ML20032B303

Submits 60-day Response to Request for Info Re Reactor Vessel Pressurized Thermal Shock

ML20032B303
Person / Time
Site:	Maine Yankee
Issue date:	11/02/1981
From:	Randazza J Maine Yankee
To:	Clark R Office of Nuclear Reactor Regulation
References
FMY-81-163, NUDOCS 8111050413
Download: ML20032B303 (6)

Text

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2.C.2.1 EDISON DRIVE MAME&SHAllHEE nmmICP0lVERCOMPARU*

Avausra, warne oa33s

[ gh (207) 623-3521

9 November 2, 1981 p3 h'~

FMY-81-163 M

N liiited States Nuclear Regulatory Commission VO 473g Washington, D.C.

20555 W

g ea, Attention: Office of Nuclear Reactor Regulation Division of Licensing b'

Operating Reactors Branch #3 67

_Q us Mr. Robert A. Clark, Chief

References:

(a) License No. OPR-36 (Docket No. 50-309)

(b) USNRC Letter to MYAPCo. dated August 21, 1981 (c) MYAPCo. Letter to USNRC dated September 29, 1981, (FMY-81-148)

(d) USNRC Letter to MYAPCo. dated September 17, 1981

Subject:

Reactor Vessel Pressurized Thermal Shock

Dear Sir:

In Reference (b), the NRC requested two sets of information concerning pressurized thermal shock, one to be submitted in 60 days and one in 150 days. The former set was requested pursuant to 10CFR50.54(f) and is responded to in this letter.

As discussed in Reference (c) more detailed information is being developed.

Provided below is the recuested information:

Question 1: Provide the RTNDT values of critical welds and plates in your vessel for (a)

Initial as-built conditions and location (e.g.1/4 T), and (b) current conditions (include fluence level) at the RPV inside carbon steel surface.

Response 1:

(a) The response to cuestion 4.16 to the Maine Yankee FSAR provided drop weight NDT data for all materials in the Maine Yankee reactor vessel.

The intermediate and lower shell courses were established as being in the belt line region.

The plate with the highest NDT was intermediate shell plate D-8406-1, with an NDT of -100F.

This plate was used in the irradiation surveillance program.

Base metal and HAZ 3

specimens were taken from the 1/4 T location.

8111050413 811102 PDR ADOCK 05000309 h'

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MAINE YANKEE ATOMIC POWER COMPANY l.

Uhites States Nuclear Regulatory Commission November 2, 1981 Attention:

Mr. Robert A. Clark, Chief Page 2 Following plant startup, a complete set of NDT, Charpy "V" notch (CVN) and mils lateral expansion (MLE) tests were conducted. These tests established an RTNDT of -100F for the basemetal and weld HAZ and -300F for the weld metal.

(b) As of Septenber 30, 1981, the Maine Yankee plant has operated for almost nine calendar years.and has genereted 43 x 106 MWH (electric).

i The corresponding vessel ID fluence 5.4 x 1018 n/cm2, Based on data from surveillance specimen tests and test reactor irradiation tests, the maximum adjusted 50 ft-lb RTNDT at the vessel ID is about 1800F.

Question 2: At what rate is RTNDT increasing for these weld and plate materials?

Response 2: Since the weld metal is governing, we will restrict our discussion to that material. The RTNDT shift is proportional to the fluence, which in turn is related to power generation.

The current maximum RTNDT is 180aF; it-will take 26 more calendar years to reach an RTNDT of 3000F.

At end of life (35 o

calendar years) the RTNDT will be approximately 2950F.

Question 3: What value of RTNDT for the critical welds and plate material do you consider appropriate as a limit for continued operation?

Response 3: Paine Yankee does not consider it appropriate to define a single value of RTNDT as an upper limit for continued aperation. The capability of the vessel to withstand the effects of pressurized thermal shock cannot be represented independent of the pressure-temperature transient which might cause the pressurized thermal shock condition. The program Maine Yankee is working on considers the many variables involved in establishing the vessel capabilities to withstand system transient responses, without violation of the pressure boundary.

Question 4: What is the basis for your proposed limit?

Response 4: See Question 3.

/

MAINC YANKEE ATOMIC POWER COMPANY Unites States Nuclear Regulatory Commission November 2,1981 Attention:

Mr. Robert A. Clark, Chief Page 3 Question 5: Provide a listing of operator actions which are required for your plant to prevent pressurized thermal shock and to ensure vessel integrity.

Include a description af the circumstances in which these operator actions are required to be taken.

Included in this summary should be the specj fic pressure, temperature and level values for: a) high pressure injection (HPI) termination criteria presently used at your facility, b) HPI throttling criteria and instruction presently used as your facility and c) criteria for throttling feedwater presently used at your facility. For each recuired operator action, give the information available to the operator and the time available for his decision and the required action.

State how each required operator action is incorporated in plant operating procedures and in training and recualification training programs.

Response 5: Pressurized thermal shock does not occur during most operational transient events either because the cooldown associated with the event will not cause unacceptable thermal gradients or repressurization is limited by reactor coolant system leakage.

However, with increased vessel wall irradiation a point could be reached where a very severe cooldown transient followed by repressurization to the HPSI shutoff head could result in a challenge to vessel integrity. Examples of such severe cooling transients include a major steam line break upstream of the steam excess flow check valves and a reactor scraq without a turhine trip coupled with a failure of the excess flow check valves. The cooling of the vessel wall as a consecuence of such transients will depend upon the amount and temperature of HPSI water injected, the injection rate and reactor coolant recirculation rate during the injection.

Such repressurization following such a severe cooldown transient could only occur if the HPSI system was not operated in accordance with approved procedures.

Notwithstanding the ve ry small probability of a pressurized thermal shock challenge to the reactor vessel at this time, Maine Yankee has adopted very conservative procedures to address this concern.

The operators are instructed, within the applicable procedures to limit reactor repressurization following a subcooling transient that results in HPSI operation.

The repressurization is controlled by stopping one of the two operating HPSI pumps and/or throttling flow.

The attached " Reactor Coolant System MPT Curve Core 6" includes a dotted lino labeled "500 F min delta T PZR to Loops (Admin)". Operators are instructed to n'datain system temperature and pressure according to this reintionship.

Maine Yankee HPSI termination criteria presently used are:

4 AINE YANKEE ATOMHO POWEd COMPANY Unites States Nuclear Regulatory Commission -

November.2, 1981

-Attention:

Mr. Robert A. Clark, Chief Page 4 oReactor coolant subcooling of 50oF or more.

oPressurizer level of 50% or more.

oSteam generator level of 50% or more.in at least one generator.

All three criteria must be achieved prior to HPSI termination.

Feedwater system operating instructions do not involve explicit criteria for throttlino because isolation where needed is provided by redundant safety grade logic and components, although cautions against causing excessive cooldown by overfeeding are presented.

Operator training programs include instruction modules on emergency procedures. HPSI termination criteria are discussed in classroom lectures and discussions.

In addition, termination criteria are addressed by control board walkthroughs in which trainees are familiarized with controls for initiating, operating, and terminating HPSI, reset of safety injection actuation logic and controls,' and with instrumentation relied upon in these activities.

It must be pointed out that the NRC staff has insisted on industry adoption of two practices which in our view are undesirable and exacerbate the pressurized thermal shock problem. The first involves operation of the high pressure safety injection system such that overpressurization concerns are relegated to secondary. level. The second involves operation of reactor coolant pumps.

The staff has stressed keeping the high pressure safety injection system on for a minimum of twenty minutes.

In at least one instance this has resulted in operators watching while a reactor coolant system was taken solid and water was passed through the plant's pressure relief valves.

The staff has also ir..'sted that reactor coolant pumps be shut off in the event of an accident and has gone so far as to suggest this action should be automated.

This action of course negates the very desirable reduction in thermal shock to the vessel afforded by the mixing action of the pumps.

In each case, suboptimum operating practices have been institutionalized.

In one case the reason was excessive concern for operator error (premature shutdown of ECCS).

In another, it is concern about an extremely unlikely event (accelerated mass depletion in LOCA due to pump operation followed by an exquisitely timed loss of AC or operator error shutting down the reactor coolant pumps causing two phase liquid level collapse and a small increment in clad heatup).

i'

MAINE YANKEE ATOMIC POWER COMPAP$Y lhites States NJelear Regulatory Commission Noverter 2,1981 Attention:

Mr. Robert A. Clark, Chief.

Page 5 Maine Yankee has t 3 sed the adoption cf operating practices proposed by the sti..' absent a-determination that such practices are truly for the best, all things considered. We hope the staff will reconsider its position regarding HPSI and reactor coolant purrp operation in this light as well.

We trust this information meets your needs.

Very truly yours, MAINE YAWEE ATOMIC POWER COMPANY w

John B. Randazza, Vice President Manager of Operations JBR/plb STATE CF MAINE

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)ss Coll'rt 0F KENNEBEC)

Then personally appeared before me, John B. Randazza, who, being duly sworn, did state that he is a Vice President of Maine Yankee Atomic Power Company, that he is duly authorized to execute and file the foregoing request in the name and on the behalf of Maine Yankee Atomic Power Company, and that the statements therein are true to the best of.his knowledge and belief.

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