ML20199E599
| ML20199E599 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 11/17/1997 |
| From: | Marissa Bailey NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Cruse C BALTIMORE GAS & ELECTRIC CO. |
| References | |
| IEB-96-004, IEB-96-4, NUDOCS 9711210257 | |
| Download: ML20199E599 (8) | |
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November 17, 1997 Mr. Charles H. Cruse Vice President - Nuclear Energy Baltimore Gas and Electric Company Calvert Cliffs Nuclear Plant 1650 Calvert Cliffs Parkway Lusby, MD 20657-4702
SUBJECT:
REQUEST FOR ADDITIONAL INFORMATION BASED ON THE NUCLEAR REGULAiORY COMMISSION'S ACCEPTANCE OF VECTRA TECHNOLOGIE5' RESPONSE TO NRC BULLETIN 96-04
Dear Mr. Cruse:
In your res)onses dated August 19 and November 4. 1996. to NRC Bulletin 96-04.
" Chemical. Galvanic, or Other Reactions in Spent Fuel Storage and Transportation Casks." you incorporated information prepr ed by VECTRA Technologies. Inc. The staff has accepted VECTRA's response for the NUHOMS-24P system.
Enclosed is the staff's safety evaluation of VECTRA's bulletin response. Also enclosed is a request for additional information (RAl) regarding your inplementation of the recommendations made in VECTRA's response. This RAI suaersedes in its entirety our RAI dated April 8, 1997.
VECTRA's bulletin response recomended adopting 60% of the lower flammability limit of hydrogen (1.t.
2.4% hydrogen by volume) as a safe upper limit for the hydrogen concentrhtion in the vapor space of a NUHOMS Dry Shielded Canister (DSC) before and during welding or cutting operations.
VECTRA also recommended monitorirg the DSC vapor space at a frequency to be assessed by each NUHOMS user.
For DSCs with flame-sprayed aluminum, the staff believes that the DSC vapor space should be monitored continuously during welding or cutting to ensure that VECTRA's recommended safe upper limit is not exceeded.
As reque. ted in the enclosed RAI. please indicate whether you agree with and how you xill implement VECTRA's response and recommendations.
If you do not plan to implement any or all of VECTRA's recommendations describe and justify your alternative course of action.
Please provide your response to the enclosed RAI within 30 days from the date of this letter. A response to our April 8. 1997. RAI is not required.
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If you have ny questions /regarding;this ' matter you may ' contact me at -
301-415 8531
-Sincerely. -
[ original signed by /s/]
Marissa G. Bailey 'Pr6 ject Manager Spent Fuel Licensing Section' Spent fuel Project Office Office of Nuclear Material Safety and-Safeguards Dockets 72-8c50-317/318
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. REQUEST FOR ADDITIONAL INFORMATION ON THE CALVERT CLIFFS NUCLEAR PLANT RESPONSE TO NUCLEAR REGULATORY COMMISSION BULLETIN 96 04-Describe in detail how the recommendations made in the VECTRA Technologies, j
Inc. (VECTRA). Report No. 31-89604-102. Revision 2. for hydrogen control and monitoring will be implemented. At a minimum, the information provided should:-
1.
Describe the methods used to control and monitor hydrogen before and during welding, grinding, or cutting operations associated wi_th loading or unloading activities.
The methods should include continuous monitoring of the hydrogen concentration in dry shielded canisters (DSCs) with flame-sprayed aluminum.
During its review of VECTRA's evaluation, the staff found that the hydrogen concentration could be highly variable in these DSCs.
In addition, results of VECTRA's computer simulation also indicate that there may be an initiating event that causes a rapid increase in hydrogen generation.
If not properly monitored, the hydrogen concentration could exceed Vr
's proposed safe upper limit of 2.4%
- hydrogen and the lower flammo? ty limit of 4.0% hydrogen in a very short time in borated water.
2.
Specify the'DSC hydrogen concentration that will be-set as the safe upper limit before and during welding or cutting operations.
VECTRA has recommended a safe upper limit of 2.4% hydrogen.
If a higher limit is used, justify that it provides an adequate safety margin between the amount of hydrogen produced and the lower flammability limit (4% hydrogen by volume).
3.
Describe the actions to be taken if the safe upper limit is er.:eeded.
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O SAFETY EVALUATION OF VECTRA TECHNOLOGIES' RESPONSE TO NUCLEAR REGULATORY-COMMISSION BULLETIN % 04 FOR THE NUHOMS 24P AND NUHOMS 7P DRY SPENT FUEL STORAGE SYSTEMS INTRODUCTION On July 5,1996, the U.S. Nuclear Regulatory Commission issued Bulletin 96 04 to notify addressees about the potential for adverse chemical, galvanic or other reactions among the materials of a swnt fuel storage or transportation cask, its contents, and the environments tie cask may encounter during use.
The potential for these reactions was identified when hydrogen gas was ignited during the welding of the shield lid on a Ventilated Storage Cask (VSC-24) system at Point Beach Nuclear Plant on May 28, 1996. The hydrogen gas was generated by a chemical reaction between the Carbo Zinc 11 coating used on the VSC 24 and the borated water in the spent fuel pool. As a result, the bulletin was issued to request information to confirm that chemical, galvanic, or other reactions are not a concern for other spent fuel storage and transportation cask designs.
VECTRA Technologies Inc., has completed its response to the bulletin for the NUHOMS-24P and NUHOMS-7P dry cask storage systems.
VECTRA's response was submitted on August 18, 1996, and supplemented on October 10,1996, and April 30, August 5, and August 6, 1997. The information requested by the bulletin is primarily contained in VECTRA Report Nos. 31-B9604-102, Revision 2, and 31-89604-103, Revision 0, which were included with the submittals.
The NRC staff has reviewed the submittals and finds that VECTRA has satisfied the requested actions and required responses to the bulletin for the NUHOMS-24P and NUHOMS-7P systems. The staff's safety evaluation follows.
SAFETY EVALUATION The NUHOMS-7P system, which stores up to seven pressurized water reactor (PWR) assemblies, is used at the H. B. Robinson Nuclear Plant under a site-specific license.
The Dry Shielded Canister (DSC) in the NUHOMS-7P has an all stainless steel shell and basket assembly, Borated aluminum neutron poison sheets, surrounded by vented stainless steel wrappers, are also used.
No general corrosion of stainle's steel is expected to occur in borated water or in a storage environment.
Laboratory tests of borated aluminum immersed in borated water showed very little hydrogen generation. Eight NUHOMS-7P units have been loaded; there are no plans to load additional units.
F The NUHOMS-24P system, which stores up to 24 PWR assemblies, is used at the Oconee Nuclear Station and Calvert Cliffs Nuclear Power Plant under site-s mcific licenses and at the Davis-Besse Nuclear Plant under a general license (Certificate of Comsliance No.1004).
Variations exist between the NUHOMS-24P units used at the t1ree different sites: however, the overall design and materials of construction are essentially the same. All NUHOMS-24P DSCs have a stainless steel shell and stainless steel fuel assembly guide sleeves.
The fuel basket spacer disks are constructed with either stainless ENCLO5URE R
steel (site-specific designs only) or carbon steel coated with flame sprayed aluminum (site-specific and general license designs).
The shield plugs are also either stainless steel (site specific designs only) or carbon steel coated with flame-sprayed aluminum (site-specific and general license i
designs). The flame sprayed aluminum coating on the carbon steel components is ap)roximately 7 mils. No neutron absorbers and no Carbo Zinc 11 or other zinc-aased coatings are used-in the NUHOMS-24P. Approximately 52 NUHOMS-24P units have been loaded.
VECTRA identified the materials in the various NUHOMS designs and evaluated the potential for adverse reactions under various operating environments (loading, drying / sealing, storage, and unloading).
Normal, off-normal, and accident conditions were considered in the evaluation.
Based on its evaluation, VECTRA concluded that the only significant reaction is the corrosion of the flame sprayed aluminum coating in a borated water environment.
In borated water, the flame-sprayed aluminum coating on the carbon steel will corrode by general corrosion and by Jalvanic corrosion between the aluminum coating and the carbon steel substrate. The corrosion will result in the generation of hydrogen and the formation of aluminum oxide on the surface of the coating. The aluminum oxide will passivate the aluminum and the rate of corrosion will decrease.
Radiolytic decomposition of wate" could also generate hydrogen, but its contribution is small compared to the corrosion reaction.
To determine the extent of hydrogen generation. VECTRA used the following sources of information: Oconee measurements on four DSCs. laboratory testing, and computer simulation. The hydrogen concentrations measured at Oconee varied from <0.2% to 2.05% hydrogen. The highest measured hydrogen concentrations were 2.05% (16.25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br /> after the shi'ld plug was installed) and 2% (after 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />).
The laboratory tests considered carbon steel coated with flame sprayed aluminum and immersed in borated water. The test data showed that the corrosion rate and hydrogen generation rate increased for the first few hours of the test and then leveled off until the temperature reached 160*F.
The corrosionrateandhydrogengenerationrateincreaseddramaticallyat_
temperatures between 160 F_and 212 F.
The test results showed higher hydrogen concentrations than the Oconee measurements.
VECTRA justified that the difference in results is due to the test set-up which took " actual instantaneous" measurements of the hydrogen generated.
The test did not account for diffusion of hydrogen in water which is about 8000 times slower than diffusion of hydrogen in air.
VECTRA also claimed that the test did not account for loss of hydrogen by passive circulation through open vents.
The computer simulation predicted a hydrogen concentration (due to corrosion and radiolysis) of 1.45% after 28.06 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> at 87.74*C (189.93*F). _The computer simulation also predicted that the hydrogen concentration would rise to 4.70% after 28.89 hours0.00103 days <br />0.0247 hours <br />1.471561e-4 weeks <br />3.38645e-5 months <br /> at a temperature of 90.37'C (194.67 F).
- However, these conditions fall outside the expected operating parameters.
Normal loading operations, from placement of the shield plug to completion of,
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welding, are expected to take less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> at temperatures ranging from 70*F to 120'F.
No boron depletion or precipitates were observed during the 1rooratory tests or Oconee measurements.
Boron depletion or precipitation is not expected to occur. -In addition, no change in the cask's thermal properties, binding of mechanical surfaces, or degradation of any safety components are expected.
The corrosion of the flame-sprayed aluminum during loading will not adversely affect the structural integrity and performance of the cask and retrievability of the spent fuel.
CORRECTIVE ACTIONS RECOMMENDED As discussed above, significant amounts of hydrogen could be generated by the corrosion of flame-sprayed aluminum in borated water.
VECTRA s evaluation indicates that the hydrogen concentration in the DSC would remain below 4% by volume, the Lower Flamacility Limit (LFL) of hydrogen in air, during normal
. loading or unloading operations.
To ensure that any hydrogen generated would not be ignited, VECTRA recomended adopting 2.4% hydrogen by volume (60% LFL) as a safe upper limit for the hydrogen concentration in the DSC before and durina welding or cutting o)erations. ThisrecommendationisbasedonANSI/ISA512.13. Parti-1986.
~)erformance Requirements, Combustible Gas Detectors." VECTRA also recomended that periodic monitoring of the DSC va>or space be performed during welding / cutting operations to ensure that tie safety limit is not exceeded. VECTRA did not recommend a frequency for monitoring the DSC air space: the frequency would need to be assessed by each NUHOMS user.
The staff acceats VECTRA's recomendation for adopting a safe upper linit of 2.4% hydrogen )y volume. However, for DSCs with carbon steel components coated with flame-sprayed aluminum, the staff believes that continuous hydrogen monitoring should be performed during welding or cutting o)erations.
The staff has found that the hydrogen concentration in the air blantet of such DSCs could be highly variable.
If not properly monitored, the hydrogen concentration could exceed VECTRA's )roposed safety limit of 2.4% hydrogen and the LFL of 4.0% hydrogen in a very s1 ort time in borated water, In addition, results of the com> uter simulation also indicate that there may be an initiating event tlat causes a rapid increase in hydrogen generation. The exact cause of the initiating event is not known (based on the data, it is possible that exceeding a temperature of 160*F could be the trigger).
For the all-stainless steel DSCs (some site-s)ecific designs), periodic monitoring is acce) table because significant lydrogen generation is not expected in these JSCs.
CONCLUSION VECTRA evaluated the materials used in the NUHOMS-7P and NUH0MS-24P systems and determined that the only potentially adverse reaction is the corrosion of the flame-sprayed aluminum coating. This reaction could generate a significant amount of hydrogen gas: although, the concentration is not w-
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expected to reach ignitable levels during nornel o>erations.
VECTRA r.oted.
ana the staff acknowledges, that.approximately 60 (UHOMS DSCs have been loaded, at four different sites, and welded without any type of ignition indications or incidents.
However, precautions should be taken to ensure that a hydrogen ignition does not occur with any casks loaded in the future. The staff agrees that use of the recommended safe upper limit, along with appropriate monitoring, would be adequate to preclude a hydrogen ignition.
-VECTRA has satisfied the requested actions and required res)enses to the bulletin for the NUKOMS-24P and NUHOMS-7P systers.
No furtier action associated with the bulletin is required from VECTRA for the Null 0MS-24P and NUHOMS 7P systems.
NOTE: VECTRA has completed its res>onse for the NUHOMS 24P and NUHOMS 7P casks only. The response for the NJHOMS-528 will be com)leted at a later date and addressed in a separate safety evaluation. Several.4UHOMS-52B casks have been manufactured, but none have Men loaded to date. Material com)atibility in the NUHOMS HP-187 system, a dual-purpose (storage / transport) cast, will be addressed in the licensing submittals for that cask.
REFERENCES 1.
ANSI /AWS C2.18 93, " Guide for the Protection of Steel with Thermal Sprayed Coatings of Aluminum and Zinc and Their Alloys and Composites."
2.
BNL NUREG 24532. " Hydrogen Release Rates from Corrosion of Zinc and Aluminum, Brookhaven National Laboratory." May 1978.
3.
Society of Fire Protection Engineers (SFPE) Handbook of Fire Protection Engineering, Section 3/ Chapter 16. Second Edition.
4-9
1 Calvert~ Cliffs Nuclear Power Plant Unit Nos. I and:2 cc:
President.-
- Mr. Joseph H. Walter, Chief Engineer --
Calvert County Board of-Public Service Comission of Comissioners Maryland
-175 Main Street Engineerine Division Prince Frederick, MD 20678 6 St. Pae" O'1tre
-Baltimore,tiJ 21202-6806 James P. Bennett. Esquire d
Counsel Kristen A. Burger Esquire
. Baltimore Gas and Electric Company Maryland People's Counsel P.O.--Box 1475 6 St. Paul Centre Baltimore, MD 21203 Suite 2102 Baltimore, MD 21202-1631
--Jay E. Silberg, Esquire Shaw, Pittman, Potts and Trowbridge
_ Patricia T. Birnie, Esquire 2300 N Street, NW Co Director Washington, DC.20037 Maryland Safe Energy Coalition P.O. Box 33111 Mr. Thomas N. Pritchett, Director, Baltimore, MD 21218 NRM Calvert Cliffs Nuclear Power Plant Mr. Lcren F. Donate 11 1650_ Calvert Cliffs Parkway NRC Technical Trair,ing Center Lusby,-HD 20657-4702 5700 Brainerd Road Chattanooga, TN 37411-4017 Resident Inspector U.S. Nuclear Regulatory Comission P.O. Box 287 St. Leonard, MD 20685 Mr. Richard I. McLean, Manager Nuclear Programs Power Plant Research Program Maryland Dept. of Natural Resources 580 Taylor Avenue Tawes State Office Building, B3 Annapolis, MD 21401 Regional Administrator, Region I U.S.- Nuclear Regulatory Comission 475 Allendale Road King of Prussia, PA 19406
.