PURPOSE: To obtain a hydrocarbon product of a low molecular weight by pouring water among storage parts containing a radioactive substance package and contained in a water decomposition tank, decomposing the water and surrounding each storage part by a fluid mixture comprising the formed hydrogen gas and a hydrocarbon oil.

CONSTITUTION: Many radioactive substance packages are placed with a plurality of storage parts 3 in a water decomposition tank 2, a water pouring system 1 is operated to pour water among the storage parts 3, and the water is decomposed into an oxygen gas and a hydrogen gas with a radiation from the packages P. The oxygen gas and the hydrogen gas are liquefied by a liquefaction means 4, sent to a gas separator 5, and separated into an oxygen gas and a hydrogen gas, and the oxygen gas is sent to an oxygen gas treatment system 6. The hydrogen gas is sent through a high-pressure pump 7 to a reactor 8 containing a plurality of storage parts 9 each containing many packages P and reacted with a hydrocarbon oil fed through a feed system 10 and a transfer pump 11 under irradiation with a radiation from the packages P to cleave the molecular chains, and the formed fragments are combined with hydrogen atoms.