ML21223A048
| ML21223A048 | |
| Person / Time | |
|---|---|
| Site: | Holtec |
| Issue date: | 08/11/2021 |
| From: | Holtec |
| To: | Office of Nuclear Material Safety and Safeguards |
| Shared Package | |
| ML21223A045 | List: |
| References | |
| 5014931, EPID L-2021-LLA-0039 | |
| Download: ML21223A048 (5) | |
Text
Holtec Letter 5014931 Attachment 2 Holtec Response to Request for Supplemental Information Docket No. 72-1014 Holtec International HI-STORM 100 Multipurpose Canister Storage System Certificate of Compliance No. 1014 Amendment No. 16 Thermal RSI 5-1 PROPRIETARY INFORMATION WITHHELD IN ACCORDANCE WITH 10CFR2.390 Confinement RSI 9-1 Demonstrate the Proposed Changes #3 and #6 will reasonably maintain confinement of radioactive material under normal, off-normal, and credible accident conditions.
In its Statement of Proposed Changes, the applicant proposed to modify vent and drain penetrations to include the option of second port cover plate (Proposed Change #3).
The applicant also stated that the addition of a second cover plate for these penetrations removes the need to do field helium leak testing of these cover plates (Proposed Change #6).
The weld on the port cover plate cannot be executed under conditions where the root pass might have been subjected to pressurization from the helium filled in the canister itself. When executing vent and drain connection cover plate welds, one should not assume that the fill and drain closure valves quick-disconnects, or similar, are leak tight without performing helium leak testing. It is assumed that mechanical closure devices (e.g., a valve or quick-disconnect) permit helium leaks.
Field experience has shown that such leaks occur and have been responsible for causing leak paths through the weld. Consequently, welds potentially subjected to helium pressure (by way of leakage through a mechanical closure device) during the welding process must be subsequently helium leakage tested in accordance with the method in ANSI N14.5. In addition, ANSI N14.5 does not allow for the elimination of leakage rate testing based on the use of multiple barriers.
This information is required to satisfy 10 CFR 72.236 (d), 72.236 (j), and 72.236 (l).
Holtec RSI Response:
Holtec concurs with the Staff that mechanical closures such as valves and quick connect fittings may not provide definitive closure against the leakage of helium required to make a leak-tight closure weld. To guard against this vulnerability and to prevent leakage of helium, Holtec employs an engineered device known as the Remote Valve Operating Assembly (RVOA) that uses a metallic (impermeable) seal ring to establish a high integrity mechanical seal whose sole purpose is to prevent loss of helium from the MPC while the welding operations are carried out.
The leak-tightness of the mechanical seal is checked by two sequential tests: the first check is performed while the Remote Valve Operating Assembly (RVOA) and related process connections are still connected to the MPC. Following closure of the port openings, the hose connections are isolated, the pressure is vented from the hoses, and then the hose vents are closed off while connected to the RVOA to check if there is leakage past the mechanical seal.
After ensuring that there is no pressure build-up, the hoses and RVOAs are removed and a second leakage test is performed with a small cover plate on the port connected to a low pressure gauge to ensure that there is no helium out-migration that would manifest as pressure build up in the cavity below the port cover plate. This two-step check for absence of helium seepage has proven to be extremely reliable in practice.
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Holtec Letter 5014931 Attachment 2
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PROPRIETARY INFORMATION WITHHELD IN ACCORDANCE WITH 10CFR2.390
]
We submit that this two-cover plate confinement system (without crediting the high integrity mechanical seal on the MPC port connections) fulfills NRCs position in the matter of leak-tightness of confinement welds set forth in ISG-18. Actually, the two welded port cover closures proposed herein emulates the process used in the closure of the Lid to shell weld system which has historically relied on the same guidance from ISG-18 to eliminate helium leak testing of the MPC loaded with used nuclear fuel. Therefore, it is logical to conclude that the closure system will provide the required reasonable assurance of confinement of radioactive material under normal, off-normal, and credible accident conditions. Finally, note that both the vent and drain port openings are enclosed by the annular Closure Ring (a mandatory design feature in Holtecs MPCs) which, as a fully qualified NB pressure boundary, provides yet another confinement protection.
The main driver for this proposed change is to minimize crew dose during the lid closure work effort where the minor (limited to the port covers) helium test requires deployment of a whole new crew, setting up of the apparatus in a high radiation environment and thus incurring significant additional radiation dose. While deleting the helium test, we have strengthened the existing weld on the first port cover plate and added another port cover plate with multiple weld passes meeting the provisions of ASME Section III Subsection NB.
Please note that the removal of the helium leak test of the port cover plates is also a part of Proposed Change #3 and not of Proposed Change #6. Proposed Change #6 is to remove the need for a pressure test of the main lid-to-shell weld joint. The port cover plate welds are not tested via hydrostatic pressure test as there is no means to access the interior of the cask to achieve the required internal pressure. We assume that its inclusion in the RSI was an editorial error.
Observations O-1 The staff recognizes that the HI-STORM 100 Amendment No. 15 (effective on June 14, 2021) and the ongoing renewal may result in additional changes to the Certificate of Compliance (CoC) and safety analysis report (SAR) which may impact Amendment No.
- 16. All application materials for Amendment No. 16 should be updated accordingly to facilitate staffs review.
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Holtec Letter 5014931 Attachment 2 Holtec RSI Response:
HI-STORM 100 Amendment No. 15 and the renewal application were taken into consideration when preparing the proposed changes and analysis used in supporting the Amendment No. 16s.
The Amendment No. 16 proposed changes to the CoC and FSAR have been further reviewed to ensure changes from Amendment No. 15 and the renewal application are incorporated. For Amendment No. 16s applications, only the proposed changes related to this applications SOPCs are provided and any changes beyond this are not tracked or shared explicitly. Only Amendment No. 16 proposed changes are tracked in the proposed CoC and FSAR documents to identify applicable edits required for the documents and prevent any confusion related to other documentation.
O-2 Provide description of Note 3 shown on Table 2.4-5a of the proposed CoC Appendix D.
The applicant marked Note 3 to the title of Table 2.4-5a, but without providing any description.
The applicant should either remove Note 3 from Table 2.4-5a or provide description on Note 3.
The staff needs this information to determine compliance with 10 CFR 72.236(f).
Holtec RSI Response:
Reference to Note 3 in Table 2.4-5a is removed. All necessary notes to support Table 2.4-5a have been previously described.
O-3 Provide cask component temperatures for MPC-32M and MPC-68M with heat load pattern No. 3 to ensure that the uniform heat load pattern is the most bounding pattern for both MPC-32M and MPC-68M.
In Report HI-2210138, Revision 0, the applicant shows in Tables 7-1 and 7-2, the uniform heat load patterns for MPC-32M and MPC-68M, respectively, resulted in highest peak cladding temperatures (PCTs) and cavity temperatures; therefore, the applicant concludes the uniform heat load pattern for the respective MPCs is the most bounding pattern.
Staff reviewed Tables 7-1 and 7-2 and found that the PCT of uniform heat load pattern is higher than the PCT of pattern No. 3 by 2°C for MPC-32M and by 1°C for MPC-68M. With such small PCT differences between uniform heat load pattern and pattern No. 3, the applicant should provide cask component temperatures, similar to Table 7-3, of the heat load patterns No. 3 for MPC-32M and MPC-68M. The staff needs the information to verify and ensure that the uniform heat load pattern is indeed the bounding heat load pattern for MPC-32M and MPC-68M.
The staff needs this information to determine compliance with 10 CFR 72.236(f).
Holtec RSI Response:
As requested by the NRC staff, the component temperatures and cavity pressures for MPC-32M and MPC-68M under heat load pattern 3 have been presented in Tables 7-22 and 7-23 of the thermal report HI-2210138, respectively. The temperatures under heat load pattern 3 are either the same or lower than those computed under the uniform heat load pattern for MPC-32M. In the case of MPC-68M, the temperatures of some components are slightly higher (~1oC) under heat load pattern 3 than those computed under uniform heat load pattern. Such a difference is extremely Page 3 of 5
Holtec Letter 5014931 Attachment 2 small and within numerical uncertainties. Additionally, the margins to design limits are significantly higher to accommodate such a small difference.
O-4 PROPRIETARY INFORMATION WITHHELD IN ACCORDANCE WITH 10CFR2.390 O-5 (A) Provide the derivations of the MPC-32M and MPC-68M annulus pressures, as shown in Report HI-2210138, Revision 0, Tables 7-8 and 7-9; and (B) explain the similarity of annulus pressures between MPC-32M and MPC-68M under long-term normal storage, extreme ambient accident, burial under debris accident, and fire accident.
(A) The applicant presented in Report HI-2210138, Revision 0, Tables 7-8 and 7-9, the annulus pressures of MPC-32M and MPC-68M, respectively, for MPC loaded in the unventilated overpack with the bounding heat load pattern in cask array under long- term normal storage, extreme ambient accident, burial under debris accident, and fire accident.
Provide the derivations of these MPC-32M and MPC-68M annulus pressures under the conditions underlined above. The derivation of annulus pressures should include the assumptions, equations (e.g., ideal gas equation), annulus temperature, and parameters (with units) used in the derivation.
(B) Report HI-2210138, Revision 0, Tables 7-3 and 7-4 present the single cask containing MPC maximum component temperatures with uniform heat load for MPC-32M and MPC-68M, respectively. The cask component temperatures for MPC-32M and MPC-68M are different, and it is expected that the difference, when stored in unventilated overpack, would have impact on the temperature and pressure of the annulus between MPCs and unventilated overpack for normal, off-normal, and accident conditions of storage.
Explain why the MPC-32M annulus pressure (Table 7-8) is identical to the MPC-68M annulus pressure (Table 7-9) under each condition of long-term normal storage, extreme ambient accident, burial under debris accident, and fire accident.
The staff needs this information to determine compliance with 10 CFR 72.236(f).
Holtec RSI Response:
(A) PROPRIETARY INFORMATION WITHHELD IN ACCORDANCE WITH 10CFR2.390 (B) The reviewer is correct that the overpack component temperatures for MPC-32M and MPC-68M reported in Tables 7-3 and 7-4 of the thermal report are different. It should be noted that these are the maximum local temperature of these components. However, for the calculation of HI-STORM UVH annulus pressure, the volume averaged temperature of air in HI-STORM cavity is the determinant parameter. The volume averaged temperature is determined primarily by 1) the aggregate MPC heat load 2) overpack thermal characteristics. Since both MPC-32M and MPC-68M have same design basis aggregate heat loads, the exact same overpack design, and the same initial backfill pressure, the annulus pressure is also expected to be similar. For further clarity, the volume averaged temperatures of air in HI-STORM UVH cavity are also included in Tables 7-8 and 7-9 of the thermal report HI-2210138.
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Holtec Letter 5014931 Attachment 2 O-6 Clarify the applicability of Proposed Change #5 to MPC types in the application (see item (A) below) and add a requirement (see item (B) below) in the Technical Specifications or SAR Chapter 9 to ensure the minimum heat load limit is adequate to maintain the water in the HI-TRAC water jacket is above the freezing point (0°C/32°F).
The applicant proposed to use water without glycol in the HI-TRAC water jacket during transfer operations below 32°F ambient (Proposed Change #5). In Report HI-2043317, HI-TRAC Thermal Modeling to Address NRC Review Comments for HI-STORM 100 LAR-9, Appendix N.5.19, the applicant stated that the temperature of water in the HI- TRAC water jacket remains above freezing point (32°F) with heat load equal to or above 10 kW, and therefore, the system of MPC-32 in HI-TRAC does not need to be filled with ethylene glycol under allowable operating short-term conditions. The applicant presented the minimum water temperature in Table N.5.23 and stated that this calculation may be performed using the above set of assumptions for any MPC type and for any site-specific ambient conditions.
(A) Clarify whether the proposed change #5 is applicable only to MPC-32 system (including MPC-32M) with heat load equal to or greater than 10 kW or also applicable to MPC-68 system (including MPC-68M) with heat load equal to or greater than 10 kW.
(B) Add a requirement in Technical Specifications or SAR Chapter 9, Operating Procedures, such as the users need to perform thermal evaluation using the site- specific heat loads and ambient conditions to determine the minimum heat load limit for using water without ethylene glycol in the HI-TRAC water jacket during transfer operations below 32°F. This is to ensure the heat load limit is adequate to maintain the water in the HI-TRAC water jacket above the freezing point (0°C/32°F).
(C) Correct the labeling for N.5.19 on page N-25.
The staff needs this information to determine compliance with 10 CFR 72.236(f).
Holtec RSI Response:
(A) The proposed change is applicable for both MPC-32 and MPC-68 systems (including MPC-32M and MPC-68M). However, as stated in Section 4.5.7 of the FSAR, the MPC decay heat of 10kW is specific to the example calculation provided in the FSAR and the actual MPC decay heat load shall be determined based on the site conditions and the appropriate MPC design using the same models and methodology described in the FSAR.
(B) Sections 8.1.5 and 8.3.2 of the FSAR have been revised to include the proposed statements. Further, it has been added that the thermal evaluations shall be performed using the models and methods consistent with those described in Section 4.5 of the FSAR. The requirement has been added in Chapter 8 of the FSAR.
(C) The title of Section N.5.19 of HI-2043317 has been corrected to Onsite Transfer using HI-TRAC at Low Environmental Temperatures, consistent with Section 4.5.7 of the FSAR.
O-7 PROPRIETARY INFORMATION WITHHELD IN ACCORDANCE WITH 10CFR2.390 Page 5 of 5