The corrosion conditions of the heat exchanger`s shell had been investigated by mean of ultrasonic test in JRR-4. According to the test conducted in 1990, it showed that the heat exchangers which were used since initial core criticality in 1965 would be corroded over the whole shell plates made of carbon steel by nearly limiting thickness of the pressure-proof strength in 1992, and then, the heat exchangers were replaced in 1991. At replacement works, the two test pieces were cut out from the old heat exchangers for the observation of corrosion condition, chemical analysis of the corrosion products and metallography examination of materials. These results were suggested the condition and depth of corrosion, and the validity of the replacement work was confirmed. This report describes the technical subject on management and replacement of the heat exchangers, including coolant management, design, manufacturing, installation, inspection and investigation of corrosion conditions. (author).