CANDU Reactivity and Power Control: Glenn Harvel Associate - - PowerPoint PPT Presentation

CANDU Reactivity and Power Control:

Glenn Harvel Associate Professor Faculty of Energy Systems and Nuclear Science, UOIT www.uoit.nuclear.ca

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Learning Objectives

• Overview of Unit Control
• Overview of Reactivity Control

Mechanisms

• Overview of Power Measurement Systems
• Overview of Refuelling Effects

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Reaction Product Energy (%) Range Time Delay Kinetic Energy of fission fragments 80 < 0.01 cm Instantaneous Fast Neutrons 3 10-100 cm Instantaneous Fission Gamma Energy 4 100 cm instantaneous Fission product β decay 4 Short delayed neutrinos 5 nonrecoverable Delayed Non fission reactions due to neutron capture 4 100 cm Delayed

ENERGY RELEASE IN NUCLEAR FISSION

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CANDU Computer Control

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End View

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• Natural uranium (.71%

U235)– no inadvertent criticality concerns

• Bundle weighs about 24

Kg

• Extensive inspection

before loading

• 28 element fuel in

Pickering and 37 element fuel in CANDU 6 and CANDU 9

• 30 pellets per pencil
• Graphite lining (CANLUB)

Spatial Control - LZC

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• 14 zones in axial pairs
• 6 zone `rods’
• Typically about 7.2 mk

(about 5mk over the range 15% to 80%)

• Can control bulk power or

spatially

• Needed because

CANDU’s are big cores so are not strongly spatially coupled

LZC – Control System

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• Normally control is at about

50% level

• Level is a function of level,

thermal power in the zone pair and local flux

• Fill and drain times are set by

water flow in and out – typically .077mk/% level with fill and drain of the order of a minute

• There are level over-rides at

the extreme ends of the range – why?

• If a reactor trip occurs, the

regulating system controls the level to go full

Reactivity Deck Shut-off Rods

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• 32 rods worth ~ 60 to

70 mk

• Safety analysis

typically assumes 2 most effective fail to go in

• Cadmium with stainless

steel sheath

• Spring assist – full

insertion in ~ 2 seconds

• Clutch controlled by

SDS logic

• Withdrawal by motor

controlled by Regulating System

Rod Position

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• Shutoff rods and Control

absorbers normally out of the flux

• Adjusters rods normally in the

flux – need to pay attention to cooling when they come out of the Calandria

Reactor Power Measurement

Two types of Neutronic Measurement

– Ion Chambers

• Out of core
• Measures `leakage’ flux

– Flux detectors

• In core
• Measures in core neutron flux

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Ion Chambers

A B C D E F Regulating and SDS1 SDS 2 Typical arrangement for Regulating and SDS1 penetrations

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Ion Chambers

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Ion Chamber Signals

• Linear N 0 to 150%
• Log N 10-5% to 150%
• Log N Rate -15% to 15%
• Class 2 power
• 3 Channels
• Leakage flux is NOT an

accurate signal – leakage flux – not suitable for high power control

• OK at low power
• Log N good at any power
• Changeover at between 5%

and 15% FP

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In Core Detectors

• Self Powered
• Signal proportional to fission neutrons ( prompt), Fission

Gamma(prompt), and Fission Decay Gamma (delayed)

• Small – can measure local conditions
• Control power from 5% to 120%

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Two Types

• Inconel with thin coating of Platinum as emitter
• Bulk signal for bulk power and Liquid zone control
• Not balanced between gamma and neutron
• Under respond to neutrons and over respond to gamma
• Not accurate – signal is somewhere between neutron and thermal

BUT

• It is immediate to measure change and linear
• Vanadium
• Vanadium emitter
• Captures neutrons in a neutron / gamma reaction (V51 becomes V52)
• V52 decays with beta /gamma emission – with half life of 3.76

minutes

• Physically small – about 30 centimeters – good for local readings
• Almost 100% neutron
• But too slow for direct reactor control ( time constant approx. 5 ½

minutes)

• Used for flux mapping

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Regulating System Flux Detectors

• 14 zones
• Each has one detector for DCC X and another for DCCY
• Some installed spares
• Class 2 – generates a linear N signal

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Thermal Power Measurement

Reactor Thermal Power

• Flow rate – venturis’ or orifice

plates

• Delta T with RTD’s inlet and
• utlet
• Delays associated with

transport of fluid ( transport lag) and time constant of the detector

• Only good if no boiling –

CANDU 9 starts to boil at about 70% Steam Generator Thermal Power

• Saturated conditions
• Feed-water flow and delta T

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Thermal Power

• Thermal power in CANDU 9 uses reactor delta T below 50% F.P and above

70% Steam Generator heat transfer is used

• In between a combination of the two is used to have a smooth transfer of

control

• Thermal energy into the PHT includes
• Neutron power – about 93%
• Decay heat – about 6%
• Pump Heat – about 1%
• If power changes ( increases)
• 93% signal responds immediately
• 6% signal responds with slow time constant
• 1% remains relatively constant

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Neutron Power – Corrected with Thermal

If reactor power is changed quickly, is the thermally corrected power accurate if near 100% full power?

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0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1 2 3 4 5 6 7 8 9 Fuel Burnup (MWd/kg (U)) k-infinity 37 EL. Nat-U, Ref. Case

k-infinity vs. Fuel Burnup: Long Term Reactivity

Next Steps

CANDU-9 and ACR-700 Simulator – Practise power maneuvers for Normal Operation Conditions – Understand basic faults

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