ML25279A174
| ML25279A174 | |
| Person / Time | |
|---|---|
| Site: | Holtec |
| Issue date: | 10/06/2025 |
| From: | Manzione K, Singh K Holtec |
| To: | Office of Nuclear Material Safety and Safeguards |
| Shared Package | |
| ML25279A172 | List: |
| References | |
| 5014992 | |
| Download: ML25279A174 (1) | |
Text
Attachment 1 to Holtec Letter 5014992 1 l P a g e Holtec Position Paper THE EQUIVALENCE OF OBLIQUE AND SLAB LEVEL INLET VENTS IN HI-STORM OVERPACKS By: Dr. Kris Singh, CTO and Kim Manzione, Director of Licensing, NPD Rev 0; October 6, 2025 In this Position Paper, a summary of the a priori admissibility of moving the location of inlet vents in the HI-STORM overpack models from slab level to an elevated configuration with an oblique pathway for inlet airflow is provided. The latter configuration is referred to as Elevated or Oblique inlet duct.
1.
Overview:
The ventilated configuration of the HI-STORM1 overpack consists of a set of inlet air vents near the bottom of the cask and another set at the top. The number of inlet and outlet vents at the top and bottom varies depending on the cask model. Of the two sets of vents, the ones at the top are more consequential to the operation of the cask because they factor in the canister loading and unloading operations and affect the local structural strength of the cask cylinder which informs its post tip-over impact (a non-mechanistic event postulated by the NRC for free standing casks) behavior. The vents at the bottom, on the other hand, are not involved in any operational loadings and their design is solely focused on providing enough compensatory shielding around the vents to minimize the level of radiation accreted from the bottom region of the cask and to prevent a direct line of sight to the stored canister from the outside of the cask.
In the beginning, all vertical ventilated casks had their inlet vents at the bottom extremity of the cask cylinder (referred to as slab level vents in this position paper), their number varying from two (in NACs VSC cask) to eight (in HI-STORM FW) distributed along the circumference.
However, this configuration has been somewhat problematic for several plants that are located on a river or are in northern regions of the country. The reasons were (i) risk of extreme floods and
/or (ii) extensive snowfall.
Plants located on the river basin have to contend with a flooding situation where the flood waters may rise and block the ground-hugging inlet vents resulting in loss of air ventilation action. This flooding event even acquired a moniker - smart flood - in the industry lexicon meaning the flooding event that blocks the inlet vents but does not wet the canister (which would help heat transfer).
The other class of plants are those located in regions where extensive snowfall may block the vents leading to their blockage and cut off of the cooling air flow.
1 The term HI-STORM in this position paper refers to both above-ground cask designs, known as HI-STORM 100 and HI-STORM FW.
to Holtec Letter 5014992 2 l P a g e Those plants vulnerable to vent blockage, whether due to flood waters or snow accumulation, must have an emergency plan to address the blockage. To deal with the pesky problem of potential inlet vent blockage, Holtec developed an elevated inlet vent design. In this design the inlet vents are located sufficiently high in the shell such that flood waters or snow (as the case may be) would not block the inlet, thus eliminating the need for an emergency response plan. The elevated vent design has distinct additional advantages:
i.
The incidence of warming of the inlet air by solar heating reflected from the ISFSI pad is alleviated providing a small boost to the casks heat rejection capacity.
ii.
The elevated inlet vents are less apt to collect air-borne dust on the vent screens, which require periodic cleaning to avoid derating the thermal performance of the cask.
From the safety standpoint, as we will discuss later, the elevated inlet vent design posed no problems making it an ideal candidate for change under 10CFR72.48. From a practical standpoint, it has been known for well over 30 years that the vents on the side of the cask cylinder lead to little or no ingress of water even in heavy rain. A recent survey of loaded HI-STORMs with elevated inlet vents, summarized in Table 1 below, corroborates this common knowledge in the industry.
Table 1 - Summary of Operating Experience with Elevated Vents Utility Operating Experience A
No elevated vent systems in use B
No evidence found of any water accumulating or draining from HI-STORMs with elevated vents that have been in service sine 2022 C
Casks with elevated vents on site since 2023. One unloaded cask was found to have water exit the drain line, no loaded casks or other unloaded casks have been found to have any water D
33 casks with elevated vents loaded since 2021, no more than two tablespoons of water found in one cask, nothing that could credibly block airflow E
73 casks with elevated vents loaded, all were checked for water and none found in any systems It should be noted that the casks with elevated vents are equipped with a bottom drain plug to allow any rainwater to exit the cask. The drain plug, however, has been essentially an ornamental feature that has not had to render its drainage function for a significant amount of water at any of the plants where the elevated HI-STORM is deployed.
To be sure, ingress of water in the cask under heavy rains in the elevated vent cask is NOT a safety matter because water, if high enough to even partially wet the canister, would help, not hinder heat rejection from the canister. The sole negative effect of a prolonged water accumulation in the cask could be possibly subject to slow corrosion of wet areas requiring maintenance work.In short, accumulation of water in the oblique vent HI-STORM has no safety significance. In what follows, a summary of the essential equivalence of the safety metrics for slab level and elevated inlet vents is considered.
to Holtec Letter 5014992 3 l P a g e
- 2.
Essentials of safety equivalence:
The equivalence of the elevated and slab level inlet vents is considered herein under the assumption that they both feature equal combined inlet flow area from multiple ducts and are properly designed to block visual sighting of the canister. The determination of equivalence rests on four safety considerations:
i.
Structural equivalence (vide Chapter 3 of the FSAR) ii.
Thermal-hydraulic equivalence (vide Chapter 4 of the FSAR) iii.
Shielding equivalence (vide Chapter 5 of the FSAR) iv.
Operational equivalence (vide Chapter 9 of the FSAR)
- i.
Structural equivalence:
As a perusal of Chapter 3 of the HI-STORM FSARs would indicate, the only structural consequence for the HI-STORM storage system is the non-mechanistic tip-over event which requires the structural safety of the casks fuel basket and its stored fuel to be established assuming the cask to strike the pad from an incipient tipping state. The maximum g-load experienced by the fuel basket due to the casks impact with the ISFSI pad (slab), which would evidently occur at the farthest location from its pivot point, i.e., its upper extremity, is the figure of merit for this loading condition. Because the inlet vents are located near the pivot point (i.e., the casks base plate), their presence has no effect on the peak g-load caused by the impact at or near the casks closure plate.
Therefore, whether the vents are elevated or at the slab level makes no difference to the casks structural response to the postulated non-mechanistic tip over event.
ii.
Thermal-hydraulic equivalence:
Thermal-hydraulic performance of the cask is considered in Chapter 4 of the HI-STORM FSARs.
The key acceptance criterion is the peak temperature of the fuel cladding which is limited to 752oF by NUREG-2215. It can be readily deduced that the fuel cladding temperature would depend on the flow rate of ventilation air which would be influenced by the cross-sectional flow area of the inlet duct. Therefore, it is necessary that to satisfy hydraulic equivalence the two candidate vent configurations should have approximately the same aggregate flow area. If the area equivalence is observed then the thermal performance would be essentially the same (neglecting second order effects such as length of inlet duct and warming of inlet air from reflected heat from the slab).
iii.
Radiation dose equivalence:
Considered in Chapter 5 of the HI-STORM FASRs, the radiation dose requirement is governed by the site boundary dose, 10CFR72.104. It is heuristically apparent that varying the elevation of the inlet duct would have a minuscule effect on the site boundary dose which is principally affected by reactor operations.
to Holtec Letter 5014992 4 l P a g e iv.
Operational equivalence:
Cask operations are described in Chapter 8 or Chapter 9 of the FSARs. Referring to this chapter, one can readily discern that the elevation of the inlet duct has NO bearing on the casks operations.
- 3.
Concluding Remarks:
The information presented in the foregoing makes it clear that:
(i)
The location of the inlet vent has no bearing on the governing safety analyses for a HI-STORM system or an array of such systems; and (ii)
The two inlet vent configurations are so akin in their function, performance, and safety metrics that a separate qualifying analysis is not even necessary to make their safety case.
Therefore, selection of the right vent configuration should be made based on the exigency of the site and a 72.48 Change evaluation is the appropriate vehicle for making such selection. In particular, there is no regulatory basis or rationale to require the selection of inlet vent location to be subject to a licensing submittal to the USNRC.