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Three
Mile Island
Report
The
accident at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near
Middletown, Pennsylvania, on March 28, 1979, was the most serious in U.S.
commercial nuclear power plant operating history, even though it led to no
deaths or injuries to plant workers or members of the nearby
community.
But
it brought about sweeping changes involving emergency response planning,
reactor operator training, human factors engineering, radiation
protection, and many other areas of nuclear power plant operations. It
also caused the U.S. Nuclear Regulatory Commission to tighten and heighten
its regulatory oversight. Resultant changes in the nuclear power industry
and at the NRC had the effect of enhancing safety. The sequence of certain
events - - equipment malfunctions, design related problems and worker
errors - - led to significant damage to the TMI-2 reactor core but only
very small off-site releases of radioactivity.
Summary
of Events
The
accident began about 4:00 a.m. on March 28, 1979, when the plant experienced a
failure in the secondary, non-nuclear section of the plant. The main feedwater
pumps stopped running, caused by either a mechanical or electrical failure,
which prevented the steam generators from removing heat. First the turbine, then
the reactor automatically shut down. Immediately, the pressure in the primary
system (the nuclear portion of the plant) began to increase. In order to prevent
that pressure from becoming excessive, the pressurizer relief valve (a valve
located at the top of the pressurizer) opened. The valve should have closed when
the pressure decreased by a certain amount, but it did not. Signals available to
the operator failed to show that the valve was still open. As a result, the
stuck-open valve caused the pressure to continue to decrease in the system.
 
World
Nuclear Association Graphic
Meanwhile, another problem
appeared elsewhere in the plant. The emergency feedwater system (backup to main
feedwater) was tested 42 hours prior to the accident. As part of the test, a
valve is closed and then reopened at the end of the test. But this time, through
either an administrative or human error, the valve was not reopened - -
preventing the emergency feedwater system from functioning. The valve was
discovered closed about eight minutes into the accident. Once it was reopened,
the emergency feedwater system began to work correctly, allowing cooling water
to flow into the steam generators.
As the system pressure in the
primary system continued to decrease, voids (areas where no water is present)
began to form in portions of the system other than the pressurizer. Because of
these voids, the water in the system was redistributed and the pressurizer
became full of water. The level indicator, which tells the operator the amount
of coolant capable of heat removal, incorrectly indicated the system was full of
water. Thus, the operator stopped adding water. He was unaware that, because of
the stuck valve, the indicator can, and in this instance did, provide false
readings.
Because adequate cooling was not
available, the nuclear fuel overheated to the point where some of the zirconium
cladding (the long metal tubes or jackets which hold the nuclear fuel pellets)
reacted with the water and generated hydrogen. This hydrogen was released into
the reactor containment building. By March 30, two days after the start of the
chain of events, some hydrogen remained within the primary coolant system in the
vessel surrounding the reactor, forming a "hydrogen bubble" above the
reactor core.
The concern was that if reactor
pressure decreased, the hydrogen bubble would expand and thus interfere with the
flow of cooling water through the core. Over the next few days, the bubble was
reduced by "degassing" the pressurizer -- adjusting air and water
pressure.
Without water to cool it, and
with the top of the reactor core uncovered, the primary damage to the reactor
occurred two to three hours into the accident. Although no "meltdown"
occurred in the classic sense of the word, in that fuel did not "melt"
through the floor beneath the containment or through the steel reactor vessel, a
significant amount of fuel did in fact melt. Radioactivity in the reactor
coolant increased dramatically, and there were small leaks in the reactor
coolant system which caused high radiation levels in other parts of the plant
and small releases into the environment. Shortly after the accident began, some
of the water, carrying fuel debris and fission products, escaped from the
reactor coolant system and flowed into the reactor building basement. By the
time the accident had ended, the water in the basement had been heated by
residual heat from the reactor vessel, evaporated, condensed on the walls, and
drained down onto the floors and back into the basement. The radionuclides then
permeated into the porous surfaces of concrete and layers of iron which later
became corroded (this area of the plant became a major focus of the subsequent
clean-up and decontamination).
Response to the accident was
swift. The NRC's regional office in King of Prussia, Pennsylvania, was notified
at 7:45 a.m. on March 28. By 8:00, the NRC headquarters in Washington, D.C. was
alerted and the NRC Operations Center in Bethesda, Maryland, was activated. The
regional office promptly dispatched the first team of inspectors to the site and
other agencies, such as the Department of Energy, and the Environmental
Protection Agency, also mobilized their response teams. Helicopters hired by
TMI's owner, General Public Utilities Nuclear, and the Department of Energy were
sampling radioactivity in the atmosphere above the plant by midday. A team from
the Brookhaven National Laboratory was also sent to assist in radiation
monitoring. At 9:15 a.m., the White House was notified and at 11:00 a.m., all
non-essential personnel were ordered off the plant's premises.
Drawing from Metropolitan Edison Co.
"Report to the Met-Ed Community" May 30, 1979
From the early stages of the
accident, low levels of radioactive gas, mostly in the form of xenon, continued
to be released to the environment. At the time, efforts to halt the releases
were unsuccessful and there was some fear of an explosion from the buildup of
hydrogen - -fortunately, this did not occur.
Richard
L. Thornburgh
However,
on Friday, March 30, Governor Thornburgh of Pennsylvania ordered a precautionary
evacuation of preschool children and pregnant women from within the 5-mile zone
nearest the plant, and suggested that people living within 10 miles of the plant
stay inside and keep their windows closed. Most evacuees had returned to their
homes by April 4 -- by that time, the situation at the reactor had been brought
under control.
The American Nuclear Insurers, an
organization made up of nuclear insurance firms, had already begun distributing
checks to evacuees to cover hotel and meal expenses, and was beginning to handle
claims for property and liability losses.
Health Effects
Detailed studies of the
radiological consequences of the accident have been conducted by the NRC, the
Environmental Protection Agency, the Department of Health, Education and Welfare
(now Health and Human Services) , the Department of Energy, and the State of
Pennsylvania. Several independent studies have also been conducted. Estimates
are that the average dose to about 2 million people in the area was about only
about 1 millirem. To put this into context, exposure from a full set of chest
x-rays is about 6 millirem. Compared to the natural radioactive background dose
of about 100-125 millirem per year for the area, the collective dose to the
community from the accident was very small. The maximum dose to a person at the
site boundary would have been less than 100 millirem.
In the months following the
accident, although questions were raised about possible adverse effects from
radiation on human, animal, and plant life in the TMI area, none could be
directly correlated to the accident. Thousands of environmental samples of air,
water, milk, vegetation, soil, and foodstuffs were collected by various groups
monitoring the area. Very low levels of radionuclides could be attributed to
releases from the accident. However, comprehensive investigations and
assessments by several well-respected organizations have concluded that in spite
of serious damage to the reactor, most of the radiation was contained and that
the actual release had negligible effects on the physical health of individuals
or the environment.
Impact of the
Accident
Today, the TMI-2 reactor is
permanently shut down and defueled, with the reactor coolant system
decontaminated, the radioactive liquids treated, most components shipped to a
licensed low-level waste disposal site, with the remainder of the site being
monitored. The owner, General Public Utilities Nuclear Corporation, says it will
keep the facility in long-term storage until the operating license for the TMI-1
plant expires in 2014, at which time both plants will be decommissioned.
The causes of the accident
continue to be debated to this day. However, based on a series of
investigations, the main factors appear to have been a combination of personnel
error, design deficiencies, and component failures. There is no doubt that the
accident at Three Mile Island permanently changed both the nuclear industry and
the NRC. Public fear and distrust increased, NRC's regulations and oversight
became broader and more robust, and management of the plants was scrutinized
more carefully. The problems identified from careful analysis of the events
during those days have led to permanent and sweeping changes in how NRC
regulates its licensees - - which, in turn, has strengthened public health and
safety.
Here are some of the major
changes which have occurred since the accident:
- Expansion of NRC's resident
inspector program - first authorized in 1977 - whereby at least two
inspectors live nearby and work exclusively at each plant in the U.S to
provide daily surveillance of licensee adherence to NRC regulations;
- Establishment of the
Systematic Assessment of Licensee Performance (SALP) program to integrate
NRC observations, findings, and conclusions about licensee performance and
management effectiveness into a periodic, public report;
- Regular analysis of plant
performance by senior NRC managers who identify those plants needing
additional regulatory attention;
- Expansion of
performance-oriented as well as safety-oriented inspections, and the use of
risk assessment to identify vulnerabilities of any plant to severe
accidents;
- Strengthening and
reorganization of enforcement as a separate office within the NRC;
- Upgrading and strengthening of
plant design and equipment requirements. This includes fire protection,
piping systems, auxiliary feedwater systems, containment building isolation,
reliability of individual components (pressure relief valves and electrical
circuit breakers), and the ability of plants to shut down automatically;
- Identifying human performance
as a critical part of plant safety, revamping operator training and staffing
requirements, followed by improved instrumentation and controls for
operating the plant, and establishment of fitness for duty programs for
plant workers to guard against alcohol or drug abuse;
- Enhancement of emergency
preparedness to include immediate NRC notification requirements for plant
events and an NRC operations center which is now staffed 24 hours a day.
Drills and response plans are now tested by licensees several times a year,
and state and local agencies participate in drills with the Federal
Emergency Management Agency and NRC;
- The installing of additional
equipment by licensees to mitigate accident conditions, and monitor
radiation levels and plant status;
- The establishment of the
Institute of Nuclear Power Operations (INPO), the industry's own
"policing" group, and formation of what is now the Nuclear Energy
Institute to provide a unified industry approach to generic nuclear
regulatory issues, and interaction with NRC and other government agencies;
- Employment of major
initiatives by licensees in early identification of important safety-related
problems, and in collecting and assessing relevant data so lessons of
experience can be shared and quickly acted upon;
- Expansion of NRC's
international activities to share enhanced knowledge of nuclear safety with
other countries in a number of important technical areas.
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