Nuclear Systems Design Chapter 8. Design of Pressurizer and Plant Control Prof. Hee Cheon NO
8.1 Sizing Problem of Pressurizer and Plant Control 8.1.1 Basic Plant Control Basic Control Scheme I : to maintain a constant average reactor coolant temperature at all power main advantages minimize the size of the pressurizer through minimizing the variations of RCS water volume small reactivity variation due to small RCS temperature variation 2
main disadvantages large steam pressure variation -> large level variation in the steam generators and minimizing load at full power 3
Basic Control Scheme II : to maintain a constant steam pressure at all power main advantages small steam pressure variation -> small level variation in the steam generators and maximizing load at full power 4
main disadvantages maximize the size of the pressurizer and the requirement on CVCS through maximizing the variations of RCS water volume large reactivity variation due to large RCS temperature excursion 5
Sliding Tavg Control Scheme : Reactor power is adjusted to maintain a programmed Tavg as turbine power is changed turbine-side control: the turbine governor valves are controlled by the error between the reference load and the actual load which is measured by the impulse pressure reactor-side control: the control rods are controlled by the errors between the reference Tavg and the actual Tavg, and between the reference load and the actual load the load is measured through the impulse pressure in the turbine 6
[1] Plant Control Scheme 7
[2] Characteristics of a Sliding Average Temperature Program 8
Practical Example #1 : Plant control in case of more demand on load More demand on load with automatic mode negative feedback without control rod movement feedback control with control rod movement 9
feedforward control with control rod movement long-term control rod movement: Xe oscillation 10
Practical Example #2 : Plant response in case of plugging of S/G tubes turbine and generator output stays the same T avg stays the same reactor power and power through SG increase lowering thermal efficiency (higher reactor power but the same turbine power) As the number of the SG tubes plugged increases, the area of the turbine throttles increases. The turbine margin, usually 5-10%, is provided to accommodate the need of the more opening of the turbine throttles so that the turbine power may remain the same. 11
Practical Example #3 : Plant response in case of degradation of condenser tubes The turbine impulse pressure stays the same : miantaining the same T avgref no control rod movement lowering Q SG and Q RX due to higher P s Turbine and generator output remains lower T avg stays the same It can be a main cause of load degradation with reactor operating time 12
8.2 Physical Response of a Pressurizer to Load Changes 8.2.1 Physical Phenomena of outsurge and insurge Outsurge : Pressure Drop Flashing in the Water Heaters On : Generating heat of vaporization to replace lost liquid volume with steam 13
Insurge : Small Insurge : Pressure increase due to adiabatic compression Large Insurge : Subcooled insurge water reaches the interface, causing rapid condensation and pressure drop Backup heaters on 14
Spray On : Condensation of subcooled droplets sprayed from the spray nozzle energy balance: Energy In from Vapor-Water Transformation = heating of sprayed water from subcooled to saturated temperature W ( h h ) W ( h h ) c g f sp f sp 15
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8.2.2 PRZ transient simulation 17
8.3 Pressurizer and Pressurizer- Interfacing System Design 18
8.3.1 Pressurizer-Interfacing System Design 19
8.3.1 Pressurizer-Interfacing System Design 20
8.3.1 Pressurizer-Interfacing System Design 21
8.3.1 Pressurizer-Interfacing System Design 22
8.3.1 Pressurizer-Interfacing System Design 23
8.3.1 Pressurizer-Interfacing System Design 24
8.3.2 Pressurizer Sizing Sizing Requirements of Pressurizer 1) Prevention of heater water uncovery during outsurge: The outsurge is to be accommodated by providing sufficient initial liquid volume so that in the final state the heaters remain submerged after restoring the initial pressure by heater operation 2) Prevention of reaching the pressurizer high level trip setpoint during insurge: : The insurge is to be accommodated by providing sufficient initial vapor volume so that in the final state the water level in the pressurizer should be below the pressurizer high level trip setpoint of the pressurizer after restoring the initial pressure by heaters and spray operation. Note that the safety valves attached to the pressurizer are qualified in the steam flow but not water nor mixture flow through them by their vendor 25
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Procedure of Pressurizer Sizing input : maximum insurge and spray, and transient insurge time, maximum outsurge and transient outsurge time, minimum water volume to cover heaters, minimum vapor volume for high level trip 28
determining the water volume to satisfy the first requirement M M M V M 2 1 outsurge f 1 outsurge / fg where M ( V V ) ( V V ) 1 f 1 f min f g1 g min g M V ( V V V ) 2 f min f f 1 g1 g min g determine the vapor volume to satisfy the second requirement M M M M M V M 2 1 insurge sp in g1 in fg M ( V V ) ( V V ) 1 f 1 f min f g1 g min g M ( V V V ) V 2 f 1 g1 f min f g min g / 29
determine the total volume of the pressurizer V V V V V PRZ f 1 f min g1 g min determine heater capacity From the first requirement: E E M h q t where 2 1 outsurge f 1 1 q [ V ( e e ) M h ]/ t 1 f 1 g g f f outsurge f 1 E ( V V ) e ( V V ) e 1 f 1 f min f f g1 g min g g E ( V ) e ( V V V ) e 2 f min f f f 1 g1 g min g g 30
From the second requirement: E E M h M h q t 2 1 insurge su sp sp 2 2 q [ V ( e e ) ( M h M h )] / t where 2 g1 f f g g insurge su sp sp 2 E ( V V V ) e ( V ) e 2 f min f 1 g1 f f g min g g pressurizer heater capacity: qprz max( q, q ) 1 2 31