Nowadays, a cybersecurity assessment is at the core of the safety and security assurance for all of software and hardware complexes. For instrumentation and control systems of nuclear facilities, the availability is proved to be the most important in the whole confidentiality, integrity, and availability triad representing cybersecurity. However, no any relevant method for the industrial control system availability evaluation is at hand now. The article presents a novel approach for the availability assessment by upper bound delay estimation with the help of a theory of deterministic queuing systems Network calculus. As an example of applying proposed approach, the availability of IntServ model under internal insider’s and external denial-of-service cyberattacks is investigated; in particular, the dependences of the maximum delay on arrival flow and server parameters are revealed. For utilizing the approach with empirical data, a technique for a linear service curve calculation, similar to a linear arrival curve regression, is proposed.