Objective of the Thesis was to design a Program of cleaning processes of the ventilation technology as soon as it is implemented in the nuclear power plant Mochovce site during the completion of units 3 and 4. The Program of post assembly cleaning operations aims to achieve the required cleanness in the internals of new pipelines that got dirty during assembly works. All pipelines of ventilation system were cleaned up according to the Program of post assembly cleaning. Subsequent testing verified and approved of the system to be properly clean. The Program was concluded by final protocols on successful accomplishment of post assembly cleaning operations. The system conditioned this way is ready to be integrated into the process of further testing, i.e. pressure and functional tests, before the nuclear power plant Mochovce being put into continuous operation in all its operation modes.