IMPROVED AXIAL FLUX SHAPE GENERATOR FOR A QUICK DEPARTURE FROM NUCLEATE BOILING TEST

Axial power shapes that develop during power-maneuvering simulations i n pressurized water reactors must be analyzed to ensure that an adequa te margin to avoid departure from nucleate boiling (DNB) is maintained during these transients. To reduce the number of flux shapes that nee d to be analyzed in detail to determine the DNB ratio (DNBR), often ge neric axial flux shapes are analyzed and maximum-allowable-peaking (MA P) limits are determined to conservatively filter those actual axial p ower shapes that are clearly safe. Current generic MAP limits, obtaine d for axial flux shapes generated by a two-parameter-based axial pur s hape generator; are overly conservative for some power shapes and are nonconservative for others, leading to unnecessary operational restric tions on conservative cases. A penalty is imposed on nonconservative c ases. To reduce the number of overly consertative and nonconservative cases, a new generic axial power shape generator that is based on thre e parameters is developed. Generic MAP limits have been developed for the new axial flux shape generator and tested using real flux shapes b y plotting the percent deviation of MAP limits for generic flux shapes from the corresponding value for actual flux shapes. A new axial flux shape generator which is clearly superior because it leads to a signi ficantly lower percent deviation, will lead to reduced man-hours for d etailed DNBR analyses and remove some of the unnecessary operational r estrictions imposed by the old flux shape generator.