Two nuclear submarine bases in Scotland and England have suffered 11 significant safety incidents in the last five years, including human exposure to radiation, the Ministry of Defence (MoD) has revealed.

Radioactive waste has been spilled, workers exposed to radiation, power supplies lost, safety valves wrongly operated and a bag of waste mistakenly dropped overboard. Six of the incidents happened at the Faslane naval base on the Clyde, and five at Devonport in Plymouth.

The incidents have been admitted by UK defence minister, Philip Dunne, in response to a parliamentary question from Angus Robertson MP, the SNP's defence spokesman and Westminster leader.

"These answers from the MoD make for scary reading," Robertson told the Sunday Herald. "They concern the most serious types of incidents, and information about them wouldn't be made public without asking."

Although the MoD described what happened in ten instances, it refused to give details of one event at Devonport because "disclosure would be likely to prejudice the capability, effectiveness or security of the armed forces". This was alarming and unacceptable, Robertson said.

"Safety should be paramount at all nuclear bases … some of these incidents do not inspire confidence about what is going on there."

According to the MoD, six incidents since 2008 at Faslane have been defined as "category B". This is the second-worst rating, involving "actual or high potential for a contained release within building or submarine or unplanned exposure to radiation".

In 2008, valves on board a submarine were shut "in error", causing a loss of power. In 2009, there were two problems with cranes at Faslane being used more often than they should be without authorisation.

American investigative journalist Eric Schlosser has recently highlighted potential safety issues with the lifting of nuclear-armed Trident missiles with cranes. "I hope in Scotland that they're very careful when loading and unloading the missiles," he said.

In 2010, the melting of an ice plug caused by the failure of a liquid nitrogen supply resulted in radioactive coolant leaking into a submarine reactor compartment at Faslane. In the same year, a bag of potentially contaminated clothing fell overboard. Last year, maintenance workers entered an area next to a reactor compartment "without the proper radiological controls in place and hence received an unplanned exposure to a radiological dose," the MoD said.

The five incidents at Devonport included a spillage of reactor coolant "into the environment" in 2008, the operation of two submarines without key safety valves in 2010 and an overflowing radioactive waste tank in 2011. Last year, a submarine at berth lost external power for 90 minutes.

UK defence minister Philip Dunne maintained that all but one of the incidents - the 2010 safety valve problem at Devonport - were not serious enough to rate on the International Nuclear Event Scale used to grade nuclear accidents. Radioactive contamination had not caused a danger to the environment or human health, he said.

But John Ainslie, co-ordinator of the Scottish Campaign for Nuclear Disarmament, said: "This catalogue of safety errors shows there is always the risk that something can go catastrophically wrong so long as nuclear submarines remain in Scotland."