Unit 1 of Kakrapar Atomic Power station was shutdown in account of small leak in the primary heat transport system.
Primary heat transport system (PHT):

Primary heat transport system is a coolant system in a nuclear reactor it uses heavy water. With the help of PHT, heavy water is made to pass through nuclear reactor core to absorb the heat generated.
Implications of leakage in coolant system:
The temperature of the reactor increased many fold. However, safety system working in the reactor suspended the nuclear fission reaction.
In case the safety system had failed, then nuclear reactor would have blasted. This would have further led to nuclear radiations leakage.