When i join my world, my zirnox reactor, who was perfectly stable when i left the game, explodes when i rejoin, destroying a lot of my stuff.
Fuel: Uranium 235
Carbon Dioxide: 10,000
Temperature: 705
Pressure: 24 bar
3 leviathan turbines and a large cooling tower
proper water recycling system
i'm 99% sure everything was configured correctly because it wasnt blowing up and the water level wasnt going down

.

The current fluid net code isn't that ideal, when a fluid source (like your tank) unloads, the connection is reset, and reconnecting takes a few seconds (it also scales with the length of the pipe) which causes your reactor to get starved of water. There is in fact a new fluid code in the works that would mostly solve this issue, but it won't be finished for a while and there's still some scenarios where it still would not provide water. Chunk loading plays a huge factor, it's also the reason the vast majority of DFCs will explode when you relog while it is still turned on.

For now all you can do is making sure that you turn off all of your reactors before leaving the world. I'll leave the issue open until the new fluid system is fully implemented, but even then you will have to be careful.