A study has been made of the relative hazards of waste arisings from the use of thorium fuels in certain power reactor systems. The hazard estimates were made using the ICRP maximum permissible concentrations for the ingestion pathway for both water-soluble and insoluble wastes. Two separate conceptual fuel cycles were considered, the one (referred to as the advanced fuel cycle) involving the use, in the THTR, of thorium and uranium bred in an LMFBR thorium blanket, the other involving the use of thorium and 235U-enriched uranium in a THTR. The waste hazards were compared with those resulting from the use of 235U-enriched fuel in HTR and PWR and the effect of recycling uranium and 239Pu-rich plutonium was also investigated. Thorium, uranium and plutonium extraction efficiencies of 99.9% were assumed wherever relevant before waste hazard estimates were computed. The uranium produced in the LMFBR blanket was found to have a high (≳96%) 233U content and the actinide waste arisings from its extraction were less hazardous than typical LMFBR core waste arisings at most cooling times. The waste hazards from THTR operation were dominated for about the first 500 yr by the fission products, with the actinides being dominant thereafter. The advanced fuel cycle THTR waste hazard was dominated by 223Ra and 226Ra for water-soluble waste and by 223Ra and 209Pb, 225Ra, 225Ac and 229Th for insoluble waste between 500 and one million yr, these isotopes being respectively, the daughters of 231Pa, 238Pu and 237Np initially present in the waste. The corresponding isotopes for the system based purely on THTR were found to be 244Cm, 241Am, 243Am and 226Ra for water-soluble waste ( 226Ra being formed this time mainly by 242Cm decay) and the above-mentioned four 237Np daughters replacing the 226Ra for insoluble waste. The THTR waste hazards were found to be comparable to those produced by HTR and PWR operating with 235U-enriched fuel, except for insoluble waste from the advanced fuel cycle, which was more than an order of magnitude less hazardous at a million years. The effect of uranium recycling on the actinide waste hazard of the advanced fuel cycle THTR was investigated, as was the effect of uranium and plutonium recycling in the THTR initially using thorium and 235U-enriched uranium. Recycling led to increases in waste hazards of up to an order of magnitude for two recycles. This is in line with corresponding increases accruing from plutonium recycling in conventional HTR and PWR.