Emergency shutdown device of pebble bed type reactor

Purpose: At the time of abnormal rise of the core reactivity, to distribute the pellets of a neutron absorber over the entire area of the core to carry out the desired reactor shutdown, and after the use of the pellets, to rapidly take the same out, thereby to easily carry out redriving of the device. Constitution: Neutron absorber pellets having a diameter larger than the minimum gap between fuel balls and pellets having a diameter smaller than the same are used. By the opening action of the device due to a reactor emergency shutdown signal, firstly larger pellets are caused to fall onto the core surface, and trapped at the minimum gap part. Then, the smaller pellets are similarly caused to fall thereon, and the core reactivity is caused to fall. (Aizawa, K.)