Two British nuclear submarines went to sea with a potentially disastrous safety problem that left both vessels at risk of a catastrophic accident, the Guardian can reveal. Safety valves designed to release pressure from steam generators in an emergency were completely sealed off when the nuclear hunter killers Turbulent and Tireless left port, a leaked memo discloses. The problem went undetected on HMS Turbulent for more than two years, during which time the vessel was on operations around the Atlantic, and visited Bergen in Norway, the Portuguese capital, Lisbon, and Faslane naval base near Glasgow. It was not noticed on...