25% of the fuel elements in a heavy water moderated carbon dioxide cooled 100 MW(e) power reactor were damaged due to operator error. The operators failed to remove silica gel pellets that has fallen into a new fuel element from a damaged pack (there was no procedure available to check the interior of fuel element, therefore only pellets from the top were removed). The silica gel packs were used to keep the unused fuel dry during storage and transport. The silica gel pellets blocked the flow of the coolant resulting in overheating of the fuel and the pressure channel holding it. As a result of overheating the heavy water leaked into the part of the reactor (the gas circuit) where the fuel elements are accommodated, the fuel cladding was subject to corrosion and a considerable amount of radioactivity leaked into the primary cooling circuit (CO2 gas). Through leaks in the steam boilers (similar basic design to a MAGNOX or AGR plant) some parts of the secondary circuit became contaminated.