Analysis and control of small breaks in PWRs and accompanying research activities in the Federal Republic of Germany

This contribution uses the example of a PWR as used in the FRG (approx. 4000 MW(th)) to describe the main safety features required for the control of accidents in the event of small breaks. Of particular importance are the four-leg high-pressure safety injection system, whose zero head is at 110 bar, and the blowdown control station on the secondary side, which ensures a shutdown at a gradient of 100 K/h in the event of a loss-of-coolant accident. In accordance with the results of the German Risk Study for the reference plant of Biblis B, uncontrolled small breaks in the primary loop occur 5.7x10-5 times a year and thus constitute the greatest contribution to the frequency of core meltdown accidents. In this context, operator errors are the decisive factor. As a result of various changes of details in systems engineering, the frequency of a core meltdown resulting from a small break should have been reduced by far more than one order of magnitude. The improvements realized at the reference plant and other PWRs refer mainly to the automatic shutdown of the secondary side, the capacity of the blowdown control station, the sump operation of the high-pressure safety injection pumps, the ensuring of energy supply and accident instrumentation. By means of an extension of the well-known RELAP 4 computer code, GRS created a program version for the analytical treatment of small breaks. The examples of two break areas of 10 cm2 and 20 cm2 are used to show that the calculated results can be explained physically in every detail. The computer code can be used to indicate trends, e.g. if the injection rate is changed or in the event of variations of measures taken on the secondary side. It is thus a valuable tool for efficiency and system studies or system optimizations. A further improved analysis of accident sequences is to be expected after completion of the GRS code DRUFAN-02