Читать книгу Industrial and Medical Nuclear Accidents - Jean-Claude Amiard - Страница 48

Of the nuclear accidents that have occurred in research laboratories, those at the Chalk River Laboratories are the most serious. The nuclear laboratories at Chalk River, Ontario, Canada, were established in 1942 as a result of British-Canadian collaboration. Their main activity is research in the field of nuclear reactions. In 1947, the first nuclear reactor outside the United States was commissioned. This nuclear research reactor, NRX (1947–1992), is moderated by heavy water and cooled by light water and was originally designed for military applications. Today, the Chalk River Laboratories are of great importance in the medical applications of nuclear energy.

The first accident occurred on December 12, 1952 in the NRX reactor. While the reactor was operating at full power, it experienced a partial loss of coolant. Operators made several bad decisions, causing a chain reaction that more than doubled the nuclear reactor’s power. In particular, operators opened valves in the cooling system to lower the containment pressure. Inexplicably, the descent of the control rods into the reactor core was not complete. This triggered an explosion that destroyed the nuclear reactor’s core and caused a nuclear fuel leak. A series of hydrogen explosions raised the four-ton dome into the air. About 370 TBq of fission products were released into the atmosphere with 4,500 tons of contaminated water. The contaminated water had to be pumped out of the subsoil and into shallow trenches near the Ottawa River. The core of the NRX reactor that could not be decontaminated had to be buried like other radioactive wastes. This accident was classified as a level 5 accident according to the INES. The Atomic Energy of Canada Company restarted the site within the year.

A second accident in 1958 involved a fuel failure and fire in the 135 MWt National Research Universal reactor (NRU) building (1957–2018). Some fuel rods had overheated. Using a robotic crane, one of the uranium metal rods was removed from the reactor vessel. But when the crane arm moved away from the core, the uranium caught fire, the rod broke and most of the stem fell into the containment. This led the whole building to be contaminated. The ventilation system valves were opened and a large area of the building’s exterior was contaminated. The fire was extinguished by scientists and cleaners wearing protective clothing by throwing buckets of wet sand.