基于离散元法的高放核废料储罐静动力稳定性初步研究

Preliminary Study on Static and Dynamic Stability of Canister for High-level Radioactive Nuclear Waste Disposal Based on Discrete Element Method

核废料储罐是核废料处理工程屏障的核心部分,其静动力学稳定性至关重要。基于碳化硅材料的核废料储罐,考虑深部岩石—储罐的相互作用特点,采用试验与模拟相结合的方法开展研究。首先对碳化硅的抗拉强度特征进行了研究,分析了深埋条件下的储罐受力特点和规律;其次,研究了自由落体与岩石撞击条件下储罐的动态受力规律和基本破坏形式,并考虑岩石破碎所带来的影响。结果显示:碳化硅是相对脆性材料,其抗拉强度存在一定变化区间;在深埋条件下,埋深、水平与竖向地应力比对储罐受力有较大的影响;运输时自由跌落的高度和倾角对储罐局部集中拉应力有较大的影响;岩石撞击时储罐内的拉应力受岩石质量和撞击发生时岩石—储罐接触类型的控制,考虑岩石撞击破碎会大幅削弱撞击附加应力,岩块间黏聚力和内摩擦角越大,岩石撞击力也越大,岩块间抗拉强度对撞击力的影响相对较小。虽然在自由跌落与岩石撞击的工况下会发生局部破坏,但通过外附一定厚度缓冲层并合理安置,可保证储罐的静动力稳定性。

Canister for high-level radioactive waste is a core part in nuclear waste disposal barrier,and its static and dynamic stability during transportation,installation,and deep buried operation is of great importance.A silicon carbide(SiC)material based canister was proposed in this paper.The material has remarkable chemical stability,but its brittleness may be the key to restrict it application.In oder to investigate the static and dynamic stability of this canister,series of numerical simulations were performed using discrete element method,considering the physical nature of rock blocks and characteristics of interactions between rock and canister.The tensile strength characteristics of SiC was first investigated via specially designed lab and numerical tests.Comparison with analytical results has proved the reliability of adopted numerical method.The influence of disposal depth and horizontal to vertical stress ratio was then investigated.The dynamic loading behaviour pattern and basic failure mode of canister under free fall and rock impact were investigated,and the influence of rock fragmentation was mainly considered.The results show that the silicon carbide material is relatively brittle,with tested tensile strength between 150 MPa and 200 MPa,compared to its very high compressive strength.The tensile strength of silicon carbide was chosen 150 MPa for safety reason in later analysis.However,this value of 150 MPa is higher than the tensile or even compressive strength of ordinary rocks.The canister can survive under 1200 m depth,horizontal to vertical stress ratio of 3 with several disposal inclination angles.Under free fall,the maximum tensile stress in canister is determined by falling height and inclination angle.Upon rock fall without rock splitting,the maximum tensile stress in canister is determined by rock weight and contact type between rock and canister.Inclusion of rock splitting in model calculation can produce stress much lower than by traditional continuum method.The tip of the rock will crack first once the rock is hitting the canister,leaving the canister safe in the first place,which is different from that in continuum analysis.This implies the energy dissipation between rock blocks due to fracturing of rock during rock impact is not negligible.As the cohesion and residual friction angle between rock blocks increase,the stress induced in canister also increases,while the tension makes limited contribution to elevated stress.Another interesting finding is that as the rock block volume ratio gets smaller,the stress induced by impacting rock decreases first but then keeps to a constant value once certain threshold is reached.This suggests by reaching certain rock block volume ratio may be enough to reproduce dynamic impact-induced cracking,instead of decreasing rock block size constantly.Although local failure is expected under dynamic impact,a soft buffer layer with certain thickness outside the canister can guarantee static and dynamic stability of SiC canister together with appropriate emplacement.