Stretch types of power uprates can be typically introduced up to around 7 % of the original licensed power levels without major plant modification by using conservative measures built into the plant.

(2) Extended type

Extended types of power uprates can increase the original rated thermal power by up to 20 % with high performance fuel, but require significant modifications to major plant machinery such as the steam turbine and main electric generator.

The following effects are expected in BWR power uprates.

• Nuclear and thermal margin (core and fuel)

• Neutron flux increase (reactor vessel)

• Decay heat increase (containment vessel)

• Steam flow rate increase (turbine system)

• Load factor increase (turbine system and electric generator)

• Turbine exhaust heat increase (condenser)

• Condenser flow rate increase (feedwater pipe and feedwater heater)

Those effects of power uprates have been evaluated as to whether they are allowable within the design criteria or not, considering reactor operation and maintenance experience and operation data. The range of improvements and facility modifications are determined based on the evaluation results.

As discussed before, the fuel lattice design of BWRs is easily changed. An increase in the number of fuel rods is one of the leading core and fuel technologies to secure the nuclear and thermal margin in uprating reactor power and to improve plant economy. As using a large number of fuel rods improves critical power characteristics and reduces average linear heat gener­ation rate of fuel rods, therefore a lattice design with more rods can be applied to a high burnup long-operating cycle core and a high power density core.

On the other hand, it causes an increase in pressure drop within a fuel assembly and reduces thermal-hydraulic stability. Hence, a practical 10 x 10 type fuel has been introduced coupled with measures against the pressure drop such as partial length fuel rod.