  Relativistic runaway electron avalanches (RREAs) are generally accepted as a
source of thunderstorms gamma-ray radiation. Avalanches can multiply in the
electric field via the relativistic feedback mechanism based on processes with
gamma-rays and positrons. This paper shows that a non-uniform electric field
geometry can lead to the new RREAs multiplication mechanism - "reactor
feedback", due to the exchange of high-energy particles between different
accelerating regions within a thundercloud. A new method for the numerical
simulation of RREA dynamics within heterogeneous electric field structures is
proposed. The developed analytical description and the numerical simulation
enables us to derive necessary conditions for TGF occurrence in the system with
the reactor feedback Observable properties of TGFs influenced by the proposed
mechanism are discussed.
