If someone were to write a procedure to make an RBMK reactor explode it would have looked a lot like what happened at Chernobyl in April 1986. The design was flawed and the political mania for secrecy in the USSR ensured the operators were unaware of those flaws. 1/

I will discuss this from a US perspective as I have never lived in a totalitarian state. In our political system I would never have performed the actions the operators took that night. I can say that knowing no one will order me or my family executed or jailed for doing so. 2/

The test they were running that night was simple. After a reactor trip the turbine still spins but slows. They wanted to see if it made enough power to keep the coolant recirc pumps going until the diesel generators kicked on. My reactor does this for 30 seconds post-trip. 3/

Had all gone as scheduled likely nothing would have happened, but the grid controller delayed their shutdown. When they continued lowering power it fell too far due to Xe (it absorbs neutrons) that formed while waiting. They pulled way too many rods to get power back up. 4/

This caused another issue. The test is now on night shift, who is not prepared for it. We practice things like this in the simulator to make sure we are ready & wouldn't throw it at the next shift and say good luck. We would train them too, or not perform the test. 5/

This would be my stopping point. I would never knowingly violate safety limits and no one has the authority to order me to do so. If I was unable to safely set conditions for the test I would have continued shutting down & not performed the test - with no pushback from anyone. 6/

So, they pulled most of their rods out and started the test. As the pumps slowed they put less water through the core and boiling began to increase. This is where the design flaw becomes apparent. RBMKs are water cooled, but graphite moderated. In a LW PWR the water does both. 7/

Moderation is the slowing of neutrons to energy levels where they will cause fission. In my core, if boiling occurs I get less moderation, so fewer neutrons & power goes down. This is a negative void coefficient. There it was the opposite. More boiling makes power go up. 8/

This is a feedback loop. More voids makes more power which boils more water, which makes more power - you get the picture. We do not operate above 70% with a + temperature coefficient for exactly this reason, & new core designs usually don't have a + temp coefficient at all. 9/

This caused power to go up rapidly. The operators did what all operators would do - they inserted control rods to shutdown the reactor. Unfortunately this added a burst of more moderation as the tips of the rods are also graphite & they displaced water which absorbs neutrons. 10/

The combination of the rods being withdrawn too far, the positive void coefficient as they went in, and the lower flow causing more boiling allowed the reactor to go prompt critical. All the water in the core flashed to steam, blowing the 1000 ton cover off the reactor. 11/

This is key - it was a steam explosion, not a nuclear one. There is not enough U235 in any reactor to form the critical mass needed to create an atomic explosion. Regardless of the type of explosion, it destroyed the reactor and threw pieces of the core everywhere. 12/

A major failing of the design was the lack of a containment structure. In the US all reactors sit within a containment dome - several inches of steel surrounded by a concrete shield building. At Chernobyl the reactors were sitting in what was basically a HS gymnasium. 13/

The containment on my reactor is designed to handle 46 psig of pressure and has filtration systems to remove any fission products that might get released in an accident and a spray system to cool the steam cloud and reduce pressure afterwards. Their HS gym didn't survive. 14/

The key takeaway for us from Chernobyl was safety culture. I am required by law to protect the core and the public. That responsibility comes before anything else and no one can order me to violate it. It is my reactor & I make the decisions to do or not do any work. 15/

My crew would never take direction from anyone else. My managers would never pressure me to perform an unsafe test. If they did I can and would report them to the NRC. Anyone at the site can do this at any time with no consequences. Bad safety culture can send you to jail.16/

There are still several RBMKs operating today. They have all been modified extensively to ensure it cannot happen again. https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/appendices/rbmk-reactors.aspx The NRC mandated our response. https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html#response
Learning from mistakes is an obsession for us & this was no different. 17/

#NuclearEnergy #Chernobyl
Disclaimer - this is simplified in a number of ways due to the limits of Twitter, and is my take on what happened and how I as a Senior Reactor Operator/Control Room Supervisor would have handled it.