Study On Optimization Of Shielding For The BDS System In The Nuclear Power Plant

The Steam Generator Blowdown System (BDS) of AP1000 nuclear power plant has two 50% thermal and fluid recovery trains, one for each steam generator. Each train includes a regenerative heat exchanger, a flow control valve, and an EDI package, containing filters and EDI modules. The BDS contributes to the control of steam generator secondary cycle water chemistry by removal of the blowdown flow from the secondary side of each steam generator during normal plant operation. Continuous running of BDS keeps the chemistry and radioactivity of secondary circuit water within the allowable limits of power plant operation.A certain margin of broken pipe is allowed by the steam generator during normal plant operation. Radioactivity accumulates within the BDS heat exchangers and the EDI packages when the steam generator tube rupture occurs. Without shielding radiation doses around the BDS heat exchangers and the EDI packages could be severe. Therefore, in accordance with national standards for radiation protection requirements it is necessary to be shielded in order to reduce the dose rate level to 2.5?Sv / h or less.Sanmen nuclear power plant Units 1 and 2 and Haiyang nuclear power Units 1 and 2(reference plant) in China is the first engineering applications of AP1000 model in the world.In these two nuclear power plants, temporary shielding is used for BDS heat exchangers and the EDI packages. Although this temporary shielding could successfully reach the requirement of does outside the equipment room, it is costing large space. At the same time,the space in the first bay is very small. So we will have troubles in daily patrolling ?overhauling and part exchanging of equipments. The aeration of equipments' room will be affected either.Xu Dapu nuclear power plant units 1 and 2 are the first self-designed AP1000 power plants in China. A number of improvements have been implemented compared to the reference power plant and the improvement of turbine building first bay is one of the large-workload enhancements. BDS shielding optimization is one of the main first-bay improvements. Based on BDS heat exchanger, EDI package and BDS piping of Xu Dapu power plant, this paper puts forword three shielding design scheme and analyze the BDS shielding using Monte Carlo method after determining radiation source ,medium, shielding material and the target value. According to the prior analysis , shielding designs more than choose optimization analysis,determine the 600mm thick ordinary concrete block for the optimal solution;Compared with the reference power plant' s defects, and considering the radiation protection design experience from operation nuclear power plant, the optimum proposal of BDS shielding design is presented. The main optimizing content are: 1)Extended the length and the span , span is adjusted 7.682m to10m, length is adjusted 50m to 57.4m,so that there could be a reserved zone for equipment operation,maintenance and repair;the maintenance of heat exchanger could be indoor reducing the risk of the radioactivity 2) a regional shielding, utilizing 600mm thick reinforced concrete walls and shielded gate,eliminate the difficulty of removing and the risk of radioactivity diffusion which is taken by the temporary shield. 3) Adding simple health entrances, helping operation and maintenance personnel have radiation detection and protection; 4) Setting separate ventilation subsystem in BDS system in order to reduce the harm to staff and the public caused by gaseous radioactive hazards; 5) Optimizing equipment installation and maintenance channel, setting installation holes and installation doors, adding equipment lifting holes in 5.000m floor, to facilitate EDI module pellets' replacement and ensure the radioactive waste pellets' transportation safety; 6)Optimizing staff channel in area with BDS system. While the power plant is under normal operating conditions, staffs get in and out the region through health entrances. In emergency situations, people can evacuate through emergency doors in every floors.By analyzing the radioactive shielding measures of BDS system in Xu Dapu nuclear power plant Units 1 and 2, we have developed a more reasonable shielding solution. It has an important effect for shielding design of BDS system in subsequent nuclear power plant.