What would prevent a reaction from yielding 100 percent products?
There are always side reactions of varying degrees and reactants are never 100% pure to start with anyway.
control rods act like brakes to slow the neutron chain reaction rate in normal operation. the SCRAM system acts in emergencies to completely bring the neutron chain reaction to an instant stop. even with the reactor stopped, the cooling system must operate to prevent overheating from the radioactive decay of the built up fission products.
Animal testing helps to prevent the usage of humans for testing. Sometimes, an animal must die in order for its' level of health, or its' reaction to a treatment, to be assessed. Animals are tested for more products than we know, and a big reason for that is that humans' emotional responses often overrule our logic.