Not all old designs were bad. And one has to understand that the USSR, UK, France, and the US all had a shared objective (by the way, these are the primary nations designing and creating nuclear reactors back in the 50's and 60's). And the goal? Plutonium for Nuclear Bombs. You can imagine how this changes design Parameters.

So now lets talk about the CANDU Reactor, designed in 1955 (or there abouts).

It's an oddity of the day - Designed for energy generation for civilian use, without the desire to actively produce Plutonium. Functionally speaking, complete fission of the material with the least degree of enrichment possible for efficient opperation was the design goal. And what you get is well, this.

Beyond this, because it is a Heavy Water Reactor (CANDU standing for Canada Deuterium Uranium), it's moderator is well, heavy water - which is interesting as two things: If it boils off, the neutron regulator (which is slowing down neutrons to encourage fission in the core) boils off. And Boiling water takes away a LOT of heat. Beyond this, heated water will naturally circulate so even if active pumps pushing the water through the system fail, natural circulation can occur until corrective action is taken.

Yes, there are newer designs that are probably safer. But don't just say "old designs bad" without understanding the design constraints created by the circumstances to which they were created. Look at also, all of the designs of the era. There is a reason pretty much everyone can name Chernobyl, 3 Mile Island, and Fukushima. And anyone with half a length of common sense would avoid putting a nuclear reactor on Japan - a place that has an active Valcano, is prone to tsunami's, and sits at the intersecting point of three tectonic plates... It's kind of a bad place for it. Not impossible to do safely, but when you use a reactor design that is basically set up for the production of plutonium by the very design constraints and such of the day: It's not surprising.

And then we can talk about SMR's.