MOSCOW (AFP) - A Russian nuclear submarine caught fire on Thursday while undergoing repairs in its northern Murmansk region dock but there was no radiation leak, officials said Thursday. The fire spread from wooden structures surrounding the dock to the submarine, whose outer hull then caught fire, Northern Fleet navy spokesman Vadim Serga told the Interfax news agency. "The burning spread to the exterior of the hull," he said, without giving full details on how this occurred. A defence ministry spokesman told Russian state television that the Yekaterinburg nuclear submarine's two reactors had been switched off in advance.