#NuclearEnergy Someone was watching the HBO Chernobyl mini-series and commenting on parts of it. If you haven't seen it I highly recommend you do. I assured him that US reactors cannot do what they did at Chernobyl. Someone said sure, that's true, but they can Fukushima. 1/

Fukushima, Japan. March 11, 2011. A magnitude 9.0 earthquake stuck off the coast. There were 6 reactors at the plant, 3 were online, 3 were S/D for maintenance. All 6 survived the EQ within normal parameters. All 6 cores & SFPs were intact. 2/

Then came the tsunami. Calling these waves is like calling the Sun hot. The largest wave was 45 feet tall and swept right over the 20 ft seawall protecting the facility. This is what caused the meltdowns, by destroying nearly all electrical power at the facility. 3/

The Great Tohoku Earthquake was a beyond design basis event. As simply as I can put it, NPPs are designed to withstand events based on their location and the geological and meteorological history there, (with a lot of conservatism.) The EQ hit .561g w/ a design of .447g. 4/

Figure is from the INPO report. http://hps.org/documents/INPO_Fukushima_Special_Report.pdf
The largest wave was ~3 times the design basis wave of 15 feet. The wave took out all AC power on Units 1-5, and left 1 EDG on Unit 6. It also destroyed the intake structure where the Ultimate Heat Sink gets it's water. 5/

Over-simplified, but all 3 online cores lost power & cooling & melted down as did SFPs. Now let's talk about the US response to the event. NRC order: https://www.nrc.gov/docs/ML1205/ML12054A735.pdf
The final plan is called FLEX. Every single NPP in the US had to comply with the order. 6/

FLEX is defense in depth for beyond design basis large scale events. It requires a 3 tier approach to mitigating the consequences of such an event on a US NPP. Tier 1 is using installed plant equipment. This is short term and can be as little as twenty minutes up to 24 hrs. 7/

Tier 2 is utilizing the onsite FLEX equipment. This is stored in a building designed to take more punishment than you can imagine. I won't give details for obvious reasons, but I was amazed at the building. It is huge & full of stuff. 8/

This includes several diesel generators of various voltages and a large supply of fuel. Hundreds of feet of hose and several portable pumps to get water from the nearest source and pump it into the cores. Demo equipment to access damaged buildings. So much stuff. 9/

This is intended to cool the core for at least 3 days. While implementing Tier 2 a plant requests Tier 3. Tier 3 is similar equipment stored in one of 2 regional response centers, 1 in TN, 1 in AZ. Resources are available to transport the equipment to affected sites quickly. 10/

All plants are designed to use the same equipment. All of this is intended to combat an event that is extremely unlikely to occur. But, we take it seriously, and given the events in Japan, the NRC took action to enhance our ability to deal with a major event like that. 11/

This is bare bones, because I am trying to not say too much. The gist is we have added several new layers of defense in depth to ensure we don't have the same outcome for a major disaster that Japan did, with plans and organizations in place to coordinate our actions. 12/

For more information, my pinned tweet explains all of the installed equipment that is there to deal with a design basis event & will likely work fine. Everything in the FLEX program is above and beyond that.
NRC page here:
https://www.nrc.gov/reactors/operating/ops-experience/post-fukushima-safety-enhancements.html
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I will try to answer questions, but might not be able to get too deep into some things. -END-