I agree that good software engineering practices are hugely important. However, there are also supposed to be additional failsafes in the system, namely trained operators and supervisory personnel, many of whom are falling down on the job as well. Even worse, in many cases, the government does not compel public disclosure of accidents, which certainly hinders investigation of recurring failure modes.

For more, see http://www.nytimes.com/2010/01/27/us/27radiation.html