IAEA Technical Meeting on Flexible (Non-Baseload) Operation Approaches for Nuclear Power Plants

Design Considerations on LWR

Holger Ludwig Germany

Increased flexible operation of NPP may have various safety related, operational or economic impacts. A systematic evaluation should include:

• Optimized mode of operation (part load diagram, I&C

concept, waste) • Core physics (e.g. PCI) • Safety analyses • Mechanical impacts (Fatigue, Erosion) • Chemical impacts • Radiological impacts

1. Requirements for flexible operation 2. Thermal load specifications 3. Operation mode of LWR 4. I&C concepts

AGENDA

Requirements (Example: Germany)

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70

Requirements from DVG(1

Maximum gradients according to load case catalog (example Konvoi)

Maximum gradients according to Operation Manual (example)

Actual range in use - till 2% PREO/min - Load step till Δ 5% PREO

(1) DVG: German abbreviation: The exploitation rules for thermal power plants, August 1982, updated 1992

Frequency stabilization

Load following operation

power change rate in % PREO/min

Valu

e of

load

cha

nge

in %

PR

EO

REO – Rated Electrical Output

Thermal load specifications

• The thermal load specifications are the basis for the fatigue- and stress- analyses

• Flexible operation is taken into account, new data (for example from fatigue-monitoring) is included into the load specifications

Thermal load specifications

Transients ► Pressure ► Temperature ► Flow rate ► possibly fluid level

Table of Loadings ► loadings ► load level ► frequency ► operational transients

Description of the loadings

• Spray line • Surge line • Volume control system • Feedwater system / injection feed water nozzles at

steam generator • Steam generator blowdown

• Main-steam system (only startup and shutdown) • Feedwater system (only startup and shutdown ) • Water separater / reheater

P W

R

B W

R

Thermal load specifications (typical fatigue sensible components)

Power Change, % RTP (Rated Thermal Power)

Number

10 (step change) 100.000 100 - 80 - 100 100.000 100 - 60 - 100 15.000 100 - 40 - 100 12.000

Frequent flexible operation can be considered in the mechanical design of the components. The thermal load specifications and following evidence would be updated.

Thermal load specifications (Example: German PWR)

Operation mode of PWR

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• Mode A: coolant temperature keeps constant with increasing reactor power; main steam pressure decreases with increasing reactor power

• Mode B: Increasing coolant temperature; constant main steam pressure

Mode A: Mode B:

• Part load diagram is an agreement between plant operation and

• Constant average temperature of coolant

• Constant main-steam pressure • In range of frequent flexible operation

(> ca. 40 % power in example on the left-hand side)

• Use of the storage and unload capacity of the NPP together with small changes in the pressurizer fill level

• Constant average temperature of the coolant (CT) lower requirements on reactivity compensation

• Lower secondary design pressure through almost constant main-steam pressure in the upper power range

• Optimum pressurizer-value

Operation mode of PWR (example: combined part load diagram)

• BWRs can be regulated by

• changing the coolant flow rate (recirculation control)

• Rapid power change • No significant effect on power

density distribution in the core • Normally applied for the

delivering of regulating power

• Maneuvering of control rods • More slowly • Strong impact on power density

distribution in the core • Normally applied to compensate

for slower changes of the reactivity (for example Xenon)

Rea

ctor

pow

er, %

Reactor flow rate, %

natural recircula-tion curve

minimal pump rotation

100% - curve for recirculation control

Operation mode of BWR

Reactivity Management

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PWR Maneuvering: • Using reactor coolant soluble boron concentration to control

reactor power is advantageous due its global affect on reactivity. However, large quantities of CVCS waste water would be generated during dilution operation, especially later in core life.

• Using existing, standard control rods (e.g., full-length / full-strength) to control reactor power has the advantage of minimizing plant modifications. However, this approach requires careful monitoring of bank worth (SDM) and prompt and accurate in-core instruments to measure core power distribution.

Reactivity Management

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PWR Maneuvering: •Using gray control rods (e.g., full-length / part-strength) to control reactor power has the advantage of minimizing core power re-distribution (relative to full-strength). In addition, gray rods are usually not included in shut down margin requirements. However, many existing NPP do not have gray rods nor excess SDM (to allow swapping out full-strength for part-strength).

• Power control concept of German PWR

• “weak” D-bank made of a few control rods for the regulating of the integral reactor power

• “strong” L-Bank made of the remaining control rods to control the axial power distribution and to compensate for the slow changes of the reactivity

• Power distribution controlled by maneuvering the L- and D-bank in a mutually compensation sequence

• “Weak” D-Bank has a significant lower influence on the power distribution than a “strong” Bank

• By rotating the choice of D-Bank, the mechanical loads are distributed to all CDRM no Exchange during Plant Lifetime necessary

“strong” control bank “weak” control bank

Control Rod Maneuvering (example: German PWR)

Fast and exact Neutron Flux Instrumentation – Why?

• Power distribution in the core • Local power density affects fuel rod cladding stress- • Flexible operation with control rods distorts power density

distribution; e.g. based on the cosinus shaped axial distribution (without offset), a power increase will generally result in increased local power density.

• The difference between the relevant maximum local value of power density and its limit defines (besides other factors) the margin available for such redistributions.

• The more reliably and precisely the local power density is measured and controlled during operation, the greater the operating margins for flexible operation

• Measurement possible via Excore-Detectors (See presentation P. Clifford) or Incore-Detectors

Incore Instrumentation • Example German PWR: Continuous and fast measurements with the in-

core power distribution detectors (PDD); 8 strings with 6 PDDs each • Precise measurement (calibration) with the help of an Aeroball flux-

measuring system (PWR) or traversing in-core probe system (BWR) • Additional Excore Instrumentation for reactor protection signals

Radial positions of PDDs

Cross section of Aeroball probe

I&C Concept • I&C concept depends on available measurement

devices and on the safety and margin management concept

• Generic statement: sequential instrumentation and control systems ensure compliance with operation and safety-related parameters (staggered level of defense) • Operation controls • Limiting conditions of operation • Reactor safety (reactor protection)

I&C concepts

• Operational controls in most cases detect the deviations from the normal

state and restore it to normal • Operator or automated limitation systems intervene and return the plant

to the control range Avoiding activation of the reactor protection system

I&C concepts

• Limitation systems may also protect the important parameters with respect to the flexible operation • PCI (Pellet Cladding Interaction) • Reactor power and power distribution

• Advantages • Separation (so possible optimization) of the control

from the functions important for the safety issues (covered by the limitation systems)

• Simplified monitoring activities • Increased availability of the plant (less reactor trips

minimize life-limiting loadings)

THANK YOU FOR YOUR ATTENTION

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Cooperation for safe and peaceful use of nuclear

energy