Thread by @NuclearHazelnut

Chornobyl (1986) & Fukushima (2011) are very well-known #nuclear disasters… but have you ever heard about Three Mile Island incident?

This coming May 4th many people will be talking about this event (thanks, @netflix), so let’s inform ourselves to know the FACTS about it.

Long story short:

In 1979 at Three Mile Island nuclear power plant in the USA, a cooling malfunction caused part of the core to melt in the #2 reactor. The TMI-2 reactor was destroyed.

Here’s what happened 👇

The Three Mile Island (TMI) Generating Station is a closed nuclear power plant located in Pennsylvania, USA 🇺🇸

It’s composed of 2 pressurized water reactors, TMI-1 & TMI-2, that started generating electricity in 1974 and 1978, respectively.

It was March 28, 1979, at 4 am, when TMI-2 was operating at 97% power.

The event was caused by a minor malfunction in the secondary cooling circuit which caused the temperature in the primary loop to raise

About 1 sec after this, the reactor was able to shut down automatically

Let’s remember that in a PWR like TMI-2, water is used as the primary cooling system for the reactor & it’s maintained at a very high pressure to keep it from boiling, & that’s the job of the pressurizer.

So, after the shutdown….

The pressurizer pressure started to exceed maximum levels so it was the job of the pilot-operated relief valve (PORV) to open and reduce that pressure.

BUT the PORV failed to close! & started leaking vital reactor coolant water used to keep it from overheating.

On top of this, the failure wasn’t revealed by the instrumentation! 😨 keeping the operators in the dark about this issue.

I know, too technical… So let’s imagine an opened champagne bottle being constantly shaken:

The system started automatically to mitigate the loss of cooling water, providing replacement water into the system.

BUT since the PORV was still open (🍾), water & steam escaped through it causing to raise the water levels in the pressurizer!

The operator's main objective was to keep the pressurizer from filling with water because if not, they couldn’t be able to control the pressure in the cooling system and it might rupture.

So they reduced the flow of replacement water…😨

Steam then formed in the system, & this caused the cooling pumps to vibrate; since this could damage the pumps, they were shut down ending the forced cooling of the reactor.

The operators still believed the system was nearly full of water due to the pressurizer levels indicated!

However, as coolant water boiled away, the reactor's fuel core was uncovered and became even hotter.
Following, the fuel rods were damaged & released radioactive material into the cooling water!

To have a better idea, check it out 👇

It was 6:22 am & in order to stop the loss of water through the PORV the operators closed a block valve between it and the pressurizer

However, superheated steam and gases blocked the flow of water through the core cooling system.🥵

Later that morning, operators tried to force more water into the reactor system to condense steam bubbles that they believed were blocking the flow of cooling water.


One hour later, operators were able to restore the forced cooling of the reactor core!👏

RADIOACTIVE RELEASES

Were there any radioactive releases? Well…
The compressors leaked & some gases went through high-efficiency particulate air (HEPA) filters and charcoal filters which removed most of the radionuclides, except for the noble gases.

TMI-2 radioactive gases released did not pose a health hazard because they were biologically inert and had a short half-life.

WONDERING ABOUT THE HYDROGEN BUBBLE?

When the reactor’s core was uncovered, there was a chemical reaction between the water & the zircaloy metal tubes holding the nuclear fuel pellets, this created hydrogen gas!

This time the instruments were able to indicate a hydrogen burn.

Operators removed this hydrogen "bubble" by periodically opening the vent valve on the pressurizer.

The @NRCgov officials believed the hydrogen bubble could explode, but it was never possible since there wasn’t enough oxygen in the system.

SHUTDOWN & ANALYSIS

After an anxious month, operators started to cool down the reactor by the natural movement of water; & almost 4yr later the head of the reactor was removed in order to access the remains of the core.

A later investigation revealed that at least 45% of the core had melted!

Most of the melted core material had remained in the core region & this was confirmed in 1988 by a multinational investigation. Samples taken indicated that there was less damage than anticipated.

PUBLIC CONCERN & HEALTH IMPACTS

As expected, the TMI incident caused concerns about the possibility of radiation-induced health effects in the area surrounding the plant.

So @PAHealthDept took action on this matter to ensure the safety of the public.

For over 18 years, @PAHealthDept kept a registry of more than 30,000 people who lived within 5 miles of TMI at the time of the accident in order to record any health effects, & it was discontinued in 1997, without any evidence of unusual health trends in the area.

Also, a lot of independent studies have assessed the radiation releases & possible effects on the 👬 & 🌿since the incident none has found any adverse health effects such as cancers that might be linked to the accident.

Wanna read more about it? 📖
world-nuclear.org/information-li…

ans.org/pi/resources/s…

allthingsnuclear.org/dlochbaum/nucl…

ydr.com/picture-galler…