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CANDU Differences with PWR

2008 January. 2. CANDU Reactor. Heavy-water moderatorNatural-uranium dioxide fuelPressure-tube reactorCANDU is a PHWR. 2008 January. 3. 2008 January. 4. CANDU and PWRReactor Coolant Systems: Very Similar. 2008 January. 5. CANDU-PWR Balance of Plant. Balance-of-plant features in CANDU and PWR are very similar:Administration and maintenance facilitiesPump houseReactor containmentTurbine and generatorDifferences between CANDU and PWR are principally in reactor-core design.

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CANDU Differences with PWR

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    1. 2008 January 1 CANDU & Differences with PWR B. Rouben McMaster University EP 4P03/6P03 2008 Jan-Apr

    2. 2008 January 2 CANDU Reactor Heavy-water moderator Natural-uranium dioxide fuel Pressure-tube reactor CANDU is a PHWR

    3. 2008 January 3

    4. 2008 January 4

    5. 2008 January 5 CANDU-PWR Balance of Plant Balance-of-plant features in CANDU and PWR are very similar: Administration and maintenance facilities Pump house Reactor containment Turbine and generator Differences between CANDU and PWR are principally in reactor-core design

    6. 2008 January 6 Differences in Reactor-Core Design

    7. 2008 January 7

    8. 2008 January 8 CANDU On-Power Refuelling Leads to: Constant global power shape, with localized “ripples” as channels are refuelled and go through their burnup cycle Constant in-core burnup Constant shutdown-system effectiveness Possibility of on-power removal of failed fuel, and therefore clean HTS

    9. 2008 January 9 Refuelling & Excess Core Reactivity

    10. 2008 January 10 Refuelling & Excess Core Reactivity

    11. 2008 January 11 Reactivity Devices In CANDU: Devices are in benign environment (moderator at low pressure and temperature) Pressure-driven ejection not possible Separate devices for control and safety Modest reactivity worth Maximum total reactivity rate <0.35 mk/s In PWR: Device worth is very high, to match high core excess reactivity Pressure-driven ejection must be considered in safety analysis Same for accidental boron dilution

    12. 2008 January 12 Reactivity Transients

    13. 2008 January 13 Reactivity Transients (cont’d)

    14. 2008 January 14 Reactivity Transients (cont’d)

    15. 2008 January 15 CANDU Caters to Void Reactivity by: Arranging heat-transport system to minimize rate of reactivity insertion on coolant voiding (e.g., subdividing the heat-transport system into 2 loops). Providing two fully capable Shutdown Systems that can individually overtake any reactivity transient.

    16. 2008 January 16 Core-Region Decoupling The CANDU core is more decoupled than a PWR core. This means that core regions or zones can behave somewhat independently of others to a greater degree in CANDU than in PWR: the spatial power distribution can be more easily tilted. Also, refuelling occurs daily, in various core regions. ? A spatial-control system is more necessary in CANDU.

    17. 2008 January 17 Fuel-Cycle Safety Natural uranium or other low-fissile-content fuel ensures that there is no potential for criticality of new or used fuel in air or light water. No need to ship new fuel in borated steel containers No need to borate the ECC System water No need to borate the fuel-bay water Simplified irradiated-fuel dry storage

    18. 2008 January 18 Inherent CANDU Safety Features Reactivity devices in cool, low-pressure moderator. Rod ejection not possible. Small core excess reactivity, because of on-power refuelling. Worth of reactivity devices in RRS is low, magnitude of reactivity-induced transients is limited. Reactivity-device worth constant over life of plant. Long prompt-neutron lifetime slows rate of transients. Nuclear lattice (lattice pitch) nearly optimized for maximum reactivity. Any event that relocates the fuel reduces reactivity. cont’d

    19. 2008 January 19 Inherent CANDU Safety Features No reactivity effect from many postulated transients, including rapid cool-down of the heat-transport system. Moderator system can remove decay heat under such severe conditions as a LLOCA coincident with ECC failure. Low radiation fields in coolant, because of on-line failed-fuel detection and removal, and absence of chemicals for reactivity control. Easy handling of new and irradiated fuel. No criticality concern, in ordinary water or air, regardless of storage configuration. Large moderator volume serves as excellent heat sink in hypothetical severe accidents.

    20. 2008 January 20 END

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