I have made a mostly stable reactor. Emergency Coolant is here. Please activate it when the big temperature bar reaches red, if you only activate it at warning 1 or 2, it's too late. 

Enjoy! 

Temperature chart

Minimum temperature:130 C

Maximum safe temperature:800 C

Overheating:900 C

Critical:1100 C

Severely overheating:1300 C

Meltdown:1600 C

Safe temperatures, stable reactor:450 C

Good luck, and don't blow it up. If it does, it's our fault for not innovating a pressure choke. 

Nuclear innovators Inc™