[en] Safety, reliability and availability of a nuclear plant can be increased by continuous monitoring, control of the significant process variables and operational parameters of the plant. These functions are usually realized by various on-line mini- and microcomputer systems which act and react in real-time. At present no method exists that guarantees computer programs to be free of errors. Several constructive means, however, exist which help to avoid errors during the program development process and tolerate certain errors during the operation. Other means allow to verify a certain cleanness from errors or a certain absence from operational failures. If the necessary effort is made, software can be made as reliable as required. 28 refs