ML20198R950

Requests Interpretation of or Rev to NUREG-1536, Std Review Plan for Dry Cask Storage Sys, Re Compliance W/ 10CFR72.236(e) & 10CFR72.122(h)(4) for Dry Fuel Storage Casks

ML20198R950
Person / Time
Site:	Rancho Seco, 07200011
Issue date:	11/05/1997
From:	Redeker S SACRAMENTO MUNICIPAL UTILITY DISTRICT
To:	Raddatz M NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
RTR-NUREG-1536 MPC&D-97-157, NUDOCS 9711130403
Download: ML20198R950 (6)

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SACRAMENTO MUMCIPAL UTiuTY DWTNCT U R O. Som 1583o, Sacramento CA 95852 1830, (916) 452 3211

, AN ELECTRIC SYSTEM SERVING THE HEART OF CAUFORNIA

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MPC&D 97157 November 5,1997 U.S. Nuclear Regulatory Commission Attn.: Document Control Desk Washington, DC 20555 Docket No. 72-11 Rancho Seco Independent Spent Fuel Storage Installation REQUEST FOR REVISION TO NUREG-1536 Attn.: Michael G. Raddatz Introduction The purpose of this letter is to request an interpretation of, or revision to, NUREG-1536

" Standard Review Plan for Dry Cask Storage Systems," regarding compliance with 10 CFR 72.236(c) and 10 CFR 72.122(h)(4) for dry fuel storage casks. As part of the requirements for spent fuel storage cask approval,10 CFR 72.236(c) requires redundant scaling of confinement systems, and 10 CFR 72.122(h)(4) requires that storage confmement systems have the. capability for continuous monitoring.

NUREG-1536 provides guidance for NRC staffin performing reviews oflicense applications for dry fuel storage systems. The principal purpose of NUREG-1536 is to establish a well-dermed basis from which NRC stafican conduct their review to ensure the quality and consistency of that review. NUREG-1536 is also open to interpretation and/or revision to allow for funher clarification regarding the acceptance criteria for designs that the NRC may not have considered prior to issuing the original NUREG.

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RANCHO SECO NUCLEAR GENERATING STATION Cl 14440 Twin Cities Road, Herald, CA 95038-9799;(209) 333-2935

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M. Raddat: MPC&D 97-1.52

'NUREG 1536, Chapter 7 "Confmement Evaluation," profides guidance to NRC staff for evaluating the confmement features and capabilities of a dry cask storage system. In its evaluation of the cask confinement system, the NRC seeks to ensure that:

1. Radiological releases to the c . ironment remain within regulatory requirements.

2.- Spent fuel cladding and fuel assemblies will be sufficiently protected during storage against degradation that could lead to gross ruptures.

NUREG 1536 currently recogrJzes as acceptable a system that monitors the pressurized region between a cask's bolted closure redundant seals. We believe that a system that monitors the redundant scaled cask volume is also acceptable, as outlined below.

Tvolcal Cask Confinement Bounderv Typically, a dry fuel storage cask confinement boundary consists of:

1. A cylindrical inner shell with welded closures.

2. A bolted top closure plate with redundant 0 ring seals.

3. Vent and drain port closure plates, also bolted with redundant 0 ring seals.

This confinement boundary, coupled with a system to monitor confinement integrity, provides a high level of confidence that licensees will be able to detect a breach in the confinement boundary, and take corrective actions to ensure that radiological releases to the environment remain within regulatory limits.

Tvolcal Confinement Bounderv Monitorina Systems Because of the high level of confidence in the inherent integrity of welded closures, the NRC has previously not required a monitoring system for welded closures.' Ilowever, due to the greater uncertainty associated with mechanical seals, the NRC has required some form of routine surveillance program and/or active instrumentation to meet the intent of" continuous monitoring," and show compliance with 10 CFR 72.122(h)(4).

For bolted closures with redundant 0-ring seals, the typical approach to monitoring confmement integrity has been to pressurize the region between the redundant seals, with a non reactive gas, to a pressure greater than the cask cavity and the atmosphere. A

' See NUREG 1536. Cluipter 7.Section IV.4.

l Af. Raddatz 3- AIPC&D 97-132 l

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' ecrease in pressure between the seals indicates that the gas is leaking either into the cask d  ;

cavity or out to the atmosphere.

Part 72 Statements of Consideration In the Part 72 Statements of Consideration for General Licensing Provisions, Comment 35 expressed a concern regarding the potential for corrosion of fuel cladding if the inert atmosphere inside a storage cask is not maintained. In their response, the NRC stated that,

" Cask atmospheres will be required to befd!ed with an inert gas andprovided with snonitoring systems to detret leaks in the <nsk sealing system. "

The NRC further stated that, "If the redundant seals and the monitoring systemfail, oxidation of thefuel cladding could occur...But, there would not be any sigmfcant increase in radioactivity because any release of radioactiw particlesfrom thefuel nxis would remain confined within the cask. If the r edundant scalsfall, and the monitoring system does notfail, the monitoring system would detect thefailure and the seals would be promptly repaired. "

In the Part 72 Statements of Consideration for the Certification of the TN-24 cask, Comment I stated that bolted closures with metal 0 rings have shown poor operating characteristics, and that all closures should be double seal welds. In their response, the NRC stated that, "Several casks with metallic 0-rings have been in operationfor at least seven years. Licensees are required tofileformal nymrts ofproblems with 0-rings occur. The NRC has not receivedam nymrts concerning 0-ringproblems. "

"The metallic O-ring seals may not necessarily last the 20 par term of the Certificate of Compliance. Ihat is why double seals are used with a higher pressure between the 0-rings with pressure monitoring equipment to quickly detect a sealfailure. Failed seals can be readily replaced. "

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MIGtD 97-1M Anarnatha Cannnarnent Boundary Monitorina Svstem

' Although the NRC has accepted conAnement monitoring systems that simply monitor the . l integrity of the redundant 0 ring seals, some cask designs may not allow for this j approach. For example, with the design of some storage casks, this approach to  !

confinement monitoring may not be possible because some seals may not accessible for the typical confinement monitoring system described above. In the case where some O ring .

i seals may not be accessible, the typical methods for maintaining redundant seals and i

providing continuous seal monitoring may be mutually exclusive.

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As an altemative to meeting the intent of the continuous monitoring requirements in 10 CFR 72.122(h)(4), and the redundant sealing requirements in 10 CFR 72.236(e), a dry i storage cask could use a system that monitors the helium pressure in the cask interior. Or, ,

for a seal welded canister stored in a cask, a system that monitors the annulus between the  !

canister and the cask inner shell. Providing direct and continuous monitoring of the +

helium atmosphere inside the cask, or canister / cask annulus, would monitor the integrity of the entire confinement boundary, as opposed to monitoring only the integrity of the ,

mechanical seals.. ,

In the unlikely event of cask confmement leakage through a path other than the mechanical seals, the alternative system would deter isuch leakage, and is superior to a between the-seal monitoring system that would not detect the leakage. The proposed l system would be consistent with the Statement of Considerations noted above, j

'llf the redurukmt sealsfall, arut the monitoring system does notfall, the mcmitoring system would detect thefailure arul the seals would be promptly repaired. "

Although there may be only a single seal boundary on the pressure transmitter associated with this type of monitoring system, the size of the seal would be small, and not as susceptible to failure as the larger diameter 0 rings used on the cask cover plates. There'  ;

would likely be a short mn of piping, with a few small fittings; however, the system would ,

operate at very low pressure, and any leak path would be very small. ' The system would  ;

. monitor the cask interior continuously and directly, and could provide a port to sample the. ,

cask atmosphere,if needed.

Based on the ad0antages of monitoring the integrity of the entire cask confinement boundary SMUD believes that this alternative provides an acceptable level of confidence in confinement integrity, and s alves the dichotomy between the requirements for providing redundar;t seals and continuous monitoring for certain types of storage cask.

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M. Rad &nz $- MPC&D 97-132 Use of an Alternative Confinement Boundary Monitorina Systern Some licensees plan on using a canister based dry fuel storage system The canister-based storage system could include a cask licensed for storage in the event of a breached canister at the ISFSt.

2 In the even; a cask is used as a storage cask , the licensee would place the breached canister la the cask, seal the cask with the bolted closure plates, evacuate the cask cavity, and backfill the cavity with helium. Ilackfilling the annulus between the canister and the cask inner shell with helium provides a non reactive environment to protect the fuel assemblics against fuel cladding degradation.

When used as a storage cask, the licentee would record the cask pressure as part of a routine surveillance program. If the cask pressure monitoring indicates the requirement to re test and replace the cask or monitoring system seals, the licensee would take actions to repair or replace the faulty seals, in accordance with approved plant procedures.

Another advantage of a system that monitors the cask volume directly is the reduced potential for unnecessarily removing the cask lid to replace seals that may not be faulty.

Minimizing the potential for unnecessary cask lid removal also minimizes the potential for a release while conducting an unnecessary 0 ring replacement operation.

When using the typical approach to monitoring con 6nement integrity, a decrease in pressure between the sals indicates that either the inner seal, the outer seal, or both seals, may be leaking if the licensee determines that the cask outer seal is leaking, the integrity of the inner seal remains indeterminate because there is no way to individually assess its status. The licensee would need to remove the cask closure plate, and replace both seals even if the inner seal is intact.

A decrease in pressure using the alternative system indicates that seal replacement is clearly required, thus avoiding unwarranted cask lid removal operations. If the leak is from the instrument seal, the licensee could easily replace that seal, without the need to unnecessarily remove the cask closure plate. Also, the alternative system would allow the licensee to sample the cask atmosphere directly, to determine the level of activity in the cask inteiior.

A routine surveillance program, along with continuous instmment monitoring of the cask helium atmosphere, would provide confidence not only in the performance of the 0 ring seals, it would provide confidence in the integrity of the entite cask con 6nement boundary, including the welded closures. This alternative monitoring system, along with operating procedures to readily replace faulty seals, would enhance protection to the environment by ensuring a non-reactive environment to protect the fuel assemblies against cladding 8 The licensec puld use the cask as a storage overpack to isolate a breached canister from the environment, if the spent fuel pool is not available.

1 M. Raddat: MPC&D 97-132

' ' ' ' degradation during storage, and ensuring that radiological releases to the environment will be within regulatory requirements.

Reauested Action Dased on the advantages of monitoring the integrity of the entire cask confinement boundary, SMUD believes that this alternative provides an acceptable level of confidence in confinement imegrity, and meets the intent of 10 CFR 72.236(e) and 10 CFR 72.122(h)(4). Accordingly, SMUD requests that the NRC consider the alternative monitoring approach described, and determine if the alternative system meets the intent of the regulatory requirements, and provides adequate assurance that radiological releases to the environment will be within regulatory requirements.

If the NRC determines that the alternative confinement monitoring system meets the intent of 10 CFR 72.236(e) and 10 CFR 72.122(h)(4), SMUD requests that the NRC clarify the acceptance criteria in NUREG-1536, Chapter 7,Section IV, by stating that storage cask confinement monitoring systems that monitor the integrity of the entire cask confinement boundary meet the intent of 10 CFR 72.236(c). For example:

Storage cask confinement monitoring systems that monitor the integrity of the entire cask confinement boundary are acceptable. Although this type of monitoring hyStem may have smallfittings, connectors, or instruments using single seals, the admntages of this methodprovide an acceptable lewl of confidence in confinement integrity, andprovide a high level of confidence that licensees will be able to take corrective actions to ensure that radiological releases to the environment remain within regida!ary lituits. Accordingly, systems that monitor the integrity of the entire cask confinement boundary meet the intcnt of10 CFR 72.236(e) and 10 CFR 72.122(h)(4).

If you, or members of your stafT, have questions requiring additional information or clarification, please contact Bob Jones at (916)452-3211, extension 4676.

Sincerely, 1 tl .4 Steve Redeker Manager, Plant Closure and Decommissioning