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I worked in the same office as a nuclear safety expert. One day he mentioned all the issues with why Chernobyl melted down. I can't remember the details (pulled

I worked in the same office as a nuclear safety expert. One day he mentioned all the issues with why Chernobyl melted down. I can't remember the details (pulled the rods partly out then re-inserted?), but I think the summary was that the people on shift were inexperienced and did everything wrong every step of the way. People in general knew how to manage reactors safely, the corners cut by a communist regime made it unsafe. Sadly, I don't have any sources, and I'm open to refutation.

marcus0x62 5y ago
We’ll, one of the corners cut was in the design of the control rods, and the Soviet government hid that from the operators. Did they screw up? Sure, but they did so believing — having been told — there was a fail safe that simply did not work as advertised.
tofof 5y ago
Your understanding is mostly correct. The issue with the fuel rods is that they were double purpose - the lower portion actually increased the reaction rate, while the upper portion decreased it. Because the team had manually fully retracted the rods, both portions were outside the reactor vessel. When it scrammed, the first portion to enter the vessel was the lower portion, which counterproductively increased the reaction rate further. This was one of several poor design choices in RBMK reactors. You are correct that the people on shift did everything wrong at every step along the way - alongside management, who pushed forward the safety test that caused the accident on a skeleton night shift who hadn't explicitly prepared for it. The test deliberately had the reactor operating in an unusual, low-power state because it was supposed to test whether you could use momentum still in the turbines to bridge a gap until emergency diesel could be brought on line in the event of a power failure affecting the pumps that circulated coolant water to the reactor. However, the shift let the power in the reactor get too low, starving the reaction, and then took increasingly dramatic steps to attempt to bring the power back to specified (low but not that low) levels. These steps included disabling automated control of fuel rods, decreasing the rate of coolant flow, the removal of 18 of the 28 'fail-safe' control rods which were never to be removed, and disabling most of the automated scram mechanisms. Obviously, these steps were not normal procedure and constituted serious flaws in judgement. Normal procedure would be to shut down completely when power levels drop that low, and then a waiting period and a several-hour-long startup procedure. The extremely low power state allowed a phenomenon called xenon poisoning to build up, which made restoring normal operation difficult - hence the normal procedure to shut down completley and allow the xenon to naturally decay. The attempt at restoring power despite this condition led to very unstable neutron flux, and with it unstable core temperatures, triggering alarms from several systems that were continually ignored. Finally, the flaw in the design of a positive void coefficient (meaning that as heat goes up, water turns to steam, and the presence of the steam causes the reaction to become more efficient and produce more heat, in a runaway) actually struck, as the unstable neutron flux presumably spiked and the coolant flow was already further decreased (they had started the actual test, so the coolant pumps were being powered only by leftover momentum in the turbines). In combination, large steam voids formed, and then thermal runaway, and then a scram that actually first increased reaction rates further. (And then fuel rods fractured and many control rods got stuck at only 1/3 insertion, with the increasing portion still within the reactor). The void coefficient is why the test was supposed to take place around 700 MW, not the ~200 the operators had managed to restore; above around 700 MW the void coefficient in RBMK reactors is negative and such a runaway would not occur.
bityard 5y ago
It's one of the most studied disasters in history, but basically the main causes boil down to: 1. Personnel who didn't understand the risks of deviating from written procedure 2. Engineers who didn't anticipate or underestimated certain failure modes and didn't design the reactor and monitoring systems with sufficient redundancy and failsafes 3. Grossly insufficient monitoring systems 4. Denial at every level once it was obvious that something was Seriously Wrong
- throwawayboise 5y ago
  The failure modes were already known before the disaster. They were kept secret from the rank-and-file reactor operators and supervisors, as the Soviet regime was loathe to admit they were imperfect in any way. The monitors and failsafes were there. Maybe could have been better, but they would have worked had they not been disregarded and deliberately overriden. It was completely preventable. RBMK reactors of the same design are still operating safely today.
Accujack 5y ago
The most critical issue was probably the reactor design's positive void coefficient. The RBMK design was created to use less enriched fuel than other designs, so it had to have performance characteristics that would permit a stable reaction when that fuel was used. To get that "feature", the design had a positive void coefficient. This meant that it to work in such a way that reactivity would increase if coolant boiled away or other voids (spaces without nuclear moderating materials) formed in the fuel assembly. To keep reactions under control, it was necessary to ensure that there was always some material present to moderate the reaction. Almost every other reactor design will essentially shut itself off if coolant is lost (negative void coefficient). This is a safety factor in case situations unexpected by the designers occur. In the event something really strange happens, the reactor turns off. With a positive void coefficient, if something unforeseen by the designers happens, the reactor ramps up to higher output. Combine that with human mistakes in a certain sequence and safety systems that don't operate 100% correctly when the reactor is overheating and running away and you can get an out of control excursion as happened at Chernobyl.
skeletal88 5y ago
Also the reactor design was flawed - the control rods that should have stopped the reactor actually increased the reactivity while moving down into the reactor. Energy spikes were observed in different plants when shutting down the reactors, but they were classified to not show the reactor design (and by proxy the soviet union) in a bad light, so the operators had no idea of this critical fault.
hindsightbias 5y ago
It's the nature of the beast, not the regime. If you read the NRC or other detailed reports on Three Mile Island, it was a non-stop litany of design flaws and making the wrong decision at every step.

marcus0x62 5y ago

We’ll, one of the corners cut was in the design of the control rods, and the Soviet government hid that from the operators. Did they screw up? Sure, but they did so believing — having been told — there was a fail safe that simply did not work as advertised.

tofof 5y ago

Your understanding is mostly correct. The issue with the fuel rods is that they were double purpose - the lower portion actually increased the reaction rate, while the upper portion decreased it. Because the team had manually fully retracted the rods, both portions were outside the reactor vessel. When it scrammed, the first portion to enter the vessel was the lower portion, which counterproductively increased the reaction rate further. This was one of several poor design choices in RBMK reactors. You are correct that the people on shift did everything wrong at every step along the way - alongside management, who pushed forward the safety test that caused the accident on a skeleton night shift who hadn't explicitly prepared for it. The test deliberately had the reactor operating in an unusual, low-power state because it was supposed to test whether you could use momentum still in the turbines to bridge a gap until emergency diesel could be brought on line in the event of a power failure affecting the pumps that circulated coolant water to the reactor. However, the shift let the power in the reactor get too low, starving the reaction, and then took increasingly dramatic steps to attempt to bring the power back to specified (low but not that low) levels. These steps included disabling automated control of fuel rods, decreasing the rate of coolant flow, the removal of 18 of the 28 'fail-safe' control rods which were never to be removed, and disabling most of the automated scram mechanisms. Obviously, these steps were not normal procedure and constituted serious flaws in judgement. Normal procedure would be to shut down completely when power levels drop that low, and then a waiting period and a several-hour-long startup procedure. The extremely low power state allowed a phenomenon called xenon poisoning to build up, which made restoring normal operation difficult - hence the normal procedure to shut down completley and allow the xenon to naturally decay. The attempt at restoring power despite this condition led to very unstable neutron flux, and with it unstable core temperatures, triggering alarms from several systems that were continually ignored. Finally, the flaw in the design of a positive void coefficient (meaning that as heat goes up, water turns to steam, and the presence of the steam causes the reaction to become more efficient and produce more heat, in a runaway) actually struck, as the unstable neutron flux presumably spiked and the coolant flow was already further decreased (they had started the actual test, so the coolant pumps were being powered only by leftover momentum in the turbines). In combination, large steam voids formed, and then thermal runaway, and then a scram that actually first increased reaction rates further. (And then fuel rods fractured and many control rods got stuck at only 1/3 insertion, with the increasing portion still within the reactor). The void coefficient is why the test was supposed to take place around 700 MW, not the ~200 the operators had managed to restore; above around 700 MW the void coefficient in RBMK reactors is negative and such a runaway would not occur.

bityard 5y ago

It's one of the most studied disasters in history, but basically the main causes boil down to: 1. Personnel who didn't understand the risks of deviating from written procedure 2. Engineers who didn't anticipate or underestimated certain failure modes and didn't design the reactor and monitoring systems with sufficient redundancy and failsafes 3. Grossly insufficient monitoring systems 4. Denial at every level once it was obvious that something was Seriously Wrong

throwawayboise 5y ago
The failure modes were already known before the disaster. They were kept secret from the rank-and-file reactor operators and supervisors, as the Soviet regime was loathe to admit they were imperfect in any way. The monitors and failsafes were there. Maybe could have been better, but they would have worked had they not been disregarded and deliberately overriden. It was completely preventable. RBMK reactors of the same design are still operating safely today.

Accujack 5y ago

The most critical issue was probably the reactor design's positive void coefficient. The RBMK design was created to use less enriched fuel than other designs, so it had to have performance characteristics that would permit a stable reaction when that fuel was used. To get that "feature", the design had a positive void coefficient. This meant that it to work in such a way that reactivity would increase if coolant boiled away or other voids (spaces without nuclear moderating materials) formed in the fuel assembly. To keep reactions under control, it was necessary to ensure that there was always some material present to moderate the reaction. Almost every other reactor design will essentially shut itself off if coolant is lost (negative void coefficient). This is a safety factor in case situations unexpected by the designers occur. In the event something really strange happens, the reactor turns off. With a positive void coefficient, if something unforeseen by the designers happens, the reactor ramps up to higher output. Combine that with human mistakes in a certain sequence and safety systems that don't operate 100% correctly when the reactor is overheating and running away and you can get an out of control excursion as happened at Chernobyl.

skeletal88 5y ago

Also the reactor design was flawed - the control rods that should have stopped the reactor actually increased the reactivity while moving down into the reactor. Energy spikes were observed in different plants when shutting down the reactors, but they were classified to not show the reactor design (and by proxy the soviet union) in a bad light, so the operators had no idea of this critical fault.

hindsightbias 5y ago

It's the nature of the beast, not the regime. If you read the NRC or other detailed reports on Three Mile Island, it was a non-stop litany of design flaws and making the wrong decision at every step.