L-75-246, Responding to Letter of 5/15/75, Letter Furnishing Analyses & Other Relevant Info to Determine Potential for Occurrence & Potential Consequences of Secondary System Fluid Flow Instability (Water Hammer)

Responding to Letter of 5/15/75, Letter Furnishing Analyses & Other Relevant Info to Determine Potential for Occurrence & Potential Consequences of Secondary System Fluid Flow Instability (Water Hammer)

ML18227B971
Person / Time
Site:	Turkey Point
Issue date:	07/03/1975
From:	Robert E. Uhrig Florida Power & Light Co
To:	Lear G Office of Nuclear Reactor Regulation
References
L-75-246
Download: ML18227B971 (24)

Text

t NRC DISTRIBUTION FOR PART 50 DOCKET MATERIAL (TEMPORARY FORM) t CONTROL NO:

FILE FROM. Florida Power & Light Co. DATE OF DOC DATE REC'D LTR TWX RPT OTHER Miami, Fla.

Robert.E Uhri TO: ORIG CC OTHER SENTNRC PDR George Lear 3 Signed SENT LOCAL PDR

'LASS UNCLASS PROP INFO INPUT NO CYS REC'D DOCKET NO:

0-250 251 DESCRIPTION ENCLOSURES:

Ltr. Re our ltr. of 5-15-75, analyses 6 other relevant info needed to determine the potenti for occurrence 6 potential consequences of secondary system fluid flow instability (wate

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" 0>@N .'-, r.-75-2<6 CO Mr. George Lear, Chief ~~'~I I

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Operating Reactors Branch No. 3 Division of Reactor Licensing Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Washington, D. C. 20555

Dear Mr. Lear:

TURKEY POINT NIT S. 3 AND 4 DOCKET NOS. 50-250 AND 50-251 WATER HAMMER I PWR F DWATER SYSTEMS Your letter of May 15, 1975, requested that we provide you with analyses and other relevant information needed to determine the potential for occurrence and the potential consequences of secondary system fluid flow instability ("water hammer") in our nuclear plants. The following input, numbered to correspond to your questions, is provided.

l.'t is expected that the following operating occurrences could cause the level of the water/steam interface in the steam generator to drop below the feedwater inlet nozzle:

A. Loss of Normal Feedwater (FSAR Section 14.1.11).

Loss of normal feedwater can be caused by a pipe break, pump failures, valve malfunctions, or loss of outside ac power. Steam generator water level will fall as steam flow continues to dissipate stored and generated heat from the reactor coolant. When the level has fallen to the low-low level point, a reactor trip will occur (if it has not already been manually initiated) and three turbine-driven auxiliary feedwater pumps will start automatically. The transient will produce a lower water level if the initial water level is low, the initial reactor power is high, or the auxiliary feed rate is low.

B. Reactor Trip (FSAR Table 7.2-1 lists various trips)

A rapid down power transient results in a collapse of secondary side steam voids and an initial decrease in steam generator level ("shrink"). If the initial water level is low and reactor power is high, the transient may result in uncovering the feedwater inlet nozzle.

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Mr. George Lear, Chief Page Two July 3, 1975

2. Feedwater piping isometric diagrams are contained in an attachment to this letter.

3. There have been three known occurrences of "water hammer" in feedwater piping at Turkey Point Unit Nos. 3 and 4.

They were reported to the Director Region II, Directorate of Regulatory Operations. on February 6, 1974, in a letter response to Regulatory Operations Information Request No.

74-1.

The first occurrence was indicated by body-to-bonnet leakage and slight elongation of some body-to-bonnet bolts on the 3B feedwater check valve.

The second occurrence was indicated by the same kind of leakage and bolt elongation on the 4B feedwater check valve.

The third occurrence was indicated by the following deformations discovered inside containment during an outage of Unit No. 4. They occurred on the "A" steam generator feedwater piping.

A. Some expansion bolts for two hydraulic pipe restraints had been pulled approximately one inch out of a concrete wall.

B. Two spring hanger mounting plates had been deformed.

C. There had been slight plastic deformation in a located in the piping leading to the steam 90'lbow generator feedwater nozzle.

The corrective action was to minimize the length of feedwater piping which could drain into the steam generator through the feed ring. This was done by shortening the horizontal length of pipe at the level of the feedwater inlet nozzle. The piping now turns'own through a approximately five feet from the steam generator.

90'lbow This modification was performed on the "A" and "B" steam generator feedwater piping for both units. The feedwater piping for the "C" steam generators was not modified because the eight foot horizontal length at the nozzle elevation was evaluated and found to be acceptable. There has been no evidence of water hammer occurrences in steam generator feedwater piping since completion of the modifications.

Mr. George Lear, Chief Page Three July 3, 1975

4. Analyses of the feedwater and auxiliary feedwater piping using dynamic forcing functions have not been performed.

Based on our operating experience following modifications to feedwater piping (described in section 3 of this letter), we feel functions.

it may not be necessary to develop forcing These modifications have been satisfactory in that there has been no further water hammer damage since their completion.

A test program designed to study the "water hammer" phenomena has not been developed specifically for Turkey Point. However, because several of their operating plants have experienced water hammer, our NSSS vendor has initiated a research effort to study the causes of this problem and determine possible remedies.

following conclusions reached so far are considered The applicable to our Turkey Point nuclear plant:

A. Water hammer can be generated by partial drainage of the feedwater line through the feed ring when steam generator water level drops below the ring. Steam enters through the ring and is condensed at the inter-face with the cold water being supplied by the auxiliary feedwater system. This generates steam flow in a direction opposite the incoming feedwater flow. Ifita water seal is then formed anywhere in the piping, could trap a steam bubble which could then rapidly collapse and force a water slug down the feed line.

B. The potential water hammer force is greater if a greater horizontal length of feedwater piping is available for acceleration of the water slug.

C. =-

Water slug formation may not occur at reduced feedwater flow rates.

D. Water slugs are most likely formed when the feed ring has been partially refilled.

In order to avoid duplication of the NSSS vendor effort, we have not initiated an independent water hammer test.

program at Turkey Point. Also, the plant modifications described in sections 3 and 6 of this letter have apparently eliminated or at least reduced the effects of water hammer because no new indications of water hammer damage have been observed since the modifications were made.

I' Mr. George Lear, Chief Page Four July 3, 1975

5. In the event of a design basis loss-of-coolant accident with concurrent turbine trip and loss of offsite power, the following is expected to occur (FSAR Sections 14.1.12 and 14.3.1):

The reactor trips on low pressurizer level. Safety injection is initiated automatically on low pressurizer level in conjunction with low pressurizer pressure. (In this case, reactor trip and initiation of safety injection can also be caused by high contain-ment pressure.) Complete loss of off-site power and turbine trip cause a loss of power to auxiliaries.

Both emergency diesels start on initiation of the safety injection signal and automatically supply power to emergency loads. Core heat is transferred to the primary coolant and then to the secondary system in the steam generators. Steam system pressure subsequently increases and the steam system power relief valves are automatically opened to the atmosphere. If steam flow through the power relief valves is not sufficient, the steam generator self-actuated safety valves may tem-porarily lift to augment steam flow. Normal feedwater flow is lost upon initiation of the accident, but steam driven auxiliary feed pumps automatically start to supply makeup feedwater to the steam generators. (Also, a safety injection signal will stop normal feedwater flow and start auxiliary flow.) Steam generator water level will decrease rapidly due to a reduction of steam generator void fraction on the secondary side and also because steam flow continues after normal feedwater flow stops. A delay in establishing full auxiliary feedwater flow could result in uncovering of the steam generator feedwater inlet nozzle. The control room reactor operator monitors steam generator water level and controls auxiliary feedwater flow with remotely operated control valves.

6. Modifications to the feedwater piping to steam generators "A" and "B" have been made on Turkey Point Unit Nos. 3 and 4.

These modifications were based on NSSS vendor recommendations.

In all cases, the modification was to reduce the horizontal run of feedwater piping entering the steam generator at the level of the feedwater inlet nozzle. Such a piping change minimizes the effect of water hammer by reducing the length of feedwater piping that can drain into the steam generator.

Subsequent vendor research has confirmed that the probability of damage from water hammer can be kept to a minimum by elimination of long horizontal pipe runs at the level of the steam generator feedwater inlet nozzle.

0 4 Mr. George Lear, Chief Page Five July 3, 1975 NSSS vendor research has also resulted in a recommendation to limit auxiliary feed flow rate to 150 gpm during recovery of a drained feedwater inlet nozzle under other than emergency conditions. This is a conservative limit to minimize the probability of water hammer.

Auxiliary feed flow is manually controlled at Turkey Point with the operator routinely limiting flow rate to the minimum necessary to meet operating requirements. Specific administrative means or other more positive means such as interlocks have not been incorporated into plant design or procedure. We feel that such means have not been definitely determined as necessary for our nuclear plants. However, we shall be reviewing subsequent results from NSSS vendor test programs and, if necessary, shall reevaluate our position based on those results.

Very t uly yours, Robert E. Uhrig Vice President REU/MAS/cpc Attachment cc: Jack R. Newman, Esquire

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