The aim of the present paper is to introduce a case study and the best practice of security vetting in relation to the critical infrastructure and in specific to nuclear security. In Slovenia the Nuclear Power Plant in Krško (the NEK) is an important part of the energy sector and the environmental safety sector of the state's critical infrastructure. The NEK is also an important source of electricity for Croatia - in that respect the plant produces and supplies electricity for both Slovenia and Croatia, each of which has the right and obligation to use 50 percent of its total output. The structure of the management board and personnel is based on the parity principle, considering the equal business shares of both partners. When there is a need for a security check of foreign citizens working in the facilities, public-Private, inter-ministerial and international cooperation is needed. The best practice presented in this paper of a comprehensive approach to cooperation regarding legislation and implementation similar to the security vetting of foreign citizens relating to nuclear security can also be used in other sectors of the critical infrastructure. It also represents an example of best practice for the risk management of potential internal threats connected with terrorist threats to the normal functioning of the critical infrastructure.