ML20027B331
| ML20027B331 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood, 05000000 |
| Issue date: | 09/09/1982 |
| From: | Tramm T COMMONWEALTH EDISON CO. |
| To: | Harold Denton, Dentony H Office of Nuclear Reactor Regulation |
| References | |
| 4946N, NUDOCS 8209200287 | |
| Download: ML20027B331 (33) | |
Text
{{#Wiki_filter:s Commonw':cith Edison Q )}, one First National Plata. Chicago. Ilhnois l/ 3 v Address Reply to: Post Othee Box 767 Chicago, Illinois 60690 September 9, 1982 i; i Mr. Harold R. Denton, Director Of fice of' Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission l Wdshington, DC 20555 I j
Subject:
Byron Station Units 1 and 2 ] Braidwood Station Units 1 and 2 Waterhammer Prevention NRC Docke t Nos. 50-454, 50-455, 50-456 and 50-457 Reference (a): July 30, 1982, letter from j B. J. Youngblood to L. O. De lGeo rg e. 1 l (b): August 5, 1982, Memorandum from S. H. Chesnut to B. J. Youngbloo d summarizing July 27, 1982, meeting with Westinghouse. i
Dear Mr. Dentan:
I j This is to provide information requested in reference (a) regarding the design features and procedural controls related to + prevention of waterhammer of the type which apparently occurred at ~the KRSK0 plant in Yugoslavia in July, 1981. i Attachment A to this letter contains our responses to each j of the questions contained in reference (a). Several of the questions requested information pertaining directly to the KRSKO plant. Those responses have been prepared with the assistance of Westinghouse'but, as indicated in the text and in the meeting i discussed in reference (b), there still remains some uncertainty in the circumstances surrounding the v95KO event. It should be noted that in Commonwealth Edison's Motion for Summary Disposition of DAARE/ SAFE Contention 9(a) concerning water hammer events, L. Bowen stated in her affidavit that Edison would be installing temperature sensors per recommendations made by 2-Westinghouse. During the intervening months since filing o f that j. a f fidavit, further research into operating guidance provided for the feedwater bypass systems has been performed which indicates that optimimum operation of this system requires constant feedwater flow through the upper steam generator nozzle. This flow precludes the ~ possibi11'ty of waterhammer such as that which occurred at KRSKO. j Hence, Commonwealth Edison no longer intends to install temperature sensors on the bypass feedwater line. gcf 1 4 8209200287 820909 PDR ADOCK 05000454 A PDR
/* 9 H. R. Denton September 9, 1982 Please direct further questions regarding this matter to this office. One signed original and fif teen copies o f this letter are provided for /qur review. Very truly yours, Yk T. R. Tramm Nuclear Licensing Administrator 1m 4946N
0 Question 1 For the KRSK0 reported event, provide KRSK0 plant and operational information which describes: a) Plant operating state, activities, (i.e. testing) underway at the time of the reported waterhammer event and the operating conditions f or the steam generator (s) f eedwater, auxiliary f eedwater, and the feedwater bypass systems.
Response
As a general comment, the exact time at which the incident occurred is not known. Tne damage was discovered several weeks after the Hot Functional Tests during which it is believed to nave occurred. The conditions at the time of the incident cannot, therefore, be established with confidence. Based on the evidence, it can only be assumed that at some time, the necessary condition for bubble collapse waterhammer did exist. Subseouent to the incident, the KRSK0 plant feedwater system was modified to address the issue of steam generator (SG) preheater tube vibration. The material whicn follows is based on the system as it existed at the time of the water nammer incident. Tne incident is believed to nave occurred during the plant Hot Functional Tests, more specifically, during tne tests of the Auxiliary Feedwater System (AFS) pumps conducted during July 1981. At tnis time, the steam generator pressure could have been as high as 900 psia and the water temperature as high as 532 F. It was during the Hot Functional Tests that a hammer or Dang was neard.
. During the course of the AFS tests, ambient. temperature water was introduced tnrough the auxiliary nozzle. The flow path included a section of 6 inch bypass piping between the junction of the 4 inch AFS pipe and the auxiliary nozzle. Tne main feedwater system was probably not in service when the incident is believed to have occurred. Question 1(b) Details of damage discovered in August, 1981, how and when damage was detected, and wnat evidence (if any) of a water hammer occurring in July, 1981 at the time of stated occurrence.
Response
A flow diagram showing the main and auxiliary f eedwater systems is presented in Figure 1. Isometric sketches of the loop 2 bypass piping inside containment and outside containment, up to the junction of the Auxiliary Feedwater System, are shown in Figure 2 and 3, respectively. I 2
3-The extent of the damage to the loop 2 bypass piping inside and outside containment, is presented in Table 1. In Figure 2, tne numbers in ovals identify tne pipe hangers wnich were affected. On botn Figures 2 and 3, the length in meters of the pipe sections are given. A bulge or blister cas observed in the bypass piping, i n the horizontal section, downstream of tne secondary shield wall, approximately midway between the shield wall and elbow in Figure 2. The bulged region was approximately six to eight inches long, witn the bulge located on tne top side of the pipe. At the bulge, tne pipe diameter was increased approximately one ouarter incn. Tne only indication of any change to loop I was the observation that the bypass line had moved down seven to nine mm at the secondary shield wall penetration. i The loop 2 bypass line and hanger damage was discovered during a final hanger inspection on August 7, 1981. Also at about this time, it was ~ observed tnat the AFS pipe paint back to the AFS pumps was blistered or discolored indicating that hot water and/or steam had at some time been present in the normally ambient temperature system. Tne basis for believing that a water nammer event did occur is the nature j of the damage and because a logical secuence of events can be postulated, i Table 2, wnicn could lead to a waterhammer, i.- .m r
- One point not included in Table 2 is that the control and isolation valves in tne discharge lines of the AFS pumps are normally open and, tnerefore, would not have prevented backleakage. Question 1(c) Plant corrective action (s) taken in terms of redesign, repair, operator instruction or procedures, etc. for avoidance in the future.
Response
Witn respect to KRSK0 plant repair, the section of bypass piping, containing the bulge, between and including the second and third upstream elbows was replaced. Also tne hanger damage was repaired. Also with respect to plant operation, the customer was instructed to maintain tne steam generator water level above the top of the auxiliary feedwater discharge pipe inside the steam generator as much as possible. With tne discharge pipe covered, only hot water and not steam could leak back into the bypass and AFS piping, Lnus greatly reducing the potential for waternammer. To reduce the likelihood of backleakage, the Auxiliary Feedwater System check valves which were known to leak, valves 11005, 11007, 11077 and 11079, on Figure 1 were refurbished. The check valves associated with the turbine driven AFS pump discharge were not inspected as a pyrometer check under hot condition indicated no leakage.
. In tne eventuality that the presence of steam is suspected in the bypass lines of all loops, based on temperature data and water level status and h i s t o ry, there would not be any operable loops with which to shut down tne plant. In this situation, the recommended course of action is to slowly refill one loop at a time with the AFS. An analytical study by the Westingnouse R&D Center shows that for the KRSK0 plant, the safe refilling flow rate is in the range of 15 to 123 gpm per steam generator. To be conservative, we recommend tne 15 gpm valve or as close to this as can be provided. Under normal conditions, between 0 and 100% power some flow is provided continuously tnrougn the auxiliary nozzle, thus effectively preventing the backflow of hot water or steam f rom the steam generator. When the feedwater flow is through the main nozzle, a tempering flow of one to two p e rc e nt is maintained through the auxiliary nozzle. Tne purpose is to maintain the auxiliary nozzle at feedwater temperature thus reducing the 4 induced tnermal stresses when the feedwater is transferred f rom the main to auxiliary nozzles, during plant unloading, for example. However, the l tempering flow also ef fectively prevents backleakage. l l At very low load or hot standby conditions, when the feedwater flow to each steam generaor is minimal, the operator is instructed to supply the feedwater continuously rather than intermittently. The reason is to l minimize the probability of f eedline cracking, but it also eliminates the possibility of backleakage. l 'L-
~ The principal modification was to provide a series of temperature measurements on the bypass piping of each loop in the first vertical leg upstream of the auziliary nozzle. Two strap on RDT's have been installed in each loop, one three meters below the first elbow and a second four meters below the elbow. The RTD's are connected to the plant's DATA-SCAN Temperature Monitoring System which allows for printing out tne temperature values in the control room on reauest. Tne system activates an alarm if the temperature values exceed predetermined setpoints. Recommended operating guidelines were provided to the customer for utilizing tne temperature data during the operations of Plant Heatup and Cooldown and f or Power Operation. Question 2 With respect to plant (KRSK0) operational states information (See Item 1), provide the following information: l (a) Water level in the steam generator relative to the auxiliary feedwater nozzle elevation. l l l
. Response During normal operation, the indicated water level is constant with power level at 488 inches above the tube sheet. At full power, the actual water level is approximately four inches higher-because of velocity head and circulation ratio effects. The differential increases and then decreases with power level; it is 0 incnes at 0 power, approximately six inches at 50 percent power and approximately four inches at 100 percent power. Tne top of the auxiliary nozzle discharge pipe is 473 inches above the tube sheet. The water level is, therefore, nominally 15 to 21 inches above tne top of the auxiliary nozzle discharge pipe, depending on power level. Accounting for normal channel accuracy of 1 five percent of span or i 12 incnes, which is believed to be conservative, the water level could be as close as three inches above the top of the discharge pipe. Question 2(b) Steam generator pressure, temperatures, and flow rates.
Response
At the t.nie wnen the waterhammer event is believed to have occurred, the steam generator pressure could have been as high as 900 psia with a saturation temperature of 532 F.
. Question 2(c) Flow rctes, temperatures and pressures in the feedwater and auxiliary feedwater. sources supplying flow to the steam generator.
Response
It is possible that the waterhammer incident occurred during testing of the AFS pumps. The pumps provide feedwater to the auxiliary nozzles through the four inch diameter AFS piping and a section of six inch diameter bypass piping. The capabilities of the motor-driven and turbine-driven AFS pumps are 350 gpm and 700 gpm, respectively, against a back pressure in the steam generator eouivalent to the set pressure of the lowest set safety valves. The water would be at ambient temperature since the pumps take suction from tne condensate storage tanks. During the testing, the pumps were started and stopped. The intervals between pump stop and restart were up to 30 minutes long. During those intervals, tnere was no forward flow through the auxiliary nozzles. The steam generator water level was not intentionally lowered below the auziliary nozzle dischcrge pipe prior to each test to accept the AFS water. l
Question 3 Provide systems design and piping details for the feedwater, auxiliary feedwater, and the feedwater bypass systems for both the Byron and KRSK0 plants. This information should be in the form of piping drawings, piping senematics or P&ID's which clearly define piping layouts, valves, and elevations relative to the steam generator feedwater and auxiliary feedwater nozzles. Valves whicn are controlled from the control room snould be identified, along with operation reouirements (or procedures) wnich are at the disposal of the operators.
Response
1. KRSK0 Tne schematic diagram, Figure 1, s, sows part of the KRSK0 plant main feedwater system and auxiliary feedwater system. The diagram includes tne feedwater system from the main feedwater neader to the steam generator as it was at the time of the waterhammer incident. It has since been modified to permit feeding simultaneously to the main and auxiliary nozzles during high power operation. As discussed earlier, based on the evidence of blistered and discolored paint, not water and/or steam leaked back f rom the steam generators througn a section of the six inch Sch 80 bypass piping and then into the AFS. The section of loop 2 bypass piping of interest is indicated in Figure 1 as between Points A and B.
l 1 ! ( Two isometric pipe sketches of the bypass piping are presented in Figures + 2 and 3. Figure 2 shows the bypass piping inside containment from the six inch auxiliary nozzle to the containment vessel penetration. Figure i 3 shows the section of bypass piping from the containment penetration back to the point where the four incn Sch 80 AFS pipe connects. } With respect to the postulated backleakage path of hot water and/or i steam, the pertinent valves are all in the AFS. The AFS motor-driven pump discharge lines are each provided with two check valves and with a pneumatically operated, normally open, control valve which is operable l locally and from the control room. i j Tne AFS turbine driven pump, by-means of a cross connect line, connects a j with tne two motor-driven pump discharge lines. The flow path from the turbine driven pump to either SG auxiliary feedline line includes two l cneck valves and a pneumatically operated control valve as is the case f f or the motor-driven pumps. In addition, each leg of the cross connect line is provided with a pneumatically operated isolation valve. The l control and isolation valves are operable locally and f rom the control room. I j 2. Byron and Braidwood (B/B) Figures b and 7 are simplified diagrams of the B/B AF and main f eedwater systems. Detailed P&ID's accompany this response. The { following table lists the valves that are controlled from the 4 control room. ~.. - _ _. _ _,
4 1 l j AF System l' 1. Suction Valves from the Essential AF007A-l&2 Auto open Service Water (ESW) System AF007B-1&2 l 2. AF Steam Generator Flow Control AF005A thru H Auto capability Valves l 3. AF Steam Generator Isolation AF013A thru H Valves l 1 Feedwater System I l 4. Feedwater Reg Valve FWOO6A(B,C&D) Isolation Valve i i a 5. Feedwater Reg Valve FW510(520,530, Auto capability 540) 6. Feedwater Reg Valve FW510A(520A, Auto capability Bypass Valve 530A, 540A) 1 J 7. Feedwater Isolation Valve FW009A (8,C&D) Auto capability j i 4 i b
. Tne remainder of the valves shown in Figures 6 and 7 are either manual valves, automatically operated valves or locally operated valves. Accompaning this response are one line diagrams that show the elevation of the upper feedwater nozzle at the steam generator and its relation to the otner piping. Question 4 Provide steaia generator " internals" design details for botn the Byron and KRSK0 plants. This information should be in the form of drawing which clearly define Feedwater and Auxiliary Feedwater nozzles, penetrations, flow distributors, etc. This information will be used to determine similarities and dif ferences between the proposed Byron Steam Generators and KRSK0 Steam Generators.
Response
- 1. KRSK0 The internals of the KRSK0 Model 04 steam generator are shown in Figures 4 and 5.
Figure 4 for the upper shell shows the auxiliary nozzle and internal extension. Also indicated is the actual water level for 100 percent power. In Table 3, the KRSK0 SG internal elevations and setpoints are tabulated.
. 2. B/B Tne internals of tne B/B models D-4 and D-5 are shown in figures 8 and 9 respectively. Figures 8 and 9 snow the auxiliary nozzle and interal extension and actual water level f or.100 percent power. In Tables 4 and 5 the B/B SG internal elevations and setpoints are tabulated. Question 5 Given the information reouested in Items (3) and (4), summarize similu ities and dif ferences between Byron and KRSK0 feedwater, auxillary feedwater, f eedwater bypass systems, and steam generators. Relate to the Byron plant operational procedures and temperature sensor installation. Response. The bypass line and AF system pipe routings are similiar for the KRSK0 and B/B plants. Tne number of cneck valves in the bypass and AF lines l are essentially the same with the exception of an additional check valve in the B/B piping at the upper steam generator nozzle. Valves in the AF system are normally open in both plant designs. l l
6 4 4 3 4 B/B Operation In contrast to the KRSK0 start up operations the B/B plants utilize a start up feedwater pump for heat up and start up rather than the AF system. Flow is through the feedwater reg. valve bypass valve, the preheater bypass valve and the upper steam generator nozzle. The feedwater reg. valve bypass valves are automatically controlled to maintain steam generator level. Leakage through the feedwater reg. valves is eliminated by closure of the upstream isolation valve.
- Hence, sufficient flow control is present to ensure flow at all times through the upper nozzle.
During hot standby conditions, SG level may be controlled by blowdown. Slug feeding of the steam generators is not recommended. 1 During power operation below a nominal 20% power, flow is directed to the upper nozzle by the preneater bypass valve. There is an automatic switchover to tne lower nozzle at higher power levels. However, flow is maintained through the upper nozzle via the tempering flow line. Therefore, during plant operation flow is present at all times to the upper nozzle. With constant flow the conditions for backleakage, either of steam or hot water, into the AF lines are eliminated. Furthermore since the steam generator level is automatically controlled at all times, i the possibility of uncovering the nozzle in the steam generator is i limits f urther the potential for backleakage of steam minimized which i even in absence of flow.
9 . During operation, on-line testing of valves in the tempering flow and bypass feedwater lines is recommended by Westinghouse. If the testing roouires valve stroking, Westinghouse recommends that the valve not be closed longer than 30 seconds. This recommendation will be followed at By ron/B raidwood. During plant hot functional tests the same precautions apply. There should be flow through the upper nozzle at all times. If flow is interrupted for any reason it will be re-initiated slowly to ensure that, if steam back leakage into the piping has occurred, water hammer will not occur. If flow is stopped and re-initiated for any reason during testing, a physical inspection of piping and hangers will be undertaken to ensure that no damage has occurred due to water hammer. 'T Because of these operating procedures, the temperature sensors recommended by Westinghouse will not be installed on the B/B feedwater piping. These sensors and associated operating procedures will not add any significant margin of protection f rom water hammer due to the procedural reouirements to maintain flow through the upper nozzle at all times. Westinghouse has further recommended that testing be performed to ensure there is no backleakage through AF system check valves. Procedures will be written to accommodate this recommendation.
c i ' Byron plant operating ano startup procedures have not been written as yet. Tne recommendations discussed above will be incorporated when these tests are written. Question 6 Provide the test plan which would be used by Byron plant for verification tnat steam generator waterhammer will not occur (as noted in the staff's Byron SER Evaluation, Section 10.4.7). Describe now this test plan will demonstrate that tne Byron plant will not experience a waterhammer such as reported at KRSKO.
Response
First of a kind steam generator waterhammer tests have been conducted for the Model 04 Steam Generator at tne KRSK0 plant. The tests verified that if the feedwater bypass system is operated in accordance with the Westinghouse recommended guidelines, SG waterhammer will not occur. Startup test procedures for Byron /Braidwood will be written in accordance with the guidance provided in NUREG/CR 1606. These tests, when written, will De available for NRC reuseur, Tne KRSK0 test plan did not include testing related to steam backleakage into tne bypass line and AFS. Any test for this purpose would be directed toward verifying that necessary steps have been taken to prevent steam backleakage.
. As stated above, preoperational tests will be written to ensure that there is no backleakage through AF system check valves as recommended by Westinghouse. .Meviences 1. Docket Numbers STN 50-454, 50-455, 50-456, and 50-457, s u bj ec t : Additional Information on Byron /Braidwood Waterhammer Prevention, B.J. Youngblood, Chief, Licensing Branch No. 1, Division of Licensing, NRC, July 30, 1982. 2. " Operating Procedures for Counter-Flow Preheat Steam Generator with Main Feedwater Bypass System", May, 1981, Westinghouse Electric Corporation. LAB /nn/2052b*
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n 6 & L - _. RESULTS Of EXAMINATION t J o LOOP 7,INSIDECONTAINNENT 0 EMBEDMENTPLATESNOVED,80LTgLOOSENED,PIPECLAMPS1.00SENED
- ANDN0yED, (AffECTED HANGERS ARE.[DENTIFIED BY NUNBERS IN OVALS),
o PJPE CilANGED LOCATION O PIPE BULGED 1/4 INCH NEAR SECONDARY. SHIELD WALL. o LOOP 2, INTERMEDIATE BulLDING o PIPE HOVEMENT WAS NEGLIGlBLE o AUXII.lARY PIPE PAINT WAS BLISTERED BACK TO Tile HDAFS PullPS o HDAFS PUMP CilECK VALVES WERE FOUND TO HAVE SOME NICKS, SCRATCllES, AND UNEVENNESS. I e 6 L-
54/e3 SEQUENCE'0F EVENTS 1 TilESTEANGENERATORWATERLEVEl-WASEELOWTHEAUXIllARYN0ZZLEINTERNAL 0 EXTENSION DISCHARGE AND THERE WAS No FEEpWATER Fl_0W TilR00611 Tile N0ZZLE. (DURINGTHEHOTpuNCT10NALTESTINGpfTHEAFS,TilEPutlPSWERESTOPPEDAND RESTARTED).. STEAM LEAKED BACK THROUGH A SECTioN OF Tile BYPASS LINE AND TilEN o THE AFS PIPING TO Tile HDAFS PUNPS,8 (Tile EXTENT OF Tile BACK LEAKAGE WAS INDICATED EY BLISTERED PAINT ON THE AFS PIPINC), o FOR THE LEAKAGE TO HAVE OCCURRED, THE TWO CilECK VALVES IN EACH OF Tile HDAFS PUNP DIScilARGE (_INES MUST HAVE BEEN l-EAKING, 1 l
T k d &.. K f C m A n _ d_.) SEQUENCE OF EVENTS (CONTINUED) i 0 WITH STEAN PRESENT IN THE SYPASS U NE PIPING, Tile AFS PUMPS WERE STARTEp, BRINGING IN COLD WATER. TliE WATER BAPiplY CONDENSED Tile STEAN RESULTING IN WATERilAMNER, o AS AN0 tiler POSSIBILITY, STEAM LEAKEp DACK TiiR00Gli A fl0RIZONTAL SECTioN OF BYPASS LINE OVER COLD WATER ALRE44Y PRESENT. AT SOME POINT A STEAM BUBBLE WAS FORMED DUE TO A SURFACE DISTURPANCE AND Tile BUBBLE CONDENSEI) BY COLD WATER CARRIED UP FROM THE E0 TION Ql Tile PIPE, o A DANQ OR liAMMER WAS llEARD DUR!NG THE il0T FUNCTIONAL TESTING. s [
1 TABLE 3 l l . KRSK0 D4 STEAM GENERATOR ELEVATION AND LEVEL DATA s ELEVATION FROM TOP PERCENT OF 0F TUBE SHEET NARROW RANGE Upper NR Tap 566" 100% Main Deck Plate Top of Swirl Vane 54P." Hi-Hi Level 530" Actual Water Level (/co % FoweM 492" 68% Indicated Water Level 488" 66% Mid Deck Plate Top of Auxiliary Nozzle Discharge 473" 60% \\ Lo Level 464" Lo-Lo Level 420" Lower Deckplate Topr of Tuba BundTe 336" Ya A Lower NR Top 333" 0% Main Feedwater Nozzle d__
.d ': l TABLE 4 l BYRON 1 D4 STEAM GENERATOR ELEVATION AND LEVEL DATA ELEVATION FROM TOP PERCENT OF 0F TUBE SHEET NARROW RANGE 5p. Upper NR Top 566" 100% Main Deck Mate Top of Swirl Vane 542" Hi-Hi Level 530" 85% ~ Actual Water LeveI_6e__3 @fM 492." 68% Indicated. Water Level. 488" 66% Mid Deck Plate Top of Auxiliary Nozzla Discharge 473" 60% Lo Level 440' 46% Lo-Lo Level 420" 37% Lower Deckplate Top of Tube Bundle 336" ZSa Lower NR Top 333" 0% MainFeedwaterNozzlEG _ _ -}}