Background: Thorium-uranium fuel cycle is attracting more and more attention because of its unique physical and chemical characteristics. Purpose: Realizing the utilization of thorium fuel in thermal reactors can save the valuable natural uranium resources and produce more fissile fuel

which is conducive to the nuclear energy sustainable development. Methods: ORIGEN code was used to simulate the build-up characteristics of 232Th

233Th

233Pa and 233U in neutron fluency rates and neutron spectra of typical Boiling Water Reactor (BWR)

Pressurized Water Reactor (PWR) and Canada Deuterium Oxide Uranium (CANDU) reactor

and the effects of multiple “irradiation-cooling” cycles on the thorium uranium conversion rate under various irradiation time were analyzed by numerical comparison. Results: Simulation results showed that the maximum inventory of 233Th and 233Pa is irrelevant to the neutron fluency rates when neutron spectrum is fixed. When the neutron fluency rates are preset

the harder the neutron spectrum is

the larger the maximum inventory of 233U will be obtained. Multiple “irradiation-cooling” cycles can improve the conversion rate of 232Th-233U

but the relative increment is becoming smaller and smaller

compared to the continuous irradiation conversion rate. Conclusion: This work provides relevant theoretical basis for thermal reactors thorium-uranium fuel cycle research. Abstract