There are many reliability and risk issues in safety-critical digital control systems. By identifying and analyzing those issues, we can suggest countermeasures and eliminate potential risk factors. Quantification of software reliability is one of the most important issues. Most digital systems are adopting software to make the system behave as designed, thus the reliability of the system is directly related to the software faults. The method of quantification of software reliability may include software testing. However, the behavior and the reliability criteria of the software used in the NPP are different from other software, so the test case generation method should be newly developed. One more challenging issue is the assessment of risk caused by safety network communication for which we are developing a new quantification method.