The reactor itself had design flaws but was still safe if run as designed. The problem is the staff ignored nearly every safety protocol developed which then exposed the flaws in its design. The changes made were adding new materials to the control rods that absorbed neutrons as well as using more enriched fuel with a higher amount of U235. These changes make it easier to control the reactor. They also automated some of the controls so that the mistakes made by the staff could not be repeated. (source)

The reactors still lack containment vessels found in western and modern reactors. These reinforced concrete vessels are the ultimate failsafe to contain the majority of the waste in the event of a meltdown. They are part of the reason why Fukushima and 3 mile island had virtually no direct casualties (Fukushima had 1, three mile island had 0).

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The critical design flaw was the tip of the control rods, which were made of graphite. The reactor was Xe-poisoned and was running at about 30MW, where it is unstable. It was ordered to increase power, but it would have been better to wait 24h to get rid of the poisoning.

So with almost all the rods out they came to about 200MW,but super instable. They tried to shut down and slammed the red button. The graphite tips actually increased reactivity and the explosion occurred (hydrogen/steam).

The fundamental flaw was to get people in charge that had no idea how the reactor worked and where and why it is dangerous.

As always - for an accident to happen, several things had to go wrong. And here they did.

>The fundamental flaw was to get people in charge that had no idea how the reactor worked and where and why it is dangerous.

I would choose to disagree on this point. They knew how reactors worked, but the design flaws and things like the positive void coefficient were deliberately hidden from them by the designers. They disobeyed the operating instructions, but did so because they were kept from knowing why those operating guidelines were put in place, and so things like the minimum number of control rods to be kept inserted at all times appeared like an arbitrary guideline, rather than something essential for safety.

It's easy to draw conclusions based on the more complete understanding we have now, but the message at the time was that it was impossible to blow up a RBMK; it was "as safe as a samovar".

In addition, added political pressure to brute force results despite running into problems.

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Well, this particular test was supposed to have been done before Unit 4 came online in late December 1983. But the unit was signed off as complete without the test being run in order to meet the end of year deadline. The planned shutdown in 1986 was the first real opportunity to take the unit offline to run the test as they had ~3 year cycles between maintenance shutdowns.

In normal operating situations the accident should never had happened, but given the culture and pressures at the time, it really was inevitable.

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I'd even back up a step further and say they knew about he coefficient, but not that the control rods had graphite tips. THAT was what was hidden from them.

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What you want is people who know how it works, and how it works when things are not working.

Summary of the changes made. They include control rod changes so that inserting the rods does not produce a reverse effect, fixed neutron absorbers to inhibit unstable low-power operation, and extra protection systems. The void coefficient is reduced, although it is still positive, which is an undesirable characteristic. A side-effect of the changes is the need for higher enrichment of the fuel.

https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/appendices/rbmk-reactors.aspx

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This theory about an all powerful nuclear industry falls apart when you consider Japan shut down its nuclear plants in 2011 and hasn’t turned many them back on. There were a lot of problems with Japans communication about Fukushima and risks, however there is also a lot of misinformation, particularly when it comes to radiation levels outside of japan.

>”I haven't forgotten the iodine-131 readings in the rain here in Portland, in March 2011.“

Here is the EPA report on iodine-131 levels: >”Boise, Idaho and Richland, Washington, showed trace amounts of Iodine-131 – about 0.2 picocuries per liter in each case. An infant would have to drink almost 7,000 liters of this water to receive a radiation dose equivalent to a day’s worth of the natural background radiation exposure we experience continuously from natural sources of radioactivity in our environment.”

>”My guess would be that an honest assessment of excess deaths would run in the tens of thousands from Fukushima and Chernobyl.”

The UN did an assessment of Chernobyl and estimated 4000 current and future deaths. I belive the estimates for Fukushima range between 500 and 2000, many of which come from the evacuation (exacerbated by the largest tsunami in Japanese history).

If reactors are so safe, why can't they get commercial insurance?

Not sure which country you are referring to but they can and do have private insurance in the US.

Do you have any sources on them being unsafe? Specifically less safe than fossil fuels?

If you had read your own link, you would realize the Price-Anderson Act subsidizes insurance with tax money. There is no purely commercial insurance available without that subsidy. The risks are too great.

They had tried to run the test a few times but it failed. It failed for the las time because they couldn’t fully shut down the reactor due to power needs and had to push the test to the next shift when power needs were less. It wasn’t about taking the reactor offline for maintenance.

The reason why the test was scheduled for this time was because they were taking the reactor offline for maintenance. It was delayed for hours due to the power needs of Kyiv.