The crashworthiness of the flask was demonstrated publicly when a British Rail Class 46 locomotive was forcibly driven into a derailed flask (containing water and steel rods in place of radioactive material) at 100 miles per hour, the flask sustaining minimal superficial damage without compromising its integrity, while both the flatbed wagon carrying it and the locomotive were more-or-less destroyed. Additionally, flasks were heated to temperatures of over 800 °C to prove its safety in a fire. However, critics consider the testing flawed for various reasons. The heat test is claimed to be considerably below that of theoretical worst case fires in a tunnel, and the worst case impact today would have a closing speed of around 170 miles per hour. Nevertheless there have been several accidents involving flasks, including derailments, collisions and even a flask being dropped during transfer from train to road, with no leakage having occurred in any case.