ML19246A352
| ML19246A352 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 06/13/1979 |
| From: | Davis W NATIONAL ACADEMY OF ENGINEERING |
| To: | |
| References | |
| ACRS-X-0001, ACRS-X-1, NUDOCS 7906180575 | |
| Download: ML19246A352 (6) | |
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STATDIENT EY W. KEN'iETH DAVIS
.U' VICE PRESIDENT, NATIONAL ACADEMY OF ENGINEERI??G
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ON THE ACCIDENT AT i
THREE MILE ISLA.O NUCLEAR Pl. ANT
._1 Because of the present and potential i=portance of neclear power in the Unit.ed States, as well as the rest c2. the world, it is essential that the ce=bers of the National Academy of Enginecring be able to plaie in per-spective the events which started on March 28, 1979, at the 900,000 kilowatt No. 2 Unit of the Ihree Mile Island Plant near Harrisburg, Pennsylvania, which is operated by the Metropolitan Edison Co=pany, a subsidiary of Gcncra.1 Public UtJ1.ities.
L'h11e considerable uncer tainty cer.ains about the actud sequence of evente as well as the specific malfunctions and actions involved, some tele-vant aspects and consequences are now reasonably clear and it is important to evaluate these as quickly ac possible.
The sequence of events was initiated by unexpected loss of norr.al feedwater flow to the two boilers while the unit was operating at full power.
This resulted in tripping off the turbo-generator. Emerge.ncy boiler feed-water pumps were started within seconda but could not p/ ovide water to the boilers because valves had been left closed which were requf. red to bc open.
The beIIere, began to dry up and lose thair ability to recove hm t frca the
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circulating hot water in the tuo high picsnure " primary" locps ling the reactor. The resulting increase in pressure in the cooling loops in turn led to 1) shutting down the nuclear reactor by insertion of the contrM.
rods (within 12 seconds of the initial event), and 2) opening of an auto-matic pressure relief valve to reduce the pressure to normal levels by
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releasing steam fres the v-.une/ pressure control vessel called a " pressurizer".
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As socetincs happens with relief valves of the type used, the vaJve failed to reclose when the pressure dropped and continued to release steam (and probably sone water) frc= the primary system.
Two things should be noted. First of all, the reactor was a pres-surized water reactor (PWR) which operates at a nor=al pressure of about 2100 pounds per square inch (osi) which is suf ficient to prevent any boil-m ing in the pri=ary systen so that it is entirely 'illed with water except for a s=all volume at the top of the pressurizer.
The pressurizer is
_r electrically heated so that the water in it is at temperatures sufficient to produce snce stecs at 2100 psi in order to ef fect voluma and pressure control. Second, once the reactor was "scrir:ed", the nuclear reaction never started again and the heat, which was.then entirely from radioactivity in the fuel caterial in the core, dropped instantly to about 7% of that at full power and then decreased rapidly to about 1% af ter one hour, about 0.4% after one day and less than half of that after a unek.
While the events described led to the eventual consequences, it is important to note that it is not likely that any of then by themselves caus. '
any significant damage or untoward consequences and that they were a=ong the events subject to normal operating procedures.
The subsequent events are difficult to reconstruct with present knowledge but ~ involved difficulties with naintaining a proper amount of water (and pressure) in the pri=ary systes including the reactor vessel (with water supplied by normal and emergency syste=s and released by the relief valve as well as a "let-down" system) and the circulation of the water through the reactor core and the syster by the four main coolant pu=ps.
So=e.further equipment malfunctione likely occurred which, when cocbined with various actions, some probably 228 5 />
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_3-taken without adequate data or erroncous data due to instrunant malfunctions, led to quite serious consequences and the necessity for what will surely be a long and costly cicanup process.
While there is only indirect evidence nou, it appears likely that the reactor core was not fully covered with cooling water for one or more periods of time, probably starting after about two hours from the initiation of the incident. A part of the circonium cladding of the fuel (itself a very high =elting point uranium oxide ceramic) reacted with water or steam to produce zirconium oxide and hydrogen. Also, part of the fission product gases, xenon and krypton (highly radioactive but very s=all phycical volumn) were released from the fucl as well as t9 aller fractions of volatile fission products, iodine and cesium.
There is no evidence of melting of the fuel or release of significant amounts of uranium or other non-volatile fission products into the water.
Substantial aaounts of the hydrogen as well as the fission product gases f ound their way into the shielded gas-tight contcinment which houses the reactor and the entire primary system through release of coolant water into the containment.
While there was speculation about hydrogen being trapped in the anvented top of the reactor vessel (and the upper pipes leading to the top of the steam generator), there was little basis-for believing this " bubble" might contain enough oxygen to explode and, as anticipated, the hydrogen to the extent it might have been there, was carried out by circulation of the coolin, later.
However, hydrogen did accumulate in the air in the contain-ment building to a level of about 27. and a " spike" in the pressure reading in the containment building seems likely to have been caused by a s=all hydro-gen detonation several hours after the incident started.
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_4-The water discharged from the primary system was collected in a sucp in the bottom of the containment building and so=e of it pumped into tanks in an " auxiliary" building where some overflowed.
This building has a con-trolled ventilation syste= which discharges gases up a stack af ter filtering to remove essentially all radioactivity except xenon and krypton. The only significant source of radioactivity discharged from.the plant during the incident was from the auxiliary building.
The operators are in the process of reducing the preseure and tem-perature in the primary system while providing adequate cooling to place the core in " cold shutdown" on a long-term basis.
Extensive measure vill be
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needed to clean up the water in the botcom of the con'.:ainnent (containing the radioactive iodine, etc.) and the residual activity in the gas in the containment structure (the xenon activity decreases by one-half every 5-1/2 days).
This will be necessary, along with the other cleanup measures, before the containment can be entered, the damage asscsced, and the damaged fuel removed which requires removing the head from the 15-foot diareter reactor vessel. Once this is done, the fuel can be re:oved and nece.ssary repairs and replaceme nts begun.
Frcm the above, it seems clear that at least in retrospect, there any immediate hazard to those living in the vicinity of the plant was not although prudence dictated esking provisions f or any hazards which might have emerged.
The exposure to radiation of those living within 50 miles of the plant has been estimated to average about I milires (crcms).
This is about the amount normally received from natural sources in 3 days of living or perhaps 1/3 of that received on a jet flight across the country. The total 228 156 W
integrated dose of all those in the area is estimated at 2000 man-rems (exposure per person tices the number of people) and it is believed that An about 10,000 can-reas are needed for one additional case of cancer.
individual staying centinually at the worst point outside the site fence would have received about 100 =ress -- the equivalent of 2 or 3 medical The exposure received by workmen in the plant was below those x-rays.
allowabic under established industrial regulations except for 3 men repor-ted to have received slightly more than the quarterly 1 Lait.
It is i=portant to recognize that the design of nuclear plants, including the Three Mile Island Unit, is based on detailed consideration of and provisions f or virtually evc.cy conceivable equip:nent or hazan n 1func-tion.
For the very unlikely chain of events (such as occurrmd at Threc Mile Isinnd), the criteria are: first, cafety for the public; second, protection of plant personnel; and third, reducing damage to the p] ant.
Clearly, the Threa Mila Unit accidant was acco==cdnted withir tha " design considerations" for the plant and achieved the first two obje.ctives. The da= age to the plant, except for the core, is not believed to be severe in the physical sense, but the cost of cluaning up the plant and restorinc it to service will undoubtedly be high and take a long rime.
In addition to the economic loss to the local businesses and to the people who coved away temporarily, there uill be a large cost for purchasing replacc=ent power at a high cost (from coal or oil-fired plants) as compared with the nuclear generating costs at Three Mile Island. This will be several hundred thousand dollars per day for Unit No. 2 and, of course, twice 1, which is down for refueling, is not allowed to start up that if Unit t:0.
again.
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Where the broad perspective and careful jud; tent is really required is as to the i= pact of the Three Mile Island accident on the future use of nuclear power.
With energy and electric power requirenents increasing re-lentles, sly, albeit at sc=cwhat lover rates than before the 1973-74 crisis, declining oil and gas production in the United States, increasing risks to L
our evergrowing oil imports, and only a distant premice of new " renewable" energy sources, the U.S. cust rely at a rapidly increasing pace on coal and nuclear if it is to retain its economic vigor, standard ci living, and political strength in the vocid. While it might be possible in time to meat our requirements with coc1 and little or no nuclear posw, this is likely to be very dif ficult physically, relatively ccstly, and at a price to the en-viron=ent, health and safety. All forna of energy production without excca-tion have an i= pact on the environment, health and saf ety.
While these ace broadly acceptable in terms of the benefits for present energy sources, including coal, it is still evident that nuclear pcwer it, one cf those sources with the least i= pact -- and the events at Three !!ile Island have not changed that fact, despite the impressions given by the T7 a nd radio prograns and the volune of speculative and sometimes distorted information in the press (some of which did present factual and well-Lalanced reports).
The lessons of Three Mlle Island will surely Jer.d to changes in the design and operation of nuclear plants, changes which vill further decrease the likelihood'of another similar accident.
I?owever, the ouestion is whether what some have called a " national disaster" wh*ch did not inj ure or kill anyone is going to result in termination or atrophy of one of the few sources of energy we can otherwise look forward to with confidence for the next few decades.
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