14.17. The general startup approach for a BWR is similar to that for a PWR except for the sequence of operations needed to bring the plant on­line. Also, for a BWR, nuclear heat is used in a cold startup to bring the coolant system to temperature and pressure. Therefore, the reactor is brought to critical by control rod removal when cold, but after thorough prestartup checks are made and recirculation at a low rate is initiated.

14.18. An extensive startup procedure calls for bringing the various systems on-line at appropriate stages. The heat-up rate is usually limited to 50°C per hour to avoid thermal stresses in the reactor walls. In the approach to full power following heat-up, there may be a rate restriction to avoid a too rapid increase in the fuel temperature, which could lead to pellet-clad interaction (§7.172) problems.

14.19. During the approach to power, thermal-hydraulic stability must be considered. Many stability studies have been made over the years to predict acceptable conditions which have led to plots using dimensionless parameters. Such variables as operating pressure, heat input, flow rate, and coolant conditions are relevant. More recently, various computer codes such as TRACG [3] have been used for stability analysis. A different type of plot, known as a power map, is shown in Fig. 14.1. This shows typical BWR operating conditions with the natural circulation and minimum re­circulating pump speed ranges indicated. A particularly important feature of such a power map is to show the role of the reactor protection system at varying core power levels and recirculating flow rates. Withdrawal of the control elements is “blocked” at the combination of power and flow indicated by the so-called block line. Conditions leading to reactor trip are also shown. Since the power is measured by several (normally four) average power range monitors, each receiving signals from many detectors through­out the core, the term APRM rod-block is commonly used.