TMI...what happened

[TMI could not happen again, not that any one was harmed or injured in
anyway. It is good, howeer, to reveiw what happened. Here a example of
an serious contribution to the debate on nuclear energy--David
Walters]

We have come a long way in the years following Three Mile Island.

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(1),
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 a partial meltdown of
the TMI-2 reactor core but only very small off-site releases of
radioactivity. - Source: nuclear Regulatory CommissionFact *** on
the Three Mile Island Accident

Without giving away possible classified information, I'll try to
explain what happened at TMI.

First, I'll try to explain is simple terms how a nuclear power plant
works.

The way a nuclear power plant works is to use the heat given off by
nuclear fission to heat water. That water, which is maintained on the
contained side of the nuclear plant is then used to heat water on the
secondary side of the plant and turn it into steam so that the steam
can propel a turbine attached to a generator.

This may be more easily conceived if put it into something that many
will understand. Imagine your car's engine is a nuclear power plant.
Extend the length of the hoses on your cars radiator, and place the
radiator in the pool. If you run your engine, the water pump causes
water to flow through the radiator. Since the radiator is in the pool,
the heat from your engine would be transferred to the pool. Notice
that the coolant from your radiator does not mix with the water in the
pool.

In a nuclear power plant we want that water to get hot enough to make
the water on the other side of the radiator turn to steam. This
presents us with two problems. In order for the water on the reactor
side to turn the water on the other side to steam, it must be hotter
than 212 degrees Fahrenheit. Water above 212 degrees is not easy to
pump, because it would be steam. To increase the temperature, and
still keep the water liquid, we need to increase the pressure on the
water. This is done in modern, pressurized, automotive cooling
systems. Since the water is liquid, and doesn't compress very well, we
need to have a way of controlling the pressure by changing the volume
of the container. This is done by using a pressurizer. Many home well
systems use a similar method to control fluctuations in pressure as
the pump turns on and off. This pressurizer maintains a steam bubble
on top of the liquid. In homes this is generally done by using an
inert gas bubble. If pressure gets too high steam is bled out thru a
valve on top of the pressurizer. This causes a very small change in
the amount of liquid and provides better control of the pressure in
the plant.

Now that you have a basic understanding of how a nuclear power plant
works, let's look at the TMI accident.

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 pilot-operated 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, cooling water poured out of the
stuck-open valve and caused the core of the reactor to overheat.

As coolant flowed from the core through the pressurizer, the
instruments available to reactor operators provided confusing
information. There was no instrument that showed the level of coolant
in the core. Instead, the operators judged the level of water in the
core by the level in the pressurizer, and since it was high, they
assumed that the core was properly covered with coolant. In addition,
there was no clear signal that the pilot-operated relief valve was
open. As a result, as alarms rang and warning lights flashed, the
operators did not realize that the plant was experiencing a loss-of-
coolant accident. They took a series of actions that made conditions
worse by simply reducing the flow of coolant through the core.

Because adequate cooling was not available, the nuclear fuel
overheated to the point at which the zirconium cladding (the long
metal tubes which hold the nuclear fuel pellets) ruptured and the fuel
pellets began to melt. It was later found that about one-half of the
core melted during the early stages of the accident. Although the
TMI-2 plant suffered a severe core meltdown, the most dangerous kind
of nuclear power accident, it did not produce the worst-case
consequences that reactor experts had long feared. In a worst-case
accident, the melting of nuclear fuel would lead to a breach of the
walls of the containment building and release massive quantities of
radiation to the environment. But this did not occur as a result of
the Three Mile Island accident.

The accident caught federal and state authorities off-guard. They were
concerned about the small releases of radioactive gases that were
measured off-site by the late morning of March 28 and even more
concerned about the potential threat that the reactor posed to the
surrounding population. They did not know that the core had melted,
but they immediately took steps to try to gain control of the reactor
and ensure adequate cooling to the core. The NRC's regional office in
King of Prussia, Pennsylvania, was notified at 7:45 a.m. on March 28.
By 8:00, 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.

By the evening of March 28, the core appeared to be adequately cooled
and the reactor appeared to be stable. But new concerns arose by the
morning of Friday, March 30. A significant release of radiation from
the plant's auxiliary building, performed to relieve pressure on the
primary system and avoid curtailing the flow of coolant to the core,
caused a great deal of confusion and consternation. In an atmosphere
of growing uncertainty about the condition of the plant, the governor
of Pennsylvania, Richard L. Thornburgh, consulted with the NRC about
evacuating the population near the plant. Eventually, he and NRC
Chairman Joseph Hendrie agreed that it would be prudent for those
members of society most vulnerable to radiation to evacuate the area.
Thornburgh announced that he was advising pregnant women and pre-
school-age children within a 5-mile radius of the plant to leave the
area.

Within a short time, the presence of a large hydrogen bubble in the
dome of the pressure vessel, the container that holds the reactor
core, stirred new worries. The concern was that the hydrogen bubble
might burn or even explode and rupture the pressure vessel. In that
event, the core would fall into the containment building and perhaps
cause a breach of containment. The hydrogen bubble was a source of
intense scrutiny and great anxiety, both among government authorities
and the population, throughout the day on Saturday, March 31. The
crisis ended when experts determined on Sunday, April 1, that the
bubble could not burn or explode because of the absence of oxygen in
the pressure vessel. Further, by that time, the utility had succeeded
in greatly reducing the size of the bubble.

We learn from our mistakes. In the many years since TMI we have done a
pretty good juob of keeping nuclear power plants safe. Solar and wind
power are good alternatives, but numerous small nuclear plants may be
our best option.

.