This Saturday marks the 30th anniversary of the partial meltdown
of the Three Mile Island (TMI) nuclear reactor. This occasion is a
good time to consider the advances in nuclear power safety since
that time and discuss the misinformation about this incident and
the 1986 nuclear accident in Chernobyl, Ukraine, which is often
associated with TMI.
Three Mile Island: What Happened
On March 28, 1979, a cooling circuit pump in the non-nuclear
section of Three Mile Island's second station (TMI-2)
malfunctioned, causing the reactor's primary coolant to heat and
internal pressure to rise. Within seconds, the automated response
mechanism thrust control rods into the reactor and shut down the
core. An escape valve opened to release pressure but failed to
close properly. Control room operators only saw that a "close"
command was sent to the relief valve, but nothing displayed the
valve's actual position. With the valve open, coolant escaped
through the pressurizer, sending misinformation to operators that
there was too much pressure in the coolant system. Operators then
shut down the water pumps to relieve the "pressure."
Operators allowed coolant levels inside the reactor to fall,
leaving the uranium core exposed, dry, and intensely hot. Even
though inserting control rods halted the fission process, the TMI-2
reactor core continued to generate about 160 megawatts of "decay"
heat, declining over the next three hours to 20 megawatts.
Approximately one-third of the TMI-2 reactor was exposed and began
By the time operators discovered what was happening, superheated
and partially radioactive steam built up in auxiliary tanks, which
operators then moved to waste tanks through compressors and pipes.
The compressors leaked. The steam leakage released a radiation dose
equivalent to that of a chest X-ray scan, about one-third of the
radiation humans absorb in one year from naturally occurring
background radiation. No damage to any person, animal, or plant
was ever found.
The local population of 2 million people received an average
estimated dose of about 1 millirem--miniscule compared to the
100-125 millirems that each person receives annually from naturally
occurring background radiation in the area. Nationally, the average
person receives 360 millirems per year.
No significant radiation effects on humans, animals, or plants
were found. In fact, thorough investigation and sample testing of
air, water, milk, vegetation, and soil found that there were
negligible effects and concluded that the radiation was safely
contained. The most recent and comprehensive study was
a 13-year evaluation of 32,000 people living in the area that found
no adverse health effects or links to cancer.
Technological Improvements and Lessons
A number of technological and procedural changes have been
implemented by industry and the Nuclear Regulatory Commission (NRC)
to considerably reduce the risk of a meltdown since the 1979
incident. These include:
- Plant design and equipment upgrades, including fire protection,
auxiliary feedwater systems, containment building isolation, and
automatic plant shut down capabilities;
- Enhanced emergency preparedness, including closer coordination
between federal, state, and local agencies;
- Integration of NRC observations, findings, and conclusions
about plant performance and management into public reports;
- Regular plant performance analysis by senior NRC managers who
identify plants that require additional regulatory attention;
- Expansion of NRC's resident inspector program, whereby at least
two inspectors live nearby and work exclusively at each plant;
- Expanded performance- and safety-oriented inspections;
- Additional accident safety equipment to mitigate and monitor
conditions during accidents; and
- Establishment of the Institute for Nuclear Power Operators, an
industry-created non-profit organization that evaluates plants,
promotes training and information sharing, and helps individual
plants overcome technical issues.
Chernobyl: What Happened
Seven years after the incident at Three Mile Island, on April
25, 1986, a crew of engineers with little background in reactor
physics began an experiment at the Chernobyl nuclear station. They
sought to determine how long the plant's turbines' inertia could
provide power if the main electrical supply to the station was cut.
Operators chose to deactivate automatic shutdown mechanisms to
carry out their experiment.
The four Chernobyl reactors were known to become unstable at low
power settings, and the engineers' experiment caused the
reactors to become exactly that. When the operators cut power and
switched to the energy from turbine inertia, the coolant pump
system failed, causing heat and extreme steam pressure to build
inside the reactor core. The reactor experienced a power surge and
exploded, blowing off the cover lid of the reactor building, and
spewed radioactive gasses and flames for nine days.
The episode was exacerbated by a second design flaw: The
Chernobyl reactors lacked fully enclosed containment buildings, a
basic safety installation for commercial reactors in the U.S.
Chernobyl was the result of human error and poor design. Of the
approximately 50 fatalities, most were rescue workers who entered
contaminated areas without being informed of the danger.
The World Heath Organization says that up to 4,000 fatalities
could ultimately result from Chernobyl-related cancers. Though
these could still emerge, as yet, they have not. The primary health
effect was a spike in thyroid cancer among children, with
4,000-5,000 children diagnosed with the cancer between 1992 and
2002. Of these, 15 children unfortunately died. Though these deaths
were unquestionably tragic, no clear evidence indicates any
increase in other cancers among the most heavily affected
Interestingly, the World Health Organization has also identified
a condition called "paralyzing fatalism," which is caused by
"persistent myths and misperceptions about the threat of
radiation." In other words, the propagation of
ignorance by anti-nuclear activists has caused more harm to the
affected populations than has the radioactive fallout from the
actual accident. Residents of the area assumed a role of "chronic
dependency" and developed an entitlement mentality because of the
Technology Improvements and Lessons
Comparing the technology of the nuclear reactor at Chernobyl to
U.S. reactors is not fair. First, the graphite-moderated,
water-cooled reactor at Chernobyl maintained a high positive void
coefficient. While the scientific explanation of this
characteristic is not important, its real-life application is.
Essentially, it means that under certain conditions, coolant
inefficiency can cause heightened reactivity. In other words, its
reactivity can rapidly increase as its coolant heats (or is lost)
resulting in more fissions, higher temperatures, and ultimately
This is in direct contrast to the light-water reactors used in
the United States, which would shut down under such conditions.
U.S. reactors use water to both cool and moderate the reactor. The
coolant keeps the temperature from rising too much, and the
moderator is used to sustain the nuclear reaction. As the nuclear
reaction occurs, the water heats up and becomes a less efficient
moderator (cool water facilitates fission better than hot water),
thus causing the reaction to slow down and the reactor to cool.
This characteristic makes light water reactors inherently safe and
is why a Chernobyl-like reactor could never be licensed in the
Given the inherent problems with the Chernobyl reactor design,
many technological changes and safety regulations were put in place
to prevent another Chernobyl-like meltdown from occurring.
Designers renovated the reactor to make it more stable at lower
power, have the automatic shutdown operations activate quicker, and
have automated and other safety mechanisms installed.
Chernobyl also led to the formation of a number of international
efforts to promote nuclear power plant safety through better
training, coordination, and implantation of best practices. The
World Association of Nuclear Operators is one such organization and
includes every entity in the world that operates a nuclear power
The circumstances, causes, and conditions of the Chernobyl
meltdown are far removed from the American experience. Important
lessons should be taken from both accidents. Thankfully, many
improvements in the technology and regulatory safety of nuclear
reactors are among them.
Spencer is Research Fellow in Nuclear Energy and Nicolas D.
Loris is a Research Assistant in the Thomas A. Roe Institute for
Economic Policy Studies at The Heritage Foundation.