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  • The nodules used in this study were collected from two cruises in the western North Pacific by box coring (KH-79-4) and dredging (GH-78-1). After being dried at 110°C each sample extracted from the top, side and bottom of the nodule at various penetration levels were radiochemically analyzed for their Thorium isotopes contents by digesting about 100mg of a nodule sample without application of the successive separation treatment. Thorium in the solution was purified with an anion exchange resin and chemical separation techniques, and electrodeposited onto a silver disc from a hydrochloric acid and ethanol solution. The alpha activities of 228Th, 230Th and 232Th were counted with a spectrometer equipped with a surface barrier silicon detector. The chemical yield was determined by counting the 0 activity of 234Th added before the analysis. The yield ranged from 80 to 95%.