If the ramjet didn't go critical it wouldn't be of much use. The key number when you are talking about an operating reactor is called the multiplication factor

If the ramjet didn't go critical it wouldn't be of much use. The key number when you are talking about an operating reactor is called the multiplication factor--when an atom fissions on average how many other atoms will be split by the neutrons released from that fission. When this number is less than one things just peacefully sit there. If an atom fissions the chain soon dies out. If this number is more than one things get hotter and hotter until it comes apart. If this were the whole story there would be no nuclear power, but there's one very useful detail--a small number of those neutrons are not emitted at once. This gives you a margin to control things. When the immediate neutrons are enough to cause more than one fission you have the state known as prompt critical, you have lost control, the reactor will run wild and destroy itself. (An atom bomb is the same idea, except rather than just edging past 1 a bombmaker pushes the ratio as high as possible so it takes a city with it in destroying itself.) I am aware of two cases where reactors had issues with control rods and crossed the prompt critical line, both were immediately destroyed. SL-1 and Chernobyl. The zone between critical and prompt critical gives you a way to control things. You have time to turn the heat up or down as needed. A nuclear ramjet is going to be designed to operate at a very high power density, it's going to be a lot closer to the line than a reactor is normally run. Stick one toe over the line and you get what SpaceX calls a RUD. Alternately, it might have run too hot, something softened or melted and the core shape changed to cross the prompt critical line, same outcome.