Can’t talk specifically about nuclear subs. But in a normal nuclear reactor once it’s taken sub-critical (fission chain reaction “stops”) still puts out a signi

Can’t talk specifically about nuclear subs. But in a normal nuclear reactor once it’s taken sub-critical (fission chain reaction “stops”) still puts out a significant amount of heat. The reactor may be sub-critical but the fuel still experiences some nuclear decay and puts off quite a bit off heat in the process. You need to continually monitor this and run cooling equipment, otherwise Bad Things happen. Interestingly this effect is used in the nuclear “batteries” in some space projects like the Mars rover, Curiosity.