Replacement of Secondary Heat Transport System Components in the Experimental Fast Reactor JOYO

A recently completed major upgrade of the JOYO experimental sodium-cooled fast reactor, to the MK-III design, increased its irradiation capability. One major change was a 40% increase in thermal power to 140 MWt, which necessitated the replacement of the cooling system. Major challenges in the replacement of secondary components were control of impurity ingress and assurance of welding integrity. Damage to existing systems was avoided during replacement operations by taking measures to prevent ingress of air into the sodium systems. The long exposure of the used pipes made of ferritic low-alloy steel to hot sodium was a concern because previous research showed that this material changes its mechanical property in sodium hotter than 673 K. Used pipe, heat transfer tubes and welds were subjected to material tests. These tests did not show notable material problems. The replacement of components was completed without major troubles, demonstrating the effectiveness of the methods used.