ML20028C411
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{{#Wiki_filter:- nosTON EDISON COMPANY. asas 6 omes. eco sov6.vn= store, s o.ro. w....c u'em. o zi o, I s. March 30, 1978
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u.n..e. auc6c......tio=. on...,-c BECo. Ler. #78-53 s s Mr. Boyce H. Crier, Director Office of Enforcement and Inspection Region I U. S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406 License No. DPR-35 Docket No. 50-293 Response to IE Bulletin No. 78-03, [ " Potential Explosive Gas Mixture Accumulations Associated with BWR Offgas System Operations" x \\ Dear Sirt The subject bulletin required the Boscon Edison Company to review the operatior and maintenance of the offgan system at Pilgrim Nuclear Power Station, Unit #11 The results of this review, which included a review of the operating, emergency 1 and daintenance procedures as well as a review of the adequacy,of the ventila-tion of spaces and areas through which offgas system piping containing explosive mixtures of gases pass, are hereby forwarded as an attachment to this letter. Very truly yours, MY ~ ~. A Attachment ' cc: U. S. Nuclear Regulatory Commission Office of-Inspectior. and Enforcement Division of Reactor Operations Inspection Washington. DC 20555 g B30110Q079 021207 PMt FOI A HIATT82-545 PDR
f db, n. ATTACHMENT s RESPONSE TO IE BULLETIN 78-03 Ite= 1 Review the operations and maintenance procedures related to the offgas system to assure proper operation in accordance with all design para-meters. Include in this review measures you have taken or will take to prevent inadvertent actions (such as are strikes) which might cause ignition of the mixture of gases contained in the offgas piping. Response Our review of the operating procedures related to the offgas system has shown that the design parameters are identified-along with what. corrective action is required in the event of a deviation from those parameters. In order to prevent inadvertent actions which might cause ignition of the mixture.of gases contained in the offgas piping, maintenance is cont-olled by written instructions or tempo-rary procedures which are identified on a Maintenance Request Form (11RF). The MRF also delineates all of the other concerns or dan-gers as identified by the Watch Engineer. Further, a welding and cutting " permit" is required when either of the operations are to occur. At that time, the area is inspected and freed of-any poten-tial hazards. Finally, in order to remove potentially explosive gases from the Augmented Off Gas System, Pilgrim Nuclear Power Sta-tion procedures require that an air purge be established whenever the system is not in service and prior to performing maintenance on the system. Item 2 Review the adequacy of the ventilation of spaces and areas through which offgas system piping containing explosive mixtures of gases pass. The review should consider ventilation losses and off-normal offgas system operation, sach as lack of dilution steam, lost loop seals, blown rupture disks, bypassing recombiners, and leakage of of f-gas into isolated portions of systems. Response The most likely causes of leakage of an explosive mixture of gases are lost loop seals and/or blovn rupture disks upstream of the steam jet compressors which provide dilution steam (and the system driving force) to maintain the hydrogen concentration below the flammable limit. The intercondenser and aftercondenser drains (loop-sealed) are piped back to the main condenser. Thus, a loss of either of these loop seals presents no hazard. A leaking or blown rupture disk would allow gas leakage into the condenser compartment of the Turbine Building. However, the Turbine Building ventilation system has a sufficient number of redundant fans to ensure that adequate ventilation is provided at all times, thus, eliminating the poten-tial for a buildup of explosive gases. Lack of dilution steam can be detected (a) by observingout abnormal temperature rise across the reco=biners or (b) with installed flow monitoring instrumentation which features a low flow alar = in the main control room and auto-matic system isolation. The isolation would lead to a loss of condenser vacuum and a reactor shutdown if the system was not returned t; service immediately. Page 1 of 3 m
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- The Augmented Off-Gas System-1's provided?with rwo (2) 100% capacity
[ recombiners with nogrovi.aions for bypassing eityer one. 4 . ?. /, ,F . ( 7- . ;s j The loop seals, in,-tha Apgmented Off Gas Buildin' g 'are pipe.d;<tolth'e j 2- ' 7'N c1han vaste sumps which are ventedato the builddng exhairst' ~ ducts 7 sE Althougli thilo'ap seafs Iin this building woyM nots nbrally conk ' s tain"combdstible' gases, the.b,uilding is}eqaipped with,two (2)~f'll" u capacity fans wiiidhle,xhaust to'3 ejedndense( compdrtmegt. If 4}ie h normally #perating fan fails, a flev's.sitchnausomatijally starts? y the s dndby f an and' preiides an ala.rm in' the 'n.in, control, ro'om. f e v Thus,fa# o '? 1oss;ofthest.loopsealspresentsno,qombusglblegas s a .y S,, ~ ~ 'hnzard. '4 .s C, ss y g+ ~- g .~- g Afthough *.he pipin p ould ndtinorm ily-contain combustible gdses, a blown lb5p. seal in th atuk Deuld present no prgblerjas ' the off- ~ gas would' not ab:umulate 19 a space'where a spark would beigenerated (inside the.stach pheL 7' i -n ts s N ~ Leakage. ct?of fgah i'nte isolated portioss; of sysiems' has cc been a pr' M em :at Pilgrim ynit.#1. Linterconne;: ting. systems are[ppovided o
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- ene'isolationjvalveOhichit, normally closed; These systems are nomally ata (positive ~) static 2 pressure which :-is greate-{
than the offgas, system _(negative) pressure; Therefor,e, because_the. ; leakage tends to'be tcwcrd the offgas system, the potential hazards '.I are minimized. ~ '~j r i g-t L ~ Item 3 Far'those spa'ces :ind areas identified, describe what action you hr.ve v taken or plan to take to assure that explosive mixtures esiinot eceu ', mul'ater that ronichring equipment willy ar,a of such an accumulat'on, i " and the e disposal of such mixt;ures will; be controlled without result-p- .ing in a damaging explosion. " Response ~ The Bos on Edison Conpeny believes tha't the accumulation of explcsive. .f f minure:7.in spaces and ' areas throu3 which the;'offgas piping passes X is high'ly unlikely base,LonDsystem design, administrative conttols y '~ ,/ ,hed p ecautions, and routirle preven ative maintenance. Therefore, i we do not plan torinstall any additionaf conitoring equipmdnt. s N i h. ^ E .Itey! 'Lttp (etls kra potent al of(gas leakag'e paths following a pressure 4 transt.cnt in the offgas system piping. Describe your design features to mialmile aad dereft the. loss of liquid frc= loop serls and des. x x,_ cribeLope. rating procedures wh'ich assure prompt detection and reseal i f of the' blown 10op.8e21s. s i ~ REspecse ' T%e loop seals opstrear c6 the stes'n jet fccmprissors are connected x to the aftercond.snscr drain.3ine. I This :line includes a drais pot and instrument.$.tien which will' provide an ' alar = in the main ' control' room when abne: mal vater' levels are' detect $. The blown seal vould i allow'a combustible mixture'to be returnec'to the main condenser, s thus ; eliminatia the potenigl h'a:ard. 7he dra'in pot ~ foi the off._
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gas holdup line drain is similasist grovided with level Jnstrumenn t ~' tion. Operator ' ctions include ' the r'eestablishing of the levels,' ~. a if necessary,-via the condensate transter s.ystem. f -r-,. y' x s ~ / , ' 'e,,, s. I ~g A x s,; page 2 of 3 t e' y t'/ - m. .,{9 g, ,1-
ili f' 1 ~.. 3 F The cooler condenser drain loop' seals include a standpipe with level instrumentation which provides annunciation in the main con-trol room. Although a loss of this loop seal would not normally-present a combustible gas hazard, the seal would be reestablished quickly via the demineralized water supply because the open loop creates an undesirable flow. path for the release of radioactive e. gases. Item 5 Review operating and emergency procedures to assure that your oper-ating staff has adequate guidance to respond properly to offgas system explosions. Response A review'of'all procedures addressing abnormal conditions within ~ the Augmented Off Gas System has been completed. Although the pro-cedures are adequate within their intent, they do not specifically address offgas systen explosions. Accordingly, an existing proce-dure will be revised or a new procedure generated. e 1 g Page 3 of 3 m
m ..c.,.., ,I) U.S. NUCLEAR REGULATORY COMMISSION 0FFICE OF.' INSPECTION AND ENFORCEMENT Region I Report No. 7p_97 Docket No. 50-293 License No. DPR-35 Priority Category C Licensee: Boston Edison Company M/C Nuclear 800 Boylston Street Boston, Massachusetts 02199 Facility Name: Pilgrim Nuclear Power Station, Unit 1 Inspection at: Boston and Plymouth, Massachusetts
- nspection conducted:
September 5-8, 1978 Inspectors: / b,o "N R. Archi tzel, Reactor Inspector cate signec $!. Y: Y / .if5 ?! A. Finkel, Reactor Inspector (September 6 only) cate signec c' ate signed Approved by: O O b~b h 912(.he E. C. McCabe, Jr., Chief, Reactor' cate signed Projects Section No. 2 RO&NS Branch Inscection St: mary: Insoection on Sectember 5-8,1978 (Recor: No. 50-293/78-22) Areas Insoected: Routine, announced inspection of plant operaticas, IE Circular 8-08 (Environmental Qualification of Electrical Equipment), IE Sulletin 78-03 (Off Gas Hydrogen Explosions), and Licensee Event Report review. A facility tour was conducted. The inspection involved 14 hours at the corporate office by two regional based inspectors and 19 hours ensite by one regional based i nspec tor. Results: No items of noncompliance were identified. i )u!!IO i Region I fc.v.12 'Rev. April 77)
-) .- J .m 6 4 Watch Engineer's Log: August 1-20, 1978 Maintenance. Requests - Selected 1978 ~ NSSS Alarm Typer: Selected August, 1978 Failure and Malfunction Reports: 78-67 through 81 Temporary Procedures: 78-29 through 36, 40 The inspection consisted of verifying adequate management review, correct identification of problem areas, completeness. and determination that conditions contrary to the Technical Specifications did not exist. The inspector noted several instances documenting difficulty in moving control rods. This problem has been analyzed by the licensee to be caused by worn internal piston seals in the Control Rod Drives (CRD) (see NRC Inspection Report-50-293/78-14 detail 6.c). The licensee has purch: sed 100 CRDs and plans replacement during the next scheduled refueling outage. These CRD prcblems will be further reviewed by the NRC together with the slow scram times identified in paragraph 6.c (293/78 01). 5. .IE Bulletin 78-03, Potential Exolosive Gas Mixture Accumulations Associated with BWR Off Gas Svstem Ooerations Licensee action concerning IE Bulletin 78-03 was reviewed by the inspector to verify that the Bulletin was forwarded to acpropriate personnel, that a review for applicability was performed, information discussed in the reply was accurate, corrective action taken was as described in the reply, and that the reply was within the time period described in the Bulletin. The inspector reviewed new procedure 2.4.55, "Aug ented Off Gas System Explosions" Revision 0, developed to address concarr.s of the bulletin. were also reviewed]>The fol owing Augmented Off Gas (A0G) System procedure 2.4.135 A0G - Loss of H2 Analyzers 2.4.136 A0G Abnormal Carbon Vault Temperatures 2.4.137 A0G Rupture of the Off Gas Ruoture Disk 2.4.138 A0G Loss of Coolant to Off Gas C:ndenscr ) 2.a.140 A0G High Charccal Tem:erature 2.4.141 Abncrmal Recombiner C:eraticn
.w, w ~ . ~- .r 7 O The inspector noted that although the recombiners cannot be bypassed within the A0G System, the entire A0G System can be bypassed. The loop seal drain for the 30 minute hold up volume. drains to the Clean Radioactive Waste System. Loss of this loop seal coupled with operation bypassing the A0G could result in release of Rydrogen cas. This item remains open (293/78-22-01) and will be re-examined during a future NRC inspection. 6. NRC Licensee Recort Review a. NRC: Region -I in' office review of the fol. lowing licensee reports has been completed with no unacceptable conditions identified. July 1978 Monthly Report (BECo Letter 78-180) 1977 Report of Changes, Tests, and Experiments Performed Pursuant to 10 CFR 50.59 (BEco Letter 78-159) b. NRC: Region I in office review of the following LERs (Licensee Event Reports) has been completed. LER Number Subject 78-029/03L Salt Service Water Pump Vibration 78-031/03L Setpoint Drift, PCIS Reactor High Pressure Switches 78-032/03L CRD 38-47 Slow Scram Time 78-033/03L Main Stack PRM Sample Pump Failure 78-035/OlX Reactor Scram-toss of All Offsite Pcwer Events surrounding LER 78-035 (Scram on Loss of Off Site Power) were reviewed onsite. Safety systems functioned as reouired following this event caused by a lichtning strike. LER 78-032 Slow Scram Times was also reviewed onsite for determination of generic implicaticns. Findings are discussed in paragraph 6.c belcw. The inspector cuesticned the licensee concerning the secuence of events surrounding LER 72-0-0, ACS Timer (Paragraph 6.d). The inspector had no further cuesticr.s in this area. s r-
Ye:-;af,2., G& u.yE i: l 114 DEC 1977 Docket No. 50-245 ~~ - Northeast Nuclear Energy Compa'ny 'ATTM: Mr. D. C. Switzer 1 President P. O. Bc:: 270 Hartford, Connecticut 06101 Gentlemen: ~ This refers to the telephone conversation between Mr. E. McCabe and Mr. A. Davis of this offica and Mr. E. Ferland of your staff on December 14, 1977, regarding the detonations of combustible gases in the Millstone Point Station, Unit 1, Offgas System and Stack Basement on December 13, 1977. W1th regard to the matters distussed, we understand that you have under-taken or will undertake the following actions: Review th'e sequence of events; determine safety significance and a. define specific measures to prevent mcurrence; b. Consider the need for additional instrumentation to detect com-bustible or explosive gaseous mixtures in enclosed spaces, and for plant equipment to renove or dilute, such mixtures; Review the adequacy of pr6cedural and management controls for c. prer.iiting and responding to events of this nature, including but not limited to the actions taken in this case to reYieW the initial explosien and to assure proper perfomance of corrective actions such as refilling loop sealsj d. Identify and correct instFumentation, equipment, piping, and structural damage resulting from the explosiens; and, Report, in writing, to this office, the results of your evaluations e. and the cocpletion of corrective actions necessary to minimize the potential for disruptive explosions. Further, we understand that you will limit Unit l's operation to less than or equal to 4 percent rated power until your cormctive actions have been reviewed and accepted by this office. If our understanding of N 'N w i-N 'y " 83 01/ OO /08 m
sa aw - r.. 14 DEC 1977 Northeast -fluclear Energy Ccapany 2 your planned actions as described above is not in acecrdance with the actual plans and actions being implemented, please contact this office icmediately. ~ Sincerely, Boyce H. Grier Director cc's: E. J. Ferland, Plant Superintendent D. G. Diedrick, Manager of Quality Assurance A. Z. Roisman, Counsel for Citizens Comittaa for the Protection of the Envircrrent s bec's: IE Mail & Files (For Appropriate Distribution) Central Files Public Document Room (PDR) Local Public Document Room (LPDR) Nuclear Safety Information Center (NSIC) Technical Information Center (TIC) REG:I Reading Room State of Connecticut G. L. Snyder ~ Lana Cobb, IE:HQ W. P. Ellis, IE:HQ o-,u,(_......___...____l...____..._____.._l__..__.._______l________........1 I __.........__..4.............. svamo-L..__...........___________.......___.........____.... o*"*L.................b.____________.....................___....__.._.....!.___...__.....j......___....- - ~. c.m,,.s s.~,, m .. 1
se. 'e P.O Box 270 NORTHEAST NUCLEAR ENERGY COMPANY 203 666-69 1 A NORTHEAST UTiuTIES CCMPANY December 22, 1977 Docket No. 50-24S Mr. Boyce H. Grier Ofrector, Region I Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406
Dear Mr. Grier:
The Northeast Nuclear Energy Company provides the attached infor- .mation in response to your letter of December 14, 1977, regarding recent hydrogen detonations in.the Millstone Nuclear Power Station, Unit l', Off-Gas system and stack base. It is our opinion that this information adequately addresses the concerns expressed in the above mentioned letter. We therefore request that an expeditious review of this material be performed so that the limitations imposed on the operation of the Millstone Nuclear Power Station, Unit 1, can be removed. Unrestricted operation of Unit I will not commence until notification of your acceptance of the specified corrective actions has been received. Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY m h C bYb:s D. C. Switzer 1 President i DCS:PJP/kwc Attachment N... ,g'
~ ^ ~ ~ ' RESPONSES TO LETTER - B.H. GRIER TO D.C. SWITZER, DECEMBER - 1977 Item a. Review the sequence of events, determine safety significance and define specific measures to prevent recurrence. a.1) Sequence of Events q On December 13,1977, at '0930 hours, a hydrogen detonation in the Unit 1 Off Gas System occurred. This hydrogen detonation resulted in a local emergency being declared to evaluate the incident and take necessary corrective actions. Following the hydrogen detonation, airborne radiation levels in the reactor building increased indicating that the off gas pipe to r.eactor building loop seal was lost as a result of the explosion. Personnel were dispatched at 0935 hours to refill the off gas system loop seals in the-reactor building and in the stack basement. e The reactor building loop seal was promptly re-established without further complications. Although action was taken to re-establish the stack locp seal and this action was initially anticipated to have been successful, subsequent investigations into the incident indicate that this attempt did not successfully re-establish this necessary barrier. Although plant operation remained stable throughout this event, erratic readout on stack gas radiation monitoring equipment prompted plant management to initiate a power reduction from the initial load of 89 percent rated output to E0 percent rated output. ~ Initiation of this-reduction occurred at 1042 hours. Following the power reduction to 50 percent rated output and with no corresponding reduction in stack gas monitor readings, it became apparent that background activity at the stack had caused this monitoring system to essentially become inoperable. At 1145 hours, with this information in hand, a management decision was made to initiate a controlled shutdown. At 1300 hours, during the controlled shutdown, a second hydrogen detonatica occurred at the base of the stack. 'This condition prompted the Supervising Control Operator'to immediately manually scram the reactor. The main steam isolation, valves were closed and a controlled cocidown was established using the Isolation Condenser system. At 1308 hours, a site emergency was declared and personnel proceeded to assemble in the designated areas. The Primary Emergency Control Center was also established to determine the magnitude of the incident and site access and egress became controlled. Inmediately after the incident, it was discovered that the second hydrogen detonation had resulted in a momentary release of radioactive materials. Additionally, an employee passing the stack at the time, was injured and contaminated as a result of the explosion and required ambulance transportation to the local hospital. Personnel in the Emergency Control Center coordinated activities to determine the magnitude of the release and minimize the possible spread of contamination. Site monitoring i teams were dispatched to determine the extent of the release. ,___-e ~.
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C . RESPONSES TO LETTER - B.H. GRIER TO D.C. SWITZER, DECEMBER - 1977 At 1513 hours, after an evaluation of. information provided by the site monitoring teams, a decision was made to commence releasing personnel -from the site. Control points were established to' survey each individual for contamination prior ~to leaving the site. At 1900 hours, all personnel had been released, or cleared for release, and the Emergency Control Center was secured. After a thorough investigation of the incident, it is suspected that the initial hydrogen detonation, occurring at 0930,-December 13, 1977, was caused by a welder welding in the area outside of the steam jet air ejector -room. An arc strike on instrument tubing, that is used by steam jet' air ejector flow instrumentation, was identified. At the~ time of the detonation,' welding was being performed adjacent to this tubing and it has been established that the arc strike that occurred would have been sufficient to cause the hydrogen detonation. a.2) Safety Significance-The health and safety of the public were never jeopardized by the ' hydrogen detonations of December 13, 1977. The initial detonation was confined within the volume of the off gas piping. The design basis for the off gas piping system adequately recognizes the hydrogen detonation potential and the resulting combination of the hydrogen and oxygen components within the piping provided no direct threat to the health and safety of either 'the public or station personnel. The second detonation, taking place in the base of the stack, did 4 provide a physical threat to personnel in the immediate vicinity. While no significant radiological threat existed, even for nearby personni.1, i the sizable energy release of the detonation oroduced missiles in the form of a metal door and its associated hardware which had the potential to cause severe injuries to personnel who may have been in the path of this debris. The location of this door in the stack precluded a trajectory which would allow impact upon nuclear safety associated plant structures. A possiblity exists that some of these components may have impacted upon adjacent 345 KV transmission lines with the potential for removing one offsite power supply from Millstone Unit 2. This event has no safety concerns as additional offsite power supplies are available in addition to onsite diesel generators. A buildup of hydrogen-oxygen to an explosive concentration, even on
- a. local basis, is considered very unlikely within the reactor building.
Reactor building construction is such that a single interconnected air space exists throughout and is constantly exhausted by a high volume i vcatilation system or by the Standby Gas Treatment system. The prompt re-establishment of the reactor building loop seal precluded any possibility of a hydrogen detonation in this volume.
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s .c w ' RESPDNSES TO LETTER - B.H. GRIER TO D.C. SWITZER. DECEMBER 1977 . c, - . Analyses of the sequence ofievents following the first detonation indicates that there was no additional offsite consequence. There was-no evidence to indicate that. radioactivity greater'than that which would nomally-be discharged was-released. The, design of the discharge piping .and the status of the stack filters preclude any consideration of the s potential for a large discharge of. radioactivity 'as-a result of this - 4 3~ event. The recombination of the hydrogen and oxygen gases and the creation of a vacuum in the delay pipe resulted in further decreases in - the amount of radioactivity released subsequent to this event. .The'second detonation did result in potential offsite = radiological ~ consequences greater than nomal since the release 'was at ground level. Using the meteorology at the time of the incident, a calculated potential dose at the downwind site boundary of 0.94 mrem to the whole body and 6.0 mr un to the thyroid was calculated. Since a number of conservatisms ' were used in the calculation, it is estimated to be conservative by-a. factor of 5 to 10.- Thus a realistic _ assessment shows the consequences F are well below one millirem. Thus, the radiological consequences of the incident are well within federal limits and guidelines and a small fraction of the average annual background dose in the area of 125 mrem. It should also be noted that the downwind direction was south, or out to the Long Island Sound. Therefore, it is unlikely that any member of the public was in the path of the radioactive release. I Dose calculations were performed to determine the dose received by the individual injured in the blast. The calculations determined that i the whole body dose and skin dose were less than 60 mrem and 323 mrem, respectively. -In addition, a whole body count was performed and it detemined that the internal uptake was negligible and the expected - maximum dose to internal organs would be less than 1 mrem. Evaluations were also made of other personnel that were involved in reducing localized increases in radiation levels or that became contaminated as a result of the hydrogen detonations. The evaluations concluded that no reportable exposures occurred and internal uptake was negligible. I Environmental radiological samples of snow and air particulates and themoluminescent dosimeters were collected from various locations after the two events. No radioactivity above normal background was observed. This further confirms that the offsite consequences were insignificant and that-the'above analyses are conservative. . As a consequence of the detonations and related events, the radwaste building ventilation was shut down. Subsecuently, moisture drainage resulted in small quantities of radioactivity from the ventilation ducts being deposited on building structures. Rainfall during the period 12/13/77. through 12/21/77 washed some of this activity into yard drains and eventually 'into the onsite quarry. Conservative calculations based on chemistry analyses of the liquid in yard drains and on building. 4
- RESPONSES TO LETTER - B.H. GRIER TO 0.C. SWITZER, DECEMBER - 1977 structures and the amount of rainfall indicate that a maximum of 2.3 millicuries total radioactivity was discharged dring this period. The-predominant isotopes are cesium-134 and cesium-137. At no time was the discharge concentration above allowable discharge limits (10CFR20, Appendix B, Table _2).
The offsite consequences of this radioactivity have been evaluated and found to be insignificant. a.3) Measures to Prevent Recurrence In order to prevent recurrence of this incident, the following corrective actions will have been completed prior to returning Millstone Nuclear Power Station, Unit 1 to service. 1. The ceiling slab at elevation 26 feet 0 inches, which was severely damaged by the hydrogen detonation, will not be replaced. Originally, this ceiling slab served as a barrier to allow personnel to enter the stack base while discharges were being made. The ceiling slab was determined to serve no structural purpose and'by administrative 1y eliminating personnel access during future operation, it is not needed. Removal of this slab will eliminate any volume in which combustible gas mixtures could form. In order to preclude any possibility of out leakage through the stack door, as a result of eliminating the ceiling slab, full seals will be installed on the new door. In addition, a second leakage barrier will be installed on the outside of the door frame. 2. The ventilation system carrying the exhaust from all plant buildings, and entering the base of the stack, has been modified to allow air flow through the base of the stack. In addition, procedure changes have beeti made to assure that at least one main exhaust fan is running at all times the plant is operating at power. These changes will eliminate any possibility of an explosive mixture of hydrogen and oxygen from fonning in the stack base. 3. The stack off gas pipe loop seals have been eliminated. The majority of the off gas pipe is sloped back to the reactor bui.1 ding and any moisture buildup in this section will be removed by the off gas pipe's reactor building loop seal. The short section of off gas piping which exists within the stack will now be exposed to the warm atmosphere provided by main building exhaust air flow, thereby eliminating the r.ced for a loop seal at this location. This modification will eliminate any possibility of hydrogen entering the stack base through an unfilled loop seal..-
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- RESPONSES TO LETTER - B.H. GRIER TO D.C. SWITZER, DECEMBER - 1977 4.
In order to preclude the possibility of the reactor building loop seal from becoming lost, a valve has been installed downstream of the loop seal. This valve will normally be closed and only opened periodically in accordance with plant procedures to drain any excess water from the off gas pipe. A sight glass will also be provided to monitor the flow from the off gas pipe during periodic draining operations. 5. All piping that required replacement as a result of the detonation and all piping that was possibly affected by the detonation has been appropriately leak tested and inspected. The results have been found to be satisfactory; all piping -is intact and no degradation has been found. 6. All piping, that is accessible during operation that could cause propagation of an ignition source to the Off Gas system has been identified. This piping has been conspicuously marked and labeled with appropriate signs. Item b Consider the need for additional instrumentation to detect combustible or explosive gaseous mixtures in enclosed spaces, and for plant equipment to remove or dilute such mixtures; In response to the concern relating to the need to evaluate enclosed spaces for the potential of retaining gaseous mixtures, which could concentrate to a combustible or explosive point, the following plant areas have been analyzed. 1. Steam det Air Ejector Room - This room contains piping and air removal components, which contain a mixture of hydrogen and oxygen. In addition to having its own ventilation system this room is connected via a passageway to the main turbine building and its exhaust ventilation system. Instrumentaion lines leaving this room are likewise in a well ventilated area. Piping and components in this area are designed to withstand the forces of a system detonation. 2. pipe Chase Below SJAE Room - The pipe chase below the SJAE room is open and freely ventilated by the Turbine Building Exhaust system. There is approximately 3 feet of off gas pipe in the chase and any possible leakage would be rapidly dispersed by existing ventilation. 3. Reactor Building - Holdup Pipe loop Seal Area - Loss of water sealing by either " blow-out" or " suck-back" following a detonation could lead to emission of an explosive gas mixture. Such mixture could not concentrate in this area as it is completely ventilated by the Reactor Building Exhaust Ventilation system and/or the Standby Gas Treatment system. Further these loop seals have been modified to preclude the loss of sealing water. f
RESPONSES TO LETTER - B.H. GRIER TO D.C. SWITZER, DECEMBER - 1977 4. Xenon-Xrypton Building - Current off gas system operations preclude the utilization of the piping and components located in this building. Tie-in points between this building and the existing off gas system do exist, and isolation valves are red tagged closed. Potential valve leakage is of no consequence since the cie-in piping and components are maintained under instrument air pressure at a level higher than the pressure in the existing off gas system. Furthermore, the building is ventilated -as described in the July 20, 1975 D.C. Switzer letter to G. Lear, titled " Interim Off Gas System." 5. Stack Gas Sampling Room - This enclosed space contains stack gas sampling equipment and off gas filter differential pressure instrumentation, which is connected to the base of the stack by instrument tubing, via sealed penetrations. The potential for penetration leakage and subsequent buildup of an explosive mixture has been eliminated by installation of ventilation equipment designed to maintain the stack sample room at a higher pressure than the instrument sensing points in the base of the stack. 6. Main Stack Base Area - Elimination of loop seals in this area results in consideration of only a short run of 4" off gas pipe and the filter casings as potential sources of input for explosive gas mixtures. Both are designed to withstand the forces of a system detonation. Should a failure occur in these components, an explosive concentration is precluded by the design changes made to improved ventilation air flow as described a.3 above. The addition of dilution air to the base of the stack is more than adequate to preclude concentrations of gases to the detonation point. The need for additional instrumentation to detect explosive gaseous mixtures in enclosed spaces has been considered. Addition of hydrogen detection equipment has been deemed unnecessary in that previously described methods for elimination of explosive mixtures, are more effective and reliable. Furthermore, such equipment measures hydrogen concentration at specific sample points and not all areas of an enclosed space. In addition, area radiation monitors in these areas are sensitive to Off Gas system leakage. Item c. Review the adequacy of procedural and management controls for preventing and responding to events of this nature, including but not limited to the actions taken in this case to review the initial detonation and to assure crocer cerformance of corrective actions such as refilling 1000 seals. A review of procedural and management controls for preventing and responding to events of this nature has been accomplished. A comparison i.
+' i RESPONSES TO LETTER - B.H. GRIER TO 0.C. SWITZER, DECEMBER - 1977 of existing procedures against license requirements and Regulatory Guide 1.33 revealed no discrepancies or inconsistancies. A review of personnel actions during and subsequent to these events indicated that conformance to the requirements of these procedures was maintained throughout. However, as a result of these reviews and of knowledge gained subsequent to these events, additional precdutions and prerequisites have been identified and will be incorporated into station procedures. These procedural changes will provide additional assurances against the recurrence of a similar event. Item d Identify and correct instrumentation, eouioment, cioing and structural damage resulting from the detonations. After the incident, thorough inspections were performed to assess the extent of the damage. These inspections were not limited solely to the Off Gas system and stack, but also included associated equipment and systems, such as instrumentation, electrical equipment, ventilation duct work and miscellaneous piping. Following is a list of items, requiring corrective action, that were discovered during these inspections. 1. Stack base door - severely damaged as a result of the hydrogen detonation. 2. Stack interior ceiling slab - excessive damage consisting of bent support beams and exposed re-bar. 3. Stack surface - vertic.al crack's on interior and exterior surfaces that may or may not have been caused by the hydrogen detonation. 4. Off Gas system isolation valve (FCV 8-5) - located in stack. Damage to valve yoke and valve operator stem. 5. Off gas discharge line (4"-0G-14) - located in stack. Damage consisted of bent pipe, distorted elbow and broken anchor bolts. 6. Gland seal exhauster discharae line (10"-0G-17) - located in stack. Danage consisted of bent and distorted pipe and broken anchor bolts. 7. Standby Gas Treatment Exhaust system discharoe line (12"-SGT-1) - located in stack. Damage consisted of bent and distorted pipe and broken anchor bolts. 8. Off cas filter inlet valves (1-0G-51a and 51b) - located in st?ck. Damage to reach rod operators. 9. Off cas filter outlet valves (1-0G-53a and 53b) - located in stack. Damage to reach rod operators. I RESPONSES TO LETTER - B.H. GRIER TO D.C. SWITZER, DECEMBER - 1977
- 10. Augmented Off Cas system return line (2"-0G-49) - located in stack. Not in service at this time. Damage consisted of distorted pipe and broken pipe hanger,
- 11. Plant ventilation system exhaust turning vane - located in stack turning vane found to be bent and distorted.
- 12. Main exhaust fan (HVE 1-C) - located in heating and ventilation room.
Internal rib broken loose.
- 13. Liquid radwaste exhaust system (HVE-7A and 78) - Minor seam split on exhaust duct.
- 14. Steam jet air ejector flow transmitter - discovered inoperable after first hydrogen detonation.
- 15. Off gas filter differential pressure indication - appeared to be damaged by first detonation.
l6. Iso-kinetic probe for the stack gas sampling system - located in stack. Supporting beam dislocated and probe mounting beam bent. 17. Steam Jet Air Ejector Ruoture Disc - ruptured by first detonation. The corrective actions for several of the above items have been specified in other areas of this report. In all other cases the corrective action required to return the above items to an operable status will have been taken prior to returning the Unit to operation. An engineering evaluation has been completed by Northeast Nuclear Energy Company and EBASCO Services Incorporated and has concluded that no stack structural damage has occurred that would reduce its capasity to withstand design loads. In addition, an inspection of the stack foundation mat did not reveal any conditions that would be indicative of a damaged base slab or support piles. l i I
t- . *.=,.
- ENCLOSURE 1 AtMLYSIS OF fl0RT11 EAST UTILITIES' FOLLOW-UP REPORT Oti MILLST0iiE I 0FF-GAS SYSTEii GAS EXPLOSIOMS A.
Licensee Description of the Event The licensee's description is basically valid. The explosions were chemical reactions. There was no damage to nuclear safety-related equipment. tio radiation exposures reached or exceeded federal limits. The most significant discrepancy noted follows. The licensee's report states "The first event was a hydrogen explosion confined to a massive underground pipe that was designed to withstand such an incident." fiRC inspection indicates that, because broken gage glass faces in the Instru-ment Building adjacent to the stack were attributed to the first explosion, that explosion's effect was not totally confined to underground piping. B. Causes of the Explosions The licensee attributes the first explosion to touching of an off-gas system instrument line with welding equipment. i!RC inspection also indicates that action is the cause of the first explosion. The licensee attributes the second explosion to buildup of explosive gases in the stack base because of inadequate restoration of the off-gas drain line loop seal after the first explosion, with detonation initiated by a spark from a leve.1 switch frca a stack base sump ptsp. NRC inspection also indicates that condition caused the second explosion. C. Licensee Reaction The licensee report and NRC inspection indicate that actions taken to combat the damaging explosion were proper. There was no hazard to the public, and no need to call for special assistance. (except to treat the injured man). The incident did identify areas where the licensee response could have been better. If the action taken to restore loop seals had been effective, the second explosion could have been prevented. Additional ccans of measuring loop seal adequacy or a hydrogen concentration would, therefore, have been valuable. Corrective actions do prevent recurrence by elimination of the loop seals as escape paths for explosive gas. !I* II I or ice > s u - r > _ _.... _ _ _ _ __..... _(_.l'v.......... l i_ l i /- o' ~cv............... L..].' R --- L s j-----------------L-----------------l l J L
r-2 A second area for improvement is licensee cocmunications in situations where incomplete knowledge and rumor could cause undue concern. Public reaction to phrases like " explosion at a nuclear power plant" or " radiation release" could be inappmpriate if licensees do not pmcptly place such incidents in perspective. Connecticut officials criticized licensee cocn:unications in this case, and the licensee cocmitted to making his cocmunications plan acceptable to the State of Connecticut. D. Licensee Analysis and Corrective Actions Tne license report and fiRC Region I inspection Report 50-245/77-33 show that the licensee identified the cause of the explosion and effected multiple correct.ive actions to prevent recurrence. iiRC concurrence in the adequacy of corrective action was cbtained before operation was resured. CFFICE > ' -7 -- L................. L............... L.............. L..... _
p: = = . 5/NBCO ~~ ' , c .0, m NARTFORC. CONNEOTIO: NORTHEAST NUCLEAR ENERGY COMPANY 203 665-6911 A No4THEAST UiluTIES COMPANY March 2,1978 Docket No. 50-245 Mr. Boyce H. -Grier, Director Office of Inspection and Enforcement U. S. Nuclear Regulatory Comission Region 1 631 Park Avenue King of Prussia, PA 19406
Reference:
IE Bulletin No. 78-03 Gentlemen: The above. referenced IE Bulletin was generated as a result of events occurring at our Millstone Nuclear Power Station Unit No.1. In your December 14, 1977 letter to NNECO, you requested detailed information concerning these events. We responded on December 22, 1977 with this information which included the concerns expressed in IE Bulletin 78-03. We therefore feel no further response is required to IE Bulletin 78-03. Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY [ D. C. Switzer U President DCS:TJD/ kwc a lY
[E ..~..a ,ef * ;.0 .i A/NECO ,C 20, m
- MTFORC CCtCE 7'C'.'T M 'C' NORTHEAST NUCLEAR ENERoY COMPANY MEG 11 A NCRINEAST UTILitlES CCMPANY April 21, 1978 MP-l- 775 Mr. Boyce H. Grier Director, Region I Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, Pennsylvania 19406
Reference:
December 1977 letter D. C. Switzer to B. H. Grier
Dear Mr. Grier:
During the hydrogen detonation in December 1977, the four inch concrete ceiling in the stack was destroyed and in the process caused considerable damage to valve actuators and piping. At that time in order to provide for ventilation to dilute any off gas leakage and expedite the plant's return to service, it was decided to leave the ceiling out. Since then a more desirable long term solution has been sought that would: 1. Warm the lower stack to prevent condensation in the stack filter. 2. Provide improved ventilation to dildte any off gas leaks. 3. Lessen the potential for ground' level releases. 4. Make access possible during plant operation. The modifications made to the stack during the 1978 refueling outage include the following: 1. The ceiling.was replaced using steel decking. 2. Two 3040 C.F.M. fans were installed, with their suction below and their discharge above the ceiling. 3. An adjustable discharge louver was placed in the lower level ventilation pipe. 4 A differential pressure switch was installed - one side sensing stack l base pressure, the other side sensing atmoscheric pressure. 5. The Stack Gas Sampling Room is no longer pressuri:ed. $3blIO M D( j
t-q ...% w .i Mr.$rierletter Page 2 The system functions with one fan operating and the second in standby. The ventilation discharge pipe louver ~has been adjusted so that there is a negative pressure in the base of the stack, preventing any outleakage. The differential pressure switch will start the standby-fan and annunciate-in the control room before zero differential pressure is reached. Pressurizing the Stack Gas Sampling Room is no longer necessary because maintaining the base of the stack at a negative pressure accomplished the same function - any leakage will be from the Sample Room to the Stack base. Therefore we' feel that these modifications are consistent with the steps outlined in D. C. Switzer's letter to B. H. Grier, December 1977, Docket No. 50-245. Yours truly, As} D C J ~. E. J. Ferland Station Superintendent ' Millstone Nuclear Power Station EJF/JPS:daa CC: E. R. Foster W. D. Romberg J. P. Stetz o 4 l! 4 m
l} ~ ww. $ Consumers i power 3 Ccmpany r ...p c..n..a ome.. sin w. uienie.n av.nw...a.c..on, wienie.a 4ones. 4,.. coe. en ves-oseo April 11, 1978 Mr James G Keppler Office of Inspection and Enforcement Region III US Nuclear Regulatory Com=ission 799 Roosevelt Road Glen Ellyn, IL 60137 DOCEET 50-155 - LICENSE DPR. SIG ROCE POINT PLANT - RESPONSE TO IE BULLETIN 78-03 IE Bulletin 78-03 discusses the potential for accu =ulation of explosive gas ri: ures in _ Boiling Water Reactor Off-Gas Syste= Operations and requests that Censu=ers Power Cc=pany reviev and respond to specific areas of concern relat-ing to this systes. The purpose of this letter is to provide the requested response. ITEM Review the operations and maintenance procedures related to the off-gas syste= to assure proper operation in accordance with all design parameters. Include in this reviev measures you have taken or vill take to prevent inadvertent actions (such as are strikes) which =ight cause ignition of the =ixture of gases contained in the off-gas piping.
RESPONSE
Big Rock Point Operating, Maintenance and Ad=inistrative procedures relating to the off-gas system have been reviewed and, where necessary, appropriate precuations were added to preclude inadve-tent ignition of potentially ex-plosive gases. ITD4 Review the adequacy of the ventilatics of spaces and areas through which eff-gas systes piping containing explcsive =irtures of gases pass. '~ne review should consider ventilation losses and off-nomal off-gas systen operation, such as lack of dilution stes=, lest loop seals, blevn rupture disks, by-passing reccabiners eakage of cff-gas into isole.ted pc-tiens of syste=s. A 6 0 .APR 141978 pf /
o*. +
RESPONSE
A review of the design of the off-gas syste= at Big Rock Point indicates that there are two potential areas that could possibly be affected by off-gas release. These areas are the pipe tunnel and the radvaste area. Both areas are well ventilated; the pipe tunnel having a flow of 5,000 cf= to lh,100 cfm (design) and the radvaste area from 1,500 cm to h,000 cfm (de-sign). In order to reach an explosive concentration in these areas, the hydrogen concentration vould have to exceed h% and, based on the ventilation flow rates, this vould correspond to a hydrogen escape rate of 60 cfm and 200 cfm for the radvaste area and pipe tunnel, respectively. Since these are significantly higher flow rates than nominany exist in the off-gas holdup line (10 cf=), it is highly unlikely that the li=it(s) can be exceeded. Note: The analysis deriving these flow rates assu=es =inimum design ventila-tion flow and unifor= "4vi q. T '- For those spaces and areas identified, describe what actica you have taken or plan to takesto assure that explosive =ixtures cannot accu =ulate, that monitoring equipme/2t vill varn of such an accu =ulation and that disposal of such =istures vill he controlled without resulting in a damaging explosion.
RESPONSE
There are no plans to alter off-gas system or ventilation systc= design at Big Rock Point. There are no hydrogen detectors in either the radvaste area or pipe tunnel and the only ventilatics flov indicatic: available in the con-trol room is ventilation supply and exhaust fan alar =s. Ecvever, because of the extremely low potential for explosion based en both the analysis performed in the previous response and Big Rock Point's 1cng cperating history vith no significant off-gas proble=s, it is concluded that current procedures, design and operation are adequate to preclude a da= aging explosien. ITD4 1 I Loop seals are potential off-gas leahage paths folleving a pressure transient l in the off-gas system piping. Describe your design features to =inid:e and j detect the loss of liquid Trc= loop seals and describe operating procedures-l vhich assure prc=pt detectics and reseal of the blevn loop seals.
RESPONSE
There is no method available to verify whether an eff-gas loep seal is full. Analysis has shown that off-gas pressure is sc=inany one to two ounces per squa e inch, necessitating a four-inch icop seal to ensure seeling integrity. Since an off-gas loop seals at 31g Rock Point are approximately two feet er longer, the possibility of seal failure is re=cte. If a leep seal shculd fe.il however, it vould autc=atically refin via =cistu e conectics frc= vithin the syste= and without a=y procedural action required. s
y
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+: 3 ITD4 Review operating and emergency procedures to assure that your operating staff r has adequate guidance to respond properly to off-gas system explosions.
RESPONSE
There are no Big. Rock Point Plant procedures that deal specifically with off-gas system explosions. However, both the Operating Procedures and the Site Emergency Plan address and adequately cover plant fires. - Further, the Plant Operating Procedures also address abnor=al off-gas and stack gas releases as well as off-normal procedures for other syste=s. Based on this, it is con-cluded that the required response to a postulated off-gas explosion is ade-- quately addressed. In sv :: mary, the operation and design of the off-gas system at 31g Rock Point has been reviewed for the potential of maintaining accu =ulations of explosive gases. The review has produced no evidence that systa= design or operation should be altered based upcn safety deficiency. Since further, and more de-tailed, review in this area vill be conducted during the Systematic Evalua-tion Program (SEP), it is requested that any other required evaluation be coordinated with this effort. . f' '- I '..{ t.
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W4114am S Skibitsky Senior Licensing Engineer CC: Director, Office of Nuclear Reactor Regulation Director, Office of Inspection and Inforce=ent ( g. i e p. e f i 6 9 .m., -. _ _,__.}}