Make temperature, and, optionally, maximum exploding/melting temperature shown in reactor interface (just like charge is shown in storage interface, for example).
Because, really, it is hardly possible to design any non-trivial without knowing temperature. Especially breeders.
Knowing exact temperature or charge of reactor elements would be helpful too, but not that critical.
Right now I'm using Nuclear Control for temperature and Not Enough Items for details of individual elements, but I dislike using any extra mods, because they are usually either too exploitable, or break immersion.