This is crazy. Why on earth wouldn’t the respective government spend money to modernize a critical infrastructure like a nuclear power plant?

Safety and reliability come from understanding the system. Something that's old but that you know everything about is far safer than something new. This risk is that the people who know move on, or the sources of replacement parts stop working, or that other things around the system change. Then the risk curve inverts and you find the old system more of a liability than a asset.

Almost always people choose to update things either too early or too late. Knowing when to do something is hard.

> This is how we would end up with nuclear disasters, not because the technology is bad, but purely because of mismanagement and human negligence.

I'm pretty sure constant upgrading would be more likely to cause disasters.

If it's not broken, don't fix it.

Same reason why MTA in NYC still operates subway using 100 y/o signaling hardware on most of the lines.

In a way, your comment and the responses you get is a perfect allegory to inexperienced vs experienced engineering.

> Why on earth wouldn’t the respective government spend money to modernize a critical infrastructure like a nuclear power plant?

Same reason they don't modernize health care infrastructure. It's too expensive, people die if they get it wrong and they get voted out of office for their troubles.

Lindy's law:

For non-perishable things - i.e technologies, ideas, books, institutions - the expected remaining lifespan is roughly proportional to how long they have already survived

Time acts as a filter: what has already withstood a long stretch of disorder is more robust, so the longer it lasts, the longer it is expected to last.

The old nuclear software running on mainframes have passed the test of time. The new software, has yet to.

Nassim Taleb talks about this at length in his book, Antifragile.

Now you know why "nuclear power may have been dangerous in the past, but with modern technology it's completely safe and Chernobyl will never happen again" is not a good argument.

There is no reason a more modern system would be better in any aspect.