启明星Ⅱ次临界非稳态热中子注量率测量

中子注量率及分布是反应堆的重要参数,本工作通过核数守恒在非稳态情况下的推导和求解,从理论上论证了次临界反应堆非稳态情况下中子注量率测量的可行性。将活化法和固体径迹法有机结合,利用固体径迹探测器标定活化片的测量数据,测量了启明星Ⅱ零功率装置的He-3管实验孔道内及反应堆外壁的中子注量率的分布,并与模拟计算结果进行了比较,利用MCNPX程序得到的模拟计算结果与实验结果的趋势一致,证明了该测量方法可以测量低通量的中子注量率,可实现反应堆不同时刻、不同位置的中子注量率测量,为CiADS技术的研发提供了实验数据与技术支撑。Neutron flux measurements were carried out at VENUS-Ⅱ lead-based zero power reactor by neutron activation method combined with solid-state nuclear track detectors (SSNTD). This experimental method was proposed based on the principle of nuclear number conservation when a foil was irradiated in an unsteady-state neutron field. By this method, thermal neutron flux distributions inside the He-3 duct were measured when VENUS-Ⅱ was operated under unsteady-state. The neutron flux distributions were also calculated with MCNPX code and were consistent with the experimental data. In addition, the neutron fluxes in the outer layer of VENUSⅡ were measured under steady-state. These results would benefit the further study of experimental methods for neutron flux measurement and provide important support for the design of CiADS.     

卵石材料在中子场下的次级放射性分析

由中国科学院近代物理研究所负责设计和建设的加速器驱动嬗变研究装置（CiADS）将建于广东省惠州市。作为一个核装置,CiADS在运行过程中会对环境造成次级放射性影响,因此评估CiADS对环境造成的次级放射性影响水平不仅对公众健康有重要意义,对屏蔽设计也具有一定的指导作用。利用241Am-Be中子源对兰州重离子加速器研究装置（HIRFL）附近的卵石材料进行了辐照实验,测量和分析了辐照后卵石样品内产生的24Na,54Mn,56Mn和27Mg的比活度等相关信息。通过对实验数据与利用蒙特卡罗程序Geant4模拟得到的各放射性核素比活度的比较,验证了Geant4模拟分析中子场下环境介质次级放射性这一方法的可靠性。进而利用Geant4模拟研究了CiADS质子加速器HEBT隧道底部屏蔽层外的中子能谱以及隧道底部屏蔽层外卵石层因中子活化产生的主要放射性核素的饱和比活度。分析结果表明,在合理的屏蔽设计条件下,CiADS运行期间泄露到屏蔽层外的中子活化环境中卵石材料造成的环境影响几乎可以忽略。The China initiative Accelerator Driven System (CiADS) is the first integrated ADS facility designed to study the safety disposal of nuclear waste. As a nuclear facility, it will cause radiological impact on the environment. Therefore, the assessment of radiological impact on the environment around this facility will not only meaningful to the public health, but also offers significant guides to the shielding design of CiADS. Pebble samplings were collected around the campus of the Heavy Ion Research Facility in Lanzhou (HIRFL) and irradiated by a 241Am-Be neutron source. Based on the γ-spectrum measured by a HPGe detector before and after the neutron irradiation, the specific activities of induced radionuclides 24Na,54Mn,56Mn and 27Mg in these samples were analyzed and compared with the Monte-Carlo simulation values obtained with the Geant4 toolkit. The comparison between the experimental and simulated results indicates that Geant4 toolkit is feasible for radiation impact assessment of nuclear facilities, such as CiADS. Then the neutron spectrum outside the concrete shielding of HEBT tunnel of the CiADS have been simulated by the Geant4 toolkit, and the saturation activities of the induced nuclides in the first 10 cm pebble layer have been predicted. The results show that the saturation activities of induced radionuclides in pebbles outside the CiADS shielding are much lower than their exemption values and their radiation impacts to the environment could be ignored.