Experience of primary cooling system modification to increase heat removal capability in the experimental fast reactor JOYO

The purpose of the MK-III program is to upgrade the irradiation capability of the JOYO experimental sodium- cooled fast reactor. As a result, the neutron flux density of the core was increased and the reactor thermal power was increased to 140 MWt from the originally designed 100 MWt. 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. During this replacement, molten sodium and fuel were retained in the reactor vessel. Consequently, the primary cooling system and cover gas boundaries had to be maintained to prevent impurity ingress to the sodium system. 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. A roller cutter was used for cutting large diameter pipes to prevent ingress of cuttings. 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. (author)