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http://www.nuclearsafety.gc.ca/eng/mediacentre/updates/march-11-2011-japan-earthquake-comparison-candu-and-bwr-reactors.cfm
Comparison of Canadian CANDU Reactors to Japanese BWR Reactors
Many Canadians are curious about the difference between Canadian nuclear power reactors and those that have been affected recently in Japan. Information that may answer those questions includes:
All Canadian nuclear power reactors use CANDU reactors, made in Canada by Atomic Energy of Canada Limited (AECL).
What kinds of reactors are used in Japan?
The Japanese nuclear power reactors at the Daiichi, Daini and Onagawa plants are boiling water reactors (BWRs), along with 11 other Japanese plants. Eight Japanese plants use pressurized water reactors (PWRs).
How are CANDU and BWR reactors different?
CANDU reactors use heavy water technology to control nuclear reactions. They have been operating in Canada since 1962, and abroad since 1972. To date, 48 heavy water moderated reactors based on the CANDU design are in operation, under construction, or under refurbishment worldwide.
In BWR reactors, the heat produced by nuclear fission in the reactor core causes circulating water to boil, producing steam. The steam, which is radioactive, drives a turbine directly, after which it is cooled in a condenser (essentially a heat exchanger cooled by sea water, lake water, a large river or cooling tower) and converted back to liquid which circulates back though the reactor. CANDUs don't permit boiling of the primary loop water. This very hot water, which is also radioactive, circulates in a closed loop entirely within containment. It passes through steam generators (heat exchangers) which transfer heat to secondary loops to power steam turbines and associated electrical generators. The residual low-pressure steam is routed back through a condenser, where it becomes water again and is returned back to the steam generators. The steam and water in CANDU’s secondary loops is not radioactive, so in the event that a heat release is required, the secondary side could be safely vented to atmosphere and new plain water introduced once the water inventories are depleted.
CANDU safety features
CANDU reactors can be refuelled while operating, and incorporate dozens of safety features to respond to the stringent safety requirements imposed by the Canadian Nuclear Safety Commission. Safety highlights include:
Canadian licensing safety conditions
All Canadian power reactors are subject to hundreds of robust safety conditions before they are permitted to be licensed by the CNSC, including the ability to:
What would happen to Canadian nuclear power reactors if there were no electrical power?
In the unlikely event of a loss of all power at a nuclear power plant, the plant's emergency power supply (EPS) would provide electrical power to certain nuclear safety-related systems that support the capability to control, cool, and contain the fuel. The EPS is started automatically when there is a loss of power or can be manually started from the EPS control room within 30 minutes of an identified need.
The EPS is seismically and environmentally qualified and has sufficient fuel stores to operate unaided for a seven-day period. Seismic qualification requires that equipment and systems retain their specified performance capability following an earthquake. Environmental qualification requires that equipment be protected against steam leaks, water flooding, high-intensity fires or other mishaps, which could disable it.
The EPS is similar to other standby generators but is remotely located from them to reduce chances of it being disabled by the same incident. Cables and control equipment involved in switching the EPS into service are routed through areas that are considered to be at lowest risk of damage.
In the even more unlikely event that EPS systems also become unavailable and all power is lost, all affected Canadian nuclear power reactors would safely shut down and stabilize using CANDU's convection cooling system design. Then, all units would be placed in guaranteed shutdown state until power was restored.
In any power outage affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
What would happen to Canadian nuclear power reactors if there were a flood?
All but one Canadian nuclear power plant are on inland bodies of water with no threat of tsunamis like those affecting Japan. The Point Lepreau Nuclear Power Plant in New Brunswick sits on the shores of the Bay of Fundy which sees tidal flows of 40- 50 feet, a factor that was built into its design capability.
All Canadian nuclear power plants have been licensed on the basis of their proven ability to withstand severe flooding with no significant damage; facilities reduce flooding risk by using engineered shoreline breakwaters, catch basins, berms and storm sewers.
In any flood affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
What would happen to Canadian nuclear power reactors if there were an earthquake?
All Canadian nuclear power plants have been licensed on the basis of their proven ability to withstand seismic events like earthquakes. Structures and systems have been designed to safely survive earthquakes to ensure that:
In any earthquake affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
Comparison of Canadian CANDU Reactors to Japanese BWR Reactors
Many Canadians are curious about the difference between Canadian nuclear power reactors and those that have been affected recently in Japan. Information that may answer those questions includes:
- What kinds of reactors are used in Canada?
- What kinds of reactors are used in Japan?
- How are CANDU and BWR reactors different?
- What would happen to Canadian nuclear power reactors if there were no electrical power?
- What would happen to Canadian nuclear power reactors if there were a flood?
- What would happen to Canadian nuclear power reactors if there were an earthquake?
All Canadian nuclear power reactors use CANDU reactors, made in Canada by Atomic Energy of Canada Limited (AECL).
The Japanese nuclear power reactors at the Daiichi, Daini and Onagawa plants are boiling water reactors (BWRs), along with 11 other Japanese plants. Eight Japanese plants use pressurized water reactors (PWRs).
CANDU reactors use heavy water technology to control nuclear reactions. They have been operating in Canada since 1962, and abroad since 1972. To date, 48 heavy water moderated reactors based on the CANDU design are in operation, under construction, or under refurbishment worldwide.
In BWR reactors, the heat produced by nuclear fission in the reactor core causes circulating water to boil, producing steam. The steam, which is radioactive, drives a turbine directly, after which it is cooled in a condenser (essentially a heat exchanger cooled by sea water, lake water, a large river or cooling tower) and converted back to liquid which circulates back though the reactor. CANDUs don't permit boiling of the primary loop water. This very hot water, which is also radioactive, circulates in a closed loop entirely within containment. It passes through steam generators (heat exchangers) which transfer heat to secondary loops to power steam turbines and associated electrical generators. The residual low-pressure steam is routed back through a condenser, where it becomes water again and is returned back to the steam generators. The steam and water in CANDU’s secondary loops is not radioactive, so in the event that a heat release is required, the secondary side could be safely vented to atmosphere and new plain water introduced once the water inventories are depleted.
CANDU reactors can be refuelled while operating, and incorporate dozens of safety features to respond to the stringent safety requirements imposed by the Canadian Nuclear Safety Commission. Safety highlights include:
- a design that can use passive convection cooling for the primary systems to keep the reactor cool in the absence of power
- the use of dousing tanks high in the reactor or containment building that work on gravity, which can be used to replenish secondary side inventory and refill the steam generators, as required, to continue heat release in the event of a loss of power
- use of ceramic uranium fuel pellets that tolerate high temperatures
- two independent and diverse shutdown systems
- calandria vessel that contains the fuel rod assembly and heavy water moderator
- high-density, reinforced concrete containment walls, almost a metre thick
All Canadian power reactors are subject to hundreds of robust safety conditions before they are permitted to be licensed by the CNSC, including the ability to:
- withstand severe flooding with no significant damage; facilities reduce flooding risk by using engineered shoreline breakwaters, catch basins, berms and storm sewers
- operate and/or shut down safely during major power failures or blackouts
- safely survive earthquakes to ensure that:
- a loss-of-coolant accident does not occur
- reactors can be shut down and remain shut down
- heat from reactors can be removed
- any required release of radioactivity is minimized
- status of nuclear steam supply system can be monitored
- no radioactive releases above allowable limits result from any damage to systems other than the core reactor
- if one power reactor fails, it will not affect the safety of adjacent systems or structures
In the unlikely event of a loss of all power at a nuclear power plant, the plant's emergency power supply (EPS) would provide electrical power to certain nuclear safety-related systems that support the capability to control, cool, and contain the fuel. The EPS is started automatically when there is a loss of power or can be manually started from the EPS control room within 30 minutes of an identified need.
The EPS is seismically and environmentally qualified and has sufficient fuel stores to operate unaided for a seven-day period. Seismic qualification requires that equipment and systems retain their specified performance capability following an earthquake. Environmental qualification requires that equipment be protected against steam leaks, water flooding, high-intensity fires or other mishaps, which could disable it.
The EPS is similar to other standby generators but is remotely located from them to reduce chances of it being disabled by the same incident. Cables and control equipment involved in switching the EPS into service are routed through areas that are considered to be at lowest risk of damage.
In the even more unlikely event that EPS systems also become unavailable and all power is lost, all affected Canadian nuclear power reactors would safely shut down and stabilize using CANDU's convection cooling system design. Then, all units would be placed in guaranteed shutdown state until power was restored.
In any power outage affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
All but one Canadian nuclear power plant are on inland bodies of water with no threat of tsunamis like those affecting Japan. The Point Lepreau Nuclear Power Plant in New Brunswick sits on the shores of the Bay of Fundy which sees tidal flows of 40- 50 feet, a factor that was built into its design capability.
All Canadian nuclear power plants have been licensed on the basis of their proven ability to withstand severe flooding with no significant damage; facilities reduce flooding risk by using engineered shoreline breakwaters, catch basins, berms and storm sewers.
In any flood affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
All Canadian nuclear power plants have been licensed on the basis of their proven ability to withstand seismic events like earthquakes. Structures and systems have been designed to safely survive earthquakes to ensure that:
- a loss-of-coolant accident does not occur
- reactors can be shut down and remain shut down
- heat from reactors can be removed
- any required release of radioactivity is minimized
- status of nuclear steam supply system can be monitored
- no radioactive releases above allowable limits result from any damage to systems other than the core reactor
- if one power reactor fails, it will not affect the safety of adjacent systems or structures
In any earthquake affecting Canadian nuclear power plants, the CNSC would work closely with any affected licensees, other government departments and appropriate safety organizations to ensure the safety of workers, citizens and the environment.
谢谢:thanks::thanks:
