14.10. Routine startups for LWRs may either be from a cold condition or from a so-called “hot standby” condition. When the reactor is started for the first time or following a shutdown that required the system to be cooled down and depressurized, we have a cold startup. Hot-standby startup refers to a return to power operation when the cooling system is still near operating temperature and pressure such as the condition following a re­actor trip resulting from a minor transient, instrument malfunction, or turbine trip.

14.11. Prior to initial “commercial” startup, extensive testing is pre­scribed by the NRC to meet licensing requirements. Such tests include not only functional tests of all components and instruments, but also physics tests at zero power and testing at various power levels.

14.12. Precriticality checks are carried out before any cold startup to assure that the plant is ready to function. In a PWR, the coolant system boron concentration is adjusted and various auxiliary systems prepared. Frictional heat from the circulating pumps is then used to raise the primary coolant temperature to about 200°C at a pressure of about 2.8 MPa over a period of about 6 hours. After the pressurizer is adjusted, systematic withdrawal of the control rods to approach criticality is initiated.

14.13. In connection with control rod withdrawal, the importance of neutron detection instrumentation must be emphasized. As pointed out in §5.216, appropriate different neutron detectors are used in the source range, intermediate range, and power range. These are monitored by con­trol room instrumentation, which has been significantly modernized during recent years (§14.25).

14.14. The control rods are withdrawn intermittently rather than con­tinuously, with the neutron level allowed to stabilize before the rods are withdrawn further. A neutron-level startup rate well below criticality of about 0.5 decade per minute, corresponding to about a 50-s period, is common. However, when criticality is approached, the neutron count rate no longer reaches equilibrium between control rod withdrawals as a result of the contribution of the core neutron source.

14.15. Reactor physics data are taken at criticality and the power is raised to about the 1 percent level, where various adjustments are made to the secondary system. Reactor power is increased by further manual control rod withdrawal and the pressure and temperature raised to oper­ating conditions while the power level is still relatively low. The reactor is now at essentially hot-standby conditions at zero electrical load. The turbine — generator may be brought up to speed and connected to the grid when the power level is about 15 percent. At this point, additional power increase is normally switched from manual to automatic control. The loading rate to full power is normally limited to 5 percent per minute, determined by turbine considerations. To compensate for the reactivity power defect, some dilution of the coolant boron concentration is needed before reaching 100 percent power. A cold startup requires a minimum of about 13 hours.

14.16. Startup from the hot-standby condition follows a similar proce­dure except rod withdrawal is initiated when the cooling system is close to operating conditions. However, the relative negative reactivity contribu­tions of the dissolved boron and control rods must first be balanced properly so that criticality will occur with the rods above a certain limit. In the event of a trip, there will then be adequate rod negative reactivity available. Hot-standby startup time can be as short as 1 hour.