钍作为一种潜在的核燃料，其储量约为铀资源的3~4倍，但尚未在商业核电中大规模利用。考虑到未来核电对核燃料的需求，钍资源的利用对于保证核能的可持续发展具有重要意义。本文基于模块化球床式高温气冷堆HTR--PM堆芯设计框架，在保证现有安全准则不变的前提下，对钍基燃料循环的特性进行了研究分析，其燃料循环模式主要包括钍铀开式循环、钍钚开式循环和钍铀闭式循环。本文首先针对钍基燃料堆芯的计算特点，对计算方法和计算工具进行了研究和验证。由于钍基燃料堆芯中会同时包含多种类型燃料球，本文提出了计算混合燃料球球床中子反应截面均匀化的碰撞概率法，相比于已有算法，对混合燃料球球床具有更好的适用性。此外，论文对快速计算球床堆平衡态的计算方法进行了推导分析，极大地降低了燃料循环优化过程的计算成本。在钍铀开式燃料循环中，本文以天然铀节省为主要优化目标，在对纯铀堆芯燃料利用率分析的基础上，对MOX和SEP型两类燃料形式的钍铀混合堆芯进行了计算分析，得出了利用钍燃料实现铀燃料节省的限值，以及钍铀混合燃料堆芯在温度反馈、进水反应性和失冷失压事故下的安全特性。在钍钚开式燃料循环中，首先以利用钍燃料提高钚燃料利用率为优化目标，在对纯钚堆芯燃料利用率分析的基础上，对钍钚混合堆芯中钚燃料利用率进行了研究分析，计算结果表明利用钍提高钚燃料利用率的效果并不显著。其次，再以增大堆芯U233产率为优化目标，在保证堆芯安全特性不变的前提下，对利用钚驱动钍生产U233进行了计算分析，并给出了钍钚混合堆芯U233产率的优化结果。在钍铀闭式燃料循环中，本文对利用Th232和U233实现堆芯增殖或近增殖进行了研究，主要对钍铀燃料球均匀混合、钍铀燃料球双区布置两类堆芯进行了计算分析。通过对两类堆芯燃料球设计参数、循环策略的优化，以及堆芯热工安全特性的评估，分析了堆芯的增殖特性。研究结果表明，燃料球分区布置堆芯会出现明显的功率畸形，进而导致稳态燃料球温度过高。钍铀燃料球均匀混合堆芯可以避免功率畸形分布，且可以实现U233卸出投入比达到90%以上。通过对多种燃料堆芯的分析，本文明确了HTR--PM采用钍基燃料循环在提高燃料利用率、实现堆芯增殖方面的特性，以及由此带来的堆芯安全特性的变化，为后续HTR--PM的燃料循环发展方向提供了参考和建议。
As a potential source of nuclear fuel, thorium is about three times more abundant in nature than uranium, but is not widely used in commercial nuclear power. Considering future demand of nuclear fuel, the use of thorium is of strategic importance to ensure the sustainable development of nuclear power. Under the design framework of HTR-PM and with its inherent safety criteria maintained, thorium-based fuel cycles are investigated in this thesis, which include thorium-uranium open fuel cycle, thorium-plutonium open fuel cycle and thorium-uranium closed fuel cycle.According to the calculation properties in thorium-based pebble-bed HTRs, calculation methods and codes are firstly studied and verified. Because multi-type fuel pebbles would mix in the thorium-based reactor core, the homogenization methods of neutronic cross sections are investigated, and a new homogenization method based on collision probability is proposed. The results show that the proposed method is advantaged to treat the pebble bed with multi-type fuels. Moreover, the fast calculation method of pebble-bed equilibrium state is also analyzed and developed, which reduces calculation time remarkably for fuel cycle optimization.In the thorium-uranium open fuel cycle, reactor properties of uranium mixed with thorium are analyzed with MOX and SEP fuel, which is mainly aimed at saving natural uranium consumption compared to the pure uranium reactor. The limit of natural uranium saving by using thorium is obtained, as well as the safety properties of temperature feedback, water-ingress reactivity and DLOFC accidents.In the thorium-plutonium open cycle, firstly, the mixed thorium-plutonium reactor is investigated to improve the plutonium utilization ratio by using thorium. The optimization results show that the plutonium saving is not significant compared with pure plutonium reactors. Secondly, with reactor safety criteria maintained, the mixed thorium-plutonium reactor is also analyzed to produce U233 by using plutonium to drive thorium, and the optimized U233 yield is obtained.In the thorium-uranium closed fuel cycle, the breeding properties of HTR-PM fueled with Th232 and U233 are studied, and this research are performed in two types of reactor core. In the first pebble-bed reactor core, uranium pebbles and thorium pebbles are mixed homogeneously in the pebble bed. In the second one, uranium pebbles and thorium pebbles are arranged in the inner and outer zones respectively. The results show that maximum fuel temperature of the two-zone reactor is much higher than that of the homogeneous reactor, which is mainly attributed to the sharper power distribution. With the optimized design parameters, the output-input ratio of U233 in the homogeneous reactor can reach more than 90%, and the reactor safety requirements are also satisfied.Based on the analyses and optimization of different fuel cycles, this thesis work figures out the thorium-based HTR-PM properties of fuel utilization, breeding and safety, which could provide reference for the further development of HTR-PM.
