One of the fundamental reasons that Chernobyl occurred lies in how that reactor was designed. Russian nuclear reactors were designed to be over-moderated, while

One of the fundamental reasons that Chernobyl occurred lies in how that reactor was designed. Russian nuclear reactors were designed to be over-moderated, while modern power reactors are under-moderated. The moderator is the substance which slows down the neutrons to a speed where they're likely to fission with the Uranium-235 in the fuel. This is typically water. Water can also absorb neutrons, so it has a poison effect as well. When the moderating effect of the water dominates its poison effects, the reactor is said to be under-moderated. When the moderator is reduced (for example, when it boils away), the reactor becomes less efficient, so it reduces power. This is called a negative temperature coefficient. The Soviets, however, designed their reactors to be over-moderated, for efficiency. Thus, when the water heated up, and started boiling away, the net result was to increase the efficiency of the reactor (a positive temperature coefficient), since the water was more of a poison than a moderator. Because the reactor is now more efficient, it got even hotter, and boiled more water away. You know the rest. We've learned a lot about designing safe reactors since Chernobyl, and it's extremely unlikely that an accident on the scale of Chernobyl will happen again (not only because of the moderation issue). I hope this helps to alleviate your concerns. If you have any further questions, please feel free to ask me.