What effect does supercritical mass have on a nuclear reaction?

The reaction rate increases.

Explanation:

To sustain a nuclear reaction, the mass of the reactor fuel should be theshold or sufficient for the reaction to carry. Critical mass is defined as the minimum quantity of the fissile material required to achieve a self-sustaining fission chain reaction under stated conditions. A critical mass is required in order to continue the chain reaction and the release of atomic energy.

    Supercritical mass is the mass of  reactor fuel which is more than needed to sustain the chain reaction. So when the state of supercritical mass is achieved, the nuclear rate increases exponentially.