I said it for to maintain undesired chemical reactions under control. My first thought after to read some papers would be below 25°C, but this is not my field so take it with a grain of salt, as the water temperature in normal operating conditions within nuclear waste water pools seems to be held below 50°C by the refrigeration systems.

In any case, personally, I would not stay in a place like that with current shielding technologies even if I were paid for it.

PS. I'm not sure if it would be trivial treating the water for to keeping it operative more than one year as effective shield.