Report Of The President's Commission On
The Accident At Three Mile Island           > TMI-2 > Kemeny

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Senior Staff







The Accident


Commission Findings:


1.  The accident at Three Mile Island (TMI) occurred as a result of a series of human, institutional, and mechanical failures.

2.  Equipment failures initiated the events of March 28 and contributed to the failure of operating personnel (operators, engineers, and supervisors) to recognize the actual conditions of the plant. Their training was deficient and left them unprepared for the events that took place. (See finding F.) These operating personnel made some improper decisions, took some improper actions, and failed to take some correct actions, causing what should have been a minor incident to develop into the TMI-2 accident.

3.  The pilot-operated relief valve (PORV) at the top of the pressurizer opened as expected when pressure rose but failed to close when pressure decreased, thereby creating an opening in the primary coolant system -- a small-break loss-of-coolant accident (LOCA).  The PORV indicator light in the control room showed only that the signal had been sent to close the PORV rather than the fact that the PORV remained open. The operators, relying on the indicator light and believing that the PORV had closed, did not heed other indications and were unaware of the PORV failure; the LOCA continued for over 2 hours. The TMI-2 emergency procedure for a stuck-open PORV did not state that unless the PORV block valve was closed, a LOCA would exist. Prior to TMI, the NRC had paid insufficient attention to LOCAs of this size and the probability of their occurrence in licensing reviews. Instead, the NRC focused most of its attention on large-break LOCAs.

4. The high pressure injection system (HPI) -- a major design safety system -- came on automatically. However, the operators were conditioned to maintain the specified water level in the pressurizer and were concerned that the plant was "going solid," that is, filled with water. Therefore, they cut back HPI from 1,000 gallons per minute to less than 100 gallons per minute. For extended periods on March 28, HPI was either not operating or operating at an insufficient rate. This led to much of the core being uncovered for extended periods on March 28 and resulted in severe damage to the core. If the HPI had not been throttled, core damage would have been prevented in spite of a stuck-open PORV.

5.  TMI management and engineering personnel also had difficulty in analyzing events. Even after supervisory personnel took charge, significant delays occurred before core damage was fully recognized, and stable cooling of the core was achieved.

6.  Some of the key TMI-2 operating and emergency procedures in use on March 28 were inadequate, including the procedures for a LOCA and for pressurizer operation. Deficiencies in these procedures could cause operator confusion or incorrect action.

7.  Several earlier warnings that operators needed clear instructions for dealing with events like those during the TMI accident had been disregarded by Babcock & Wilcox (B&W) and the Nuclear Regulatory Commission (NRC).

a.  In September 1977, an incident occurred at the Davis-Besse plant, also equipped with a B&W reactor. During that incident, a PORV stuck open and pressurizer level increased, while pressure fell. Although there were no serious consequences of that incident, operators had improperly interfered with the HPI, apparently relying on rising pressurizer level. The Davis-Besse plant had been operating at only 9 percent power and the PORV block valve was closed approximately 20 minutes after the PORV stuck open. That incident was investigated by both B&W and the NRC, but no information calling attention to the correct operator actions was provided to utilities prior to the TMI accident. A B&W engineer had stated in an internal B&W memorandum written more than a year before  the TMI accident that if the Davis-Besse event had occurred in a reactor operating at full power, "it is quite possible, perhaps probable, that core uncovery and possible fuel damage would have occurred."

b.  An NRC official in January 1978 pointed out the likelihood for erroneous operator action in a TMI-type incident. The NRC did not notify utilities prior to the accident.

c.  A Tennessee Valley Authority (TVA) engineer analyzed the problem of rising pressurizer level and falling pressure more than a year before the accident. His analysis was provided to B&W, NRC, and the Advisory Committee on Reactor Safeguards. Again no notification was given to utilities prior to the accident.

8.  The control room was not adequately designed with the management of an accident in mind. (See also finding G.8.e.) For example:

a.  Burns and Roe, the TMI-2 architect-engineer, had never systematically evaluated control room design in the context of a serious accident to see how well it would serve in emergency conditions.

b.  The information was presented in a manner which could confuse operators:

(i)    Over 100 alarms went off in the early stages of the accident with no way of suppressing the unimportant ones and identifying the important ones. The danger of having too many alarms was recognized by Burns and Roe during the design stage, but the problem was never resolved.

(ii)   The arrangement of controls and indicators was not well thought out. Some key indicators relevant to the accident were on the back of the control panel.

(iii)   Several instruments went off-scale during the course of the accidentr, depriving the operators of highly significant diagnostic information. These instruments were not designed to follow the course of an accident.

(iv)   The computer printer registering alarms was running more than 1-\ hours behind the events and at one point jammed, thereby losing valuable information.

c.  After an April 1978 incident, a TMI-2 control room operator complained to his superiors about problems with the control room. No corrective action was taken by the utility.

9.  In addition to the normal instrumentation present in the control room at the time of the accident, TMI-2 was equipped with a special data recorder that B&W had temporarily installed during the plant start-up and never removed. This data recorder, called a reactimeter, preserved a large amount of information useful in post-accident analysis. This type of data recorder was not required as standard equipment by the NRC.

10. Those managing the accident were unprepared for the significant amount of hydrogen generated during the accident. Indeed, during the TMI-2 licensing process which concentrated on large-break LOCAs, the utility represented and the NRC agreed that in the event of a large-break LOCA, the hydrogen concentration in containment would not be significant for a period of weeks. In the first 10 hours of the TMI accident (a small-break LOCA), enough hydrogen was produced in the core by a reaction between steam and the zirconium cladding and then released to containment to produce a burn or an explosion that caused pressure to increase by 28 pounds per square inch in the containment building. Thus, TMI illustrated a situation where NRC emphasis on large breaks did not cover the effects observed in a smaller accident.

11. Iodine filters in the auxiliary and fuel handling buildings did not perform as designed because the charcoal filtering capacity was apparently partially expended due to improper use before the accident. Required testing of filter effectiveness for the fuel handling building had been waived by the NRC. There were no testing requirements to verify auxiliary building filter effectiveness.

12. The nature and extent of damage to the core is not likely to be known with assurance until the core materials are recovered and carefully examined. However:

a.  We estimate that there were failures in the cladding around 90 percent of the fuel rods. The interaction of the very hot cladding with water generated somewhere between 1,000 and 1,300 pounds of hydrogen gas and converted 44 to 63 percent of the zirconium to relatively weak zirconium oxide. As a result of oxidation and embrittlement of the fuel rod cladding, several feet of the upper part of the core fell into the gaps between the fuel rods, causing partial blocking of the flow of steam or water that could remove heat from the damaged fuel.

b.  Fuel temperatures may have exceeded 4,000F in the upper 30 to 40 percent of the core (approximately 30 to 40 tons of fuel). Temperatures in parts of the damaged fuel that were not effectively cooled by steam may have reached the melting point of the uranium oxide fuel, about 5,200F.

 c.  An NRC study suggests that some of the fuel may have become liquid at temperatures above 3,500F by dissolving in a zirconium-zirconium oxide mixture. The study estimates that the amount of fuel that may have melted by this process is from zero to a few tons. An independent analysis by Argonne National Laboratory suggests that the formation of such a mixture was unlikely.

d.  Substantial fractions of the material in the reactor control rods melted.

e.  There is no indication that any core material made contact with the steel pressure vessel at a temperature above the melting point of steel (2,800F).

13. The total release of radioactivity to the environment from March 28 through April 27 has been established as 13 to 17 curies of iodine and 2.4 million to 13 million curies of noble gases. (The health effects of the radiation released are described in finding B.)

a.  Five hundred thousand times as much radioactive iodine (7.5 million curies) was retained in the primary loop. On April 1, 10.6 million curies of iodine were retained in the containment building's water and about 36_^000 curies in the containment atmosphere. Four million curies were in the auxiliary building tanks. Almost all of the radioactive iodine released from the fuel was retained in the primary system, containment, and the auxiliary building. Since the accident, most of the short-lived radioactive iodine has decayed and is no longer a danger.

b.  No detectable amounts of the long-lived radioactive cesium and strontium escaped to the environment, although considerable quantities of each escaped from the fuel to the water of the primary system, the containment building, and the auxiliary building tanks.

c.  Most radioactivity escaping to the environment was in the form of fission gases transported through the coolant let-down/ make-up system into the auxiliary building and through the building filters and the vent header to the outside atmosphere.

d.  The major release of radioactivity on the morning of March 30 was caused by the controlled, planned venting of the make-up tank into the vent header. The header was known to have a leak.

14. The process of recovery, cleanup, and waste disposal will be lengthy, costly, and presents its own health dangers. Cleanup of the reactor and auxiliary buildings and disposal of approximately one million gallons of radioactive water, a substantial amount of radioactive gases, and the solid radioactive debris within the reactor vessel remain to be done.

15. The cost of the accident, including this cleanup and a portion of the waste disposal, will be between $1 billion and $1.86 billion, if the plant can be refurbished. If it cannot be refurbished, the total cost will be significantly higher. An independent study prepared for the Commission estimates these costs as follows:

    Low       Medium      High
    (Millions of dollars)
Refurbish TMI-2      
  Emergency Management   $ 120 $ 160 $  225
  Replacement Power*/    678    966  1,128
  Plant Refurbishment    249    306     503
  Total**/ $1,047 $1,432 $1,856

*/ The low case assumes TMI-2 will be returned to service in January 1983, the medium assumes January 1984, and the high assumes January 1985.

**/ The costs associated with health effects have been deleted from this table. The costs projected by the study had a minimal effect on the total costs projected. The Commission believes that the analysis of health effects costs was insufficent to reach the conclusion set out in the study.

16. The 1974 WASH 1400 Reactor Safety Study (the Rasmussen Report) analyzed events, equipment failures, and human errors that could happen during reactor accidents, including those associated with the TMI accident. However, NRC has not made systematic use of WASH 1400, a major study commissioned by the Atomic Energy Commission (AEC), in its design review analyses. WASH 1400 showed that small-break LOCAs similar in size to the accident at TMI were much more likely to occur than the design basis large-break LOCAs, and can lead to the same consequences. Further, the probability of occurrence of an accident like that at Three Mile Island was high enough, based on WASH 1400, that since there had been more than 400 reactor years of nuclear power plant operation in the United States, such an accident should have been expected during that period.

17. The Commission tried to determine what would have happened if certain additional events had occurred during the accident. For a discussion of these scenarios, see the Commission Overview and the technical staff analysis report on "Alternative Event Sequences."

 Note: For a definition of loss-of-coolant accident and other technical terms used in the Commission's report, see the Glossary.