British Submarines Go to Sea with Safety Valves Isolated: Root Cause?

Two British Trafalgar-Class submarines were at sea for an extended period of time with their steam generator safety valves isolated.  One was at sea for over two years, the other for over one year.

For those that are unfamiliar with a submarine or nuclear systems, these valves are very important in the event another problem occurs.  They allow the steam generators to relieve excess pressure overboard in the event of an over-pressure condition.  On these two submarines, test blanks were left installed after maintenance, blocking the safety valve overboard discharge path.

The report cites “poor discipline” at the shipyard that performed repair work.  Do you think maybe we could come up with better root causes?