热堆中钍铀转化规律

Simulation study on ^(232)Th-^(233)U conversion in thermal reactors

钍铀燃料循环以其优异的物理和化学特性,受到核能界的广泛关注。本文利用单群的点燃耗计算程序ORIGEN,分别研究了钍燃料在沸水堆(Boiling Water Reactor,BWR)、压水堆(Pressurized Water Reactor,PWR)和加拿大重水铀反应堆(Canada Deuterium Oxide Uranium,CANDU,又称坎杜堆)能谱中辐照时,232Th、233Th、233Pa、233U等核素生成量随中子注量率和中子能谱的变化规律,并探索了多次"辐照-冷却"循环对钍铀转化率的影响。计算结果表明,能谱相同时,233Th和233Pa存量的最大值与注量率有关;233U存量的最大值与注量率无关,大概在注量(注量率×时间)为4×1016 n·cm-2左右;注量率相同时,能谱越硬,233U存量的最大值越大。采取循环"辐照-冷却"可以提高233Th-233U的转化率,对于相同的总辐照时间,每次循环周期内的辐照时间越短,相对于总辐照时间相同的单次辐照,转化率增量提高越明显;当总辐照时间超过两个月时,循环辐照对转化率增量的作用较小,与单次辐照不冷却相比,转化率相对增量不超过1倍。

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.