A Russian nuclear-powered icebreaker that developed a radiation leak while on an Arctic mission was due Tuesday to arrive in the Barents Sea port of Murmansk after a five-day trek.

The 21,000-tonne Taimyr vessel shut down its 171-megawatt nuclear reactor on Saturday and will rely on diesel engines to make its way into port some time before 2000 GMT, a Rosatomflot nuclear fleet official said.

"The exact arrival depends on sea conditions," the nuclear fleet official told AFP by telephone from Murmansk.

"The reactor takes seven to 10 days to cool and then we will be able to assess the situation," said the official.

"That is why the decision to shut down the reactor was taken -- to give us a head start."

The 23-year-old icebreaker developed the leak on May 5 after completing a mission on the frozen Yenisei River in Siberia and starting its return trip through Arctic waters.

The agency attributed the slight jump in radiation to "a leak in the primary coolant system" and stressed the event could be registered as a zero on the seven-point nuclear safety scale.

But neither the fleet nor its parent company Rosatom that oversees nuclear power had released any information about the incident since the initial reports came out.

National television has also avoided any mention of the episode.

The Norwegian-based Bellona Foundation that monitors nuclear safety issues in the region said the leak was the third such accident to strike Russian nuclear icebreakers in recent years.

A similar leak was reported on board the same vessel one year ago while the Arktika nuclear icebreaker had nuclear coolant problems a few years earlier, the Bellona Foundation said.

"But this is the first such situation to occur during the reactor's performance at sea," the group's report added.

The Taimyr is one of two nuclear icebreakers built for the Soviet Union by Finland in the late 1980s.

The US-based GlobalSecurity.org research organisation said the two Finnish ships were "originally designed for 100,000 hours of reactor life, but this was extended first to 150,000, then to 175,000."

Rosatomflot said local reporters will be invited on board the vessel on Wednesday to ensure that the vessel was safe.