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 1

Learning Objectives • Overview of Unit Control • Overview of Reactivity Control Mechanisms • Overview of Power Measurement Systems • Overview of Refuelling Effects 2

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

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CANDU Computer Control 11

End View • 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) 12

Spatial Control - LZC • 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 13

LZC – Control System • 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 14

Reactivity Deck Shut-off Rods • 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 15

Rod Position • 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 16

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 20

Ion Chambers SDS Regulating and SDS1 2 A B C Typical arrangement for Regulating and SDS1 penetrations D E F 21

Ion Chambers 22

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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 24

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% 25

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 26

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 27

Thermal Power Measurement Reactor Thermal Power • Flow rate – venturis’ or orifice plates • Delta T with RTD’s inlet and outlet • 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 28

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 29

Neutron Power – Corrected with Thermal If reactor power is changed quickly, is the thermally corrected power accurate if near 100% full power? 30

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k-infinity vs. Fuel Burnup: Long Term Reactivity 1.13 1.12 1.11 37 EL. Nat-U, Ref. Case 1.10 1.09 1.08 1.07 1.06 1.05 k-infinity 1.04 1.03 1.02 1.01 1.00 0.99 0.98 0.97 0.96 0.95 0.94 0 1 2 3 4 5 6 7 8 9 Fuel Burnup (MWd/kg (U))

Next Steps CANDU-9 and ACR-700 Simulator – Practise power maneuvers for Normal Operation Conditions – Understand basic faults 35