The chain reaction is easy to stop. Just add a neuron absorbing material. The Control Rods in rectors can do that You just SCRAM (put the rods all the way in) or add something like Boron and the chain reaction stops.
The problem is the radioactive waste. Those isotopes break down and release heat spontaneously, no neutrons required. The only known way to stop or slow radioactive decay down is to slow time down by moving at relativistic speed or near orbit to a black hole.
