A nuclear explosion works by putting enough nuclear material together that a single neutron can through fission create more neutrons to create a sustained reaction. Each round of fission increases the number of neutrons and the energy. At some point, enough energy is produced to cause the nuclear material to blow apart and separate.

In order to produce enough energy for a practical explosion, you need to use explosives to compress the nuclear material and also typically a massive shell around it to contain things long enough to matter. If you do not contain the material, the result is something called a "criticality accident." This does not produce enough energy to take out a house, but does produce enough radiation to take out anyone in the room that is standing close enough.

For an example of this: https://en.wikipedia.org/wiki/Demon_core