1. EP4P03 Nuclear Power Plant Systems and Operation
Unit Transients – Event 3

2. Event 3
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
After one minute the main indications of a problem on the Simulator are decreases in ROH and Pressurizer Pressure, and Pressurizer Level.

3. Event 3 (continued) Time after malfunction was initiated: 1 minute

4. Event 3 (continued)
Q1: Checking the PHT Feed and Bleed display, what are your main observations?

5. Event 3 (continued) Time after malfunction was initiated: 1 minute

6. Event 3 (continued)
Q1: Checking the PHT Feed and Bleed display, what are your main observations?
A1: Decreases in ROH and Pressurizer Pressure, and also Pressurizer Level are seen, with ROH Pressure lower than Pressurizer Pressure, with a resultant flow of coolant from the Pressurizer to the Main Circuit through MV1.
Pressurizer level has decreased, possibly due to the above noted outflow, while Pressurizer Level setpoint has remained constant, since Reactor Power also hasn’t changed.
The PHT Inventory control system has responded to the Pressurizer level error by increasing Feed flow and decreasing Bleed flow.

7. Event 3 (continued)
Q2: What are the possible explanations for the continuing decreases in ROH and Pressurizer Pressure, and also Pressurizer Level?

8. Event 3 (continued) Time after malfunction was initiated: 2 minutes

9. Event 3 (continued) Time after malfunction was initiated: 2 minutes

10. Event 3 (continued)
Q2: What are the possible explanations for the continuing decreases in ROH and Pressurizer Pressure, and also Pressurizer Level?
A2: A check of the Plant Overview display 2 minutes after the malfunction was inserted shows Reactor Neutron Power staying constant while there is a small decline in Reactor Thermal Power and a similar decline in Turbine Power.
Steam Generator Pressure and Level are constant, so the problem does not appear to be due to changes in the ability of the SGs to facilitate the regular transfer of heat from the PHT system. Hence the loss of pressure is not due to either a decrease in heat from the reactor, nor an excess transfer of heat to the SGs
Since the ROH Pressure is lower than Pressurizer Pressure, the loss of pressure must be in the Main Circuit. But a loss of pressure from the Main Circuit can only happen if there is a net outflow of coolant from the Main Circuit.

11. Event 3 (continued)
Q2: What are the possible explanations for the continuing decreases in ROH and Pressurizer Pressure, and also Pressurizer Level?
A2 (cont): Outflow of coolant from the Main Circuit can be verified as not taking place, since the pressure relief valves and the Bleed valves are closed, while there is substantial Feed flow into the Main Circuit
The fall in Pressurizer Level while the Level setpoint is constant, and the apparent correct functioning of both the PHT Pressure Control and the PHT Inventory control systems, suggest that there is an uncontrolled loss of coolant from the Main Circuit.
Checking the Main Circuit flows on the PHT Main Circuit display, we see that the flow at the outlet of Pump 1 is constant at 2700 kg/s, while the flows are lower and vary in the other three pump outlets.

12. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 2 minutes

13. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 2 minutes
In the case of such a LOCA at an operating plant, there would be alarms in response to moisture being detected in the sumps of the floor of reactor building.
Q3: What key parameters should the operator be monitoring?

14. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4 minutes

15. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4 minutes

16. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4 minutes
Q3: What key parameters should the operator be monitoring?
A3: Need to ensure that the fuel continues to be cooled, i.e. monitor the Overall Plant parameters checking for continued energy balance between the Reactor and the Turbine, and monitor the rates at which ROH Pressure and Pressurizer level are falling.

17. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4minutes
Q4: What actions should the operator initiate (in terms of control room operations), and in the absence of such actions, what are the likely consequences on the operation of the unit?

18. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4minutes
Q4: What actions should the operator initiate (in terms of control room operations), and in the absence of such actions, what are the likely consequences on the operation of the unit?
A4: The operator would need to call for make-up of D2O to the Storage tank (cannot be done on the Simulator)
Once the LOCA has been confirmed, the unit should be shut down in an orderly manner.

19. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 4 minutes
Q5: What would be the likely consequences of no operator action on the operation of the unit?

20. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 8 minutes

21. Sim Ex 6.5 RIH #1 Small Break Time after malfunction was initiated: 8 minutes
In the case of such a LOCA at an operating plant, there would be alarms in response to moisture being detected in the sumps of the floor of reactor building.
Q5: What would be the likely consequences of no operator action on the operation of the unit?
A5: The reactor would trip on Low Pressurizer Level. The time of the trip would depend on the size of the leak, it is about 7.5 minutes on the Simulator.
The operator should select Solid mode of PHT Pressure Control and close the Pressurizer Isolation Valve (MV1).