activated and coolant flow was established, carrying heat out of the reactor through one of the steam generators to the condenser, bypassing the turbine. The primary system remained essentially in this mode for a month, until natural circulation was finally achieved on April 27.

Thursday-March 29

On Thursday morning, a team of seven specialists from NRC headquarters arrived at the site. At that time the radiation readings at and near the plant were not negligible but also were not alarming. No significant iodine releases were detected. These would be considered especially hazardous because radioactive iodine, should it enter the human food chain, tends to accumulate in the thyroid and can cause cancer of that gland. The Congress evinced immediate and urgent interest in events at the plant: Chairman Hendrie was called to explain the situation before the House Subcommittee on Energy and the Environment, and Senators Heinz and Schweiker and Congressmen Ertel and Goodling-all of Pennsylvania-were briefed by the utility and the NRC. During the afternoon, some waste water from the plant was discharged by the licensee into the Susquehanna River. Because it contained only slightly radioactive material, the release did not constitute a violation of NRC regulations, but, with all the uncertainties still surrounding the scene at TMI, the NRC Chairman ordered the discharges stopped. Late in the day, analyses of coolant samples confirmed the presence and showed something of the extent of the core damage that took place during the periods that the core was uncovered on Wednesday. (It was later determined that there had been three periods when a significant portion of the core was being cooled by steam rather than fluid coolant.) First concerns about the presence of a hydrogen bubble in the reactor vessel arose on Thursday, and the fact that there had been a hydrogen explosion outside the vessel in the containment building early Wednesday afternoon was brought to light.

Friday-March 30

Friday was the day when it became clear to all concerned that the event was far from over; that radiation releases from the auxiliary building were not under control and were increasing; that there was a large gaseous bubble in the reactor vessel which could conceivably expand, forcing the level of coolant below the top of the core, uncovering it again; that, according to some analyses and expert judgments, the bubble might become flammable as oxygen evolving from the decomposition of water by radiation made its way into the upper part of the vessel; that radiation was emanating from the facility in a manner neither planned nor controlled.

Early in the day, reports of a 1200 mr/hr reading above TMI-2 precipitated serious discussion at the NRC Operations Center in Bethesda of the possibly urgent need to evacuate the residents of Goldsboro, Middletown and other communities and areas around the plant, even out as far as Harrisburg. The fact that there was a consensus favoring such a recommendation at the Operations Center was relayed to State officials in Pennsylvania, occasioning considerable anxiety and confusion, since the judgment was not shared by people at the plant site. The NRC position was clarified when Chairman Hendrie spoke with Governor Thornburgh about 10:00 a.m., and counseled against full-scale evacuation of the population, suggesting instead that the Governor recommend that people stay indoors for awhile until the true situation could be better defined. The Governor did so. About 40 minutes later, President Carter contacted Chairman Hendrie and directed that a senior NRC official be dispatched to the TMI site as his personal representative; the President also assured that the White House staff would see to it that an adequate and dependable communications system would be set up as soon as possible between the site, the White House and the NRC. Prior to this, communications between the plant and the NRC had been unreliable and had even been lost for a time. The Director of NRC's Office of Nuclear Reactor Regulation, Harold Denton, left NRC headquarters for the TMI site with a support staff of 12 to serve as the President's representative and as the primary NRC official on the scene. Shortly after noon, Chairman Hendrie indicated to Governor Thornburgh by telephone that a recommendation by the Governor that pregnant women and pre-school aged children within five miles of the plant leave the area temporarily was advisable. The Governor made this recommendation soon afterwards.

Discussions and assessments of the possible need for total evacuation of the population near TMI continued throughout the day among NRC, other Federal and State officials. About an hour after the former's arrival at TMI and a first assessment of conditions in and around the plant, NRR Director Denton and Chairman Hendrie reviewed various possible courses the accident might take-or that licensee personnel might take in their effort to gain control of events-and the implications of each for a judgement on whether and when to move people out of the area. Within an hour of their conversation, Chairman Hendrie was in contact with Governor Thornburgh, at which time he advised the Governor that, though the bubble in the reactor vessel could cause trouble later in keeping the core cooled, there was no appreciable amount of oxygen in it and the chance of a hydrogen explosion such as took place in the containment on Wednesday was "close to zero." The Chairman also appraised the chance of a core meltdown as being extremely low, but the possibility of a significant radiation release as being somewhat higher.

Additional contingents of NRC personnel were sent to TMI during the day and by 4:00 p.m. there were 83 NRC staff people at the site. Other Federal agencies-DOE, EPA, FDAA and others-and State officials responsible for emergency management and radiation protection were also present in force. In a press release issued around 6:00 p.m., the NRC Chairman declared that there was "no imminent danger of a meltdown" of the reactor core. By 8:30 on Friday evening, Governor Thornburgh decided, having consulted with NRC officials on the site, to lift the advisory that people within five miles of the plant should stay indoors but, with NRC concurrence, continued to recommend that pregnant women and young children leave and/or stay out of the area.

By day's end, there was deep uncertainty among all concerned as to the potential hazard represented by the hydrogen bubble in the reactor vessel. National laboratories and industrial experts, as well as NRC research personnel, were at work calculating how long it might be until the amount of oxygen finding its way into the hydrogen bubble would produce a flammable

mixture in the upper portion of the vessel. Preliminary estimates of that time-frame varied. Later on it was realized that there was no appreciable build-up taking place because the oxygen resulting from the radiolytic decomposition of water was combining with free hydrogen in the reactor coolant.

Saturday-March 31

On Saturday the focus of concern had shifted from periodic uncontrolled radiation releases to potential explosion of the hydrogen in the reactor vessel. Radiation readings were very low everywhere but inside the containment. The NRC and other Federal presence at the site was expanding. The NRC Commissioners meeting in Washington, D.C., continued discussions of what changes in the situation might warrant a recommendation that people leave the TMI area, or whether such a recommendation should be made immediately, as a precaution. The conditions at TMI-2 were improved in virtually every respect, except for the hydrogen problem, and the Commissioners were

conscious of the hardships an evacuation would impose upon the population. There was also the matter of range to consider, whether to evacuate out to five miles or 10 miles or more, and of how much time would be available, if core conditions began to deteriorate, before the defensive barriers of the plant would be breached.

Around noon the NRC Chairman and NRR Director at the site discussed the situation at length, considering both the kinds of events that would signal a need to begin moving people out and also various means by which to reduce the hydrogen hazard. Soon afterwards, estimates were received from various research teams that the conditions necessary for hydrogen combustion or explosion in the reactor vessel were perhaps days away, and it appeared that there would be amply sufficient time to vent the vessel into the containment or otherwise defuse the danger. In mid-afternoon, Chairman Hendrie held a press conference at the NRC Operations Center in Bethesda, Md., at which he affirmed that a precautionary evacuation of the TMI area was still a possibility, especially if it were decided to try to force the hydrogen bubble out of the reactor vessel. Soon after, the Chairman and the Governor conferred by phone. Responding to the latter's query, the Chairman advised that, since some low-level releases of radiation were still coming from the auxiliary building, it would be prudent to continue the recommendation on pregnant women and pre-school aged children and to keep emergency planning personnel and resources in readiness.

Following a brief meeting with the staff at Bethesda, Chairman Hendrie left Washington to go to the TMI site. President Carter was to arrive there in the early afternoon for a tour of the scene and briefings on the status of the reactor. During the morning, the NRC personnel at the site had augmented the radiation monitoring equipment by placing 37 thermoluminescent dosimeters within a 12-mile radius of the plant. By mid-afternoon the bubble in the reactor vessel seemed to be dissipating and the system stabilizing, though intense discussion of the evacuation question continued among Commissioners and staff in Washington. Chairman Hendrie communicated the favorable change in the situation to the group in Washington and characterized the next phase in management of the accident as a choice between moving at once to activate decay heat removal from the reactor or moving slowly and letting the reactor cool at its own rate.

Reactor cooling was maintained by the action of one of the main coolant pumps providing the flow through the reactor core, and heat removal through one of the steam generators to the condenser, until about 2:00

Governor Thornburgh and President Carter are escorted into the TMI Unit 2 control room by a Metropolitan Edison employee on Sunday, April 1, 1979.

p.m. on April 27 when the reactor coolant pump was intentionally shut down and core cooling by natural circulation was achieved.

A bulletin was transmitted Sunday afternoon to all NRC licensees operating reactors of the B&W design to make an immediate review of plant conditions and to implement a number of precautionary measures derived from the TMI experience. NRC inspectors were also sent out to confirm that the prescribed actions were taken. The bulletin was the first in a series issued by NRC to licensees as analyses of the TMI accident revealed both necessary and prudential actions to be taken to prevent recurrence of the event (see "Bulletins and Orders Task Force," below).

Later in April, licensees for the other nuclear power plants employing B&W nuclear steam supply systems indicated that they would voluntarily shut down until prescribed alterations in design and procedures were completed. Confirmatory orders to that effect were subsequently issued by NRC for several of these units.

By the end of May, "dedicated" telephone lines had been established between the NRC Operations Center in Bethesda and 68 of the 70 licensed nuclear power plants and 14 licensed fuel cycle facilities. The lines make it possible for operations personnel in these facilities to communicate immediately and directly with members of the NRC's technical staff any time of the day or night on any day of the year. The system also provides for instant communication with any one of the five NRC regional offices.

The accident at TMI-2 generated investigations, reports, findings and recommendations literally too

Water from the industrial waste treatment systems of the plant (TMI), designed to be non-radioactive, is checked to insure that is has not been contaminated.

Plotting wind direction to assist in monitoring operations.

numerous to mention. The balance of this chapter attempts only to describe the major NRC undertakings in the matter and to cover the findings and recommendations of the commission appointed by President Carter to conduct an independent investigation of the accident and its implications, together with NRC's responses to those recommendations. At the time this report was prepared, the work of the NRC Special Inquiry Group-an investigatory body set up by the NRC under independent directorship-was not yet complete, nor had the various Congressional reviewers reported their results.

RADIOLOGICAL CONSEQUENCES TO PERSONS AND THE ENVIRONMENT

Individual and Collective Doses. NRC staff members participated in an interagency study to determine the individual and population doses associated with the TMI accident. The results of the study are presented and discussed in the NRC report, "Population Dose and Health Impact of the Accident at the Three Mile Island Nuclear Station" (NUREG-0558). Based on environmental measurements performed during the accident, it was estimated that the maximum individual off-site whole body dose was about 83 millirem, which is approximately one-sixth the NRC's allowable maximum whole body dose of 500 millirem-per-year. The population within 50 miles of the TMI site received an estimated integrated dose of 3,300 person-rem. This population dose is expected to result in less than one additional fatal cancer among the exposed population, in which 325,000 fatal cancers can be expected to occur as a result of other causes.

Radiation doses to licensee employees have also been estimated. Occupational whole-body doses accumulated from the date of the accident through May 31, 1979, totaled 225 person-rem. These doses were received by employees in performing recovery operations after the accident, such as changing filters in the cleanup systems for air leaving the auxiliary building and fuel-handling building, sampling of air and primary coolant, decontamination and radioactive waste processing operations, and routine inspection and maintenance activites. In the days immediately following the accident, four persons received exposures exceeding NRC regulatory standards. Two persons involved in taking a primary-coolant sample received doses substantially in excess of the standards. One person received a total body dose of 4.1 rem (the regulatory limit is 3.0 rem), an extremity (finger) dose of 147 rem (the limit is 18.75 rem) and skin dose to the top of the head of 13 rem (the limit is 7.5 rem). The second person received extremity doses of 54 rem. Two other persons received whole body exposures of 3.2 rem and 3.1 rem, which are slightly higher than the NRC limit of 3.0 rem.

Environmental Protection at Three Mile Island. During the accident at Three Mile Island there was concern that a core meltdown might occur. This could have led to the contamination of the groundwater of the island and ultimately of the Susquehanna River and beyond. The staff developed contingency plans to mitigate the effects of groundwater contamination by isolating the immediate plant area from the regional water supplies. The plans provide for blocking groundwater movement, for withdrawing the potentially contaminated water, and for monitoring and temporarily storing the contaminated water. Working with the U.S. Army Corps of Engineers, the NRC staff formulated a plan to construct a bentonite-cement cut-off wall, dewatering wells, and a pumping system. Availability of equipment needed to carry out the plan was verified. Plans for monitoring and on-site storage were also completed. It did not prove necessary to implement planned isolation of the area.

Another problem encountered in the accident was the need for the staff to produce estimates of the transport or diffusion of gaseous releases, in order to plan for possible evacuation of the population and for assessment of the consequences thereof. These estimates were made by staff meteorologists assigned to the NRC Operations Center. Well into the accident, the staff ascertained that meteorological data were available from the TMI meteorological tower by remote access and made use of this information. In addition, the staff arranged for National Weather Service (NWS) to provide supplementary meteorological instrumentation at the site. The staff established communications with and utilized the forecasting services

of the NWS Harrisburg River Forecast Center and NWS Philadelphia Area Weather Forecast Center. The staff's estimates of the transport and diffusion of TMI releases were used in estimating doses for the locations of dose-rate instrumentation both on the TMI site and off. Because the magnitude of the release was unknown during the early stages of the accident, data from environmental monitors and meteorological estimates were used to calculate releases. Atmospheric transport estimates were used to advise evacuation planners.

In further protection of the environment, the possible non-radiological consequences to the aquatic biota and recreational fisheries of the Susquehanna River from the accident at Three Mile Island Nuclear Station in late March of 1979 were investigated up through the post-accident period (through June). Data used in the investigation included site-specific biological and water quality information collected by the license. under the Environmental Technical Specifications and National Pollutant Discharge Elimination System monitoring programs, and also information from State and Federal agencies, knowledgeable persons, and