A core for a nuclear weapon needs a near-spherical shape for any kind of runaway chain reaction, and depending on size and material may also need a neutron reflector. A nuclear reactor simply cannot cause a full-scale nuclear explosion: fuel assemblies are arranged into long, thin columns separated by cladding the large surface area causes a significant percentage of the available fission neutrons to dissipate into the moderator rather than causing further fission events, preventing a critical mass from forming. Similarly, fictional nuclear reactors will meltdown or go up in gigantic nuclear explosions at the slightest thing going wrong.This is why most of Japan's nuclear reactors shut down safely during the 2011 earthquake, as the failsafes for their SCRAM systems kicked in (or were activated from the control room) at the appropriate time. If the power to the safety systems is interrupted even for a moment, the mechanism stops resisting and the reactor shuts down. Usually, the SCRAM mechanism has to actively prevent the shutdown from happening - for instance, by constantly pushing against a spring, or holding up control rods with an electromagnet. What's more, even the failsafe have "dead-man" failsafes. A switch that usually exists in multiple redundant locations both near and far away from the reactor room, so that you can always reach at least one during an emergency.
This is as opposed to real life, where it's typically an automatic safety feature which engages if the reactor shifts outside a certain set of safe operating parameters and where a manual reactor SCRAM is as simple as turning a switch.
note The standard setup for a nuclear bomb is a sphere of weapons grade fissile material surrounded by conventional explosives. Shooting, or even blowing up a real-life nuclear weapon with conventional explosives is likely to disable the warhead, not set it off. to achieve a full-scale explosion (mainly a sphere of conventional explosives being set off in unison around the nuclear mass, compressing it to supercriticality and initiating a nuclear reaction) while fictional nukes act like spheres filled with mega-nitroglycerin. In real life, a nuclear weapon requires precise conditions note The precise engineering of a nuclear weapon makes the best Swiss watch look like a flint knife in comparison.It doesn't matter if it's designed not to do that, it doesn't matter if it's not fissile enough to be used for an atomic bomb, it doesn't matter if it hasn't got enough material for critical mass, it's gonna blow. Related to Reliably Unreliable Guns and Stuff Blowing Up, if something is nuclear, and something, anything happens to it, it's Going Critical and gonna blow up like an atomic bomb.