EP4P03
Nuclear Plant Systems and Operation
Unit Transients
Causes of Unit Transients
• Initiated by Operator
• Initiated by an Abnormality or Malfunction
Diagnosing an Abnormality
• Initial indication is a key `clue’
• May have time from first alarm to correct before
defensive measures progress
• Sometimes a lot happens in a short time
• Focus is first on ensuring the Plant is in a safe
condition and then diagnosis starts
• In this case, alarm logs record what happened in
time chronology
Diagnostics (cont’d)
• Cause and effect
• Look at first indication and deduce what could
cause that – then eliminate options until
converging on the cause
• Note that in diagnostics you also need to look for
secondary problems throughout the transient to
ensure that response of systems and components is
correct
Problem Solving Strategies on the Simulator
• What are the initial indications of the problem?
- alarm?
- deviation of one or more parameters from normal
(must know what is “normal”)?
• How are the main plant parameters responding to the
problem and why (cause and effect)?
- reactor power versus turbine-generator power
- steam generator pressure, heat transport pressure
- steam generator level, pressurizer level, bleed
condenser level
Problem Solving Strategies on the Simulator (cont)
• How (and why) are the main plant control systems
responding (or contributing) to the problem?
- RRS, UPR, SGPC, SGLC, HTPC, HTIC, Bleed
Condenser pressure and level control
- for each control system, is the set-point changing
as a result of reactor power change or is it staying
constant?
• Operator action (having identified the fault):
- remove the fault
- mitigate the fault
- place unit into safe state
Problem Solving Strategies on the Simulator (cont)
• From Overall Unit page identify likely system where
malfunction exists (alarm windows may identify cause)
• Select system display to check if initial diagnosis is
correct
• Select a second system display if necessary to identify
or confirm malfunction
• Identify control system(s) responding to malfunction
• Identify parameter(s) that triggered reactor or turbine
trip
• Execute response to malfunction
Event 1
• 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 Authorized Nuclear Operator (ANO) was monitoring
the Plant Overview page, and noticed changes in Reactor
Thermal Power, Boiler Levels, PRZR Level Setpoint and
Level.
Event 1 (continued)
Time after malfunction was initiated: 20 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
The ANO decided to initially concentrate on understanding
the response of the PHT system.
Q1. Why did the ROH and Pressurizer Pressures and also
Pressurizer Level increase suddenly?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q1. Why did the ROH and Pressurizer Pressures and also
Pressurizer Level increase suddenly?
A1. Because of the sudden stoppage of feedwater flow, the lack of
feedwater flowing into the steam generators lowered the
ability of the steam generators to absorb heat from the PHT
coolant, resulting in the average temperature of the coolant
increasing, the coolant swell resulting in a rapid increase in
ROH pressure, which in turn raised Pressurizer pressure, as
excess coolant flowed into the Pressurizer. This excess coolant
flow into the Pressurizer raised Pressurizer level.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q2. Observing the responses of parameters on the Plant Overview
display, it was noted that the displayed value of Reactor
Thermal Power decreased for the first minute of the
disturbance, although Reactor Neutron Power remained
constant at 100%FP. Why?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q2. Observing the responses of parameters on the Plant Overview
display, it was noted that the displayed value of Reactor
Thermal Power decreased for the first minute of the
disturbance, although Reactor Neutron Power remained
constant at 100%FP. Why?
A2. The displayed value of Reactor Thermal Power is a value
computed based on the energy transfer across the Steam
Generators above 70%FP, and as can be seen from the Plant
Overview page, both Steam generator pressure (from which
steam temperature is derived) and Steam Generator levels are
dropping, likely caused by a drop in feedwater flow.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q3. Why did Pressurizer Level Setpoint decrease?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q3. Why did Pressurizer Level Setpoint decrease?
A3. Since Pressurizer Level Setpoint is based on the calculated
value of Reactor Thermal Power, the apparent decrease in
Reactor Thermal Power resulted in a decrease in Pressurizer
Level Setpoint.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q4. What was the response of the PHT Inventory Control system to
the Pressurizer Level error?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q4. What was the response of the PHT Inventory Control system to
the Pressurizer Level error?
A4. The PHT Inventory Control system responded to the
Pressurizer Level error by increasing the Bleed flow and
decreasing the Feed flow.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q5. Why was the PHT Inventory Control System not able to correct
the Pressurizer Level error?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q5. Why was the PHT Inventory Control System not able to correct
the Pressurizer Level error?
A5. Since the flow of coolant into the Pressurizer through MV1
exceeded the net flow of inventory via feed and bleed,
Pressurizer Level error could not be corrected in the first two
minutes of the event.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q6. While the ANO was considering the observed parameter trends
on the PHT system, the Shift Operating Supervisor (SOS) was
monitoring the Plant Overview display. Which other system
display should be checked beside the PHT? Why?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q6. While the ANO was considering the observed parameter trends
on the PHT system, the Shift Operating Supervisor (SOS) was
monitoring the Plant Overview display. Which other system
display should be checked beside the PHT? Why?
A6. On the Plant Overview display the most prominent parameter
deviation was the drop in Steam Generator levels, and
therefore the SOS directed the ANO to check the Steam
Generator Level Control display.
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q7. What should be the ANO’s diagnosis and why?
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Event 1 (continued)
Time after malfunction was initiated: 35 seconds
Q7. What should be the ANO’s diagnosis and why?
A7. The ANO should diagnose the malfunction:
ALL FEEDWATER LEVEL CONTROL VALVE MOTORIZED VALVES
HAVE FAILED CLOSED
by recognizing that all LCV MVs, which are normally open are
all closed, and resulting in zero feedwater flow.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Q8. What automatic actions by the SG Level Control Systems should
the ANO observe?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Q8. What automatic actions by the SG Level Control Systems should
the ANO observe?
A8. The ANO should observe that as the level in each SG dropped
below the VT point, the changeover of LCV valve responsible
for controlling feedwater flow has taken place.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Q9. Which additional display should be checked to further verify
the diagnosis of a loss of SG heat sink and to foresee the likely
evolution of the event?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 35 seconds
Q9. Which additional display should be checked to further verify
the diagnosis of a loss of SG heat sink and to foresee the likely
evolution of the event?
A9. The Steam Generator Level Trends will show the rate of level
loss, the timing of the valve transfer, and the imminent SG
Low Level alarm, followed by a Reactor Setback on Low SG
Level.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 00sec
• At time = 39 sec the “Stm Gen Level Lo” alarm was observed
• At time = 42 sec the “Setback Required” alarm was observed
• At time = 51 sec the “PRZR Level Hi” alarm was observed
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 00sec
Q10. What should be the next checks that the ANO should make?
A10. Verify that each of the observed alarms, namely:
“Stm Gen Level Lo”
“Setback Required”
“PRZR Level Hi”
is valid, and that the relevant systems are responding in the
expected manner, by checking the SG Level Trends, SG Level
Control, RRS and PHT Feed and Bleed system displays
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 00sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 00sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 00sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 00sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 00sec
Q11. What could the control room operating staff do to correct or
mitigate the malfunction?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 00sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 00sec
Q11. What could the control room operating staff do to correct or
mitigate the malfunction?
A11. While efforts are under way to find and remove the cause of
the valves tripping closed, need to verify that all control
systems concerned with plant safety are operating correctly,
including RRS, HTP&IC, SGPC. As long as the MVs are closed,
the reactor trip on Low SG Level cannot be avoided.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 40sec
Q12. What observations can be made in terms of controlling the reactor
and cooling the fuel?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 40sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 40sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 40sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 1min 40sec
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 1min 40sec
Q12. What observations can be made in terms of controlling the reactor
and cooling the fuel?
A12. At 1 minute 40 seconds after the insertion of the malfunction,
• Reactor Neutron Power and Reactor Thermal Power can be seen
decreasing as determined by the Reactor Setback function (Plant
Overview display),
• Steam Generator Large Level Control Valves have completed the
Valve Transfer operation,
• Steam Generator Level Trends are approaching the SDS#1 Trip on
Low SG Level,
• PHT Pressure Control is keeping Pressurizer Steam Bleed Valves
open to control ROH Pressure, but both remain above the
setpoint, with flow from the ROH to the Pressurizer
• PHT Inventory control continues to try reducing Pressurizer Level
error by increasing Bleed over Feed
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 2 minutes
Q13. The SDS#1 Reactor Trip on Low SG Level was observed at 1 min 42 sec.
What are the key actions that the ANO should be taking following the
Reactor Trip?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 2 minutes
Q13. The SDS#1 Reactor Trip on Low SG Level was observed at 1 min 42 sec.
What are the key actions that the ANO should be taking following the
Reactor Trip?
A13. At 2 minutes after the insertion of the malfunction,
• Reactor Neutron Power and Reactor Thermal Power have
decreased to the expected low levels, as observed on all of the
displays
• All shutdown rods, both banks of Mechanical Control Absorbers
have dropped into the reactor core, and AZL has filled, as observed
on the Shutdown Rods and Reactivity Control displays
• ROH and Pressurizer Pressure have dropped below the setpoint,
and all the heaters are now fully ON.
• Flow through MV1 is now from the Pressurizer to the main Circuit,
reducing Pressurizer Level from its high value towards the setpoint,
• Steam Generator Levels continue to fall
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 2 minutes
Q14. What is the next likely key event for the unit?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 2 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 2 minutes
Q14. What are the next likely key events for the unit?
A14. A short time after SDS#1 Reactor Trip the SG levels fall below the
SDS#2 Reactor Trip level, guaranteeing that the reactor will
poison out and cannot be restarted until the Xenon
concentration decreased to the point where the reactor can be
made critical (approximately two days, but factors other than
Xenon may extend the outage).
Also the Turbine has tripped, Generator breaker opened, Class IV
transferred so that all station loads are supplied by the System
Service Transformer.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction initiated: 6 minutes
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 6 minutes
Q15. With both SDS#1 and SDS#2 having been triggered, the reactor is
in a deep shutdown state. What key concerns should the ANO and
the SOS be addressing?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 6 minutes
Q15. With both SDS#1 and SDS#2 having ben triggered, the reactor is in
a deep shutdown state. What key concerns should the ANO and
the SOS be addressing?
A15. Need to ensure cooling of the fuel.
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 6 minutes
Q16. What are the options for ensuring that the fuel in the reactor,
which is producing decay heat, continues to be cooled?
Sim Ex 6.1 FAIL CLOSED ALL FW LCV MVs (cont)
Time after malfunction was initiated: 6 minutes
Q16. What are the options for ensuring that the fuel in the reactor,
which is producing decay heat, continues to be cooled?
A16. If the SGLC MVs can be reopened, cooling of the fuel can be done
using the steam generators. Otherwise, or if it is desired to cool
down the PHT system, Shutdown Cooling would need to be
placed into service.
Sim Ex 6.1 Clear FAIL CLOSED ALL FW LCV MVs
Time after malfunction initiated: 6 minutes
Q17. At about 6 minutes 10 seconds after the start of the event the
malfunction was cleared. What actions should the operator take
to restore feedwater flow to the Steam Generators?
Sim Ex 6.1 Clear FAIL CLOSED ALL FW LCV MVs
Time after malfunction initiated: 6 min 10 sec
Q17. At about 6 minutes 10 seconds after the start of the event the
malfunction was cleared. What actions should the operator take
to restore feedwater flow to the Steam Generators?
A17. The operator should recognize that control of SG level (if in DCC
control mode) was automatically transferred to the small SGLC
valves as reactor power dropped below 18%FP, and the operator
should manually open the MVs that are in series with the small
SGLC valves.
Sim Ex 6.1 Clear FAIL CLOSED ALL FW LCV MVs
Time after malfunction initiated: 6 min 10 sec
Sim Ex 6.1 Clear FAIL CLOSED ALL FW LCV MVs
Time after malfunction initiated: 6 min 10 sec
Q18. What other automatic action by the SGLC system should the
operator verify?
Sim Ex 6.1 Clear FAIL CLOSED ALL FW LCV MVs
Time after malfunction initiated: 6 min 10 sec
Q18. What other automatic action by the SGLC
system should the operator verify?
A18. When control of SG level (if in DCC
control mode) was automatically
transferred to the small SGLC valves as
reactor power dropped below 18%FP, if
SGLC is under DCC control, automatic
transfer from 3 element to 1 element
control should have taken place.
Sim Ex 6.1 FAIL CLOSED ALL FEEDWATER LEVEL
CONTROL VALVE MOTORIZED VALVES
Event Summary
• 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 the “FAIL CLOSED ALL FEEDWATER LEVEL
CONTROL VALVE MOTORIZED VALVES” malfunction was
inserted
• At time = 39 sec the “Stm Gen Level Lo” alarm was observed
• At time = 42 sec the “Setback Required” alarm was observed
• At time = 51 sec the “PRZR Level Hi” alarm was observed
• At time = 1 min 42 sec the Reactor Tripped on Low SG level