The purpose of the MK-III program is to upgrade the irradiation capability of the liquid sodium-cooled experimental fast reactor JOYO. As a result, the neutron flux density of the core was increased, and the reactor thermal power was increased to 140 MW(thermal) from the originally designed 100 MW(thermal). To accommodate the increased thermal power, the flow rates of sodium coolant in the primary and secondary systems were increased by 20 and 10%, respectively. Also, all intermediate heat exchangers and dump heat exchangers were replaced with new ones. The replacement of these large sodium components was carried out over an [approximately]1-yr period with both fuel and molten sodium still in the reactor vessel (RV).
Major challenges in the replacement were the control of impurity ingress to existing systems and protection from radiation exposure in the high-dose-rate regions inside the containment vessel. During the replacement, the seal bag method, impurity concentration monitoring of cover gas, and low-pressure control of cover gas were applied to prevent damage to existing components and systems, such as the RV, fuel subassemblies, sodium piping, and tanks. The measures taken to reduce the radiation exposure were a lowering of the surrounding dose rate through the use of temporary shielding, shortening of the operation time near the high-dose-rate area by first doing thorough training, and the employment of protection equipment to avoid contamination. The replacement of components was completed without major trouble, and methods applied for the replacement proved to be effective in the operation and maintenance of sodium-cooled reactors.