This article introduces an integrated methodological model (IMM) for assessing the software quality of systems important to nuclear power plant safety. The model integrates five interrelated components: formal verification and model checking, AI/ML-based anomaly detection, digital twin modeling, DevSecOps/CI/CD practices, and independent V&V audit, within a unified framework covering all software lifecycle stages. A fivelevel qualitative scale of organizational and technological maturity and a mathematical framework for integrated quality assessment with differentiated weighting factors by safety category, in accordance with IAEA SSG-39 guidelines, are presented. Experimental testing in an NPP information and control systems modernization project resulted in a 27−45% increase in identified defects, a 28% increase in requirement coverage, a 32% reduction in defect identification labor intensity, and a twofold increase in early-stage defect detection.