A neutronic analysis has been performed to assess a prospective utilization of light water reactor (LWR) spent fuel in Canada deuterium uranium (CANDU) reactors mixed with thoria (ThO2). The study is conducted for mixture grades with 50%, 60% and 100% LWR spent fuel and 50%, 40% and 0% thoria, respectively. Burn-up grades are evaluated for alternative fuels to reach a bundle criticality of k(infinity) = 1.06, which are calculated as similar to28,000, similar to14;000, similar to8000 and 8800 MW d/MT with 100%, 60% and 50% LWR spent fuel content and for natural uranium fuelled CANDU after plant operation periods of 690, 340, 200 and 205 days, respectively. The presence of even plutonium isotopes with higher neutron absorption cross sections in the LWR spent fuel obliges starting with a higher cumulative fissile inventory in the initial charge compared to natural uranium fuel. Extended utilization of worldwide disposed spent nuclear LWR fuel in CANDU reactors in a symbiotic system opens prospects with respect to environmental concerns as well as to energy economics. After separation of the fission products, further utilization of the actinides in nuclear waste becomes possible as a valuable nuclear fuel. (C) 2003 Elsevier Ltd. All rights reserved.