ML20203D486
| ML20203D486 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 12/09/1997 |
| From: | Huffman W NRC (Affiliation Not Assigned) |
| To: | Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| References | |
| NUDOCS 9712160209 | |
| Download: ML20203D486 (3) | |
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Mr. Nicholas J. Liparulo, Manager.
Nuclear Safety and Regulatory Analysis--
Nuclear and Advanced Technology Division Westinghouse Electric Corporation P.O Box 355 Pittsburgh, PA 15230
SUBJECT:
- OPEN ITEM ASSOCIATED WITH THE AP600 SAFET) EVALUATION REPORT (SER) ON BORON DILUTION
Dear Mr. Liparulo:
The Reactor Systems Branch of the U.S. Nuclear Regulatory Commission has provided an SER input to the Standardization Project Directorate on the AP600 boron dibtion concoms that may occur during loss-of coolant accidents and associate recovery procedures (such as resttt of reactor coolant pumps). This GER will be part of the Chapter 15 Final Safety Evaluation Report -
(FSER). The input has an open itein which has been extracted and designated as an FSER Open item in the enclosure to this latter, e
. If you have any questions regarding this matter, you may contact me at (301) 415-1141.
Sincerely, -
criginal signed by:
William C. Huffman, Project Manager Siandardization Project Directorate Division of Reactor Program Management Office of Nuclear Reactor Regulation Docket No.52-003
Enclosure:
As stated cc w/ encl; See next page N
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Mr. Nicholas J. Liparuto Docket No.52-003 Westinghouse Electric Corporation AP600
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Mr. B. A. McIntyre Mr. Russ Bell Advanced Plant Safety & Licensing Senior Project Manager, Programs Westinghouse Electric Corporation Nucleai Energy Institute Energy Systems Business Unit 1776 i Street, NW P.O. Box 355 Suite 300 Pitsburgh, PA 15230 Washington, DC 20006 3706 Ms. Cindy L Haag Ms. Lynn Connor Advanced Plant Safety & Licensing Doc-Search Associates Westinghouse Electric Corporation Post Office Box 34 Energy Systcms Business Unit Cabin John, MD 20818 Box 355 Pittsburgh, PA 15230 Dr. Craig D. Sawyer, Manager Advanced Reactor Programs
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Mr. Sterling Franks GE Nuclear Energy U.S. Department of Energy 175 Curtner Avenue, MC-754 NE-50 San Jose, CA 95125 19901 Germantown Road Germantown, MD 20874 Mr. Robert H. Buchholz GE Nuclear Energy Mr. Frank A. Ross 175 Curtner Avenue, MC-781 U.S. Department of Energy, NE-42 San Jose, CA 95125 Office of LWR Safety and Technology 19901 Germantown Road Barton Z. Cowan, Esq.
Germantown, MD 20874 Eckert Seamans Cherin & Mellott 600 Grant Street 42nd Floor Mr. Charles Thompson, Nuclear Engineer Pittsburgh, PA 15219 AP600 Certification NE-50 Mr. Ed Rodwell, Manager 19901 Germantown Road PWR Design Certification Germantown, MD 20874 Electric Power Research Institute 3412 Hillview Avenue Palo Alto, CA 94303 w
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+c SER CHAPTER 15 OPEN ITEM ASSOCIATED WITH AP600 BORON DILUTION CONCERNS 440.748F -
In the AP600 design, the steam generator functions as a "best source" as the RCS depressurizes rather than a " heat sink" as it does in a conventional PWR. Consequently, the differential temperature across the primary and secondary side of the generator is always such that steam from the reactor will not condense on the tubes. However, in the AP600, the passive residua! heat removal heat exchanger (PRHR) rapidly becomes the dominate RCS heat sink following the generation of an "S" signal during postulated i
SBLOCA events. Consequently, the FRHR may become a potential source for generating a volume of unborated coolant during a small break LOCA (SBLOCA).
j Any condensate generated in the PRHR is delivered to the Loop 1 steam generator outlet plenum during a SBLOCA event. However, since the AP600 has no loop seejs, only a small amount of condensate, approximately 21 ft', can accumulate in the casing before the excess will drain from the steam generator outlets into the loop 1 cold legs, and eventually down the downcomer. Thus, an unborated slug (s) cannot stagnate in the RCS loop cold legs of a AP600 design. The diluted slug from the steam generator outlets must pass through highly borated water from the safety injection CMT tanks and/or the accumulators, before reaching the lower plenum.
Condensate from the PRHR system flows frota the steam generator outlets, through the cold leg into the reactor vessel, directly above the entry of the safety injection line into the vessel. During a LOCA event, an "S" signal actuat.es passive safety injection from the CMT's and/or the accumulators, injecting 240,000 lbm of water at a boron concentration of 3400 ppm, (the minimum Technical Specification value), into the downcomer. Westing-house stated that the relatively low flow rate of condensate down the downcomer and into the core, following the post RCP trip " natural circulation" phase of a AP600 SdLOCA event, enables mixing to occur in the core and the lower plenum. Westinghouse also stated that because there are no loop seats in the AP600 design, no unborated " slugs" of condensate can form in the PRHR loops and thus no unmixed slugs can enter the downcomer, or the core, during LOCA design basis scenarios. However, the applicant's responses to RAI 440.724 regarding the degree of mixing in the down comer and the lower plenum did not provide adequate technicaljustification to satisfy the staff concems about mixing in the downcomer and the lower pienum.
In a telephone conversation with Westinghouse on November 3,19%. fne staff requested Westinghouse to provide additional quantitative or qualitative technical justification on the mixing in the down comer. Westinghouse could attematively consider the possibility of a recriticality (including prompt critical) as the assumed " unmixed" coolant moves through the core, if recriticality occurs, Westinghouse should provide quantitative and qualitative technical analyses of the consequences. Consequently, this issue remains an Open item.
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Enclosure