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CANDU & Differences with PWR B. Rouben McMaster University EP 4P03/6P03 2011 Jan-Apr
CANDU Reactor • Heavy-water moderator • Natural-uranium dioxide fuel • Pressure-tube reactor • CANDU is a PHWR
CANDU and PWR Reactor Coolant Systems: Very Similar
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
Differences in Reactor-Core Design • CANDU • Natural-uranium fuel • Heavy-water moderator & coolant • Pressure tubes; calandria not a pressure vessel • Coolant physically separated from moderator • Small/Simple fuel bundle • On-power refuelling • No boron/chemical reactor control in coolant system • PWR • Enriched-uranium fuel • Light-water moderator/coolant • Pressure vessel • No separation of coolant from moderator • Large, more complex fuel assembly • Batch (off-power) refuelling • Boron/chemical reactor control in coolant system
CANDU On-Power Refuelling Fuelling machines at both ends of the reactor: One inserts new fuel, oneremoves irradiated fuel.
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
Refuelling & Excess Core Reactivity • What the previous slide means, in words: • In CANDU, a little bit of fuel is replaced each day. The reactivity change is small. The excess reactivity of the core is always small, a few milli-k (except at the very beginning of life, when all the fuel is fresh). This small excess reactivity is continuously compensated for by varying the amount of light water in liquid zone-control compartments. The low excess reactivity is a safety feature of the CANDU lattice. • In PWR (LWR generally), batch refuelling is used. About 1/3 of the fuel in core is replaced every 12-18-24 months. The reactivity change is very large. At the beginning of cycle (BOC), there is very high excess core reactivity (100 milli-k?), which must be compensated for with large amounts of boron in the moderator.
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
Reactivity Transients (cont’d) In CANDU, Large Loss of Coolant (LLOCA) is the accident which is the most challenging in terms of positive reactivity insertion. PWR lattice has very high negative fuel-temperature (Doppler) and power coefficients, which cater to device ejection and short prompt-neutron lifetime. In CANDU, the fuel-temperature and power coefficients are much less negative, but the transients are generally milder and slower.
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.
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 an absolute necessity in CANDU.
Comparison of Some Features • PWR • High fuel burnup • Simple PHT (small number of pipes) • Pressure boundary is not open every day • Expensive fuel • High excess reactivity • Large pressure vessel • Short neutron lifetime CANDU • Natural uranium • Simple, inexpensive fuel bundle • Excellent neutron economy • Unpressurized calandria • On-power fuelling • Long neutron lifetime • Automatic plant operation • Large amounts of cool water • Expensive heavy water • Large number of pipes • Tritium production • Positive coolant void reactivity (?)
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
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
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.