1 Soft errors induced by radiation are an increasing problem in the microelectronic field. Although traditional models estimate the reliability of memories suffering Single Event Upsets (SEUs), Multiple Bit Upsets (MBUs) are becoming more and more important as technology scales. In this paper, a model that deals with MBUs in memory systems, which allows calculating reliability in a fast way similar to the SEU case, has been used to analyze the Mean Time To Failure (MTTF) of a 150 nm device under radiation. This analysis illustrates the importance that physical factors, as the energy, have on the system reliability. Categories and Subject Descriptors B.8.1 [Performance and Reliability]: Reliability, Testing, and Fault-Tolerance. General Terms Reliability. Keywords Multiple Bit Upsets (MBUs), reliability, memory, radiation.