Reactor Containment Decompression And Filtration In Qinshan NPP Phase Ⅱ

The reactor containment building is the last barrier to prevent the radioactive substance from releasing to the environment. After the core meltdown accident, the molten core reacts with the containment's concrete base and produces large amounts of noncondensing gas; pressure within the contaiment will increase by the accumulation of the gas, which might jeopardize the integrity of the containment, and the radioactive substances should be released into the atmosphere. The risk and consequences of the reactor containment overpressure is not taken into consideration in the initial design bases of the Qinshan NPP Phase II in 1996. The experts organized by the National Nuclear Safety Agency (NNSA) required the utility to install a filtrating and exhausting system as early as the project was primary evaluated, so that the pressure would not exceed the bearing capacity of containment vessel through active pressure-relief, and the integrality of containment would not be destroyed.Based on the integration of theoretical analysis and engineered practice, and according to the practical operation of Qinshan Phase II and with reference to both domestic and foreign advanced design experiences, this paper proposed practical and useful scheme and design method on filtrating and exhaust system which is matched for the actual conditions of Qinshan Phase II NPP. Thermodynamic conditions of containment atmosphere are analyzed with the MELCOR code, and effects of the filtrating and exhaust system are calculated. Meanwhile,the commissioning test scheme and documents of the filtrating and exhaust system is worked out. Furthermore, operation and control principle, safety analysis and principle of maintenance and periodic test of the system are given in this paper. Finally, an overpressure U5 procedure special for Qinshan Phase II NPP is developed based on the Adobe PageMaker 6.5C Windows.With the up-mentioned work, the system has been checked and accepted by NNSA. After dosing and nitrogen filling, the system has been at the standby condition. The pressure of containment vessel will increase under the severe accident condition. With the depressurizing system equipped, the plant operation mode will be guided by the operator follow the U5 procedure, the filtered gas will be released to the atmosphere through the EUF system without causing containment overpressure. Most of the radioactive substance will decrease in releasing through filtration. (10 time less at least, to make sure it is lower than source term S3 level). The established system satisfies the source term requirement of emergency plan and can ensure the radioactive products against escaping from the containment or EUF system, and the radioactive products will not release into environment.