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A NORfMEAST UTIUTIES COMPANY 4
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. April 21, 1978 MP 1-775.
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 8. 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 tha teiling 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.
t 2.
Provide improved ventilation to dilute any off gas leaks.
.i 3.
Lessen the potential for ground level releases.
l 4
Make access possible during plant operation.
l The modifications made to the stack during the 1978 refueling outage ' include I
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 base pressure, the other side sensing atmospheric pressure, 5.
The Stack Gas Sampling Room is no longer pressurized.
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Mr. Grier letter 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, d
Q E. J. Ferland Station Superintendent Millstone Nuclear Power Station EJF/JPS:daa CC:
E. R. Foster W. D. Romberg J. P. Stetz e
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NUCLEAR REGULATORY COMMisslON
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WAsHs NG TON. Q. C. 20555 c
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j AUG 141978
', MEMORANDUM FOR:
Chairman Hendrie Comissioner Gilinsky
[
Comissioner Kennedy Comissioner Bradford I
Comissioner Ahearne i
FROM:
- 0. Gene Abston, Acting Director Office of Inspector and Auditor
SUBJECT:
GA0FINALREPORTTOTHEHONORABLETOMBEh!L"LONOFF-GAS EXPLOSIONS In accordance with our August 24, 1975 memorandum concerning coordination of GAO activities within NRC, the subject report is being sent for your information.
It should be noted that a recommendation directed to the Chairman, NRC',
is contained on page 8 of this letter recort.
As you know, Section 236 of the Legisla.tive Reorganization Act of 1970 requires the Chairman to submit a Written statement on actions taken on GA0 recomendations to the House and Senate Committees on Government Operations not later than 60 days after the date of the report and to the House and Senate Committees on Appropriations with the NRC's first request for appropriations made more than 60 days after the date of the report.
This response on NRC's actions will be coordinated and drafted by EDO.
Should you have any views or coments on the subject report, we will be happy to pass them on to GA0.
Enclosure:
Cy subj rpt dtd 8/4/78 cc: LL. Gossick, w/ encl.
S. Chilk, w/ encl.
J. Kelley, w/ encl.
C. Kamerer, w/ encl.
K. Pedersen, w/ encl.
J. Fouchard, w/ enc 1.
H. Shapar, w/ encl.
J. Davis, w/ encl.
C. Smith, w/ encl.
R. Minogue, w/ encl.
S. Levine, w/ encl.
H. Denton, w/ encl.
Contact:
Fred Herr OIA, 27051 Wgf V
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3-164105 AUGUST 4.1973 The Honorable Tom Sevill Chairman, Subcommittee on Public Works Committee on Appropriations House of Representatives
Dear Mr. Chairman:
In a June 21, 1978, letter you asked us to look into those gas explosions 1/ which have occurred at boiling water nuclear powerplants and to assess the actions of the Nuclear Regulatory Commission in connection with these explosions.
On December 13, 1977, a gas explosion at the Millstone nuclear powerplant in Connecticut became videly publicized.
An employee at Millstone was injured and slightly contaminated with radioactive gases, a building was damaged extensively, and the powerplant was shut down for 10 days.
In our view, the explosion at Millstone, as well as other gas explosions might have been prevented if the Commission had acted decisively and aggressively to require licensees to take certain precautions.
However, only recently has the Commission initiated actions which should =inimize the possibility of these explosions.
We believe these actions, when implemented, will minimi:e the chances of explosions, but under the Commission's present plans these measures will not be implemented for at least 1 year.
Because the measures are available, we believe the Com-mission should accelerate the process for requiring and imple-menting the actions aimed at minimizing gas explosions at nu-clear powerplants.
1/In the nuclear industry jargon they are referred to as off-gas explosions.
IMO-73-99 (30140)
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3-164105 As you requested, we-d id not. obtai,n the Commission's
.. formal comments-on this report.- However, we me t with Com-mission 'of ficials and received informal comments which -have been incorporated into the report..
The rest of _ this report discusses the details of gas ex-plosions which led us to the above conclusion.
WHAT ARE GAS EXPLOSIONS AND THEIR HAZARDS?
Gas explosions are potential hazards only,co the 25 boil-ing water power reactors now licensed to operate in the United States.
In a boiling water reactor, nuclear fuel
- converts water to steam to drive electric generating turbines.
The steam then condenses and returns to the reactor to be reheated.
As it creates steam, the reactor also produces gas which cannot be condensed.
Some of the gas is radioactive.
An o f f-
. gas system is designed to remove this gas f rom the steam.
Through a delaying and filtering process, the radicactivity is reduced and then released to the environment.
The off-gas system consists of equipment located away from the reactor building, which separates the gas from the steam, and piping to carry the gas to a release stack.
The stack usually houses fans and filters.
The delay time designed into the o f f-gas system is impor-tant because most of the radioactive gas loses its radioactive properties rapidly.
Very small radioactive particles carried oy the gas are collected by the system filters.
Af ter being delayed and filtered, the remaining gas, containing a limited amount of radiation, is diluted with air and released to the environment through the stack.
The amount of radiation that can be released is limited by Federal regulations.
At present, 20 of the 25 operating boiling water reactors use a new off-gas system which releases much less radiation than the older system. 1/
Although the new system is not re-quired by the Commission, nearly all of the remaining boiling water reactor licensees, according to the Commission, are com-mitted to installing the system soon to meet new radiation standards, 1/The older system is designed to release about 20 percent of
~ the amount allowaole by the Commission regulations.
Tne newer system, releases less than 1 percent of the allowaole amount.
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u B-164105 Cver the past several years, about 50 gas explosions have been reported at nuclear powerplants.
According'to the Com-mission, none of the explosions resulted in offsite releases of radioactivity which exceeded the limits specified in the Federal regulations.
But in some cases, these explosions have caused property damage and injuries to workers.
Explosions occur both internally and externally to the system.
In internal explosions, gases contained within the system are ignited.
Internal explosions are not, viewed by the Commission as a problem' because the systems "are designed to withstand these explosions.
Neither personal injury nor property damage is known to have resulted from an internal explosion.
External explosions are-generally caused by equipment failure which permits gas leakage.
They can follow an in-ternal explosion which damages or weakens a part of the sys-tem.
The failure of equipment or an internal explosion may permit combustible hydrogen, oxygen, and radioactive gas to escape from the contained system.
If the gas escapes into a closed area, the buildup of hydrogen 'in the presence of an ignition source could result in an explosion.
The severity and consequences of a gas explosion depend on whether the explosion is internal or external.
Of the approximately 50 explosions reportad, 5 were external and all of these resulted in physical. damage or personal injury.
The following table shows the consequences of the five external explosions.
3 i
N
Q i
us Iteactor faci 1ity I) ate of 4
and location explosion Probable cause (note a)
Damage p
Dresden 2 4/1973 Welding are from welders working Two workers received Horrin, Illinoin near an of f gan system l'llter.
minor burns.
Cooper 11/1975 Improperly installed valve in a One worker received llrownville, drain area allowed gas to escape.
major burns and five Nebraska The gas was ignited by a spark others received minor from an air sampler.
radiation exposure.
Cooper 1/1976 Ice blocked the top o f system's Off gas building de-Ilrownville, release stack, backing up gas molished, no injuries Nebraska in the off-gan building.
The to people, gas was ignited by electrical equipment.
tironswick 2 1/1976 Ilecause of equipment failure, Minor damage to equip-4 Southport, North excessive moisture.iccumulated ment, no injuries to Carolina in a filter.
It froze and
- people, blocked gas flow.
Increased i
pressure drained loop seal al-lowing gas to escape.
Hillstone 1 12/1977 l'irst explosion caused by One worker suffere'da Waterford, welding arc.
I.oop seals were concussion, multiple Connecticut defeated allowing gas to es-abrasions, and was cape into base of stack.
Ig-slightly contaminated.
nited by electrical equipment.
', Building sustained
- major damage.
4
~a/The probable cause is determined and documented by the Commission, or its predecessor, the Atomic Energy Commission.
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B-154105 At the Mills tone plant, an internal explosion weakened the system and resulted in a much more severe external explo-sion.
The internal explosion weakened seals in the system and allowed the hydrogen gas to escape.
The explosion was ignited by heat from a welding arc.
Attempts to replace all.the seals in the system were un-successful.
As a result, combustible gas escaped into an un-ventilated area at the base of the stack.
According to the Commission, the probable ignition source of the second explo-sion, which occur red about 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> af ter the fir,st explosion, was a spark from an electrical switch.
The second explosion blew a metal door from the stack base into a warehouse about 200 feet away.
The impact of the door bent a 12-inch structural beam approximately 8 inches.
The blast also removed two 2,000 pound shield plugs f r om the floor of the stack base and did extensive damage to the ceiling.
WEAT RAS THE COMMISSION DONE TO ELIMINATE OR REDUCE THE CHANCE OF GAS EXPLOS IONS ?
The Commission and its predecessor, the Atomic Energy Commission, were aware of gas explosions as early es 1971 when the first one was reported.
Since that time, about 49 more explosions have been reported.
In a 197 4 report entitled " Review of Explosions in Boiling.
Water Reactor Off-Gas Systems," the Atomic Energy Ccmmission discussed the causes of six explosions and the actions that could be taken to prevent future explosions.
The report stated that because of the diverse causes of the explosions, they would continue at frequent intervals unless system :odifica-tions were made to minimize ignition sources.
The report did not call for corrective actions to be required at all boiling water reactors.
However, since 1974 the Commission has re-quired all boiling water reactors in the design or construction stage to impr ove the integrity of seals in the of f-gas system.
The General Electric Company, the prime vendor of boiling water reactor off-gas systems, as early as 1973 sent ccmmuni-cations to all boiling water reactor owners emphasi:ing the potential ha:ard of the gas explosions and identifying opera-ting and engineering actions which might be used to prevent them.
In 1976 the General Electric Company recommended, among other things, that reactor owners (1) label pipes and ecuip-
=ent containing detonable mixtures of gas, (2) provide fo r adequate ventilation to prevent buildup of detonacle mixtures, 5
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3-164105 and (3) assure the integrity of all seals.
Commission officials could not tell us how many reactor owners may have taken these actions.
In the past, the Atomic Energy Commission and the Commis-sion requested reactor owners who had' reported external ex-plosions to take steps aimed at preventing a similar explosion at the same powerplant.
For example, an explosion occurred at the Cooper nuclear powerplant in early 1976, which completely demolished a 32-foot by 48-foot metal building.
Immediately before the explosion occurred, plant personnel had entered the building to perform their duties but noticed an Gn. usual oder and an abnormal reading of air monitoring equipment.
These employees vacated the building which exploded shortly there-after.
During cleanup, a partially melted piece of ice was found and it was postulated that the ice had formed at the top of the stack through which the gas is released, thus causing the gas to backup in the stack building.
Ignition was prob-
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ably caused by a spark f rom one of the electrical devices in the building.
The Commission requested the owner of the Cooper power-plant to heat and insulate the upper 10-foot section of the stack pipe to prevent another ice buildup.
Fan positioning and piping changes which may have prevented the explosion were also made.
However, the Commission did not require any other plant owners to assess and correct the possibility of hydrogen gas buildups in stack areas.
The Millstone explosion is antther example of the Commis-sion's action in response to an external explosion.
In this case, the Comm'issi6n required the owner to improve seal integ-city, identify and label pipes, and improve ventilation.
This cost about 510,000 and was completed in less than 10 days.
Again, the Commission required these corrections only at Mill-stone and did not apply them to other plants.
The first' action taken by the Commission that was directed at all operating boiling water reactor owners was to issue a bulletin on February 8, 1978.
The bulletin instructed every
, owner to
--review measures taken to prevent actions which may ignition of gas in off-gas piping:
cause
--review the adequacy of ventilatien of spaces and areas through which pipes containing the explosive gas pass; 6
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--describe actions to prevent ~ accumulation of explosiva gas, and describe monitoring equipment that.will warn
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of the accumula' tion and disposal of such accumulations
--describe' procedures for assuring seal integrity; and
--review operating and emergency procedures to respond.
properly to gas explosions.
All owners have responded, and the Commission has begun its review of the responses.
The Cc3 mission's review will first involve inspecting each reactor to verifyt.the information in the response.
Then the Commission will determine what addi-tional steps, if any, are_needed to assure that every reactor owner has taken or will take actions to minimize: the possibil-ity of exteraal explosions.
Commission of ficials told us that this-inspection and review will take about' 6 months, and that another 6 months will be given to the reactor owners to comply.
CONCLUSIONS Since 1971 about 50 gas explosions have been reported at.
boiling water. reactors.
These explosions have caused degrada-tion and failure of off-gas systems, radiation exposure and
-other injuries to personnel, ' physical damage to f acilities,
.and lost revenues during reactor. shutdowns for repair.
The Commission and its predecessor, the Atomic Energy Commission, have known about the-problem, its hazards, and -
actions which would minimize the problem for some time.
The Commission has required some actio*n on plants under design and construction, however, no action has been taken on opera-ting planti except on an af ter-the-f act, case-by-case basis.
Measures to eliminate or reduce explosions, such as ventilat-ing closed areas, have been known for years by both industry and the Commission.
In most cases, these measures appear to be relatively simple and are not as time-consuming as one might believe.
At the Millstone f acility, for example, the total cost was about $10,000, and the work could have been com-pleted during a normal shutdown,.
A direct cause of the Mill-stone explosion was lack _ of ventilation in a closed area.
The explosion at Millstone may not have ' occur red if the stack area had been ventilated.
Although the Commission has failed to act promptly, we
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believe the recent actions, when implemented, will minimi:e the chances of gas explosions.
However, according to the Com-mission's present plans, it will take up to a total of 1 year
.before corrective actions are implemented at all facilities.
Tnese corrective actions include such measures as providing 7
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B-164105
' ventilation in closed areas where gas may accumulate, assuring seal integrity, and protection of piping from ignition sources.
In the past, corrective actions have been taken at individual powerplants in relatively short time frames; therefore, the Commission should accelerate its process and promptly require all powerplants to take those measures necessary to provide a greater margin of safety.
In commenting on this report, the Commission stated that its efforts are timely and disagrees with accelerating the actions.
Commission officials said that off-gas systems are not reactor-safety related and therefore present no danger to the public.
However, the Commission did recognize that injuries to plant workers, property damage, and lost revenues due to reactor shutdown can occur.
Further, although Commission of-ficials recognize that gas explosions are an occupational safety.
issue that could be considered the responsibility of the Occu-pational Safety and Eealth Administration, the Commission be-lieves it should be the lead agency on this problem because the Commission has the experience and expertise needed to deal with the problem.
RECOMMIMDATICM GAO recommends che Chairman, Nuclear Regulatory Commission:
--Accelerate the process for requiring and implementing the actions aimed at minimizing gas explosions at nuclear powerplants.
These, actions should includa ventilation of closed areas where explosive gas may accumulate, assuring seal integrity, and protection of piping from ignition sources.
As arranged with your office, unless you publicly announce its contents earlier, we plan no further distribution of this report until 7 days from the date of the report.
At that time we will send copies to interested parties and make copies avail-aole to others upon request.
Sin
_f yours, f
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' Comptroller General of the United States 8
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