The explanation of what could happen in this scenario is quite long but this article explains it extremely well using Japan's nuclear crisis caused by the earthquake of 2011.
How Japan's Nuclear Crisis Works
Commercial nuclear reactors are a fact of life in many parts of the developed world. Because they do make use of a radioactive fuel source, these reactors are designed and built to the highest standards of the engineering profession, with the perceived ability to handle nearly anything that nature or mankind can dish out. Earthquakes? No problem. Hurricanes? No problem. Direct strikes by jumbo jets? No problem. Terrorist attacks? No problem. Strength is built in, and layers of redundancy are meant to handle any operational abnormality.Shortly after an earthquake hit Japan on March 11, 2011, however, those perceptions of safety began rapidly changing. Explosions rocked several different reactors in Japan, even though initial reports indicated that there were no problems from the quake itself. Fires broke out at the Onagawa plant, and there were explosions at the Fukushima Daiichi plant.
So what went wrong? How can such well-designed, highly redundant systems fail so catastrophically? Let's take a look.