They're easy to shut down, but when the reactor is first turned off the core is full of unstable isotopes that are byproducts of the fission reaction. Those isotopes continue decaying even with the reactor "off" until the become stable, and they put out enough heat to cause a meltdown on their own. This means the reactor needs active cooling for a couple of weeks until it reaches a safe state. That's what happened at Fukashima; the reactors were shut down when the earthquake hit, but then the tsunami disabled the backup generators that were powering the cooling pumps, and they weren't able to restore cooling before the core melted.

I did not know about that aspect of nuclear plants and in my non-expert view it would be a major point against nuclear plants in the discussion how to solve climate change (at least until we learn how to secure them in case of a war). Am I wrong?

EDIT: I would also like to ask if you can recommend some source where curious layman with no knowledge of physics could learn a bit more about this.

You are not wrong that it's a major point in the discussion, but even with that it can still be better than the alternatives for baseline production (the part that intermittent sources like solar and wind can't cover). Remember that coal power add insult to injury by continuously releasing radioactive pollution.

You’re right about old nuclear design.

The new designs are built with the expectation that power can be cut off. The new design shut themselves off passively without any damage (that we know of) when the power is gone.

Seems to me like it should have been this way from this start. Catastrophic meltdown caused by power loss, no matter how many power backup systems there are should have been a non-starter. Basically requiring a 100% SLA, 6 nines not enough, with catastrophic consequences for failing to adhere to 100% uptime. That is scary as hell.

> I did not know about that aspect of nuclear plants and in my non-expert view it would be a major point against nuclear plants in the discussion how to solve climate change

It's a reasonable criticism to make, but I think it tends to be a bit overblown. Modern reactors have redundant layers of systems to prevent such a disaster from occurring. In the ~60 year history of nuclear reactors Fukashima is the only time this type of failure has happened (Three Mile Island and Chernobyl were very different), and even then it was due to a cascading series of problems:

* In hindsight it was a really poor idea to locate a nuclear plant by the ocean in a region known for strong earthquakes and tsunamis.

* The backup generators flooded because they were located in the basements, another poor decision for an area at risk of tsunamis.

* TEPCO had additional generators and batteries on site within 6 hours of the earthquake, which was in time to prevent a meltdown, but they were unable to connect them to the plant's electrical system (I don't recall the details of why).

It's also worth mentioning that this is only a problem with large reactors. Smaller designs like those used in ships and submarines still have decay heat, but the reactor core is physically smaller so that it doesn't contain enough isotopes to cause a meltdown. Many nuclear advocates believe that we should be building more nuclear plants with smaller reactors for this reason as well as smaller reactors being easier to operate without getting into a dangerous state.

As far as reading, I highly recommend Atomic Accidents by James Mahaffey [0]. It's very well written, aimed at a total layman, and covers basically everything that has gone wrong since we as a species started seriously messing with nuclear materials in the 19th century.

[0]: https://www.amazon.com/gp/product/1605986801

Russia most likely designed and built that reactor and the others in the country, back when it was part of the Soviet Union. I think it's safe to say they fully understand and are capable of operating nuclear reactors like it.

edit: Construction started on it in 1980. This is a Soviet designed and built reactor. Russia should have no problem maintaining it.

It is highly unlikely there are be nuclear scientists with that knowledge amongst the armed forces currently invading Ukraine. The type of reactor may be known but each reactor and its instruments, controls and protocols will be unique in some way. The only people who are qualified to run this reactor safely are the staff who work there every day.

It's not impossible, though. Russia seems to have pressed a lot of civilians into service. Ukraine recently captured an entire squad of school teachers.

Per Wikipedia, all 6 units are VVER-1000 [0]. So, unlike RBMK, water-moderated / negative void coefficient.

[0] https://en.m.wikipedia.org/wiki/VVER#Design

Usually they have fail safes that will more or less immediately shut them down. On any reactor designed in the last 50 years, they pretty much default to a safe state, as they require power to keep the failsafes open.

Starting back up after is the slow part.

The one snag is that even with the reactor shut down (just drop all the control rods in) you still have to deal with the decay heat.

Which, in the case of a controlled manual shut down shouldn't be a problem.

It's not a problem if you can continue to run the cooling system for quite some time. If you bomb the plant or its connections to the outside, that might be a problem - just as in Fukushima, where a key cause of the disaster was that the cooling system could not be properly powered when required.

No clue how old the Ukrainian reactor (and its design) is. Perhaps more than 50 years?

My understanding is that it's relatively easy to shut down, but costly/time consuming to start back up.