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  • Tom Hess

Safety Features of Pickering Nulcear Generating Station

History

Pickering Nuclear was constructed in 2 stages. Pickering A was built on experience gained from earlier prototype reactors at Nuclear Power Demonstration (NPD) in Rolphton, and the larger commercial scale Douglas Point reactor. Pickering A Units 1 and 2 employed the same pressure tube alloy used in the forerunner reactors, but Units 3 and 4 were built with the new alloy that remains unchanged to the current generation of CANDU reactor pressure tubes. It was this older alloy that led to the Unit 2 pressure tube failure.


All pressure tubes/calandria tubes (fuel channels) are designed to detect leaks.


Early manufacture of the new alloy tubes led to end fitting roll joint cracking that was originally discovered in Unit 3. This same manufacturing defect was also detected in similar vintage Unit 4, as well as Bruce A Units 1 and 2, but was subsequently mitigated.


The four Pickering A reactors were retubed from 1986-1993, as were Bruce A Units 1 and 2 before their restarts in 2012. Subsequent unit builds at Pickering B employ the same revised pressure tube standards used at Bruce B and Darlington, as well as all CANDU 6 reactors.


Physical Safety Features

Ontario CANDU sites are unique in that they employ an extra level of defence in the event of a reactor mishap causing a release or overpressure in any of the reactor reinforced concrete containment buildings. A site connected vacuum building is designed to expand containment volume by evacuating pressure from any of the eight, now six, operating reactor containment buildings. It is also equipped with a roof mounted douse tank to cool any heated contaminants pulled into the building. No other reactors in the world employ this extra level of protection.


Support systems include normal and emergency power sources, a service water system, a recirculated cooling water system, and an emergency water system. Emergency mitigation equipment and hookup are also in place, a lesson learned from the Fukushima event. Portable generators, pumps, etc. are stored onsite for immediate deployment if needed.


Pickering A

Fuel pellets are encased in a tube called a pencil. Pencils are bonded together to form a fuel bundle which is then inserted in a pressure tube filled with heavy water, encased in a calandria tube that is also surrounded by heavy water within the calandria assembly. The calandria is encased in a vault also housing a dump tank, filled with air, situated in its containment building.


The heavy water heat transport system includes a shutdown cooling system, a heavy water recovery system to recover heavy water, and the emergency coolant injection system for any loss of coolant accident. There is also a boiler emergency cooling system.


Safety systems include a fast shutdown system, a moderator dump tank shutdown system, an emergency coolant injection system, and negative pressure containment system (vacuum building).


Pickering B

Fuel pellets are encased in a tube called a pencil. Pencils are bonded together to form a fuel bundle which is then inserted in a pressure tube filled with heavy water, encased in a calandria tube which is also surrounded by heavy water within the calandria assembly. The calandria is encased in a vault filled with light water, situated in its containment building.


The heavy water heat transport system includes a shutdown cooling system, a heavy water recovery system to recover heavy water, and the emergency coolant injection system for any loss of coolant accident. The calandria heavy water tank may also be utilized as an additional heat sink. There is also a boiler emergency cooling system as well as a shutdown cooling system that can contribute to cooling.


Safety systems include two fast shutdown systems, an emergency coolant injection system, and negative pressure containment system (vacuum building).


Systems are arranged in two physically separated groups, one controlled from the main control room, the other from an emergency control center.


The Pickering B safety features are very similar, almost identical, to those of CANDU 6 reactors deployed within Canada and worldwide. However, the stand-alone CANDU 6 sites do not employ the additional safety feature of a vacuum building that Pickering has.


In 2016, the CNSC issued Pickering the highest possible rating, “Fully Satisfactory” in its Regulatory Oversight Report,26 and the World Association of Nuclear Operators (WANO) reconfirmed for a second time Pickering’s exemplary safety performance.


Irradiated Fuel Handling

Irradiated field is handled the same way it is on all other CANDU reactors. Automated fuelling machines remove the spent fuel bundles from the operating reactor. These spent fuel bundles are moved via a water immersed conveyance system to the spent fuel pool, where they will cool for 6 to 10 years. The bundles are then moved into dry storage casks, which are stored on site. By this point in time, it is safe to move within close proximity to, or stand near, these dry storage containers.


Tom Hess