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Questions and Answers
What is the primary advantage of the Basic Control Scheme I?
Which disadvantage is associated with Basic Control Scheme II?
How does the Sliding Tavg Control Scheme manage turbine power adjustments?
What is maintained constant in Basic Control Scheme I regardless of load changes?
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What is a disadvantage of maintaining constant steam pressure in Basic Control Scheme II?
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What happens to the impulse pressure when there is an increase in load demand without control rod movement in automatic mode?
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Which factor primarily drives the movement of control rods in feedback control systems during increased load demand?
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In the event of S/G tube plugging, what is the expected effect on heat transfer?
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What is a consequence of increasing reactor power while turbine output remains unchanged?
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What feedback mechanism results from a decrease in Tavg during increased load demand?
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What is a key characteristic of the Basic Control Scheme I?
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Which statement accurately describes a disadvantage of Basic Control Scheme II?
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What is the main advantage of the Sliding Tavg Control Scheme?
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Which of the following outcomes is associated with Basic Control Scheme I?
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How does Basic Control Scheme II impact reactor temperature variation?
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What is the result of applying feedforward control in response to increased load demand?
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In the context of reactor control, what is the effect of plugging S/G tubes?
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What occurs after approximately two hours of increased QRX in the reactor?
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During the automatic mode under increased load demand, what impact does the impulse pressure have?
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How does maintaining constant Tavg affect the overall reactor response during load fluctuations?
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What are the main disadvantages of maintaining a constant average reactor coolant temperature in Basic Control Scheme I?
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In Basic Control Scheme II, how does maintaining constant steam pressure affect reactivity variation?
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What is the impact of a Sliding Tavg Control Scheme on reactor and turbine interactions?
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Why is minimizing variations in RCS water volume important in Basic Control Scheme I?
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What consequence does maximizing load at full power have in Basic Control Scheme II?
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Explain the role of impulse pressure in measuring actual load in a turbine during increased load demand.
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What is the impact of control rod movement during a load increase in a feedback control system?
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Describe the long-term effect of Xenon oscillation in relation to control rod movement.
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What happens to overall reactor efficiency when S/G tubes become plugged?
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How does the plant control scheme respond to changes in demand while maintaining load?
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How does Basic Control Scheme I ensure minimal reactivity variation despite load changes?
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What is the impact on pressurizer size when using Basic Control Scheme I, and why?
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Explain the trade-off in maximizing load at full power when using Basic Control Scheme II.
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Describe the relationship between turbine power changes and reactor power adjustments in the Sliding Tavg Control Scheme.
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What causes large reactivity variations in Basic Control Scheme II, and what are the consequences?
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Basic Control Scheme I aims to maintain a constant average reactor coolant temperature at all power levels.
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Basic Control Scheme II results in small steam pressure variations which is advantageous for maximizing load at full power.
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A major disadvantage of Basic Control Scheme I is a large reactivity variation due to significant RCS temperature fluctuations.
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The Sliding Tavg Control Scheme adjusts reactor power in response to changes in turbine power.
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Basic Control Scheme II minimizes the size of the pressurizer by maximizing variations of RCS water volume.
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Basic Control Scheme I aims to maximize the variations of RCS water volume.
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Large steam pressure variations in Basic Control Scheme II lead to small level variation in steam generators.
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The Sliding Tavg Control Scheme requires adjustments in reactor power as turbine power is altered.
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Basic Control Scheme II provides advantages such as large reactivity variations due to significant RCS temperature excursions.
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The main disadvantage of Basic Control Scheme I is that it minimizes the size of the pressurizer.
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Study Notes
Basic Plant Control
- Basic plant control scheme contains Reactor (Rx), Steam Generators (SG) and Turbine (Turbine)
- Reactor power is related to temperature in reactor coolant (Th)
- Steam Generator output is related to temperature of the steam (Tc)
- Turbine receives steam and produces power
Basic Control Scheme I
- Maintains constant average reactor coolant temperature (Tavg)
- Minimizes variations on reactor coolant system (RCS) water volume
- Results in small reactivity variation due to small RCS temperature variation
- Leads to large steam pressure variation and large level variation in the steam generators
Basic Control Scheme II
- Maintains constant steam pressure (Ps)
- Minimizes steam pressure variation and level variation in the steam generators
- Maximizes load at full power
- Maximizes the size of the pressurizer, and the requirement on CVCS (Chemical and Volume Control System)
- Results in large reactivity variation due to large reactor coolant temperature excursion
Sliding Tavg Control Scheme
- Reactor power is adjusted to maintain a programmed Tavg as turbine power is changed
- Turbine-side control: turbine governor valves are controlled by the error between the reference load and the actual load measured by the impulse pressure
- Reactor-side control: control rods are controlled by the errors between the reference Tavg and the actual Tavg
- Actual load is measured through the impulse pressure in the turbine.
Practical Example #1
- More demand on load can be achieved with automatic mode
- Negative feedback without control rod movement:
- Increased demand (Qdemand ↑) leads to increased turbine reference pressure (pimpref ↑)
- Positive error in (pimpref - pimp) increases steam flow (ms ↑) which lowers steam pressure (Ps ↓)
- Lower steam pressure leads to lower steam temperature (Ts ↓)
- Lower steam temperature increases heat transferred from reactor coolant to steam (QSG ↑)
- Increased heat transfer lowers reactor coolant temperature (TCL ↓)
- Lower reactor coolant temperature causes negative reactivity which decreases reactor power (QRX ↑)
- Decreased reactor power leads to increased reactor coolant temperature (Tavg ↑)
- Increased reactor coolant temperature increases heat transferred from reactor coolant to steam (QSG ↑)
- Increased heat transfer increases steam pressure (Ps ↑)
- Increased steam pressure, leads to increased steam flow (ms ↑).
Practical Example #1 (continued)
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More demand on load can be achieved with automatic mode
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Feedback control with control rod movement:
- Increased demand (Qdemand ↑) leads to turbine reference pressure (pimpref ↑)
- Positive error in (Tavgref - Tavg) results in the control rod moving up (control rod ↑)
- Positive reactivity increases reactor power (QRX ↑)
- Increased reactor power increases reactor coolant temperature (Tavg ↑)
- Increased reactor coolant temperature increases heat transferred from reactor coolant to steam (QSG ↑)
- Increased heat transfer increases steam pressure (Ps ↑)
- Increased steam pressure, leads to increased steam flow (ms ↑).
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Long-term control rod movement: Xe oscillation
- Increased reactor power (QRX ↑) leads to decreased Xenon (Xe ↓)
- Decrease in Xenon results in a positive reactivity after about two hours
- Control rod moves down (control rod ↓) to maintain Tavg = Tavgref and negative reactivity
- Xenon (Xe) increases again after about four hours to build up to equilibrium value
Practical Example #2
- Plugging of steam generator tubes results in lower heat transfer
- Lower heat transfer (U ↓ or A ↓) leads to lower steam pressure (ps ↓)
- Lower steam pressure leads to lower steam temperature (Ts ↓) and lower turbine impulse pressure (pimp ↓)
- Lower turbine impulse pressure leads to increased throttle opening (A throttle ↑)
- Despite same load demand, increased heat transfer from reactor coolant to steam occurs (QSG ↑)
- Increased heat transfer from reactor coolant to steam results in increased reactor power (QRX ↑)
- Despite same power demand, reactor power and power through steam generator increases
- This leads to lower thermal efficiency (higher reactor power but the same turbine power)
- As the number of plugged steam generator tubes increases, thermal efficiency further decreases.
Basic Plant Control
- Plant control systems maintain the power output of the reactor and turbine, ensuring steady operation.
- The reactor (RX) produces heat (QRX) which raises the reactor coolant temperature (Th).
- Steam generators (SG) utilize the heat from the reactor coolant to produce steam (QSG), impacting steam pressure (Ps) and temperature (Ts), which drive the turbine (ms).
- The turbine drives the generator, producing electrical power (load) and rotational speed (Pimp).
Basic Control Schemes
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Control Scheme I: Focuses on maintaining a constant average reactor coolant temperature (Tavg).
- Advantages: Minimizes pressurizer size and reactivity variations.
- Disadvantages: Leads to large steam pressure fluctuations and generator load variations.
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Control Scheme II: Prioritizes maintaining a constant steam pressure.
- Advantages: Minimizes steam pressure fluctuations and maximizes generator load.
- Disadvantages: Requires a larger pressurizer and increases reliance on the Chemical and Volume Control System (CVCS) to manage reactor coolant volume variations.
Sliding Tavg Control Scheme
- Utilizes a programmed Tavg to adjust reactor power in response to turbine power changes.
- Turbine-side control: Uses the turbine governor valve to adjust steam flow based on the difference between the reference load and the actual load.
- Reactor-side control: Utilizes control rods to maintain the desired Tavg by adjusting reactivity based on the difference between the reference Tavg and the actual Tavg.
Practical Example 1: Increased Load Demand
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Automatic Mode: The control system responds without control rod movement, increasing steam flow and reactor power to meet the increased demand.
- Increased load demand leads to a higher turbine pressure setpoint.
- This results in increased steam flow, decreasing steam pressure and temperature.
- Reactor power automatically increases to compensate, leading to a rise in reactor coolant temperature.
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Feedback Control with Control Rod Movement:
- The control rods are adjusted based on the difference between the reference Tavg and the actual Tavg.
- Increased demand leads to increased reactor power and eventually an increase in Tavg.
- Control rods are inserted to reduce reactivity and maintain the desired Tavg.
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Long-term Control Rod Movement:
- Xenon oscillation influences the control rods, requiring adjustments to compensate for the changing reactivity.
- Increased reactor power leads to a reduction in Xenon concentration, increasing reactivity.
- Control rods are withdrawn to decrease reactivity.
- Eventually, Xenon builds up, requiring control rod insertion to compensate.
Practical Example 2: Plugged Steam Generator Tubes
- If steam generator tubes become plugged, heat transfer efficiency decreases.
- This leads to reduced steam pressure and temperature.
- The turbine and generator output remain the same, but reactor power increases to meet the demand.
- This results in lower thermal efficiency due to the higher reactor power output achieving the same turbine power.
- The increased heat transfer required to maintain the same load results in higher reactor power.
- This highlights that even though the turbine load stays the same, the control system must adjust reactor power to compensate for the reduced heat transfer efficiency caused by plugged tubes.
Basic Plant Control
- Plant control can be viewed as a system managing load and power from startup to shutdown.
- The system involves components like the reactor (QRX -> Th), steam generators (QSG -> Tc, Ps, Ts, ms), turbine (ms -> Pimp), and generator (load -> power).
Basic Control Scheme I
- Maintains a constant average reactor coolant temperature (Tavg) at all power levels.
- Ensures a constant temperature in the primary system while adjusting the secondary system.
- Advantages:
- Minimizes the size of the pressurizer by minimizing variations in the reactor coolant system (RCS) water volume.
- Reduces reactivity variation stemming from slight RCS temperature variations.
- Disadvantages:
- Causes significant steam pressure variations, potentially leading to substantial level changes in the steam generators.
Basic Control Scheme II
- Maintains a constant steam pressure at all power levels, with a constant temperature difference between the primary and secondary systems.
- Advantages:
- Minimizes steam pressure fluctuations, resulting in minimal level changes in the steam generators.
- Maximizes load at full power.
- Disadvantages:
- Requires a larger pressurizer to accommodate fluctuations in RCS water volume.
- Places greater demands on the Chemical and Volume Control System (CVCS).
- Leads to increased reactivity variation due to wider RCS temperature swings.
Sliding Tavg Control Scheme
- Reactor power adjusts to maintain a programmed average temperature (Tavg) as turbine power changes.
- Uses turbine-side control, where the turbine governor valves are adjusted based on the difference between reference load and actual load, measured as impulse pressure.
- Uses reactor-side control, adjusting control rods based on the difference between reference and actual Tavg.
Practical Example #1: Increased Load Demand
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Automatic Mode: Responds to increased demand by triggering a sequence of changes:
- Increased demand (Qdemand ↑) leads to higher reference impulse pressure (pimpref ↑).
- Difference between pimpref and actual impulse pressure (pimp) opens the main steam (ms) valve.
- Increased ms flow reduces steam pressure (Ps ↓) and temperature (Ts ↓).
- Lower Ts increases heat transfer in the steam generator (QSG ↑).
- Increased QSG cools the reactor coolant loop (TCL ↓), leading to lower average reactor coolant temperature (Tavg ↓).
- This temperature drop induces a positive reactivity change (+ρreactivity), increasing reactor power (QRX ↑).
- Rising QRX eventually increases Tavg, leading to a new equilibrium where QSG, Ps, and ms stabilize.
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Feedback Control: Uses control rod movements to adjust reactivity for better control:
- Increased demand signals a difference between reference and actual Tavg (Tavgref - Tavg), triggering control rod insertion (+ρreactivity).
- The added reactivity increases reactor power (QRX ↑), leading to higher Tavg, QSG, Ps, and ms.
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Long-Term Control: Addresses Xenon oscillations:
- Increased reactor power (QRX ↑) leads to a decrease in Xenon (Xe ↓) during the first two hours.
- Reduced Xe prompts a positive reactivity increase, requiring control rod withdrawal (↓ρreactivity) to maintain Tavgref.
- After about four hours, Xe builds up again to a new equilibrium value.
Practical Example #2: Plugging of S/G Tubes
- A decrease in the steam generator's heat transfer coefficient (U) or surface area (A), caused by plugged tubes, leads to lower steam pressure (Ps ↓) and temperature (Ts ↓).
- This triggers a reduction in impulse pressure (pimp) and a corresponding increase in throttle valve opening (Athrottle ↑) to maintain the same load (Qload).
- Reduced U or A also increases heat transfer from the reactor coolant to the secondary side (QSG ↑), leading to higher reactor power (QRX ↑).
- Despite constant turbine and generator output and Tavg, the reactor power and power through the SG increase due to plugging.
- The final result is reduced thermal efficiency (higher reactor power for the same turbine output).
- As more tubes get plugged, the effect becomes more pronounced, leading to higher reactor power and lower thermal efficiency for the same turbine output.
Basic Plant Control
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Basic control scheme I:
- Maintains a constant average reactor coolant temperature (Tavg) at all power levels
- Minimizes pressurizer size by reducing variations in Reactor Coolant System (RCS) water volume
- Results in less reactivity variation due to small RCS temperature fluctuations
- Disadvantages: Large steam pressure variations lead to significant level changes in steam generators, limiting load at full power
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Basic control scheme II:
- Maintains constant steam pressure at all power levels
- Advantages: Minimizes steam pressure variations and level changes in steam generators, allowing for maximum load at full power
- Disadvantages: Requires a larger pressurizer and increased demands on the Chemical and Volume Control System (CVCS) due to larger RCS water volume variations; also leads to significant reactivity changes caused by wider RCS temperature excursions.
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Sliding Tavg control scheme:
- Reactor power is adjusted to maintain a programmed Tavg as turbine power changes.
- Turbine-side control: Turbine governor valves are adjusted based on the difference between the reference load and actual load, measured by impulse pressure.
- Reactor-side control: Control rods are adjusted based on the difference between reference Tavg and actual Tavg, measured by the impulse pressure in the turbine.
Practical Examples of Plant Control
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Increased Load Demand (Automatic Mode):
- Increased demand results in a higher reference impulse pressure (pimpref)
- The positive difference between pimpref and actual impulse pressure (pimp) increases main steam flow (ms)
- Increased ms reduces steam pressure (Ps)
- Reduced Ps decreases steam temperature (Ts)
- Decreased Ts increases heat transfer from the steam generator (QSG)
- Increased QSG reduces reactor coolant temperature (TCL)
- Reduced TCL decreases Tavg
- Decreased Tavg has a positive reactivity effect, increasing reactor power (QRX)
- Increased QRX raises Tavg, which increases QSG, raising Ps and ms
- The system eventually reaches a new equilibrium with a higher load
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Increased Load Demand (Automatic Mode):
- Increased demand results in a higher reference Tavg
- The positive difference between reference and actual Tavg raises control rod position
- Raised control rods increase reactivity, increasing reactor power (QRX)
- Increased QRX raises Tavg, which increases QSG, raising Ps and ms.
- System reaches a new equilibrium with a higher load.
- Xe oscillation: As QRX increases, Xe concentration decreases, adding further positive reactivity
- Control rod position is reduced to maintain Tavg
- Over time, Xe concentration builds up, requiring further rod movement
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Steam Generator Tube Plugging:
- Plugged tubes reduce heat transfer area (A) or overall heat transfer coefficient (U)
- This leads to lower steam pressure (Ps) and temperature (Ts)
- Lower Ps results in higher turbine throttle opening (Athrottle) to maintain the same Qload
- Even with the same generator output, QSG increases, necessitating higher reactor power (QRX)
- Although reactor power increases, turbine power remains the same, resulting in lower thermal efficiency.
Fission Process and Criticality: Negative Feedback Effect
- The negative feedback effect helps stabilize the reactor.
- Increased reactor power leads to:
- Higher coolant temperature
- Increased neutron absorption
- Reduced reactivity
- Reduced reactor power
- This feedback loop helps to prevent runaway chain reactions.
Basic Plant Control
- Plant control schemes manage reactor power and load by regulating various system parameters.
- Key parameters include reactor power (QRX), primary coolant temperature (Th), secondary coolant temperature (Tc), steam pressure (Ps), turbine mechanical speed (ms), and generator load.
- Control schemes aim to maintain a balance between power production and system stability.
Control Scheme I
- Focuses on maintaining a constant average reactor coolant temperature (Tavg) across all power levels.
- Advantages include minimizing pressurizer size and reactivity variations due to smaller Tavg changes.
- Disadvantages include significant steam pressure fluctuations, leading to larger steam generator level changes and limited load at full power.
Control Scheme II
- Prioritizes maintaining constant steam pressure at various power levels.
- Advantages include minimizing steam pressure fluctuations, resulting in stable steam generator levels and maximizing load at full power.
- Disadvantages involve a larger pressurizer requirement and increased demand on the Chemical and Volume Control System (CVCS), due to larger Tavg variations.
Sliding Tavg Control Scheme
- Employs a programmed Tavg as a target for reactor power adjustments during turbine power changes.
- Turbine-side control uses impulse pressure to regulate turbine governor valves based on the difference between reference and actual load.
- Reactor-side control utilizes control rods, driven by the difference between reference and actual Tavg, to adjust reactor power.
- Turbine impulse pressure provides a measure of the actual load.
Practical Example #1: Increased Load Demand
- Negative feedback mechanism without control rod movement: Increased load demand (Qdemand ↑) triggers an increase in turbine reference pressure (pimpref ↑). This initiates a chain reaction: increased mechanical speed (ms ↑), reduced steam pressure (Ps ↓), decreased secondary temperature (Ts ↓), increased steam generator heat transfer (QSG ↑), decreased reactor coolant temperature (TCL ↓), decreased Tavg (↓), increased reactivity (+ρreacivity), increased reactor power (QRX ↑), ultimately leading to a rise in Tavg back to the desired level.
- Feedback control with control rod movement: Increased load demand triggers a control rod insertion (+ρreacivity) to increase reactor power. This raises Tavg, leading to increased steam generator heat transfer (QSG ↑), higher steam pressure (Ps ↑), and eventually a rise in mechanical speed (ms ↑).
- Feedforward control with control rod movement: This approach utilizes the difference between reference and actual Tavg to directly manipulate control rods, ensuring a rapid response to power changes.
- Xe oscillation: After a prolonged period of high reactor power (QRX ↑), Xenon (Xe) concentration decreases, resulting in increased reactivity. To maintain the target Tavg, control rods need to be withdrawn, reducing reactivity and allowing Xe to build up. This cycle repeats with Xe buildup and depletion, impacting control rod adjustments.
Practical Example #2: Plugged Steam Generator (SG) Tubes
- Reduced heat transfer area (U ↓) or reduced tube conduction (A ↓) in the SG leads to decreased steam pressure (Ps ↓) and secondary temperature (Ts ↓). This reduces turbine mechanical speed (ms ↓), which activates a throttle increase to maintain load demands.
- Though turbine output stays the same, the decreased heat transfer in the SG necessitates an increase in reactor power (QRX ↑) to maintain thermal efficiency, resulting in higher reactor power and the same turbine output which lowers overall efficiency.
- The number of plugged SG tubes directly impacts reactor power changes.
Basic Plant Control
- Plant control scheme manages power output in a nuclear power plant.
- Reactor (Rx) controls heat production (QRX) and primary coolant temperature (Th).
- Steam Generator (SG) controls steam pressure (Ps) and temperature (Ts), and heat transfer from primary to secondary coolant (QSG).
- Turbine controls steam flow (ms) and responds to pressure changes (Pimp).
- Generator converts mechanical energy to electricity.
Control Schemes
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Scheme 1:
- Maintains constant average reactor coolant temperature (Tavg) at all power levels.
- Advantages: minimized pressurizer size due to minimal RCS volume variations, small reactivity variation due to minor Tavg fluctuations.
- Disadvantages: high steam pressure variation, significant steam generator levels at full power.
-
Scheme 2:
- Maintains constant steam pressure at all power levels.
- Advantages : minimized steam pressure variation, optimized level in steam generators and maximized load at full power.
- Disadvantages : increased pressurizer size, higher demands on Chemical and Volume Control System (CVCS) due to significant RCS volume changes, high reactivity variation due to extensive Tavg fluctuations.
Sliding Tavg Control Scheme
- Adjusts reactor power to maintain a programmed Tavg as turbine power changes.
- Uses turbine-side control to regulate turbine governor valves based on the difference between reference load and actual load measured by impulse pressure.
- Uses reactor-side control to regulate control rods based on the difference between reference Tavg and actual Tavg.
- Actual load is measured through the impulse pressure in the turbine.
Practical Example #1
-
Increased load demand:
-
Automatic mode:
- Increased demand (Qdemand) leads to increased reference impulse pressure (pimpref).
- Positive difference between pimpref and actual impulse pressure (pimp) increases steam flow (ms).
- Reduced steam pressure (Ps) and temperature (Ts) increase the heat transfer from primary to secondary coolant (QSG).
- Lowered secondary coolant temperature (TCL) and Tavg increase reactivity and reactor power (QRX).
- Increased QRX raises Tavg, which increases QSG, Ps, and ms.
-
Feedback control:
- Increased load demand increases pimpref.
- The difference between reference Tavg and actual Tavg triggers control rod insertion.
- Increased reactivity from rod insertion increases QRX, raising Tavg and consequently QSG, Ps, and ms.
-
Long-term control:
- Increased QRX reduces Xenon (Xe) concentration.
- Increased reactivity (Xe decay) causes control rod withdrawal after about two hours.
- Decreased reactivity from rod withdrawal allows Xe to build up and reach equilibrium after about four hours.
-
Automatic mode:
Practical Example #2
-
Steam generator tube plugging:
- Reduced heat transfer surface area (U) or plugging (A) leads to decreased steam pressure (Ps) and temperature (Ts).
- Reduced impulse pressure (pimp) triggers increased throttle opening (Athrottle).
- To maintain the load demand, QSG increases, leading to an increase in QRX.
- Result:
- Reactor power and power through SG are increased.
- Although there is increased reactor power, turbine power remains the same, leading to lower thermal efficiency.
- As the number of plugged SG tubes increases, these effects become more pronounced.
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Description
Test your knowledge on basic plant control systems, including the relationships between reactors, steam generators, and turbines. This quiz covers key aspects of reactor coolant temperature regulation and steam pressure management. Evaluate your understanding of control schemes and their impacts on reactor performance.
