Question: what happens if a particle accelerator explodes when used in full-power?
Would it comparable to a nuke?
And please, don't give that strangelet/black hole doomsday answers.
I really need a realistic one (to destroy the facility)
Precisely jack and shit will occur

, the things have no damn explosive parts.
If one catastrophically breaks the particle beam will drill a small but
very long hole in a wall and people standing
right next to the machine might get electrocuted/hurt by bits of shrapnel from the equipment rupturing. The large magnets might do some damage to electronic systems
in the building and produce a little more shrapnel.
I don't think you seem to understand what particle accelerators do - they impart energy to a few particles, with what
relative the particle is an enormous amount - a proton with a trillion electronvolts, the best we can do right now still has had less than a single joule of energy imparted to it. A guy* has literally been shot through the brain by the full beam of a particle accelerator and merely got headaches and a weird scar.
A gas leak and cigarette is more likely both to destroy a building and give you cancer.
*Those wacky soviets!