In the case of a high radionuclideUnstable nuclide spontaneously decaying without external eff... concentration in a system, the production of decayThe spontaneous conversion of a nuclide into another nuclide... heat may exceed the heat removal from the system. Self-heating is present in this case and is to be avoided e.g. for the storage of spent fuelSee 'nuclear fuel'. elements and high active waste solutions by means of cooling.