Abstract
Laser-cutting feasibility for the safe and efficient dismantling and retrieval of Control Rod Drive Housing (CRD-H) units from the damaged units at the Fukushima Daiichi nuclear power plant was thoroughly investigated in this paper. To facilitate this exploration, two distinct types of mock-ups were designed and fabricated: one representing an elementary CRD-H, and the other replicating the bottom of the vessel. These mock-ups also included crucial components such as the CRD-H flange, grid, and support bars. They exhibited various outer diameters, ranging from 152 mm to 168 mm and were modular in nature, composed of several cylinders with variable thicknesses, along with a central rod. Each cylinder within the mock-ups was designed to emulate the internals of a CRD-H unit. In certain configurations, zirconia pebbles and gravels were incorporated to simulate the conditions of damaged CRD-H mock-ups. To address the risk of falling objects during CRD-H segmentation, different scenarios were rigorously tested, including piercing for inserting a 10 mm diameter pin and U-shaped cutting for a plate insertion. This approach aimed to simulate field conditions and constraints, as accurately as possible and assess the effectiveness of lasercutting technology in handling the complexities of dismantling CRD-H units in Fukushima Daiichi's challenging environment.
Laser cutting tests were conducted on elementary mock-ups in DELIA(CELENA) facility at CEA Saclay, while remote handling operability with a robotic arm and cutting feasibility of the bottom of the vessel mock-up were assessed in HERA facility at CEA Marcoule. Results demonstrated the capability to cut 168 mm diameter 304L mock-ups, filled with zirconia pebbles and gravels, using 8 kW laser power in less than 40 minutes. Components of the bottom vessel, including a 152 mm diameter CRD-H mock-up of an equivalent thickness of 141 mm and the upper part of the flange, were efficiently cut using a 14 kW laser power in less than 15 minutes. Additionally, a hanger rod was cut at a distance of 300 mm, despite accessibility constraints, in less than 35 seconds. Furthermore, demonstrations of scenarios involving the cutting and removal of the grid, along with remote handling operations for pin and plate insertion, were also successfully carried out, showcasing the versatility and efficiency of the laser-cutting process in this intricate dismantling process.
