弥散型燃料的等效性质及棒状元件的辐照力学行为的研究

【作者】 姜馨；

【导师】 霍永忠；

【作者基本信息】 复旦大学 ， 流体力学， 2009， 博士

【摘要】 弥散型燃料与传统的燃料元件相比具有高燃耗和高热导的优点,已广泛应用于研究试验堆,且在核动力舰船和核废料处理方面有着良好的应用前景。棒状弥散型燃料由弥散型燃料芯体和包壳构成。芯体是由核裂变颗粒弥散分布在金属基体中构成的。在核反应堆中,燃料棒处于非常苛刻的环境之中,裂变热使燃料棒内部产生较大的热应力；裂变产物又会使燃料颗粒发生辐照肿胀而加剧其与基体之间的相互作用；基体在中子的冲击下会发生硬化变脆和蠕变现象。所以如何保证燃料棒的安全可靠性是一个非常值得研究的问题。本文首先对弥散型燃料的等效热学性质和等效力学性质进行了研究。运用细观力学理论结合有限元法计算了弥散型燃料的等效热传导系数、等效热膨胀系数和等效弹性系数,分析了燃耗、温度、颗粒体积含量、颗粒排列方式和尺寸大小变化对等效性质的影响。研究结果表明,等效热传导系数的计算值和Maxwell模型及Brailsford模型的预测值最为接近；等效热膨胀系数和线性混合模型的预测值最为接近;而颗粒随机排列时的弹性系数的计算值和Mori-Tanaka法以及自洽法的预测值最为接近。其次,针对棒状弥散型燃料,在基于一定假设的基础上,建立了一个简化的模型,用有限元软件Ansys计算其内部的温度场。并计算了燃耗初期和燃耗发展时基体和包壳中的应力应变场,考察了燃耗、颗粒产热率、颗粒体积含量等因素的影响效应。对温度场的计算结果表明,颗粒产热率的增大或体积含量的增大会显著提高燃料棒中温度。外围流体的热交换系数不应低于0.005 W/mm2K,否则棒内温度急剧上升。颗粒的半径大小的变化对温度场影响较小。在燃耗初期,裂变热导致的热应力场是影响燃料棒安全性的主要因素。颗粒产热率的增大、体积含量的增多都会显著增大燃料棒中的应力水平和塑性应变。颗粒半径大小的变化则对其影响较小。当燃耗发展时,颗粒的辐照肿胀会导致其体积膨胀。通过虚拟温升的方法来模拟颗粒的辐照肿胀行为。此时,燃料棒中的应力水平较燃耗初期大大地升高了,塑性应变也急剧增加。当燃耗超过15%时,基体中的应力和塑性都已经非常大颗粒的增多也会使基体和包壳中的应力值和塑性应变迅速增大,当颗粒含量为30%时,10%的燃耗水平就可能使基体发生破坏。颗粒的半径不应过小,在半径值为150μm时,基体中的第一主应力要比其它半径时的值高,有可能发生拉伸破坏。本文可以为弥散型燃料棒的研究工作提供一个理论上的参考,减少试验的盲目性,并为其优化设计提供一个依据。更多还原

【Abstract】 Compared with the traditional nuclear fuel rods, the metal matrix dispersion nuclear fuel rods have higher burnup and higher thermal conductivity, so they have been extensively usd in the research reactors, and they have good prospects in the nuclear waste disposal and nuclear power vessels.The metal matrix dispersion fuel rod consists of the metal cladding and the dispersion nuclear fuel meat. The meat is distinguished by having the nuclear fuel particles dispersed through the metal matrix. Inside the demanding environment of the nuclear reactors, the fuel particles generate heat by nuclear fissions, the fission product accumulation results in irradiation-induced swelling of particles; the metal matrix creeps with time and becomes brittle. In order to use the fuel rod safely, the integrity of the matrix and cladding must be guaranteed and the stability of the rod should be kept. Thus . the relative researches are necessary to the safety and optimal design of the fuel rod.In the present work, the effective thermal properties and the effective mechanical properties of the dispersion nuclear fuel are investigated firstly. The effective thermal conductivity coefficient, the effective thermal expansion coefficient and the effective elastic coefficient are studied by the finite element method using the micromechanics. The study indicates that the numerical results of the effective thermal conductivity coefficient are in good agreement with the law of Maxwell and Brailsford; the numerical results of the effective thermal expansion coefficient are in good agreement with the law of mixtures; and the numerical results of the effective elastic coefficient are in good agreement with the law of Mori-Tanaka and self-consistent.A simplified model is developed to simulate the fuel rod and the temperature field is calculated by FEM. The stress field and strain field at lower burnup and higher burnup are investigated also. The effect of burnup and microstructure on the stress field and strain field are studied.The results of the temperature field indicate that increment of the heat generation rate or the volume fraction of the fuel particles will induce higher temperature in the rod; the convection coefficient can not be lower than 0.005 W/mm~2K; the effect of the size of the fuel particles on the temperature field is little.At lower burnup level, the thermal stress is the principal factor affecting the safety of the rod. The increment of the heat generation rate or the volume fraction of the fuel particles will increase the stress level and plastic level in the rod, and the effect of the size of the fuel particles on the temperature field is little.At higher burnup level, the irradiation-induced swelling induces the volume expansion of the fuel particles. A virtual temperature increment is applied to the particles to simulate the irradiation-induced swelling. The stress level is much higher than that at the lower burnup level and the plastic strain increase quickly. At 15% FIMA burnup level, the stress and plastic strain level in the matrix is very high. Especially, 10 FIMA burnup may induce the damage of the matrix while the volume fraction of the fuel particles is 30%. The first principal stress in the matrix will be very large if the radius of the fuel particles is 150μm, the material may be damaged by the higher tensile stress.This study could supply the simulation methods for the mechanical behavior analysis of the dispersion nuclear fuel rods, and it could provide numerical reference basis for the actual operation and optimization of the fuel rod.更多还原