Operational issues involving use of supplementary cooling towers to meet stream temperature standards with application to the Browns Ferry Nuclear Plant

Abstract

A mixed mode cooling system is one which operates in either the open, closed, or helper (once-through but with the use of the cooling towers) modes. Such systems may be particularly economical where the need for supplementary cooling to meet environmental constraints on induced water temperature changes is seasonal or dependent upon other transient factors such as stream- flow. The issues involved in the use of mixed mode systems include the design of the open cycle and closed cycle portions of the cooling system, the specification of the environmental standard to be met, and the monitoring system and associated decision rules used to determine when mode changes are necessary. These issues have been examined in the context of a case study of TVA's Browns Ferry Nuclear Plant which utilizes the large quantity of site specific data reflecting conditions both with and without plant operation. The most important findings of this study are: (1) The natural temperature differences in the Tennessee River are of the same order of magnitude (50F) as the maximum allowed induced temperature increase. (2) Predictive estimates based on local hydrological and meteorological data are capable of accounting for 40% of the observed natural variability. (3) Available algorithms for plant induced temperature increases provide estimates within 1F of observed values except during periods of strong stratification. (4) A mixed mode system experiences only 10% of the capacity losses experienced by a totally closed system, (5) The capacity loss is relatively more sensitive to the environmental standard than to changes in cooling system design. (6) About one third of the capacity loss incurred using the mixed mode system is the result of natural temperature variations. This unnecessary loss may be halved by the use of predictive estimates for natural temperature differences.