OPTIMIZATION OF THE OPERATING-CONDITIONS OF PRESSURIZED-WATER REACTORS LIMITED BY THE DNB PHENOMENON

The replacement of steam generators in a pressurised water reactor (PW R) requires an adaptation of the normal operating conditions imposed b y modified characteristics of the replaced steam generators (RSGs). To cover the cost of steam generator replacement, plant uprate is often considered. This is made possible by a significant improvement of the heat exchange characteristics of the SGs, and also by modification of the position of the turbine control valve. The present paper discusses the methodology that has been developed by Tractebel to solve this pr oblem for most of the up-to-date PWRs that are limited by the departur e from nucleate boiling (DNB) phenomenon (boiling crisis). A code has been developed, validated and applied to two units in Belgium, respect ively Doel 3 and Tihange 1, for which it has been decided to replace t he steam generators. This code is founded on the basic principle that for most DNB-limiting class-2 accidents, the maximum reactor power and reactor inlet temperature may not violate either the overpower limit (currently 118% of nominal power), or the DNB-ratio criterion in the c ourse of this accident. The code allows the designer to assess the inf luence on the operating conditions of the various key safety parameter s, as well as of the basic assumptions that are necessary to conduct t he thermalhydraulic design work. Examples of such investigations are g iven in this paper. Special emphasis is put on the thermal-hydraulic ( TH) design procedure, the plant principal TH key parameters, as well a s the features of the new SGs. The development of the DNBOPT code was also motivated by the wish to perform audit and sensitivity calculatio ns, as well as to confirm calculations performed by the SG supplier.