[en] Pressurized water reactors WWER-440 and WWER-1000 are and will remain the basis of the nuclear power of the CIS (Common-wealth of Independent States) and some East-European countries for the nearest future. In this connection much attention is paid to an increased utilization of nuclear fuel and NPP safety, including irradiation safety at a NPP site and off-site. One of the aspects of this job is to increase fuel element and assembly serviceability under normal operational conditions including a higher fuel burn-up. The paper analyzes the experience gained in WWER-440 and -1000 fuel operation both in design fuel cycles and on conversion to more efficient four- and three year fuel cycles. The WWER fuel operational conditions are discussed. The results of the fission gas activity measurements primary circuit coolant and the control of the fuel cladding tightness in a shut-down reactor were used to analyze the loss tightness by fuel elements and assemblies vs normal operation conditions and the fuel burn-up. The data given using mainly as an example 3^d unit of Kola NPP with WWER-440 and 5th unit of Novovoronez NPP and 1st unit of Kalinin NPP with WWERs-1000 and their analysis show that the WWER fuel is highly reliable, the amount of leaky (gas) fuels (e.g. for WWER-1000 is less than 0.01% of the total number) meets the world-wide level. No increase in the amount of damaged fuels with burn-up to ∼50 MW.day/kg is observed. (author). 5 refs, 8 figs 5 tabs