Research On Technical Method Of Detecting Plume Levels Of Real-time Monitoring System Of Ambient Environment Of Nuclear Power Plant

Real-time monitoring ofγradiation levels in the ambient environment of Nuclear Power Plant is the most extensive used method of monitoringγradiation levels of the radioactive inert gases that emitted by the Nuclear Power Plants. The main purpose of that is putting into early emergency preparedness and response of nuclear accidents, assessing the potential impact on the public from nuclear power plant's emissions. The research of the plume detection level for the monitoring system can improve the ability of distinguishing the rising of levels in the ambient environment of Nuclear Power Plant are due to the affects of plume or nature factors, and the ability of monitoring the abnormal emissions of nuclear facility or accident release. This paper can be used for monitoring system of nuclear power plants, and also can be used for reference for other gamma-radiation alarming and monitoring system as well.This research take the real-timeγ-radiation monitoring system of Qinshan Nuclear Power Base as the research object and analyze the exterior factors which impact on the monitoring results of the system. Commanding the disturbing rule of background changing to monitoring results, the experiment confirmed that the changing trend of the radon concentration and dose rate was in agreement, the impact of rainfall on the dose rate value of all sites is primarily analyzed in the research. Confirm the detection limit and detection threshold of the monitoring system by analyzing the substantive data of recent years got by the real-time monitoring system of Qinshan Nuclear Power Base.Through theoretic analyses and experimental results, shielded model was designed and confirmed, the shape and thickness of the shielded layer were also designed and confirmed. Then, the long-term comparative monitoring of two detectors in the same site was carried out, the detection limit and detection threshold of the monitoring system were according to 3 times standard deviation and background plus 3 times standard deviation separately. The monitoring results showed that the detection limit of shielded ionizing chamber and unshielded ionizing chamber are same on the whole, and shielded ionizing chamber can reduce 53% of detection threshold compared with unshielded ionizing chamber when it's sunshine. The shielded ionizing chamber can reduce 45% of the detection limit and 57% of the detection threshold compared with unshielded ionizing chamber when it's rainy.The two detector measuring system is composed of shielded ionizing chamber and unshielded ionizing chamber, the two ionizing chamber got proportioned data when the system measured environmental radiation dose rate. It could distinguish abnormal emissions by the principle that the additional plume rate would breach this relationship. The experimental results show that, the monitoring system could discriminate 5.4nGy dose rate per hour of radioactive plume produced by monitoring site in rainy day, and 44nGy dose rate per hour of radioactive plume produced by monitoring site in rainy kurtosis period of time. In practice, it could discriminate the dose of radioactive plume by the dose relationship of shielded ionizing chamber and unshielded ionizing chamber in different interval.According to the operation and research results of the Real-time Monitoring System of Ambient Environment of Qinshan Nuclear Power Base, this paper suggested three plume estimating programs of nuclear power plant refer to three dose levels-below 10% of the background, 10~100% of the background and above 100% of the background respectively, then advise the monitors on duty to take proper action.