Safety analysis of the software of the power density limiting computer (DNB-module) at Grafenrheinfeld

This report starts with a brief description of the computer system used in Grafenrheinfeld for limiting the power density in the reactor core (DNB-Module) and is then followed by a discussion on methods for verification of the systems' software. The different language levels on which analysis may be performed are presented and criteria for choosing the appropriate language level with regard to verification are given. The program analysis splits the software into sections which are manageable and testable. Then the detailed structure of control flow and data flow as well as the functional properties of these sections are investigated. On certain premises completely analysed sections can be regarded as errorfree, i.e. their failure probability can be assumed to be zero. In addition, test cases for the various software sections are established on the basis of the program analysis. The test strategy comprises the module (sectional) test, partially performed with the test cases mentioned above, as well as an integration test of the entire DNB software. Both tests may be performed automatically - the test embedment is already established to a certain extent - thus allowing execution of a great number of tests; a feature which is extremely important in view of an assessment of quantitative software parameters. (orig.)