Study On The Characteristics And Release Amounts Of Radioactive Sources In Severe Nuclear Power Plant Accidents

When a serious accident occurs in a nuclear power plant,if not handled in time,the core will melt,the pressure vessel will fail,and the fission products will be released to the containent.With the increase of temperature and pressure in the containment,the containment will fail,resulting in the release of fission products to the environment.By studying the characteristics of source term and the distribution,release and deposition of radioactive fission products in containment,the release amount of source term to the environment can be reasonably evaluated,which is conducive to the development of emergency plans to reduce hazards.Therefore,it is very urgent and necessary to study the characteristics of source term and the release quantity in serious accidents.Taking the AP1000 reactor as the research object,the source analysis calculation model,accident analysis calculation model and multi factor variance analysis model are established.Based on the Source Item Analysis Program and Integrated Accident Analysis Program,the source calculation input card and severe accident analysis input card are developed.The core source term and its influencing factors are calculated by Source Item Analysis Program,and the multi factor variance analysis method is used to analyze the sensitivity of the factors affecting the core source term.The fission product behavior of a serious accident with a large break is calculated by Integrated Accident Analysis Program,and the reliability o.f the calculated results is verified by comparing with the results of other programs.Finally,the basic mechanism of fission product deposition in containment is obtained.According to the calculation of Source Item Analysis Program,under the influence of various core source terms,the radioactivity of 238Pu,239Pu and 240Pu,which are typical actinide nuclides,reaches a stable state when the operation time of the core increases to a certain amount,and the radioactivity of 241 Am increases with the operation time of the core,which is in direct proportion.The typical short-lived fission products,89Sr and 91Y,increase in a short period of time,and decrease in an exponential trend after reaching the maximum.The typical]ong-lived fission products,137Cs and 90Sr,showed an increasing trend in a short period of time and a stable decreasing trend after reaching the maximum value.The total activity of fission products is higher than that of actinides,generally one order of magnitude higher.The photon source in the low energy region is stronger than that in the high energy region,with a maximum difference of ten orders.The influence of burnup,specific power,enrichment and operation mode on the source term of the reactor core is analyzed by using the method of multi factor variance analysis.The results show that the sensitivity of the factors affecting the generation of core source term are specific power,enrichment and burnup in order,while the operation mode has little effect on the generation of core source term.Based on Integrated Accident Analysis Program,the behavior of fission products under the serious accident of large break initiation is calculated and analyzed.The results show that inert gas,volatile source term and non-volatile source term are mainly distributed in the core and containment space,a few in the pressurizer,and a few in other locations,Inert gas is mainly released in the form of gas,and the release share is usually larger,more than 90%.volatile fission products and non-volatile fission products are mostly released in the form of aerosol,and the release share is usually smaller,less than 1%.The aerosol deposition process in containment can be divided into internal and external deposition processes.The internal deposition process is mainly that particles collide with each other,and the external deposition process is mainly that particles migrate to the surface of deposition structure by gravity,thermophoresis and diffusion electrophoresis.