Trends of availability performances in light water reactor units

Availability performances of the LWR units have heen analysed. The annual load factor has heen used as the basic measurement in quantifying availability performances of the LWR units. The whole operating history, in the sense of the annual and cumulative load factors achieved by all Western commercial PWR and BWR units the end of 1985, has been taken into consideration. Furthermore, the annual load factors achieved by the WWER-440 units in the CMEA countries until the end of 1984 have now been included in the ordinary data base of the study, which updates compactly the results of the preceding study published by the Technical Research Centre of Finland in Research Report 382. The year 1985 can be regarded as the best availability performance year of the LWR units up to date. The total mean of all the annual load factors (which is equal to the mean of all the cumulative load factors weighted by the unit age) of the PWR units since the beginning of their commercial operation was 66 % up to and including 1985 (1984 by WWER-40 units in the CMEA countries), while that of the BWR units was 62 %. The generic load factor averages, however, conceal wide variations from unit to unit and from year to year. In the preceding study some contributing factors (unit size, vintage and age) had been found that inter-acting with each other correlate with nuclear power plant availability performance. Accordingly same trends in the availability performance of LWR units have been identified. In this updating study the same exponential regression model was used, and the coefficient of determination, R2, increased both in the PWR units and BWR units along with the updated data base: the dependence between the availability per-formance and unit size, vintage and age has become still stronger. The updated regression analysis resulted additionally in that the later installed duplicate (twin) units of PWR type have generally performed better at the same age than their first-installed sister unit on the same site and better than the respective single PWR units generally. The LWR units form an inhomogeneous population in respect to their achieved load factors. The load fac-tors of LWR units supplied by same contractors and/or located in certain countries deviate clearly and regularly from general load factor averages. The individual cumulative load factors of all the PWR units and the BWR units at the end of 1985 (at the end of 1984 for the WWER-440 units in Bulgaria, Czechoslovakia and the Soviet Union) are presented as functions of unit size and vintage wiht informatian of site country and reactor contractor. Furthermore, the individual values of the annual load factors achieved by the WWER units in the CMEA countries up to and including 1984 are given separately.