Document: NRC Regulatory Guide
Document ID: 086612c4-a8a7-4f50-a166-6f1cb05bcdaf
Document Type: regulatory_guide
Title: Guidelines for Environmental Qualification of Safety-Related Computer-Based Instrumentation and Control Systems in Nuclear Power Plants
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0630/ML063040591.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.209
CFR Part: 
CFR Title: 

Content:
completion of digital data communication or error detection before continuation of discrete functional steps. The capability of digital system design accommodates the potentially cumulative effects of environmental stress and is an important consideration for qualification of computer-based I&C systems. From a hardware standpoint, one significant difference between analog and advanced digital systems is the radiation tolerance of different integrated circuit (IC) technologies. The analog technology historically used in nuclear power plants includes discrete bipolar devices. Advanced digital systems tend to use metal oxide semiconductor (MOS) technology, particularly complementary MOS (CMOS) technology. The radiation threshold for MOS devices is generally lower than those for bipolar (analog) devices. However, MOS technology is preferred for ICs because of its technical superiority in other areas (such as higher input impedance, fewer manufacturing processing steps, better temperature stability, and lower noise). Commercial MOS devices are very sensitive to ionizing doses, in contrast to their relative insensitivity to neutron fluence. Ionizing dose radiation hardness levels for MOS IC families range from about 10 gray (Gy) or 1 kilorad (krad) for commercial off-the-shelf (COTS) circuits to about 105 Gy (104 krad) for radiation-hardened circuits. The threshold fluence hardness level for MOS devices is about 1014 neutrons per square centimeter (n/cm2), 1 million electron volts (MeV) equivalent. In contrast, the ionizing radiation hardness level range for bipolar devices begins around 104 Gy (1000 krad). The threshold fluence hardness level for bipolar devices ranges on the order of 10 14 to 1015 n/cm2 (1 MeV equivalent). Another significant difference between analog and advanced digital systems arises from the potential effect of the more rapid evolution of digital technology; in particular, the ever-increasing density and complexity of ICs at the wafer level make