JOYO MK-III modification work on heat transport system. Working plan and plant control

The MK-III project to improve the irradiation capability of the experimental fast reactor JOYO have been in underway since 1987. The increase of fast neutron flux and the enlargement of that field increase the reactor thermal rate from 100 MWt to 140 MWt. To increase cooling capacity of heat transport system, intermediate heat exchangers (IHXs), dump heat exchangers (DHXs), piping connecting to IHXs and DHXs, main motors on primary and secondary main circulation pumps were replaced. The replacement of these large components was carried out under following hard conditions. 1) Limitation of work space, 2) Fuel subassembly and molten sodium in the reactor vessel, 3) high radiation circumstances for primary cooling system, 4) treatment of radioactive sodium (radioactive sodium and corrosion product such as 60Co, 54Mn). There are little experiences of this kind of work in the world. Therefore the organization, working plan and safety management points were carefully examined and established, based on the previous experience of JOYO operation and maintenance, research and development results of safety treatment of sodium, experience of previous work on sodium facilities. Followings results were obtained and effectiveness was confirmed in the work. (1) Development of most suitable working plan derived from elements and full size mock up experiments, reduction of exposure time by workers training, reduction of radiation dose by installation of temporal radiation shielding were useful to reduce radiation dose. The usage of seal bag was useful to prevent the contamination spreading over. (2) The usage of seal bag, oxygen concentration monitoring in the seal bag, nitrogen concentration monitoring in the cooling system cover gas, low pressure control of cover gas were useful to reduce the inflow of oxygen to cooling system. (3) The bite cutting method for piping in air and press down cutting by roller cutter in the seal bag to prevent inflow of cutting piece, stopper insertion in the pipe to prevent dropping of tools were useful to prevent foreign material entering. (4) Mechanical sodium removal and sodium wiping by cloth wetted with alcohol containing water were useful to remove residual sodium. (5) Structural analysis for releasing force of cold spring at piping cut, installation of temporal piping support to stop exiting pipe movement, material integrity evaluation of weld joint of used and new pipe by material test were useful to secure the structural integrity during the work. (6) Reducing pressure of cooling system cover gas that was used as back seal gas during the welding, residual sodium removal and temperature monitoring near the weld joint were useful to increase the safety during the work. MK-III modification work started on October 30th 2000 and completed successfully on September 21st 2001 without major troubles by using above mentioned procedures. (author)