ML20151C040

Responds to NRC Re Reactor Vessel Supports.Encl Info Demonstrates That Ample Margin Exists in Support to Perform Satisfactorily Under All Design Conditions

ML20151C040
Person / Time
Site:	Yankee Rowe
Issue date:	07/08/1988
From:	Papanic G YANKEE ATOMIC ELECTRIC CO.
To:	Fairtile M NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM), Office of Nuclear Reactor Regulation
References
FYR-88-096, FYR-88-96, NUDOCS 8807210206
Download: ML20151C040 (8)

Text

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~ TeMpMne (617) S72-8100 TWX 7103807619 YANKEE ATOMIC ELECTRIC COMPANY e- -,

5a 1671 Worcester Road. Framingham, Massachusetts 01701

'YAN KEE - July 8, 1988 FYR 88-096 United States Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 Attention: Mr. Morton B. Fairtile, Project Manager Project Directorate I-3 Division of Reactor Projects I/II

References:

(a) License No. DPR-3 (Docket No. 50-29) '

(b) Letter, NYR 88-86, NRC to YAEC, dated May 9, 1988 (c) Letter, NYR 87-140, NRC to YAEC, dated July 16, 1987

Subject:

Reactor Vessel Supports

Dear Sir:

In your letter of May 9, 1988, Yankee Atomic Electric Company was requested to provide information concerning the Yankee Nuclear Power Station (YNPS) reactor vessel supports. The attachment to this letter provides our response.

We understand that the NRC is researching the issue of potential reactor vessel support embrittlement on a generic basis. At an ACRS subcommittee.

meeting held on February 17, 1988 the NRC indicated that they would begin screening nuclear plants to determine if any are susceptable to this concern.

During that same meeting a representative of NRR stated that in order for embrittlement to affect the safe operation of a plant, three conditions must be present:

o Susceptable Materials Such as Those Which Have an Initial Flaw o Irradiation Environment o Tensile Stress None of these parameters are significant for the YNPS reactor vessel supports. Furthermore, the attached information demonstrates that ample margin exists in our supports to perform satisfactorily under all design conditions.

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s Unitsd States Nucisar Rsguistory Commission July 8, 1988 Attention: Mr. Morton B. Fairtile, Project Manager _Page 2 FYR 88-096 We also wish to point out that the YNPS react- vessel is not supported by the neutron shield tank but instead by a highly redundant steel ring located above the top of the core.

The attached information, we believe, is sufficient to resolve this issue for YNPS. A summary of this material will be included in the July 1989 update of the FSAR, Section 232. Should you have any questions, please contact,us.

n' Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY G. Pap ic, Jr.

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Senior Project Engineer Licensing Yankee Project GP/25.694 Attachment ec: USNRC Region I USNRC Resident Inspector, YNPS

. ATTACINENT Yankee Nuclear Power Station Reactor, Vessel Support Response to Request for Additional Information i

Ques. tion (a)

Provide a sketch / isometric of the reactor vessel supports.

Response

The Reactor Pressure Vessel (RPV) and its support system are illustrated on three sketches which are enclosed herewith. The RPV is supported by 28 equally spaced lugs which are shown on FSAR Figure 101-2. Each is welded to the outside of the vessel upper shell section. The lugs are made of,SA-212 Grade B material. They are seated on a 3.75 inch by 6 inch continuous steel ring or pillow block. Centered at the lug / block interface, a 4 inch diameter hole was drilled to receive a pin of the same size. The pins are AISI A9310, case hardened. Each of these pins is thus positioned horizontally and aligned radially with the RPV. This design allows thermal growth of the RPV but prevents undesirable translational movement.

The 3.75 inch x 6 inch ring is an integral part of the steel Reactor Support Ring (RSR). The RSR is seated on reinforced concrete which is part of the 5 foot thick biological shield. This entire arrangement is shown in Figure 1.

The RSR is a circular structure consisting entirely of 1.25 inch thick sclid steel plates welded together. The plate material is AS"X A201, Grade B, fire box quality. The inside and outside radii are 68.25 and 113 inches, respectively. It is 24 inches high. Details of the RSR are presented in Figure 2.

The principle elements of the RSR are an upper circular plate, or flange, which is 42.875 inches wide and a bottom flange, also circular, which is j 24 inches wide,. Separating these flanges are 28 main vertical stiffeners, or i webs, and 56 additional intermediate stiffeners, each aligned radially from I the RPV center. There are also two vertical circumferential stiffeners. l The Neutron Shield Tank (NST) hangs f reely f rom the RSR. It is a water-filled hollow cylinder which surrounds the RPV on its side and bottom. It provides biological shielding and inhibits neutron activation of plant components 1 inside containment. {

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. ATTACHMENT

-(Continued)

Yankee Nuclear Power Station Reactor Vessel Support Response to_ Request for Additional Information Question (b)

Identify the support designer. ,

Response

.; The RSR and NST were designed and fabricated as a single, integraliunit. It was designed by Stone and Webster Engineering Corporation using performance criteria specified by llestinghouse. Fabrication was performed by The Portland Company to the requirements of ASME,Section VIII, 1956. .The Stone and Webster Fabrication Specification, YS-844, required that all welds were to be completely radiographed wherever possible. Welds which could not be radiographed were to be inspected by the magnetic particle method. The completed tank and ring assembly were inspected and pressure tested.

Question (c)

Report the calculated maximum support design stresses and their location relative to the core beltline.

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Response

The original design of the RSR and NST did not include dynamic loads resulting from an earthquake. This issue was addressed through SEP Topic III-6. Seismic Design Considerations. The NRC developed a site-sper.ific spectrum for YNPS, and required that it be used for all seismic analyses.

4 In 1987, Yankee performed a finite element analysis of the RSR using the following sources of loads:

l o Dead weight of the RPV and intervals. i o Dead weight of the NST/RSR and water. l i

o Contributory weight of the main coolant loops. I o RPV nozzle reactions.

o NRC site-specific spectrum seismic inertia.

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, ATTACHMENT (Continued)

Yankee Nuclear Power Station Reactor Vessel Support Response to Request for Additional Information These loads were combined by absolute summation. LOCA loads were not ineladed because Yankee has successfully demonstrated leak-before-break detection capability. T is issue was resolved through SEP Topic III-5.A (Reference (c)).

For the RSE, the maximum bending plus membrane stress is 21.5 kai which occurs in an innermost element of the upper flange. The maximum stress in the RSR, without seismic loads, is 10.9 ksi. The minimum specified yield strength of the plate material is 32.0 ksi.

The upper flange of the RSR is approximately 8 inches above the top of the reactor core and 53.8 inches above the core centerline. The plaae of maximum neutron flux lies below the core centerline.- <

Qoestion (d)

Indicate whether maximum design stresses are low enough to preclude brittle fracture. Provide details.

Response

, The RSR is a highly redundant structure fabricated of ductile materials. It behaves fundamentally as a compression ring, even under seismic loading.

Primary stresses under all loads are bending and shear. No element within the RSR is subjected to pure tensile stress which would be more susceptable to flaw propagation.

Vertical loads, including seismic, are equally distributed to the RSR by the 28 RPV lugs. Horizontal seismic loada are approximately equally distributed with not more than a !1 0 percent variation around the ring.

l The comprehensive non-destructive examination performed after fabrication i precluded the existence of significant initial flaws when the RSR/NST was  !

placed into service. This is further substantiated by 28 years of I satisfactory performance. '

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