The Effect Of Primary Water Chemistry On The Integrity Of Nuclear Fuel Cladding Tube

During the operation of a pressurized water reactor power plant, corrosion, cracking and surface deposit of the fuel element cladding tube seriously affect the performance and safety of the reactor. In the case of cracking of cladding tube, leakage of fission products may occur, which may lead to an accident involving release of radioactivity. The integrity of cladding tube is directly affected by the primary water chemistry control, therefore, it is very important to optimize water chemistry control, in order to mitigate the corrosion and maintain the integrity of cladding tube.At first, Qinshan phase II was designded as the followings: cycle length (280-290EFPD), using AFA 2G fuel assembly in which Zircoloy-4 was used to fabricate the cladding tube. For achieving better performance, Qinshan phase II improved the method of the core and fuel management. According to the new method. A high performance fuel assemblies such as AFA3G was used, in which the cladding tube material was changed to M5 zircoloy, and cycle length was added to about 340EFPD, the u-235 concentration of the fuel was also added to 3.7%. Then the reactor core will undergo 3 to 4 cycles of the mixing core, which is composed of AFA 2G and AFA 3G fuel assemblies. And because the mixing core has a higher excess reactivity, it also requires a higher boron concentration, and adjusted pH value in primary water chemistry control.According to property of mixed core, this article research the coordinate pH control by the boron acid with lithium hydroxide on the basis of performance and mechanism of corrosion of the zirconium alloys. In order to optimize pH management and as possible as decrease the corrosion rate of the primary and fuel cladding. This paper propose a advanced coordinate control program by boron acid with lithium hydroxide when AFA2G and AFA3G assembly was adopted at Qinshan phase II.This article also research the origin of the dissolved oxygen and aggressive impurity base on the corrosion mechanism of the oxygen hydrogen and aggressive impurity on fuel cladding. Put forward the control method for removing dissolved oxygen. Aggressive ion and deposits. And find a optimize range for control of hydrogen.Through oxygenation operation performance evaluation and trend analysis of previous outage at Qinshan phase II, this article conclude the variation rule of the radioactivity and dose rate and also propose a solution for solving some problems occur during outage which is very difficult to deal with. It was valuable for optimize oxygenation operation and assure a better performance.Optimized parameters for primary water chemistry control were proposed after the research, which helped to mitigate corrosion and of cladding tube, to protect cladding tube from cracking. Results also showed that the dose rate was well controlled during refueling outage. The present research has a profound meaning to the operation of the mixed fuel assembly core of Qinshan phase II reactors.