On March 28, 1979, the most significant commercial nuclear power accident in the U.S. to date
occurred at the 3-mile island power plant in Dauphin County, Pennsylvania
and resulted in a partial meltdown of the number 2 reactor core
and the release of radioactive coolant water and some gas.
The accident occurred just 12 days after the release of the US thriller “The China Syndrome”
about a potential meltdown at a US nuclear power plant which only heightened the concern
when 3-mile island occurred and increased the calls to halt nuclear power in the US.
42 years on and we are in a situation where nuclear will be required to fill in the gaps
as our reliance on renewables as they increases and fossil fuels such as gas and coal are reduced
but what happened to 3-mile island and how will it affect us in the future .
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From 1963 to 1979 the number of nuclear power plants built globally almost doubled every
year. The nuclear age of the 50s and 60s promised power too cheap to meter and as a result, nuclear
power in the 70s was going through a boom time. 3-Mile island was one of these new nuclear power
stations. It had two pressurized water reactors, TMI-1, an 800 MW reactor going online in 1974
and TMI-2, a 906 MW reactor in 1978. TMI-1 was at the time and up until 2019
one of the best performing units in the US. At the time of the accident in 1979,
TMI-2 was in terms of age was almost brand new, so what happened to cause its failure.
At 4am on the 28th March 1979, TMI-2 was working at 97% capacity when a minor malfunction in the
secondary cooling circuit caused the temperature of the primary cooling circuit to rise.
Within 1 second, this triggered an automatic reactor shutdown but unbeknownst to the operators,
a relief valve stuck open and was not shown by the instrumentation. This allowed most of the coolant
from the reactor core to drain away and even though the reactor was shut down it was still
very hot and without the pressurised coolant the core suffered a partial meltdown.
That is the simplified version but in reality, things were a bit more complicated with systems
design and human error is as much to blame as the initial mechanical fault.
As the relief valve stuck open, water surged into the pressuriser, a tank that is part of
the primary cooling circuit. The operators acting on their training which told them the level in the
tank was the only reliable measure of water in the system, saw the rise in the levels of pressuriser
tank and reduced the flow thinking that the system might rupture through having too much water in it.
Steam formed in the primary circuit which was pumped around the system causing the pumps to
vibrate violently and thinking the system was full anyway they turned off the pumps.
This stopped the flow of water which because of its low level then boiled away causing the fuel
rods to be exposed and heat up releasing radioactive material into what water was left.
At 6:22 am, 2 hours and 22 minutes after the reactor had shut down,
the operators closed a block valve and stopped the loss of water from the primary circuit
but by then super-heated steam and gases were blocking the flow of coolant water.
Throughout the day they tried to force more water into the system to condense and
collapse the steam bubbles and eventually were able to restart one of the pumps.
There was a build-up of hydrogen gas when the zircaloy metal tubes holding the nuclear fuel
pellets were exposed and a high-temperature reaction with the remaining water occurred,
shortly afterwards there was a limited hydrogen burn but there wasn’t enough oxygen in the system
to cause an explosion. The remainder of the unburnt hydrogen was gradually released.
On the 29th and 30th of March, the operators using a system of pipes and compressors
removed the remainder of the unburnt hydrogen and other gases to a decay tank.
All the radionuclides except some noble gases including krypton had
been removed by the filters and this was also released into the atmosphere, but because of
its short half-life and being biologically inert, these did not pose a health hazard.
After a month of careful control and settling down the reactor
they eventually established a natural convection circulation where the reactor was said to be in a
cold shut down with the coolant water at under 100 degrees celsius and at atmospheric pressure.
In the subsequent investigation, it was shown that there was insufficient instrumentation to tell the
operators what was going on and some of the ones that were important to them making the correct
decisions in the heat of the moment were in places that were out of sight or behind control panels.
The operators also followed a book of procedures that seemed to best fit the events they saw but
without going through a Yes, No checklist and with missing instrument information many of the actions
they took were incorrect and made matters worse. It wasn’t until July 1984 over 5 years later
when the head of the reactor was removed was the damage was seen first-hand. Over 45 % of the core,
about 62 tonnes of material had melted with 19 tonnes reaching the lower part of the reactor head
pressure vessel but it wasn’t seriously damaged. The cleanup which started in August 1979 was
also equally long-winded and very complicated and expensive. It took almost 12 years ending in 1991,
cost nearly a billion dollars and was carried out by over 1000 skilled workers.
Shortly after the start of the clean up, all connections with the TMI-1 reactor were severed,
at the time of the incident the TMI-1 reactor was shut down for
refuelling and it stayed that way until 1985. All the plant surfaces had to be decontaminated
and the 10.6 million litres of water used had to be stored and then ultimately evaporated.
Approximately 100 tonnes of damaged uranium fuel rods had to be removed and processed
and the remainder stored in concrete and the whole reactor de-fueled.
Today, TMI-2 reactor is in long-term monitored storage.
Although over a third of the fuel in the reactor melted there was no china syndrome and the
containment building worked as designed, there was a limited release of some radioactivity but with
little to no health hazards, this was confirmed by the thousands of samples and measurements
that were taken during and after the accident. Now whilst the damage to the reactor might
not have been as bad as first thought, the nuclear industry itself was badly damaged by
the 3-mile island incident which saw a sharp decline in the 80s and 90s.
Much of this was this was down to the sense of panic that ensued with poor communications between
the power company and the Department of Energy and between state and federal agencies and the media
which drove a policy based on what they thought could happen when they
didn’t actually know the status of the plant. The partial hydrogen burn which was indicated
by a rapid pressure rise, led those in the government agencies in charge to think there
could be a hydrogen explosion like those that occurred at Fukushima in 2011 and that there
could be a meltdown that breached the containment building, the so called China Syndrome. Both of
these scenarios were proved incorrect in 1985 the access was finally gained to the core.
That and the mixed messaging for the media only fueled the sense of panic, but in the end the
damage was nowhere near the scale of Fukushima and Chernobyl was just a whole different thing.
More than a dozen major studies have been carried out since the incident at 3-mile
island on the people living in the area around it and the environment and none found any adverse
effects such as increased cancers beyond the initial stress of the incident itself.
What it did bring about was a much greater emphasis on training for when things go wrong
and keeping the cooling systems working no matter what cause of an incident. This and
better instrumentation not just in the US but in the rest of the world made nuclear safer overall.
The number of significant events decreased from 2.38 per reactor unit in 1985
to 0.10 at the end of 1997. Fukushima on the other hand
wasn’t a failure of the reactor which was a US design, but a failure of planning by allowing the
placement of the backup generators on ground level behind a seawall when the tsunami hit.
In 2019, 3-mile island was shut down completely 10 years earlier than expected due to the commercial
pressures from cheap gas but as we have seen that situation won't last forever and until we have
worldwide 24/7 renewables and not just ones that rely on the sun shining and the wind blowing,
like it or not nuclear will be our fallback. So I hope you enjoyed the video and if you did
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