反应堆核测系统通过实时测量中子通量监控反应堆功率水平，是反应堆安全稳定运行的关键子系统。现有的商用反应堆普遍采用“堆外测量”，即将探测器置于堆芯外混凝土屏蔽层中的探测器孔道内，根据测量堆芯外经慢化后的热中子注量率实现对堆芯功率水平的监控。然而对于部分特殊反应堆，由于其对结构紧凑性要求较高，无法设置中子慢化层，探测器必须直接对快中子进行测量且不能有太大的尺寸，这就对探测器的快中子灵敏度要求较高。本文主要根据甲方提供的一种特殊反应堆的设计输入，设计用于测量中子能量大于100KeV的探测器并进行实验验证。根据不同工作介质对快中子的反应截面和反应能比较，选取甲烷电离室、硼电离室和裂变室三种探测器设计方案，分别进行结构设计和理论计算，对比三者在等灵敏体积下的探测性能，并结合探测器性能测定实验验证理论计算的准确性。提供设计输入的特殊反应堆还在设计研发中，无法直接以此直接进行探测器性能的验证实验，因此基于实验室现状采用241Am-Be中子源进行实验。由于源生产时间较早，缺少源尺寸与位置信息，为确保后续实验的准确性，首先通过结合实验与MCNP模拟验证确定源的尺寸与位置。根据已确定的源信息，利用MCNP模拟，设计用于探测器性能验证实验所需的最优中子场，以此布置实验，并确定探测器灵敏体积所处位置的中子能量与注量率的具体分布信息。裂变室所用丰度90%的235U难以获得无法开展实验，本文主要对甲烷电离室和硼电离室开展探测器性能测定实验。由于中子场并非单能，无法直接根据实验的输出电流计算探测器电流灵敏度，因而将MCNP模拟得到的探测器灵敏区内的中子场能量和注量率分布，结合理论计算得到的探测器电流灵敏度进行计算得到理论输出电流，并用理论输出电流和实验测得实际输出电流进行比较，验证探测器灵敏度计算的准确性。根据实验结果，甲烷电离室和硼电离室实际输出电流和理论输出电流的误差分别为5.79%和11.52%，则证明电流灵敏度的估算是合理可信的。在此基础上，对设计的三种电离室比较，最终确定了甲烷电离室和裂变室可满足设计输入，甲烷电离室性能最优。本文设计制造的甲烷电离室，漏电流<0.5pA,对100KeV中子的电流灵敏度为1.77×10^-15 A/(n?cm^(-2)?s^(-1) )，饱和电压为500V。
The nuclear instrument system monitors the power level of the reactor by measuring the neutron flux level in real time, which is the key subsystem for the safe and stable operation of the reactor. The existing commercial reactors generally use "ex-core nuclear instrument system ", that is, the detector is placed in the detector channel in the concrete shielding layer outside the core, and the monitoring of the working state of the core is realized according to the measurement of the thermal neutron flux rate after slowing outside the core. However, for some special reactors, due to its high requirement of compact structure, it is impossible to set neutron moderating layer, so the detector must directly measure the fast neutron, which requires high sensitivity of the detector. According to the design input of a special reactor, a detector for measuring neutron energy greater than 100keV is designed and verified by experiments.According to the comparison of reaction cross section and reaction energy of different working medium for fast neutrons, three detector design schemes of methane ionization chamber, boron ionization chamber and fission chamber are selected to carry out structural design and theoretical calculation respectively, and the detection performance of the three detectors under the same sensitive volume is compared, and the accuracy of theoretical calculation is verified by combining with the experiment. The special reactor which provides the design input is still in the process of design and development, so it is impossible to directly carry out the verification experiment of detector performance. Therefore, based on the current situation of the laboratory, 241Am- Be neutron source is used for the experiment. Due to the early production time of the neutron source and the lack of source size and location information, in order to ensure the accuracy of the subsequent experiments, first determine the size and location of the source by design experiment measurement and MCNP simulation verification. Based on the determined source information and MCNP simulation, the optimal neutron field for the detector performance verification experiment is designed to arrange the experiment and determine the specific distribution information of neutron energy and flux rate where the detector sensitive volume is located. It is difficult to obtain 235U with 90% of the abundance for fission chamber. In this paper, the detector performance of methane ionization chamber and boron ionization chamber is tested. Because the neutron field is not single energy, the detector current sensitivity can not be calculated directly according to the experimental output current, so the neutron field energy and flux distribution in the detector sensitive area simulated by MCNP are combined with the detector current sensitivity calculated by theoretical calculation to obtain the theoretical output current, and the theoretical output current is compared with the actual output current measured by experiment to verify the accuracy of detector sensitivity calculation. According to the experimental results, the errors of the actual output current and the theoretical output current of the methane and boron ionization chambers are 5.79% and 11.52% respectively, which proves that the calculation of the current sensitivity is reasonable and reliable. On this basis, compared with the three designed ionization chambers, the methane ionization chamber and fission chamber can meet the design input, and the performance of methane ionization chamber is the best. The experimental of methane ionization chamber designed and manufactured in this paper has a leakage current of less than 0.5pA, a current sensitivity of 1.77×10^-15 A/(n?cm^(-2)?s^(-1) ), and a saturation voltage of 500V.
