They can stop the reaction with the control rods.
>The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1:1 ratio of produced to used up neutrons is achieved. At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators.

Read more: Nuclear Reactor - Control Rods - Fission, Reaction, Neutrons, Core, Produced, and Chain [link to science.jrank.org]


Yes it is best to have water circulating but there are many safety features built in to nuclear reactors since Chernobyl.