Does nuclear containment building has gates or how can employers get inside?

Yes, a nuclear containment building has pneumatically operated entrance doors with an in-between airlock compartment so that the outside environment is not mingled with the containment's inner atmosphere.

The containment is always maintained at a negative pressure than the normal atmosphere in order to keep the vicinity safe against any untoward airborne radiation escape.

Also the containment itself is designed, constructed and maintained according to the stringent safety design guidelines and standards authenticated and governed by the International Atomic Energy Agency (IAEA).

The employees working in a nuclear environment have to undergo radiological safety training to be qualified to have access to the containment. Their radiation exposure is continually monitored as per the safety guidelines issued by IAEA.

For more detailed information, you may visit the IAEA website. Please see related link.