Sustainable clean energy for an ever expanding global population...

Sustainable clean energy for an ever expanding global population in the immediate & near future is unrealistic. That's the reality. The transition from fossil fuels to successful implementation of clean energy (nuclear isn't clean) is going to take a long time.

Regarding your opening question: "What part of retrospective reactor safety upgrades don't you understand? Tell me something about explosive hydrogen in a reactors energy cycle."

Well, there's lots of things I don't understand about retrospective reactor safety upgrades, but one thing I do understand is that they don't always happen for one reason, or another, or a combination of reasons.

http://www.asahi.com/ajw/articles/AJ201809060062.html

You might consider heading over to Japan to offer Tepco a helping hand with the cleanup & share your nuclear expertise with them. The last report I read about one of the probes Tepco sent into reactor number 2 is that it intersected a region near the chamber where the radiation was 530 sieverts/hour.

https://www.japantimes.co.jp/news/2017/02/03/national/fukushima-radiation-level-highest-since-march-11/#.XAdE9tszZtA

Let's hope another earthquake or tsunami doesn't occur to make things even worse than they already are.