The NRX reactor incorporated a calandria, a sealed vertical aluminium cylindrical vessel with a diameter of 8 m and height of 3 m. The core vessel held about 175 six-centimetre-diameter vertical tubes in a hexagonal lattice, 14,000 litres of heavy water and helium gas to displace air and prevent corrosion. The level of water in the reactor could be adjusted to help set the power level. Sitting in the vertical tubes and surrounded by air were fuel elements or experimental items. This design was a forerunner of the CANDU reactors.