Convoys carrying nuclear bombs and hazardous radioactive materials by road through Scotland and across the UK have suffered 70 safety lapses in five-and-half-years, according to the Ministry of Defence.
A new log of incidents obtained from the MoD reveals vehicles have suddenly broken down, fuel has leaked, brakes have overheated, alarms have malfunctioned and many other vital systems have failed in convoys on the move between July 2007 and December 2012.
The convoys, which ferry Trident nuclear warheads to and from the Royal Naval Armaments Depot at Coulport on the Clyde, have also gone the wrong way, been delayed, been diverted and lost communications. Incidents have happened on average more than once a month, with by far the highest number - 23 - logged in 2012.
The revelations have drawn fierce criticisms from leading Scottish Nationalist and Labour politicians, as well as campaigners concerned that the convoys' cargoes pose unique and unacceptable dangers, but the MoD insists they are safe.
Perhaps the most serious incident occurred late in the afternoon of July 25, 2011, when a convoy command vehicle broke down on the northbound carriageway of the M6 near junction 20 in Cheshire.
The commander's official report of the incident, released with large sections of text blacked out by the MoD, gave a vivid description. The vehicle "suffered a sudden and dramatic loss of power and was forced to pull onto the hard shoulder of the motorway together with the rest of the convoy assets", he wrote. Nuclear warhead convoys can include up to 20 vehicles.
This blocked the busy road and, according to a truckers' website, closed two lanes and caused 10-mile tailbacks. The MoD said the vehicle had suffered a "fuel system failure" that turned out to be a manufacturing fault which had to be rectified across the whole fleet.
During a convoy trip in January 2012, five incidents were reported by the MoD, including a "fuse-box failure" and "security system air leak" on the heavy-duty nuclear warhead carrier; a "fire tender brake fault"; and "reduced braking" on a command vehicle. The gun port flap of an escort vehicle also "opened inadvertently".
A June 2012 convoy ran into problems after it was halted because of a "suspension system defect" in an armoured escort vehicle. "During an unplanned stop to investigate above incident," the MoD reported, "a manhole cover collapsed under a further escort vehicle."
In January 2009, a nuclear warhead carrier suffered an "unrepairable" fuse-box failure, meaning a spare truck had to be used. In November 2010, the spare truck itself suffered an "unspecified break down".
During a rest break in April 2008, a fuel leak was discovered from an escort vehicle as well as an oil leak from a warhead carrier. An escort vehicle's brakes reportedly overheated in September 2008.
In July 2010, a convoy strayed off route due to a "commander error". The convoy was delayed by 45 minutes until it found an approved route to bring it back on course.
In March 2012, the convoy had to be diverted because of the "proximity of low-flying at MoD establishment". According to the MoD log, it was often diverted or delayed because of bad weather, traffic congestion, road works or accidents.
Computer software also had to be upgraded after four false alarms wrongly suggested the warhead carrier was overheating. In 2010 and 2011, the convoy's blue lights, speed sensors, sirens and warning lights failed 11 times.
The locations of most of the incidents have not been disclosed by the MoD, but many will have been on the regular convoys that carry nuclear weapons for maintenance between Coulport and the bomb plants at Aldermaston and Burghfield in Berkshire. The convoys have been seen travelling through Glasgow twice this year, on January 29 and early on July 11.
As well as the M6 and the M74, convoys have used eastern routes including the A1, the A68 and the M9. Other cargoes transported around England as part of the UK's nuclear weapons and submarine programme include plutonium, highly enriched uranium and tritium.
Details of the nuclear convoy mishaps, divided into 56 "engineering incidents" and 14 "operational incidents", were released to the monitoring and campaign group, Nukewatch, in response to Freedom of Information requests. Details of a further 67 safety incidents during nuclear convoys between January 2000 and June 2007 were previously provided to the Sunday Herald.
"Some of the safety incidents on the MoD's list were relatively serious and, had bad luck caused events to play out in a different way, could have resulted in harm to motorists, or the convoy crew, or damage to the deadly cargoes," said Jane Tallents from Nukewatch.
"It only takes a moment's thought to see that, far from being a benign insurance policy which keeps the public safe, nuclear weapons actually increase the risks that we all face. The MoD should not be moving nuclear weapons around the country if it can't guarantee to do so safely."
The SNP's Westminster leader and defence spokesman, Angus Robertson MP, angrily condemned the MoD's safety record. "Any one of these incidents should be of huge concern - a catalogue of 70 is utterly unacceptable," he said.
"In the same month we find out that nuclear bombs trundled through Scotland's biggest city under cover of darkness, it is revealed that previous convoys have actually gotten lost, suddenly lost power, suffered brake failures and breakdowns."
He added: "It is dangerous to pull over on any busy road, but unquestionably more dangerous if you are transporting nuclear warheads. If the MoD cannot read a map properly or do basic maintenance, I have virtually no faith that it could respond in a serious emergency."
Glasgow Labour councillor and former MSP Bill Butler, who convenes Scotland's nuclear-free group of local authorities, welcomed the Sunday Herald's revelations. He also highlighted the 20-vehicle nuclear warhead convoy seen going through Glasgow two weeks ago.
"I shudder to think what would have happened if this convoy had been involved in a serious traffic accident or a malicious incident. We now know that convoys are regularly involved in incidents that could easily have become more serious," he said.
"I urge the MoD to improve its safety record, talk to council emergency planning officers more regularly and reduce the number of convoys running through Scotland. It is another reason why we need to get rid of nuclear weapons."
A major emergency exercise run by the MoD and other public agencies in September 2011 imagined a nuclear convoy becoming embroiled in a "catastrophic" pile-up on the M74 at Bellshill, near Glasgow. The scenario envisaged a warhead carrier overturning, catching fire, leaking plutonium and uranium, killing two people and contaminating 100 with radioactivity.
In July 2005, the Sunday Herald revealed an internal MoD report warning nuclear warheads could accidentally explode if involved in a major crash. A bomb's key safety feature could be disabled, enabling a nuclear reaction to unleash a burst of lethal radiation, termed an "inadvertent yield" by the MoD.
But last week the MoD denied that public safety was under threat from nuclear convoys. "The safety of the public is our priority during any movements of our nuclear material, and we monitor all convoys closely," said an MoD spokeswoman.
"Any incidents, however minor, are recorded and investigated thoroughly and action is taken where appropriate. At no point has the public been put at risk and our nuclear convoys continue to operate safely accompanied at all times by military police."