ML20198S683

Forwards Open Items Associated W/Ex Vessel Steam Explosion SER for AP600

ML20198S683
Person / Time
Site:	05200003
Issue date:	12/08/1997
From:	Joseph Sebrosky NRC (Affiliation Not Assigned)
To:	Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
References
NUDOCS 9801260256
Download: ML20198S683 (5)

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December,8, 1997J

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Mr. Nicholas J. Liparulo, Manager.

. Nuclear Safety and Regulatory Analysis

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=j 1 Nuclear and Advanced Technology Division.

(Westinghouse Electric Corporation;

- P.O. Box 355' 3

W Pittsburgh, PA 15230 i

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SUBJECT:

1 OPEN ITEMS ASSOCIATED WITH EX VESSEL STEAM EXPLOSION SAFET t EVALUATION REPORT (SER) FOR THE AP600 l

Dear Mr. Liparulo:

Jntainment Systems and Severe Accident Branch has provided an SER for their review

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- m sociated with ex vessel steam explosion for the AP600. However, the SER contained some open items. These open items have been extracted from the SER and can be found in ther enclosure to this letter.

You have requested that portions of the information' submitted in the June 1992, application for design certification be exempt from mandatory public disclosure. While the staff has not -

completed its review of your request in accordance with _the requirements of 10 CFR 2.790, that =

' portion of the submitted information is being withheld from public disclosure pending the staff's _

final determination. The staff concludes that these follow on questions do not contain those

- portions of the information for which exemption is sought. However, the staff will withhold this letter from public disclosure for 30 caleMar days from the date of this letter to allow Westing.

1 house the opportunity to verify the staff's concluslons, if, after that time, youl do not request that "sil or portions of the information in the enclosures be. withheld from public disclosure in accor-L

dance wi'.h 10 CFR 2.790, this let'st will be placed in the Nuclear Regulatory Commission Public Document Room.

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if you have any questions regarding this matter, you may contact me at (301h 415-1132.

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Sincer'ely, =

Original signed by:

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_ Joseph M. Sobrosky, Project Manager 9901260256 971209 Standardization Project Directorate PDR ADOCK 05200003 Division of Reactor Program Management j

E PDR Office of Nuclear Reactor Regulation Docket No.52-003 1 Enclosure;; As stated -

NRC 51BB M CRW

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- DISTRIBUTION.

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g)r'O M DOCUMENT NAME;1A:SCSBJCl.RAll, _1..

1T# receive a copy of this document,' indicate in the box: "C" = Copy without attachment / enclosure "E" = Copy

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Iwith attachment / enclosure ? "N" = No copy OFFICE PM:PDST:DRPM:l o JiiCSB:DSSA l5 D:PDST:DRPM j

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Mr. Nicholas J. Liparuto Docket No.52-003 Westinghouse Electric Corporation AP600 1

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Mr. B. A. McInty.e Ms. Cindy L. Haag j

Advanced Plant safety & Licensing Advanced Plant Safety & Licensing Westinghouse Electric Corporation Westinghouse Electric Corporation Energy Systems Business Unit Energy Systems Business Unit P.O. Box 355 Box 355 Pittsburgh, PA 15230 Pittsburgh, PA 15230 Enclosure to be distributed to the following addressess after the result of the proprietary evaluation is received from Westinghouse:

Mr. Russ Bell Ms. Lynn Connor Senior Project Manager, Programs DOC Search Associates Nuclear Energy Institute Post Office Box 34 1776 l Street, NW Cabin John, MD 20818 Suite 300 Washington, DC 20006-3706 Mr. Robert H. Buchholz GE Nuclear Energy Dr. Craig D. Sawyer, Manager 175 Curtner Avenue, MC-781 Advanced Reactor Programs San Jose, CA 95125 GE Nuclear Energy 175 Curtner Avenue, MC-754 Mr. Sterling Franks San Jose, CA 95125 U.S. Department of Energy -

NE 50 Barton Z. Cowan, Esq.

19901 Germantown Road Eckert Seamans Cherin & Mellott Germantown, MD 20874 600 Grant Street 42nd Floor Pittsburgh, PA 15219 Mr. Charles Thompson, Nuclear Engineer AP600 Certification Mr. Frank A. Ross NE-50 U.S. Department of Energy, NE-42 19901 Germantown Road Office of LWR Safety and Technology Germantown, MD 20874 19901 Germantown Road Germantown, MD 20874 Mr. Ed Rodwell, Manager PWR Design Certification Electric Power Research Institute 3412 Hillview Avenue Paio Alto, CA 94303 4

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DISTRIB JTION: Letter to Mr. Nicholas J. Lloarulo. dated: December 8,1997 g Dedtetl'#e

• Enclosure to be held for 30 days 1

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-F Open items Associated with Ex-Vessel Steam Explosion for the AP600 Bacieround r

A request for additional information (RAI) was transmitted to Westinghouse by letter, dated June 3,1997 conoeming ex-vessel steam explosions. Westinghouse responded to the staffs request via its June ig,1997 submittal. This submittal contained further information on TEXAS calculations and addressed most of the staff's comments. However, two RAls,720.407 and 720.40g, remain unresolved. The staff's follow on questions to these RAls are discussed in -

detail below.

It should also be noted that the Civil Engineering and Geoscience Branch is currently evaluating the structural aspects of this assessment. Their evaluation of the structural capacity of the -

reactor cavity is necessary to close out the DSER open item associated with this issue. If questions regarding this aspect of the evaluation are deemea necessary they will be provided in a separate letter.

t Open item 720.456F '

RAI 720.407 refers to the issue of breach size for the localized failure case, used by Westinghouse in the TEXAS calculations. The localized failure occurs at the transition between the hemispherical lower head and the cylindrical section of the vessel, presumably due to thermal attack of the vessel wall. It is not clear if the breach size was chosen arbitrarily or was based on calculations of thermalload generated by the molten pool. The basis for assuming a 0.06 m diameter breach for the localized failure case is considered an open item pending a response from Westinghouse, it is noted that the overall conclusion for the localized failure case, i.e., that the containment integrity would not be challenged, is not expected to change as a result of the resolution of this RAl.-

Open item 720.457F RAI 720.40g refers to the choice of melt temperature and superhea: for the hinged failure case, noting the values used for these parameters in the TEXAS calculations were those of steel, i.e.,

a metallic molt. Westinghouse assumed that the initial fuel-coolant interactions would involve molten steel and water in the hinged failure configuration, as shown in Figure B-1 of the PRA. It is not evident that would be the case if the melt poolis well mixed. Even in a stratified situation, there are two possible scenarios in which the oxidic melt may be released before the metallic melt in the event of a vessel breach.

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The first possibility arises from a breach at a location slightly below the metallic upper layer of the melt pool. Work by the staff, T. G. Theofanous, et al., " In-vessel Coolability and Retention of a Core Melt," DOE /1D 10460, and T. N. Dinh, J.- A. Green, and e. R. Sehgal, at the Royal Institute of Technology, as described in a paper titled, "On Mechanism-that Govem the Vessel Melt Source for Ex Vessel FCis," which was presented at ICONE !, r4 ice, France, May 26-30,1997, shows that the highest heat flux region may be at the transition point just below the metal layer.

The second possibility arises from a layer inversion process in the melt pool whereby the metal

!ayer initially on top of the pool may become more dense than the oxidic layer below it due to.

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2-the presence of a small mole percent of heavy metals and, as a result, flips over and stratifies undemeath the oxidic layer, in Section 19.2.3.3.1 of this FSER, the staff concluded that layer inversion, N it occurs, could result in host fluxes that exceed the critical heat flux which according to the ROAAM study is a necessary and sufficient criteria for RV failure.

a)

Westinghouse should demonstrate why oxidic melts need not be considered in the investigation of ex-vessel steam explosions. Absent this demonstration, Westinghouse needs to recalculate the steam explosion loads with a revised set of material properties representative of a predominanty oxidic melt. This is an open item.

b)

Westinghouse should also confirm if the metallic component of the melt includes unoxidized zirconium which is known to release a substantial amount of additional energy upon exothermic chemical reaction with water. The additional energy in the localized failure caso does not appearlarge enough to warrant concem. However, for the hinged failure case, Westinghouse needs to consider this additional energy release on their i

assessment. This is an open item.

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The one-dimensional TEXAS code calculates loads in the explosion zone, it is not clear how the cavity floor and wallloads are calculated from the explosion zone loads. This is

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an open item.

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