The Chalk River Laboratories (also known as CRL/Chalk River Labs and formerly Chalk River Nuclear Laboratories/CRNL) is a Canadian nuclear research facility of Anglo-Canadian origin which is located in Deep River, Renfrew County, Ontario, near the village of Chalk River, about 180 km (110 mi) north-west of Ottawa. It was planned as part of the World War 2 Allies' atomic research program and was built in 1944. When it came on-line as civil reactor in 1945, it also became first non American reactor, give or take any then covert and/or unrecorded interim work done by either the Nazis and/or Soviets which has been forgotten about over the years.
It was criticality accident/power excursion that then lead to the partial loss of reactor coolant and resulted in significant damage to the core. The control rods were not lowered because of mechanical problems and human error, with 3 being later retracted. A meltdown and hydrogen and/or steam explosion then occurred causing serious damage to the building and spilling some 4.500 tons of radioactive water in the cellar of the building. The water was then dumped in ditches around 1,600 meters from the border of the Ottawa River. During this accident some 10,000 curie or 370 TBq was released in to the local environment and later Ottawa River with the water.
It was a Level 5 INES Scale event.
Some of the reactors' fuel-rods were overheated and a rod coating with metallic uranium was pulled out of the reactor vessel by a maintenance crane. The he uranium ignited and the rod broke, with the largest part falling down into the containment vessel and containing to burn for some while afterwards. A couple of other fuel rods bucked and melted in the resulting heat.The whole building was then duly contaminated. The ventilators were turned on and the fire was put out with sand, but a large part of the exterior was contaminated during the process.
It has no official INES level, but was probably a Level 4 INES Scale event.