EP4P03 Nuclear Plant Systems and Operation

EP4P03 Nuclear Plant Systems and Operation
Unit Transients – Event 2

Event 2 The Simulator was initialized to 100%FP, and ALTERNATE MODE was selected. The SG1 SGLC and SG4 SGLC Large LCV Selections were both set to LCV #1. At time = 0 a malfunction was inserted The immediate response to the malfunction were: Window display: Main BFP(s) Trip Window display: Reactor Trip Window display: Step Back Req’d

Event 2 (continued) Q1: What is the diagnosis of the event, and what caused the Reactor Trip?

Event 2 (continued) Time after malfunction was initiated: 1 second

Event 2 (continued) Time after malfunction was initiated: 1 second
Q1: What is the diagnoses of the event, and what caused the Reactor Trip? A1: The event is easy to diagnose as caused by the Trip of All Main Boiler Feedpumps. The original cause may have been a loss of Class IV power that supplies the pumps, although that would likely have caused other effects as well; or human error. The cause of the SDS#1 Reactor Trip is not specifically displayed on the Simulator, but is almost certain to have been due to the “Low Boiler Feed- line Pressure” signal.

Sim Ex 6.2 All Main BFPs Trip Time after malfunction was initiated: 1 second
Q2: What key parameters should the operator be monitoring?

Q2: What key parameters should the operator be monitoring? A2: Need to confirm that reactor trip is completed (neutron and thermal power decreasing, shutdown and MCAs fully inserted, AZL at 95%) Need to confirm that Auxiliary Feed Pump is in service and supplying feedwater to the Steam Generators

Q3: What is the immediate response of the Steam Generator Feed Pumps system to the Trip of All Main BFPs?

Sim Ex 6.2 All Main BFPs Trip Status of SG Feed Pump system prior to malfunction

Sim Ex 6.2 All Main BFPs Trip Time after malfunction was initiated: 10 seconds

Sim Ex 6.2 All Main BFPs Trip Time after malfunction was initiated: 20 seconds

Q3: What is the immediate response of the Steam Generator Feed Pumps system to the Trip of All Main BFPs? A3: One Auxiliary BFP starts up. Valves FCV153, 253, 353, 453 that control Main BFP recirculation flows open FCV553 that controls Auxiliary BFP recirculation flow closes PCV555 that supplies flow to the SGs via the now operating Auxiliary BFP opens After a brief period of zero feedwater flow, it is reestablished at the required low value

Q4: Having confirmed that the reactor trip has been correctly executed, after checking the main system parameters, which system display should the operator check next?

Sim Ex 6.2 All Main BFPs Trip Time after malfunction was initiated: 1 minute

Q4: Having confirmed that the reactor trip has been correctly executed, after checking the main system parameters, which system display should the operator check next? A4: Since the PHT Pressures and Inventory levels have not deviated beyond what is expected on a reactor trip, the operator should respond to the Steam Generator Low alarm by selecting first the SG Level Trend display, followed by the Steam Generator Level Control display.

Q5: What system and equipment was responsible for controlling Steam Generator pressure?

Q5: What system and equipment was responsible for controlling Steam Generator pressure? A5: The SGPC system controlled SG Pressure by reducing the opening of the Governor valve, and as a consequence reducing Turbine Generator power.

Sim Ex 6.2 All Main BFPs Trip Time after malfunction was initiated: 2 minutes
Q6: What major unit event should the operator expect to see next?

Q6: What major unit event should the operator expect to see next? A6: Subsequent to the reactor trip, a turbine trip should be anticipated, and the operator should verify that the Steam Generators will continue to act as heat sinks, by confirming that the ASDVs and/or CSDVs have opened and confirming the continued operation of the SGPC system, the Feedheating system and the SGLC system.

Q7: Why did neither the ASDVs nor the CSDVs open following the Turbine Trip?

Q7: Why did neither the ASDVs nor the CSDVs open following the Turbine Trip? A7: Subsequent to the reactor trip the Turbine continued to operate taking steam from the Steam generators, resulting in a decrease of SG pressure, which was continuously below the setpoint (4.7 MPa) until, and for some time following, the Turbine Trip.

Note, although not covered in class, that on a Reactor Trip there is an automatic “fast turbine runback” that takes control of the governor valves, and pro- actively ramps them closed, reducing turbine output in an essentially linear manner as shown by these Trend Plots.

Q8: How does the SGLC system respond to this event?

Q8: How does the SGLC system respond to this event? A8: The level drops quickly in the beginning because there is steam flow out but no feed flow into the SGs. Once the turbine has tripped and the Auxiliary Boiler Feed Pumps are supplying feedwater, the SG levels stabilize. The SG level control setpoint has been reduced due to the reactor power reduction. The small SGLC valves are controlling SG level in single-element mode. The SGs will continue to be the heat sink for the decay heat being produced by the reactor.

Q9: What are the main parameter responses of the PHT system?

Q9: What are the main parameter responses of the PHT system? A9: The decrease in heat into the PHT coolant from the reactor is greater than the loss of the SGs as heat sink: steam flow continues out of the SGs, although at a reduced rate due to the fast turbine runback, and the Auxiliary feed pumps are replenishing the feedwater supply after a few seconds. The coolant volume shrinks as the average coolant temperature decreases, resulting in a flow of coolant from the Pressurizer into the Main Circuit.

Q9: What are the main parameter responses of the PHT system? A9 (cont): The Pressurizer Level Setpoint is lowered as a function of reactor power more quickly than the decrease in Pressurizer Level due to the outflow from the Pressurizer to the Main Circuit, resulting in an increased Bleed flow. This further reduces the volume of coolant in the Main Circuit. The increased Bleed flow raises Bleed Condenser level and pressure, the latter resulting in an increase in Reflux flow. Within 4 minutes steady state is essentially reached in PHT Pressure and Pressurizer Level.

Q10: How was feedheating accomplished following the turbine trip?

Q10: How was feedheating accomplished following the turbine trip? A10: The Deaerator Pressure control (aka Poison Prevent) valve opened to provide feedheating and pressure control to the Deaerator.

EP4P03 Nuclear Plant Systems and Operation

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