13.47. The length of a fuel cycle in a BWR can be extended beyond that normally available from reactivity limitations by reducing the reactor feedwater temperature and hence its enthalpy. As can be seen from equa­tion (13.1), a decrease in hf will be accompanied by a decrease in the quality X, assuming other conditions are not altered. The thermal power of the reactor would thus remain unchanged, but the reactivity would be increased. This means that the useful life of a BWR fuel loading would be extended by reducing the feedwater temperature toward the end of the normal operating cycle, a practice known as “coast-down.”

13.48. The decrease in feedwater temperature is, however, accom­panied by a decrease in steam flow and in the electric power generated, for a given thermal power. For example, a reduction in feedwater tem­perature from the normal 216°C (420°F) to 12ГС (250°F) toward the end of a cycle permits an extension of 6 weeks to the regular fuel cycle length of about 18 months. However, during this period, the steam flow decreases to 82 percent of the normal value and the electrical power output to 91 percent. There is, nevertheless, a net cost benefit as a result of a reduction in the enrichment required for the reload fuel to achieve the same cycle length.