[en] The main source of radioactive contamination in nuclear power stations is the primary coolant containing radioactive corrosion products, and when the cladding of fuel rods is leaking, it is fission products in nuclear fuel. The most dangerous thing is the fission products, therefore the radioactivity of the fission products in primary coolant should not exceed the level decided by the design. In order to solve such subject, one of the basic measures is to find out a cassette containing broken fuel rods, and take it out of a reactor core. For the purpose, the conditions of the core of a reactor in operation are measured periodically, and as an occasion demands, the cassette in question is inspected after the reactor has been shut off. The detection of the cassette containing broken fuel rods in a stopped reactor is carried out with a special stand for inspecting broken fuel rods. There is some correlation between the measured values obtained with the special stand and the degree of break of fuel rod cladding according to the inspection results. In nine plants of same type with VVER-440 power reactors, 24 fuel loadings have been studied in detail so far. Some data concerning the monitoring of the conditions of cores in the plants are given in this report. The monitoring during operation and in stopped reactors is described. (Kako, I.)