NRX was an experimental heavy water moderated nuclear reactor at the Canadaian Chalk River Laboratory which experienced a partial core meltdown accident on 12 December 1952. The reactor began operation on 22 July 1947.
A heavy water moderated reactor is governed by two main processes. First, the water slows down (moderates) the neutrons which are produced by a nuclear reaction, allowing the high energy neutrons to cause further reactions. No water moderator and the reacton stops. Second, control rods absorb neutrons and adjust the power level or shut down the reactor in the course of normal operation. Either inserting the control rods or removing the heavy water moderator can stop the reaction. A nuclear explosion isn't possible because the rapid heat release from the accelerating reaction turns the moderating water to steam and removes the water and its moderating effect, as demonstrated in the US boiling water reactor Borax experiments and SL-1 accident.
The NRX reactor design was an early version of the CANDU reactor, with a sealed vertical aluminium calandria with a diameter of 8m and height of 3m. The calandria held about 175 6cm diameter vertical tubes in a hexagonal lattice, 14,000 litres of heavy water and helium gas. The level of water in the reactor could be adjusted to help set the power level. Sitting in the calandria tube and surrounded by air were fuel elements or experimental items.
The fuel elements contained fuel rods 3.1m long, 3.1cm in diameter and weighing 55kg, containing uranium fuel and sheathed in aluminium. Surrounding the fuel element was an aluminium coolant tube with up to 250 litres per second of cooling water from the Ottawa river flowing through it.
Twelve of the calandria tubes contained control rods made of boron powder inside steel tubes. These could be raised and lowered to control the reaction, with seven inserted being enough to absorb so many neutrons that no chain reaction could happen. The rods were held up by electromagnets, so that a power failure would cause them to fall into the tubes and terminate the reaction. An air pneumatic system could use air pressure from above to quickly force them into the reactor core or from below to slowly raise them from it. Four of these were called the safeguard bank while the other eight were controlled in an automatic sequence. Two pushbuttons on the main panel in the control room activated magnets to seal the rods to the pneumatic system and the pushbutton to cause the pneumatic blowdown of the rods into the core was located a few feet away.