry and NRC are seeking to effectively manage future dry storage needs. A clear definition of the safety basis for what constitutes gross rupture is an important precedent step in establishing relevant and actionable metrics for fuel integrity and assurance of safety. However, the current metrics do not provide an actionable definition. In the interest of developing more safety focused metrics in this area, NEI offers the following observations for consideration by PIRT team.
Gross rupture appears once in the Part 72 regulations, at §72.122(h):
Confinement barriers and systems. (1) The spent fuel cladding must be protected during storage against degradation that leads to gross ruptures or the fuel must be otherwise confined such that degradation of the fuel during storage will not pose operational safety problems with respect to its removal from storage. This may be accomplished by canning of consolidated fuel rods or unconsolidated assemblies or other means as appropriate.
Industry complies with this requirement in two ways
- 1. By assuring that fuel loaded into the dry storage system is either undamaged or by placing fuel assemblies that are considered to be damaged, including those that have gross ruptures, in damaged fuel cans
- 2. By preventing degradation of the fuel during storage that leads to gross rupture When loading a cask, industry relies on fuel sipping records, visual inspections, and reactor operating records to assess the condition of the fuel and determine if it is damaged or undamaged. If these measures indicate the presence of possible defects in the cladding in excess of pin hole leaks or hairline cracks, the fuel is placed in a damaged fuel can to provide an additional containment boundary. Fuel cladding integrity during storage is ensured by using conservative thermal limits derived from testing in evaluations that use conservative thermal models to establish confidence that fuel temperatures will not reach those limits, by drying the system internals, and by backfilling with inert gas to prevent in situ degradation.
In both cases, the goal of preventing gross rupture is to prevent the release of fuel material from inside to outside the cladding in an amount that could either cause established radiation exposure limits to be exceeded or that could be reconfigured in a way that could result in an unintended criticality. This is summed up in the
Ms. Andrea Kock May 13, 2020 Page 3 May 27, 1986 Federal Register notice accompanying the Part 72 rulemaking, which states:
A definition of gross rupture is based on the potential consequences of chemical and radioactive releases and their effect on handling of the fuel rods during loading and unloading operations.
This definition is commonly understood to have two safety objectives:
- 1. Prevent the release of significant quantities of fuel material
- 2. Avoid safety problems with removing the fuel from storage The question of what role cladding integrity plays in meeting these objectives could be answered by the designers radiation shielding and criticality safety analyses. In this approach, the allowable size of gross rupture can be virtually anything the cask designer determines would provide adequate radiation protection and criticality safety. But to accomplish this, the safety analyses would have to cover an enormous number of different permutations for the escape of fuel material from the cladding - which can be an unmanageable number of analysis runs. Understandably, designers typically do not go to those lengths, and instead have defaulted to the 1 mm defect standard originally set forth in ISG-1 (which was more recently incorporated into NUREG-2215).
While this approach is certainly conservative - a 1 mm or smaller defect is commonly accepted to be well below the level of damage that would compromise the safety of the system - it becomes problematic because it is not a fully actionable parameter. It can neither be directly observed nor mitigated. This defect standard does not represent a threshold above which the safety objectives would not be met. Yet it has driven industry to apply highly conservative thermal limits to fuel loaded in the dry storage systems. These limits can constrain industry operations, sometimes in ways that are counterproductive. Examples include the following:
- Designers often replace more protective radiation shielding with a less protective material having better heat transfer characteristics, while fuel loaders often maintain heat transfer by forgoing temporary shielding - which would normally be used to reduce doses.
- Dry storage licenses typically require periodic inspections for vent blockages in an elevated dose environment.
- Thermal considerations often limit spent fuel pool offload capabilities.
Finally, assuring compliance with these restrictive limits typically requires significant analytical effort and regulatory review which diverts both industry and NRC resources away from more safety significant activities.
We believe that the PIRT teams should revisit the safety objective, for which gross rupture currently serves as a proxy, by looking at three aspects of this requirement.
Ms. Andrea Kock May 13, 2020 Page 4
- 1. Radiation Protection and Shielding
- 2. Criticality
- 3. Retrievability Radiation protection and shielding and criticality requirements are well understood. In contrast, our understanding of retrievability has evolved over time. ISG-2, Revision 2 in April 2016 recognized canister-based retrievability in addition to fuel assembly-based retrievability as a means of compliance with the applicable regulations. This recognition has never been rolled back into ISG-1. Thus, the highly conservative 1 mm limit continues to be in use, even though a dry storage system design having a licensing basis that meets the retrievability requirement on a canister basis makes fuel rod gross rupture moot from a retrievability standpoint.
In light of this evolution, the PIRT teams should consider the extent to which retrievability should factor into the safety objective and consider the possibility that, for many dry storage systems, this objective may be established based entirely on criticality and radiation protection criteria.
Industry believes the PIRT team which the NRC and EPRI will form should thoroughly evaluate the radiation protection and shielding, criticality protection, and retrievability aspects of dry storage to develop a safety objective that can, in turn, be translated into a definition of gross rupture that is readily actionable. We believe that the result of this effort will be of fundamental importance to establishing a more safety focused regulatory framework. We are hopeful that this effort can be completed in a timely manner to enable much needed near term improvements. Please contact me or Mark Richter of my staff (mar@nei.org) with any comments or questions regarding these insights.
Sincerely, Rod McCullum c: Mr. Christopher Regan, NRC/NMSS/DFM Mr. Meraj Rahimi, NRC/NMSS/DFM Ms. Tara Inverso, NRC/NMSS/DFM