Deposition of boron on fuel rod surface under sub-cooled boiling conditions; An Approach toward understanding AOA occurrence

Uchida, Shunsuke; Asakura, Yamato*; Suzuki, Hiroaki*

In PWR, it has been assumed that Li and B ions deposited on fuel under sub-cooled boiling conditions. Accumulated B on the fuel led to AOA. The amount of B deposited on the fuel was evaluated from two directions. The first calculated the amount with the extended MED model and the other estimated it from the viewpoint of reactor reactivity. It was concluded that: (1) the calculated B deposition amount on the fuel, which was one or two orders larger than measured amounts and Ni oxides compounds, was seldom measured in the fuel deposits due to its high release rate; (2) its hideout return during the reactor shutdown period was seldom observed due to its high concentration in the primary coolant; (3) one of the most promising approaches to evaluate its accumulation on the fuel during plant operation was the MED model calculation; and (4) control of Ni concentration in the primary coolant resulted in decreased Ni oxide deposition and then mitigation of AOA occurrence.