UN核芯TRISO燃料颗粒破损概率模型研究

TRISO燃料颗粒由核芯和4层包覆层组成,具有良好的裂变产物包容能力。TRISO燃料颗粒破损概率是表征TRISO燃料事故安全特性的关键参数。本文基于修正的PANAMA破损概率计算方法,在考虑UN核芯裂变气体释放导致的气体内压以及内外致密热解炭层辐照蠕变和收缩作用的基础上,开发了UN核芯TRISO燃料颗粒压力壳式破损概率计算方法,并采用IAEA基准题6和基准题9对模型进行了验证;基于开发的UN核芯TRISO颗粒破损概率计算方法,采用随机抽样统计方法分析了事故工况下UN核芯和包覆层设计参数(包括包覆层尺寸及密度)对UN核芯TRISO燃料颗粒破损概率的影响。研究结果显示,疏松热解炭(Buffer)层设计参数是影响TRISO颗粒破损概率的关键因素,可通过降低Buffer层尺寸及密度分布设计标准偏差的方法降低UN核芯TRISO燃料颗粒的破损概率。     

包覆燃料颗粒尺寸及其标准偏差对失效概率的影响

三结构同向性型(Tristructural isotropic,TRISO)包覆燃料颗粒是目前高温气冷堆和固态燃料熔盐堆采用的燃料元件。TRISO包覆燃料颗粒破损会导致裂变产物不可接受的释放,由此影响反应堆的安全运行。基于TRISO包覆燃料颗粒压力壳式破损模型,分析了TRISO包覆燃料颗粒核芯和各包覆层的尺寸对失效概率的影响,研究了TRISO包覆燃料颗粒核芯半径、疏松热解碳(Buffer)层厚度和碳化硅(Si C)层厚度的合理设计范围。同时,利用随机抽样统计的方法分析了TRISO包覆燃料颗粒核芯半径分布和各包覆层厚度分布对颗粒失效概率的影响。研究发现,降低Buffer层厚度分布的标准差至16μm可以使TRISO包覆燃料颗粒的失效概率降低一个数量级。     

10 MW高温气冷堆首次装料的包覆燃料颗粒研制

10 MW高温气冷堆采用全陶瓷TRISO型包覆颗粒燃料元件.TRISO型包覆燃料颗粒由燃料核芯、疏松热解炭层、内致密热解炭层、碳化硅层和外致密热解炭层组成.本工作研究用于生产包覆燃料颗料的具有多气体入口的新型喷动流化床和4层连续包覆工艺.采用化学气相沉积方法在150 mm直径流化床沉积炉中生产出10 MW高温气冷堆的包覆燃料颗粒.用扫描电镜观察研究了包覆燃料颗粒的微观结构.包覆燃料颗粒的制造破损率为3.4×10-6.包覆燃料颗粒的辐照考验结果(包覆燃料颗粒的裂变产物85Krm释放率为10-6)表明,包覆燃料颗粒的性能可以满足我国10 MW高温气冷堆的设计要求.     

UN核芯TRISO包覆燃料颗粒性能分析

为分析致密热解碳层、内压等因素对TRISO包覆燃料颗粒热-力学性能的影响,基于多物理场耦合软件COMSOL建立了以UN为核芯的TRISO包覆燃料颗粒三维热-力学耦合模型,并通过IAEA CRP-6基准题进行了验证。利用本文模型对稳态运行及反应性引入事故（RIA）工况下典型TRISO包覆燃料颗粒的性能进行了分析,结果表明,正常运行工况下SiC层能维持结构完整性,但IPyC层存在失效风险,需进一步优化TRISO包覆燃料颗粒的设计方案,而RIA工况下热膨胀是造成TRISO包覆燃料颗粒发生结构失效的主要原因。该模型能对轻水堆运行环境下的TRISO包覆燃料颗粒进行复杂的多物理场耦合性能分析,为进一步优化FCM燃料元件设计打下基础。     

钍基氟盐冷却高温堆TRISO包覆燃料颗粒结构优化分析

钍基氟盐冷却高温堆(Thorium-based Pebble Bed Fluoride Salt-cooled High-temperature Reactor,PBTFHR)作为第四代核反应堆的堆型之一,其燃料元件由TRISO(TRi-structural ISOtropic)包覆燃料颗粒组成,具有较好的中子性能和安全性。本工作采用SCALE 6.1程序开展PB-TFHR的临界和燃耗性能计算,结合PANAMA模型研究包覆燃料颗粒的破损率,分析了PB-TFHR中TRISO包覆燃料颗粒的kernel半径、包覆层的厚度和密度对堆芯中子学性能、裂变气体氪、氙和碘产量及包覆燃料颗粒破损率的影响,给出优化的包覆燃料颗粒结构,为其物理设计提供参考。研究发现:当保持包覆层的厚度和密度不变时,较大的kernel半径(≥0.01 cm)可使堆芯处于欠慢化区,且堆芯温度反应性系数均为负值;在相同的燃耗下,kernel半径越小,堆芯中裂变气体的生成量越少,且包覆颗粒的破损率越小;当保持包覆层密度不变,只改变包覆层的厚度时,疏松热解炭层和内致密热解炭层的厚度对keff有较大影响;而当保持包覆层厚度不变只改变包覆层的密度对keff影响较小。     

10 MW高温气冷堆包覆燃料颗粒辐照考验

TRISO型包覆燃料颗粒由燃料核芯、疏松热解炭层、内致密热解炭层、碳化硅层和外致密热解炭层组成.在冷态性能检验合格的基础上,进行了10 MW高温气冷堆包覆燃料颗粒的静态辐照试验和动态回路辐照试验.在辐照温度1 000 ℃、累积快中子注量1.28×1025 m-2和燃耗(以金属铀计)达到95 GW·d·t-1时,包覆燃料颗粒的放射性裂变产物85Krm的释放率为1.02×10-6,辐照后检验未发现包覆燃料颗粒破损.辐照考验结果表明,包覆燃料颗粒的性能可以满足我国10 MW高温气冷堆安全运行的要求.     

多物理场耦合TRISO颗粒堆内行为研究

三向同性燃料(TRISO)颗粒是高温气冷堆元件和弥散微封装燃料最核心的组成部分,在反应堆运行过程中,TRISO颗粒在辐照-热-力多物理场的作用下发生变形、产生温度梯度及颗粒内部裂变产物扩散等行为,为研究TRISO颗粒在高温气冷堆环境下的堆内行为,本文通过设置边界条件,定义燃料材料物性模型,建立了辐照-热-力耦合作用下TRISO颗粒的多物理场计算方法,应用三维有限元平台对TRISO颗粒的堆内行为进行分析。结果表明,TRISO颗粒核芯温度随核芯功率增大而增大,但相应的温度梯度绝对值变化较小;颗粒中疏松热解碳层(Buffer层)与内致密热解碳(IPyC)层产生间隙,且寿期末间隙尺寸随核芯功率增大而降低;TRISO颗粒中IPyC层受到较大拉应力,而SiC层只有在较高的核芯功率下,才会受到拉应力,且最大拉应力随核芯功率增大而增大,这导致高核芯功率下SiC层的失效概率达到2.2×10^-6。SiC层对¹¹⁰Ag、⁹⁰Sr、¹³⁷Cs等裂变产物具有优良的包容能力,在寿期末,SiC层以外几乎不存在裂变产物,这验证了T...     

碳化硅基新型包覆燃料颗粒的设计及制备

TRISO型包覆燃料颗粒可将核裂变产生的气体、固体裂变产物束缚在燃料颗粒内部,是高温气冷堆安全性的重要保障。为满足未来超高温气冷堆在更高温度及更高燃耗条件下对燃料元件的要求,需对传统TRISO颗粒进行优化和改进。基于包覆颗粒的破损机制,设计了两种SiC基新型包覆颗粒,一种采用疏松SiC层替代疏松热解炭层,包覆层由内而外依次为疏松SiC层、内致密热解炭层、致密SiC层、外致密热解炭层;另一种为全SiC包覆结构,包覆层由内而外依次为内层疏松SiC层、SiC过渡层、外层致密SiC层。根据结构设计,采用流化床化学气相沉积法实验探索了疏松SiC的形成机制及包覆工艺条件,并利用SEM、XRD等进行材料分析,最终成功实现了两种新型包覆颗粒的大规模制备。更进一步,提出了全SiC基燃料元件的概念,并制备了球形和柱形全SiC基模拟燃料元件。     

10MW高温气冷堆包覆燃料颗粒的研制

我国10MW高温气冷堆采用全陶瓷型包覆颗粒球形燃料元件。TRISO型包覆燃料颗粒由燃料核芯、疏松热解碳层、内致密热解碳层、碳化硅层和外致密热解碳层组成。采用丙烯和乙炔混合气体制备致密热解碳层以及四层连续包覆的新工艺,开展生产工艺条件试验,系统地研究了生产工艺和性能之间的关系,摸索出最佳生产工艺条件。用化学气相沉积方法在150mm流化床沉积炉系统中批量生产出TRISO型包覆燃料颗粒。用扫描电镜观察分析了包覆燃料颗粒的微观结构,包覆燃料颗粒的制造破损率为3.4×10-6,冷态性能达到我国10MW高温气冷堆设计要求。包覆燃料颗粒辐照考验结果(放射性裂变产物释放率R/B为1×10-6左右)表明,包覆燃料颗粒的质量可以满足10MW高温气冷堆安全运行的要求。     

包覆燃料颗粒几何参数统计规律对破损率的影响

将包覆燃料颗粒的碳化硅包覆层看作模型中的承压壳,利用蒙特卡罗方法,将颗粒几何参数的统计分布应用到压力壳式模型,考虑颗粒包覆层几何参数的统计规律对破损率的影响。研究结果表明,包覆燃料颗粒核芯半径、疏松层厚度、内致密热解炭层厚度以及碳化硅层厚度等参数对破损率有较为明显的影响;其中,核芯半径和疏松层厚度是影响破损率相对关键的因素。     

Modeling spent TRISO fuel for geological disposal: corrosion and failure under oxidizing conditions in the presence of water

High temperature gas reactors (HTGRs) are being considered for near term deployment in the United States under the GNEP program and farther term deployment under the Gen IV reactor design (U.S. DOE Nuclear Energy Research Advisory Committee, 2002). A common factor among current HTGR (prismatic or pebble) designs is the use of TRISO coated particle fuel. TRISO refers to the three types of coating layers (pyrolytic carbon, porous carbon, and silicon carbide) around the fuel kernel, which is both protected and contained by the layers. While there have been a number of reactors operated with coated particle fuel, and extensive amount of research has gone into designing new HTGRs, little work has been done on modeling and analysing the degradation rates of spent TRISO fuel for permanent geological disposal. An integral part of developing a spent fuel degradation modeling was to analyze the waste form without taking any consideration for engineering barriers. A basic model was developed to simulate the time to failure of spent TRISO fuel in a repository environment. Preliminary verification of the model was performed with comparison to output from a proprietary model called GARGOYLE that was also used to model degradation rates of TRISO fuel. A sensitivity study was performed to determine which fuel and repository parameters had the most significant effect on the predicted time to fuel particle failure. Results of the analysis indicate corrosion rates and thicknesses of the outer pyrolytic carbon and silicon carbide layers, along with the time dependent temperature of the spent fuel in the repository environment, have a significant effect on the time to particle failure. The thicknesses of the kernel, buffer, and IPyC layers along with the strength of the SiC layer and the pressure in the TRISO particle did not significantly alter the results from the model. It can be concluded that a better understanding of the corrosion rates of the OPyC and SiC layers, along with increasing the quality control of the OPyC and SiC layer thicknesses, can significantly reduce uncertainty in estimates of the time to failure of spent TRISO fuel in a repository environment.     

Performance modeling of Deep Burn TRISO fuel using ZrC as a load-bearing layer and an oxygen getter

The effects of design choices for the TRISO particle fuel were explored in order to determine their contribution to attaining high-burnup in Deep Burn modular helium reactor fuels containing transuranics from light water reactor spent fuel. The new design features were: (1) ZrC coating substituted for the SiC, allowing the fuel to survive higher accident temperatures; (2) pyrocarbon/SiC “alloy” substituted for the inner pyrocarbon coating to reduce layer failure and (3) pyrocarbon seal coat and thin ZrC oxygen getter coating on the kernel to eliminate CO. Fuel performance was evaluated using General Atomics Company’s PISA code. The only acceptable design has a 200-μm kernel diameter coupled with at least 150-μm thick, 50% porosity buffer, a 15-μm ZrC getter over a 10-μm pyrocarbon seal coat on the kernel, an alloy inner pyrocarbon, and ZrC substituted for SiC. The code predicted that during a 1600 °C postulated accident at 70% FIMA, the ZrC failure probability is <10^?4.     

泡沫不锈钢层TRISO颗粒的堆内行为模拟

三向同性燃料（TRISO）颗粒中疏松热解碳层堆内辐照收缩产生间隙后,会导致TRISO颗粒热导恶化。为解决该问题,本文采用泡沫不锈钢替代TRISO颗粒中的疏松热解碳层。对泡沫不锈钢TRISO颗粒的堆内行为模拟结果表明,采用泡沫不锈钢可以避免疏松层堆内密实化,提高疏松层的传热效率,有效降低核芯运行温度;不论采用泡沫不锈钢还是疏松热解碳作为疏松层,内层致密热解碳层（IPyC层）和外层致密热解碳层（OPyC层）的应力均会超过包覆层强度;碳化硅（SiC）层的环向应力随泡沫不锈钢层弹性模量的减小而减小,通过降低泡沫不锈钢弹性模量可以有效控制SiC层应力,保证其结构完整性。因此,应选取气孔率高、弹性模量低的泡沫不锈钢作为TRISO颗粒的疏松层,可在改善热导恶化问题的同时保证SiC层的结构完整性。该研究为TRISO颗粒在工程应用中的优化设计提供了指导。      

PANAMA程序及其在10MW高温气冷实验堆安全分析中的应用

ＰＡＮＡＭＡ程序是德国在高温气冷堆安全研究中开发的一个实用程序，可以用来计算ＴＲＩＳＯ－包覆燃料颗粒在事故条件下的破损率，本文简介ＰＡＮＡＭＡ模型，着重开发了ＰＡＮＡＭＡ程序中ＳｉＣ压力容器失效模式，并利用１０ＭＷ高温气冷实验堆（ＨＴＲ－１）包覆燃料颗粒的设计参数，计算了燃耗，温度，核芯直径以及各包覆层厚度对颗粒破损率的影响，结果分析表明破损率阻燃耗，温度和核志直径的增大面而增长较快，对缓冲层和Ｓ     

Optical anisotropy measurements of TRISO nuclear fuel particle cross-sections: The method

The analysis of two-modulator generalized ellipsometry microscope (2-MGEM) data to extract information on the optical anisotropy of coated particle fuel layers is discussed. Using a high resolution modification to the 2-MGEM, it is possible to obtain generalized ellipsometry images of coating layer cross-sections with a pixel size of 2.5 μm and an optical resolution of ∼4 μm. The most important parameter that can be extracted from these ellipsometry images is the diattenuation, which can be directly related to the optical anisotropy factor (OAF or OPTAF) used in previous characterization studies of tristructural isotropic (TRISO) coated particles. Because high resolution images can be obtained, the data for each coating layer contains >6000 points, allowing considerable statistical analysis. This analysis has revealed that the diattenuation of the inner pyrocarbon (IPyC) and outer pyrocarbon (OPyC) coatings varies significantly throughout the layer. The 2-MGEM data can also be used to determine the principal axis angle of the pyrocarbon layers, which is nearly perpendicular to the TRISO radius (i.e., growth direction) and corresponds to the average orientation of the graphene planes.     

THETRIS: A micro-scale temperature and gas release model for TRISO fuel

The dominating mechanism in the passive safety of gas-cooled, graphite-moderated, high-temperature reactors (HTRs) is the Doppler feedback effect. These reactor designs are fueled with submillimeter-sized kernels formed into tristructural-isotropic (TRISO) particles that are imbedded in a graphite matrix. The best spatial and temporal representation of the feedback effect is obtained from an accurate approximation of the fuel temperature. Micro-scale models of TRISO particles are necessary in order to obtain accurate predictions during fast transients or when parameters internal to the TRISO are needed. Most accident scenarios in HTRs are characterized by large time constants and slow changes in the fuel and moderator temperature fields. In these situations, a meso-scale, or pebble- and compact-scale, solution provides a good approximation of the fuel temperature as the fission thermal energy transports out of the kernel and into the surrounding matrix with a much shorter time constant. Therefore, in most cases, the matrix can be assumed to be in quasi-static equilibrium with the kernels. These models, however, fail to provide accurate information on the state of the various components of the TRISO during the early stages of transients. Since the coated particles constitute one of the fundamental design barriers for the release of fission products, it becomes important to understand the transient behavior inside this containment system. An explicit TRISO fuel temperature model named THETRIS has been developed and incorporated into the CYNOD–THERMIX-KONVEK suite of coupled codes. The code includes gas-release models that provide a simple predictive capability of the internal pressure during transients. The new model yields similar results to those obtained with other micro-scale fuel models of TRISO particles, but with the added capability to analyze gas release, internal pressure buildup, and effects of a gap in the TRISO. Analysis of bounding benchmark transients yield good agreement with other codes in which the TRISO particles are modeled explicitly. In addition, a sensitivity study of the potential effects on the transient behavior of high-temperature reactors due to the presence of an inter-layer gap is included. Although the formation of a gap occurs under special conditions, its consequences on the dynamic behavior of the reactor can yield responses during fast transients that depart significantly from those in which no gap is present in the model. The new model was applied to an extreme (beyond design basis) scenario in order to observe the behavior of the fuel during a large prompt critical reactivity insertion. Although a large amount of fission energy was deposited rapidly into the fuel, the kernel temperature is shown to stay well below the melting point and the silicon carbide layer remained well below the temperature above which failure is expected to occur. The explicit treatment of the TRISO particle geometry leads to much lower estimations of power peaking during the transient and a greater degree of negative Doppler feedback.     

Verification of Fission Product Release Model from High Temperature Engineering Test Reactor Fuel

A fuel assembly of the High Temperature Engineering Test Reactor (HTTR) is composed of fuel rods and a hexagonal graphite block. A fuel rod is composed of the fuel compacts and a graphite sleeve. The coated fuel particles are incorporated into a graphite matrix to form a fuel compact. The fuel consists of microspheres of low-enriched U02 with a TRISO coating. The TRISO coatings consist of a porous pyrolytic carbon (PyC) buffer layer followed by an isotropic PyC layer, a SiC layer and a final (outer) PyC layer.

In order to evaluate amounts of fission products released from the HTTR fuel rods during normal operation, analytical models have been developed. Fractional releases of noble gases and iodine are calculated based on release data of ⁸⁸Kr which are obtained by irradiation tests with failed coated fuel particles. The transport of the metallic fission products through the kernel, coatings, fuel compact and graphite sleeve is modeled as a diffusion process. These analytical models have been verified by comparison with measured fractional releases in in-reactor tests and have been concluded to be applicable to the HTTR fuel condition.