三维功率分布是最基本的堆芯状态量，应对其准确、连续地监视，以确保反应堆操纵运行的灵活性和有经济的安全保护裕量。而为了能反映堆芯真实状态，监视的芯内三维功率分布必须是“实测的”。尽管芯外电离室是目前大多数反应堆上唯一实时敏感元件，而且其安装和维护比芯内探测器容易得多，但当用它进行堆芯状态监视时，涉及到人们通常所认为的如下困难：1)芯外电离室仅响应最靠近它的一到二盒组件。2)它们对分布的变化是否有足够的灵敏度？3)堆内有多种物理扰动在电离室读数变化中引起的效应是否可分开？4)一般，电离室的数目不足以重构分布。然而，根据我们的研究，即使是芯外电离室读数中的中子信号也可以包含堆芯边缘大量组件的功率信息，而读数中的成分则使其含有更大范围芯内功率分布的信息；更重要的是，芯内功率分布的变化一定是由可理解的物理因素引起的，而适当布置的芯外电离室，其读数改变是足以辩识的。本文利用仿真计算寻找分布变化的规律，找到了包含较少堆芯状态参数的精确的约束公式。因它直接用对应于物理因素的三维特征分布的线性组合来表达分布变化，所以还具有如下特点：堆芯状态参数有明确的物理意义；公式对新的未考虑到的物理因素是开放的。从而可以在实现对误差不灵敏的分布监视的同时，在线判断堆芯内是否存在“异常”扰动。因为扰动堆芯中子截面参数的各种物理因素差别很大，我们发展了两种特征分布抽取方法：一是谐波分组方法，另一个是满足系统算子可加性的高阶微扰方法。由于三维谐波的全堆芯变化性质，谐波分组方法只能用于发生在全堆芯大范围的截面参数扰动。为了分析局部强扰动，提出了满足算子可加性的高阶微扰方法。两种互补的特征抽取方法涵盖了所有可能的物理因素。以200MW供热堆为例，由仿真计算给出了燃耗、硼浓度、冷却剂温度、各组控制棒移动及其相互作用下的大量堆芯状态的芯外电离室读数，并用这些读数进行了分布重构。归纳得出了如下结论：通过合适地设计，可以仅由为数不多的芯外电离室读数在线重构出芯内功率分布。对读数中1％的误差，可以将分布的重构误差控制在5％之内。从而可以消去燃耗引起的最大约4%、控制棒引起的最大约20％的对功率水平的估计偏差。可以预料，堆芯状态监视还会在减小芯内测量频率、提供操纵员支持和修改运行技术规范等方面发挥作用。本论文首次表明了：仅用芯外电离室，实现实时、实测的监视芯内三维功率分布是可能的。

The in-core 3D (three-dimensional) power distribution (IPD), the essential state variable in a reactor core, should be monitored in all time to guarantee the economic protection-margin and the flexibility of the reactor operation. And the monitored IPD must be a “measured” result in order to agree with the real states in the core. Though ex-core ion-chambers (EIC) are the sole real-time radiation sensors on the reactor and their mounting and maintain are much easier than that for the fixed in-core detectors, they have not been used in surveillance of the IPD yet，because they become involved in such following difficulties as the routine of speech. 1) An EIC responds only to its nearest one or two fuel-assemblies. 2) Are their sensitivities to the variances in IPD high enough? 3). Since there are several kinds of perturbation in reactor, can the effects in the reading of ion-chamber be separated from each other? 4). Generally, the number of the EIC is not large enough to reconstruct IPD.According to our study, not only the neutron ingredient in the readings of the EIC can give out a certain of power-information of a lot of assemblies around the periphery of the core, but also the  ingredient contains the information in the IPD over a larger area of the core. More importantly, the variance in the IPD must be driven by reasonable physical causes, and their variances in the readings of the EIC with proper configuration are notable enough to be identified. We look for the regular patterns of variance in the IPD by using simulations and find out an accurate restricted equation to the reasonable variance in the IPD. Because a certain linear combination of the 3-D characteristic distributions corresponding to the specified physical causes is proposed to express the restricted variance in the IPD, there exist the following features. Firstly, the core parameters have explicit physical meanings. Secondly, the equation will open for any new physical causes that can be included by adding new characteristics conveniently. Then the IPD can be estimated error-insensitively by the readings of the EIC as well as the determination of the in-core abnormality can also be performed simultaneously.Since there is a great difference between different physical causes that disturb the neutron cross-sections in the core, two kinds of extraction methods for the characteristics have been developed. One is the harmonics grouping method and the other is the higher order perturbation method satisfying the additivity for the system operators. Because of the whole core property of the 3-D harmonics, the harmonics grouping method can only be applied to the perturbations to the cross-sections occurring throughout the core area. In order to analyze the local and intense perturbations, the higher order perturbation method satisfying the additivity of the system operators is proposed. These two characteristic extraction methods can be applied to analyzing all reasonable physical causes.The readings of the EIC for a number of core states with burn-up, changing of coolant temperature and movements of five bands of control rod were created with simulations for examples on the 200MW heating reactor and the re-constructions of the IPD were performed with them. It is can be concluded that with proper design of the EIC, the IPD can be reconstructed on-line only with the readings of the EIC, while the error of the reconstructed IPD can be limited in five percent with one percent error in the readings of the EIC. Then about four percent maximum offset cause by burn-up and about twenty percent maximum offset caused by movement of control rod while estimating total-power can be eliminated. It is hopeful that the core status surveillance can be applied to reduce the frequency of the routine flux mapping, to provide extensive operator aids, and to modify the technique specification in reactor operation and etc. in the future. The first success of the technique demonstrates that the EIC can give out the “measured” power distribution in a reactor core continuously.