By: Don Jones, retired nuclear industry engineer, 2018 September 27
This is an overview of how a present day CANDU unit (excludes Pickering Generating Station that does not have turbine steam bypass capability and Bruce A that does not have adjuster rods – see later) responds to the loss of the power grid, a load rejection. The design intent is to keep the unit operating to supply its own service loads (house load) while disconnected from the grid and to be ready to reconnect to the grid when the grid becomes available.
All actions are automatic. When the loss of the power grid is detected the circuit breakers connecting the turbine-generator to the grid open. If house load is normally supplied by the unit service transformer the unit service loads (reactor heat transport circulating pumps, boiler feed pumps, condenser cooling water pumps, and other loads) are not affected by the de-energized grid. If the system service transformer were supplying half the unit service loads an automatic transfer would occur to transfer those loads to the unit service transformer. Reactor power is quickly stepped back to around 60 percent of full power by dropping the four mechanical control absorbers (rods) part way into the core, the valves supplying steam to the turbine-generator are closed, and the condenser steam discharge valves (CSDVs) are fully opened to prevent a rise in boiler pressure sufficient to cause opening of the boiler main steam safety valves. The CSDVs allow the steam that would have gone through the turbine-generator to go directly to the condenser. As the boiler pressure drops because of the fully open CSDVs the CSDVs are partially closed by the boiler pressure control system to maintain boiler pressure at setpoint. The turbine governor adjusts to supply just enough steam to take care of the house load, around 7 percent of full power.
Keeping the reactor at 60 percent of full power avoids a poison-out due to build up of Xenon 135, a fission product that absorbs neutrons, that could take the unit off line for several days. The unit can remain in this poison-prevent mode of operation until the grid becomes available. When the grid becomes available the output from the already hot turbine-generator can be quickly raised until the CSDVs fully close at 60 percent full power and then reactor power can be gradually increased to full power.
The sudden drop in power from 100 percent to 60 percent full power would more than likely require withdrawal of adjuster rods to provide positive reactivity to compensate for Xenon 135 build up (Ref.1). Present operating procedural constraints with regard to adjuster rod use could possibly delay adjuster withdrawal and result in a reactor shutdown and poison outage. Note that the Enhanced CANDU 6 (EC6) will have 100 percent steam bypass capability (Ref. 2) so unit could remain in poison-prevent mode with reactor at 100 percent power enabling a very rapid return to full power when the grid becomes available.
1. Why Ontario’s CANDU nuclear reactors don’t load follow, Don Jones, 2018 July 21, https://thedonjonesarticles. wordpress.com/2018/07/22/why- ontarios-candu-nuclear- reactors-dont-load-follow/
2. Enhanced CANDU 6 and NuScale SMR have capability to easily integrate wind and solar, Don Jones, 2016 August 17, https://thedonjonesarticles. wordpress.com/2016/08/17/ enhanced-candu-6-and-nuscale- smr-have-capability-to-easily- integrate-wind-and-solar/