Der Tschernobyl-Unfall : Hergang, Ursachen und Nachrüstungen

The accident in the fourth unit of the Chernobyl nuclear power station was able to occur because of the major deficits in engineered safeguards design of the RBMK line of reactors : faulty core design ; unsuitable shutdown system ; absence of automatic shutdown criteria ; insufficient protection against reactor protection measures being blocked by the reactor operators. The sequence of accident events was started and aggravated by a faulty experimental program ; insufficient experience in, and preparation for, the experiment of the personnel; and violations of important rules by the operating crew. The decisive causes of the reactor accident were known to the plant planners responsible for safety, and to some scientific institutions, already many years before the event. Although measures had been planned, and their implementation considered, to repair the design flaws of the reactor core and of the control and shutdown system, they never came to anything. The plant crew were commissioned to perform a job whose failure was likely to have catastrophic consequences. In view of the grave deficiencies in design, the existence of adverse burnup conditions and, consequently, a higher void coefficient, an experiment like this should never have been approved without a detailed safety analysis. The plant crew were only insufficiently supported by suitable information and control elements, analytically backed information about plant behavior, and by reactor protection systems designed with safety in mind. The crew were out of their depth in meeting their responsibility for safety. In the meantime, all RBMK plants have been upgraded in reactor physics design, shutdown systems, and operating instructions. As a consequence, it is now deemed to be nearly impossible that the explosion like accident sequence will be repeated. This is the gist of Russian studies confirmed in principle by the GRS. However, a number of details still need to be investigated further, especially inasmuch as safe operation is still highly dependent on the operating instructions being observed. Because of persisting major safety deficits, not directly associated with the causes of the Chernobyl accident, the RBMK plants are still in need of improvements in engineered safeguards. Their urgency is a function of the plant generation and of the backfitting measures implemented so far.