ML19056A191
| ML19056A191 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 02/18/2019 |
| From: | Entergy Nuclear Operations |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| 2.19.011 | |
| Download: ML19056A191 (4) | |
Text
Attachment 1 2.19.011 Response to Request for Additional Information - Exemption from the Requirements of 10 CFR 50.47 and Appendix E to 10 CFR Part 50, Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station
2.19.011 Page 1 of 3 Response to Request for Additional Information - Exemption from the Requirements of 10 CFR 50.47 and Appendix E to 10 CFR Part 50, Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station A U.S. Nuclear Regulatory Commission (NRC) request for additional information (RAI) regarding a request for exemptions from portions of Title 10 Code of Federal Regulations (CFR) 50.47 and 10 CFR Part 50, Appendix E for the Pilgrim Nuclear Power Station (PNPS) was received by Entergy Nuclear Operations, Inc. (Entergy) via electronic mail (email) dated February 4, 2019. An Entergy response to the RAI request is provided below.
NRCREQUEST
Background
By letter dated July 3, 2018 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML18186A635), Entergy Nuclear Operations, Inc. (Entergy) requested an exemption from specific emergency planning requirements of Title 10 of the Code of Federal Regulations (10 CFR) Part 50 for the Pilgrim Nuclear Power Station (PNPS),
based on the proposed permanent cessation of power operations and removal of fuel from the reactor vessel, which is expected no later than June 1, 2019. The exemption request has been reviewed against the requirements in 10 CFR 50.47, "Emergency plans," and Appendix E to 10 CFR Part 50, "Emergency Planning and Preparedness for Production and Utilization Facilities," using the 'guidance provided in Interim Staff Guidance (ISG) NSIR/DPR-ISG-02, "Emergency Planning Exemption requests for Decommissioning Nuclear Power Plants" (ADAMS Accession No. ML14106A057).
Applicable Regulation and Guidance The current 10 CFR 50 regulatory requirements for emergency planning, developed for operating reactors, ensure protection of the health and safety of the public. However, once a power plant is permanently shutdown and defueled, some of these requirements exceed what is necessary to protect the health and safety of the public. Therefore, pursuant to 10 CFR 50.12 "Specific Exemptions", Entergy requested exemptions from certain emergency planning regulations in 10 CFR 50.47 and 10 CFR Part 50, Appendix E, for PNPS.
Guidance for the staff review of Emergency Plan Exemption Requests can be found in Interim Staff Guidance (ISG) NSIR/DPR-ISG-02, "Emergency Planning Exemption Requests for Decommissioning Nuclear Power Plants." This guidance notes that the provisions of 10 CFR 50.12 permit the NRC to grant exemptions from the requirements of 10 CFR Part 50 regulations in circumstances where the application of the regulation is not necessary to achieve the underlying purpose of the rule. The staff concluded that a minimum of 1O hours would provide adequate time to initiate mitigative actions to cool the fuel or, if needed, for offsite authorities to implement protective actions using a comprehensive emergency management plan (CEMP) approach. Thus, a formal offsite radiological emergency plan would not be necessary for permanently shutdown and defueled nuclear power reactor licensees when at least 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> would be necessary for the fuel to heat-up to the cladding ignition temperature following a complete loss of coolant.
2.19.011 Page 2 of 3 By letter dated December 4, 2018 (ML18341A219), Entergy responded to a request for additional information related to heat transfer within the adiabatic analysis boundary.
Specifically, considering the construction of the GNF2 fuel assembly, the staff requested a description of how the heat up rates of the upper and lower plenums would be the same as the rest of the fuel assembly, as was modeled in the Adiabatic Heatup Analysis for Drained Spent Fuel Pool provided by Entergy as Attachment 2 to the exemption request dated July 3, 2018.
The RAI response states:
The adiabatic heatup analysis is conservatively based on the limiting (lowest enrichment, highest burnup) GNF2 fuel assembly discharged from the last cycle at Pilgrim Nuclear Power Station .... The analysis defines an adiabatic envelope that incorporates the entire GNF2 fuel assembly, including the metallic mass of the upper and lower plenum, as well as the partial mass of the GNF2 channel. GNF2 is a 1Ox10 array that utilizes the classical tie rod construction for structural support of the bundle. This design utilizes 8 tie rods to connect the upper and lower tie plates providing the structural support of the bundle. The fuel rods, including the 8 tie rods, are all in direct metal contact with the upper and lower plenums, which are in turn in direct metal contact with the channel and the water rods .... Direct metal contact provides a thermal pathway for the decay heat generated in the active region of the fuel rods to be conducted via the fuel cladding to these components. Under adiabatic conditions, there is no radiative, convective, or conductive heat transfer to the surrounding environment. Therefore, temperature differences between fuel assembly components are negligible and the components within the adiabatic envelope, including the upper and lower plenum regions, heat up at the same rate ....
The NRC staff found that the response does not justify an implicit assumption used in the analysis that all fuel components heat up uniformly because direct metal contact doesn't provide instantaneous heat transfer. While it is common to assume components within the adiabatic envelope are at the same temperature, this assumption must be justified or the envelope boundary should be moved to ensure the assumption is valid. The upper and lower plenums are complex components with much of their mass separated from the heat source (i.e., the fuel pellets) by metal with small cross-sectional area and by significant distance.
Without rapid convective heat transfer and effective radiative heat transfer vertically, the bulk of the upper and lower plenum masses appear to have only weak thermal connectivity with the remainder of the fuel assembly. Therefore, the NRC staff does not agree that the direct metal contact described in the above response adequately justifies the conclusion of the response that all components within the adiabatic envelope, specifically the upper and lower plenums, heat up at the same rate.
The adiabatic analysis is meant to establish the minimum time for cladding to reach a temperature that could support runaway oxidation leading to extensive fuel damage.
Therefore, heat sinks considered in the analysis must be shown to be thermally coupled with the cladding in the fuel region.
2.19.011 Page 3 of 3 Request The NRG staff requests the licensee to provide additional justification and/or description as to how the heat transfer could occur to all components within the proposed adiabatic envelope, specifically the upper and lower plenums, considering the complex design of the fuel assembly components, without significant delay in reaching 900 C within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />.
Entergy Response The adiabatic heatup analysis (Reference 1) was revised to incorporate a redefined adiabatic envelope boundary that removes the upper and lower plenums and includes only the fuel rods, water rods, spacers, and part of the mass of the GNF2 channel box surrounding the fuel over the active fuel length. The revised analysis also incorporates temperature dependent material properties from References 2 and 3. In the revised adiabatic heatup analysis, only the masses within the active fuel region (fuel rods, water rods, and spacers) are initially credited. When the bulk temperature reaches 580°F, the channel mass in the active fuel region is added to the adiabatic envelope. Radiative heat transfer between the fuel rods and the channel is a function of the surface area of the fuel rods that is viewed by the channel, the emissivity of the fuel rods and the difference in temperatures to the fourth power between the fuel rods and the channel (~!~ -* fi~N'~lii~). With multiple rods in the 1Ox1 O fuel array providing an adequate viewing factor of the channel, and emissivity values typical for Zircaloy-2, the entire bundle decay heat can be transferred to the channel at temperatures significantly less than 580°F. As such, the assumption that the channel box is thermally connected to the active fuel region and can be included in the adiabatic envelope at temperatures exceeding 580°F is justifiable. Heat transfer can occur to all components within the proposed adiabatic envelope, considering the complex design of the fuel assembly components, without significant delay in reaching 900°C within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />.
References
- 1. Entergy Calculation PNPS-EC-81416-M1418, "Adiabatic Heatup Analysis for Drained Spent Fuel Pool."
- 2. NUREG/CR-6150, Vol. 4, Rev. 2, "SCDAP/RELAP/MOD3.3 Code Manual: MATPRO-A Library of Materials Properties for Light-Water-Accident Analysis", January 2001 (ADAMS Accession Nos. ML010330363, ML010330400, and ML010330422).
- 3. NUREG/CR-7024, "Material Property Correlations: Comparisons between FRAPCON-3.4, FRAPTRAN 1.4 and MATPRO" March 2011 (ADAMS Accession No. ML11101A012).