Doppler, moderator temperature, and void coefficients<\/strong>. It is expressed as a change in reactivity per change in percent power,\u00a0\u0394\u03c1\/\u0394% power<\/strong>. The value of the power coefficient is always negative in core life. Still, it is more negative at the end of the cycle primarily due to the decrease in the moderator temperature coefficient.<\/p>\nLet\u2019s assume that the reactor is critical at 75%<\/strong>\u00a0of rated power and that the plant operator wants to increase power to\u00a0100%<\/strong> of rated power. The reactor operator must first bring the reactor supercritical by inserting a positive reactivity (e.g., by\u00a0control rod<\/a>\u00a0withdrawal or\u00a0boron<\/a>\u00a0dilution). As the thermal power increases, moderator temperature and fuel temperature increase, causing a\u00a0negative reactivity effect<\/strong> (from the power coefficient), and the reactor returns to the critical condition. Positive reactivity must be continuously inserted<\/strong>\u00a0(via control rods or\u00a0chemical shim<\/a>) to keep the power increasing. After each reactivity insertion, the reactor power stabilizes itself<\/strong>\u00a0proportionately to the reactivity inserted. The total amount of feedback reactivity that must be offset by control rod withdrawal or boron dilution during the power increase (from ~1% \u2013 100%<\/strong>) is known as the\u00a0power defect<\/strong><\/a>.<\/p>\nLet assume:<\/p>\n
\n- the power coefficient: \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u0394\u03c1\/\u0394% = -20pcm\/% of rated power<\/strong><\/li>\n
- differential worth of control rods: \u00a0 \u00a0\u0394\u03c1\/\u0394step = 10pcm\/step<\/strong><\/li>\n
- worth of boric acid: \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0-11pcm\/ppm<\/strong><\/li>\n
- desired trend of power decrease: \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a01% per minute<\/strong><\/li>\n<\/ul>\n