球床高温气冷堆燃料元件多次通过堆芯，卸料燃耗深，常采取不经过后处理的一次通过燃料循环方式。从提高天然铀利用率和改进废物管理方面考虑，研究球床高温气冷堆乏燃料中铀钚再利用的可行性和不同闭式燃料循环的特性。从乏燃料中提取铀和钚，制成新的燃料元件再返回到反应堆中裂变，构建闭式燃料循环方案。利用设计程序VSOP进行球床高温气冷堆闭式循环的模拟，研究了堆芯物理中关键性参数和事故下特性，分析了不同燃料循环的性能。应用蒙特卡罗方法MCNP程序验算了VSOP程序对纯钚堆芯和MOX燃料堆芯的适用性。论文以250MW球床高温气冷堆核电站示范工程HTR-PM为参考堆芯结构，从平均卸料燃耗为90GWd/tHM的铀钚循环乏燃料中回收钚，建立纯钚燃料循环模型。研究了卸料燃耗、反应性温度系数等关键特性，分析了失冷失压事故下燃料最高温度和堆芯进水引入的反应性，验证了纯钚燃料循环的可行性。为进一步优化堆芯性能，借鉴轻水堆MOX燃料概念，同时利用乏燃料中的铀和钚，构建全堆芯装载MOX燃料循环，研究了MOX循环的堆芯特性，并优化设计独立的铀球和钚球，使得钚球在堆芯内滞留时间更长，尽可能提高钚卸料燃耗。研究高温气冷堆不同闭式循环的资源利用率和废物嬗变，包括纯钚燃料循环、MOX燃料循环和轻水堆级钚燃料循环等三种方案，天然铀利用率分别提高了6%、8%和20%，超铀元素嬗变率分别为40%、41%和63%。结果显示，1个纯钚堆芯或MOX堆芯可焚烧16个或13个同功率铀钚循环的乏燃料中的钚。一个1GWe轻水堆核电站产生的钚可为两个250MWth钚燃料循环的高温气冷堆提供燃料。高温气冷堆闭式燃料循环能适度提高天然铀利用率和有效嬗变超铀元素。VSOP程序缺乏对于钚燃料和MOX燃料的使用和验证经验。利用计算方法和截面数据库完全不同的MCNP程序分析了不同燃料循环的高温气冷堆零燃耗堆芯，重点比较了有效倍增因子keff。结果显示，对于高温气冷堆铀钚循环，VSOP结果和MCNP结果符合较好；而对于纯钚循环和MOX循环，两者有1%-2%的偏差。说明VSOP程序计算钚燃料和MOX基本可行，本论文分析结果可信；同时分析了偏差的原因，提出了VSOP程序改进的方向。
The pebble-bed High-Temperature Gas-cooled Reactor (HTR) in a multi-pass shuffling scheme was designed to have a high fuel burn-up, so the Once-Through-Then-Out (OTTO) fuel cycle were adopted. In terms of resource usage and waste minimization, it is necessary to study the possibility of closed fuel cycles based on HTGR spent fuel and the characteristics of different closed fuel cycles.Closed fuel cycle means that Uranium and/or Plutonium are recycled from the spent fuel, new fuel elements are made of the recycled Plutonium and Uranium and are used to build up a new core with same geometry of the original reactor. Closed fuel cycle for pebble bed HTR is simulated with VSOP program package, some key physics features and accident characteristics are investigated, performance of different fuel cycles is analyzed. The applicability of VSOP program to pure plutonium fuel core and MOX fuel core has been verified by Monte Carlo package MCNP.Based on the reference geometry configuration of the 250MWth pebble bed modular high temperature gas-cooled reactor HTR-PM, Plutonium is recycled from spent fuel with burnup of 90GWd/tU from the U-Pu fuelled core, pure plutonium fuel cycle model is established, the main characteristics of discharging burnup, temperature coefficients are investigated, the maximum fuel temperature under depressurized loss of forced cooling accident and the reactivity of water ingress accident are analyzed, the feasibility of pure plutonium fuel cycle is verified. In order to improve the reactor performance, the concept of MOX fuel cycle like that in light water reactor is proposded, both the uranium and plutonium from spent HTR fuel is used, whole core can be loaded with these MOX fuel elements, the core performance is studied. The discharging burnup of Plutonium is optimized by adopting sparate Uranium pebbles and Plutonium Pebble and increasing the irradiation time of Plutonium pebble in the core..Resource utilization and waste transmutation of different HTR closed fuel cycles are analyzed, including the pure Plutonium from HTR, MOX, and Plutonium from spent LWR fuel, the result is that the natural Uranium utilization is increased by 6%, 8% and 20%, transuranic elements transmutation rates are 40%, 41% and 63% respectively. And a HTR reactor with pure Plutonium or MOX can burn the Plutonium produced by 16 or 13 HTRs with the U-Pu fuelled core. The Plutonium from one 1000MWe PWR can supply fuel for two 250MWth HTR. HTGR closed fuel cycles can effectively burn transuranium isotopes and appropriately increase in the utilization of natural uranium.The experience to use and verify VSOP with Plutonium fuel is limited. The initial HTR core with different fuel cycle are modeled with MCNP which use different methodology and cross section library, and are comparied with the result form VSOP, especially the effective multiplification factor keff. The results show that the calculated results from VSOP program coincide with MCNP results for uranium-plutonium cycle, and have the deviation of 1%-2% for pure plutonium and MOX fuel cycle. This implies that VSOP is basically capable to calculate plutonium based core, the fuel cycle analysis result presented in this thesis is reasonable. Also the reasons of this deviation are analysised, and the improved direction of the VSOP program are suggested.
