"One of our nuclear warheads is missing" - The 1971 THROSK Incident

On a dark February day in early 1971 off the coast of Plymouth, there was rising concern about the fact that the unthinkable had happened. The small elderly Naval Armaments Vessel (NAV) THROSK, on passage to the Royal Navy Armaments Depot Coulport, in Scotland, and carrying Polaris nuclear warhead sub-assemblies, had vanished. Warships put to sea, and helicopters from RNAS Culdrose were launched in a desperate search operation to find her. Despite this, no sign could be found. At 0824 that morning, Flag Officer Plymouth sent a SECRET signal initiating the procedures for a possible ‘Nuclear Weapon Accident’ and the Royal Navy prepared for the previously unthinkable reality that one its strategic nuclear warheads was missing…

This sounds the stuff of good Cold War fiction, or a scenario from a disaster planning exercise. In fact it is completely true and represents an all but forgotten story of the early days of the Polaris force and the challenges faced by the Royal Navy as it set up the continuous at sea deterrent (OP RELENTLESS) in moving strategic nuclear warheads for the SSBN force from the south of the England to Scotland, where the facilities in the Faslane area housed the submarines, missiles and warheads to be sent on patrol. This article sheds light on a previously unknown part of this story and brings to life some of the challenges faced during this period.

Copyright MOD

In the early days of the British nuclear weapons programme, nuclear warheads were air launched (either as free fall bombs or via stand off missiles) and intended for delivery via the V Force of Valiant, Vulcan and Victor bombers as well as tactical delivery through the Canberra force and Thor missiles using US supplied warheads.  In the event of a war, should nuclear retaliation have been ordered by the Prime Minister then the V Force would have been scrambled to launch attacks on the Soviet Union as part of a wider plan with Strategic Air Command, with RAF bombers playing a vital role in delivering the ‘SIOP’. During this period, which lasted until the late 1960s, nuclear weapons were road moved around the UK from the main Atomic Weapon Establishment sites in Aldermaston and Burghfield to the RAF bases around the UK where the V force operated from.

The decision to move British strategic nuclear forces to sea as part of a submarine based deterrent meant that change was required. Under the terms of the deal between the US and UK, the US Navy would provide the Royal Navy with Polaris missiles, while the UK would provide the nationally designed and produced nuclear warheads to go on the missile. The Royal Navy is believed to have deployed three warheads per missile in the early days, dropping to two warheads in the 1970s and 80s as the Chevaline upgrade programme was introduced to service. With four submarines, this generated a theoretical requirement of around 192 warheads to fit the fleet out (although in reality it is unlikely that this was ever achieved).  By introducing Polaris to service, the Royal Navy had to overcome two specific challenges. Firstly, how to get the missiles to the UK in the first place, and secondly, how to get the warheads from AWE facilities in the south to Scotland.

The first question is one that is often forgotten – although the US sold Polaris to the UK, it still needed to get there and have the UK warheads attached to it. This could only realistically be done by a journey at sea, and for that purpose in 1967 the elderly Royal Fleet Auxiliary FORT LANGLEY was converted into a missile carrier. This ship, completed in 1944 and of the same design as the venerable RFA RAME HEAD, for many years a feature of Portsmouth harbour where she was used for special forces training, was used by the RFA to ferry ammunition around the world where it was needed. Having served in both the Korean War and for many years as part of the Far East Fleet, she was the ideal low profile ammunition carrier to transport Polaris missiles around the world.

To that end, in 1967 she underwent conversion at Chatham Dockyard to be able to transport Polaris missiles from the US to the UK. In this role she carried out several voyages between Coulport and Charleston South Carolina, a major US naval base, and one that over many decades became closely linked to the Royal Navy. Indeed at the magnificent SSBN memorial in  the city, there are plaques dedicated to the RN Submarine Service and a reminder that for many years, RN personnel were permanently based there. Over the course of around five voyages, RFA FORT LANGLEY sailed the Atlantic, picking up multiple Polaris missiles loading them into her holds and then sailing to Scotland where they were taken under the charge of the RNAD Coulport. It is not an exaggeration to say that without the RFA, the RN would not have been able to mount OP RELENTLESS.

RFA FORT LANGLEY - Copyright unknown

But even with the missiles safely in the UK, they remained useless without their warheads, and those weren’t being supplied by the US. Although the UK did use some US derived ‘dual key’ weapons for the tactical elements of the armed forces, it was clear that Polaris had to be a British warhead, albeit mounted on a US missile. This in turn posed a challenge – how do you move the warheads from the south of the UK all the way up to Scotland safely and with minimal risk?

The requirement was higher than may first be imagined – given the scale of the build up of the Polaris force and the need to regularly deliver warheads to Coulport, the ask was considerable. According to a 1968 MOD paper, the requirement was for: “a delivery to be made once or twice per month to and from RNAD Coulport”

The first option appears to have been to look at doing road moves from Burghfield to Portsmouth or Portland, where the warheads would have been loaded onto the FORT LANGLEY. The operation involving the FORT LANGLEY was known as OP CADDIS HEAD, although no records can be found about it in the National Archives.

The challenge with this option was that she was, particularly during the late 1960s, employed on the transatlantic shipping of Polaris missiles, meaning she wasn’t available to help. It was agreed that:

“In 1967 Board approval was given to the transport of sub-assemblies to be undertaken by road to Portland and thence by sea in RFA FORT LANGLEY with a Royal Marines armed guard embarked.

If this didn’t work then the next option considered was the ‘road move’ – essentially send the warheads up by convoy to Scotland as required. The problem is that this wasn’t seen as being a particularly safe option. A paper written in 1969 noted that:

“Road transport must in case be used for Polaris warheads between the factory and port of embarkation and regularly for other nuclear warheads being used by the RN and RAF between inland storage points and ports and operational airfields. It is therefore not suggested that road transport should be dropped but the hazards from such transport be recognised’…”

The problem with using road transportation is that in the 1960s road safety was not as advanced as it is today (e.g. over 6000 people were killed in road accidents in the UK, 88,000 seriously injured and over 253,000 injured in 1968 alone, compared to 1,633 fatalities, 29,000 seriously and 133,000 injured in 2023, despite there being 10m cars in the UK in 1968 and 41m in 2023). Proportionately the roads were not a good place to be moving nuclear weapons securely, even before you consider the challenges of the weather. The same report went on to note that:

“In February 1969 a road convoy (carrying nuclear warheads) was unexpectedly snowbound on the Pennines for 3 days; the risk in icy conditions needs no stressing. In April 1969 an oncoming vehicle which went out of control as the result of a dangerous overtaking by a private motorist collided with and badly damaged a load carrier in an RN convoy. Although routes are changed regularly, main roads must be used because of the size of the load carriers and increasing congestion in the summer is causing DGST (N) considerable anxiety. Under heavy traffic the vehicles making up a convoy are often separated from each other by rows of other road users. Although RN road convoys are civilianised to attract as little attention as possible, they cannot avoid it entirely in the way that a sea-going vessel can. The risks of the road can be, and are reduced as much as possible by planning and choice of the right weather conditions, but some difficulties are inherent in road transport”.

Its worth considering for a moment what this paper was suggesting. Imagine for a moment that you are in 1969 driving a convoy containing nuclear warheads intended to be used to destroy Moscow, travelling via the Pennines and then find yourself snowbound high on the Pennine hills for 3 days. That’s a fairly impressive (if mildly concerning) course of events that seems like a great basis for a cold war thriller. On the plus side, if they’d stayed warm, they could have cracked open the crate and tried to light a fire – doesn’t the old saying go “build a man a fire on a nuclear warhead, and stay warm for the rest of his very short life”?  

Of perhaps more concern was the idea that convoys of nuclear warheads were travelling around the UK, broken up by traffic and disrupted by dangerous civilian driving. The fact that a vehicle which may or may not have been carrying a nuclear warhead was hit in a collision due to separate overtaking suggests that the risk to nuclear warheads in those early years may have been far greater than publicly realised.  Given this, it is perhaps no surprise that the Royal Navy wanted to consider a further contingency option as a final fall-back option.

NAV THROSK - Copyright unknown

The decision was taken to use the elderly Naval Armaments Vessel ‘THROSK’ to meet the task as a contingency. THROSK was built in Dartmouth, on the south coast of England in 1943 to carry munitions between various naval dockyards and depots. In the mid 1950s she was converted to support the work carried out by HMS GIRDLE NESS, the first RN vessel to fire missiles. She was typical of the myriad of small vessels built during the war and post war years to support the more glamorous ‘grey hulls’ of the main fleet. Working for many decades without glory or glamour, the THROSK was, in theory, the ideal platform to convert in the background to carry Polaris warheads when required.

The proposals to use THROSK for this role seem to have initially dated back to around 1967 as a contingency, with the idea being she would conduct the mission with a discrete naval escort (usually a Coastal Mine Sweeper – TON class or similar) would be used to provide a reassuring naval presence to deter problems.  The problem though was that OP BASCOTE was unlikely to be a planned evolution, being intended as a final back up for when FORT LANGLEY was busy and the weather was too bad for road moves. It was very much a ‘last ditch’ which would be called at the last minute when it was clear no other option was available. The problem for the Royal Navy though was in finding an escort ship at this notice to work with her – TON class hulls were busy enough as it was, and retasking one at short notice for escort duties would not always be possible.

This meant that the RN and MOD in the ‘Nuclear Weapons Safety Committee’ were concerned though that  this may not be enough protection for a strategic nuclear warhead in transit, and asked for an analysis of the likely security threats facing THROSK on this mission (known as OP BASCOTE).  The Security Service (more popularly known as MI5) was asked to produce an assessment of the threats to the warhead. Their findings were that the threat stemmed from three different vectors:

“Threat from UK Subversive Organisations: There is no evidence of such a threat. To mount a threat an organisation would need considerable notice of the route and timing, and considerable advance publicity to alert their supporters. There is always the possibility of a local demonstration by ‘canoe-born’ demonstrators or of a demonstration with the primary objective of handing leaflets to crew members and to preach to them about the wickedness of their work.

Threat From Foreign Powers: Russian and Communist intelligence services would include RFA FORT LANGLEY and THROSK, and their crews among their targets should they discover the Polaris use they are put to; but there is no indication of the Russians using their trawlers for direct action against these vessels….

Conclusion: Although the threats of interference, whether directed by British subversive organisations or foreign powers, are small, the possibility of interference cannot completely be ruled out. It would therefore seem prudent that any vessel carrying nuclear weapons has a Royal Marine guard and adequate ship to shore communications.”

To mitigate these threats then the decision was taken to embark a small armed party of Royal Marines comprising an Officer, NCO and four men as well as two RN communications ratings, whenever Polaris warheads were embarked. This would provide a level of assurance and ‘final denial’ capability to prevent the warhead being taken. Ever the stickler for protocol though, the RN noted that to embark the RM Party, an austere deckhouse would need to be constructed for the detachment, with the cabin previously used for the RN comms ratings passed to the Officer Commanding for his use! There was some financial arguments in MOD over the cost, noting that the deckhouse would cost £2,500 (£35,000 today), which led to push back over the value of investment in a ship with barely 3 years of life remaining.

While this analysis and debate on the size of the protection party was being undertaken, wider events increased the importance of the THROSK. the decision was taken in 1969 to pay off the FORT LANGLEY completely, meaning that THROSK found herself as the sole vessel in the UK intended for use to carry Polaris warheads to Coulport. This led to serious concerns in the MOD, which had previously seen THROSK purely as a last resort, and an analysis on her suitability to carry out the role on a full-time basis. This concluded that the risks facing THROSK were unchanged, but serious:

“It can reasonably be argued that the risks of piracy, which was very much in the minds of the Nuclear Weapons Safety Committee in their 1967 discussion is met by carrying a Royal Marines armed guard on THROSK… The principal danger to safety is accordingly that represented by collision or fire or engine failure occurring simultaneously with bad weather. In such circumstances either the vessel might founder and the cargo sink, or THROSK might be stranded on either the UK or Irish coast. In either of these eventualities serious embarrassment would result; but it would be magnified if a foreign government were involved, most particularly if there were some release of radioactive material. The possibility of an accident at sea involving nuclear weapons is of course a risk faced inevitably in the case of SSBNs if the deterrent is to operate…  In addition, there are contingency plans in existence for the provision of immediate assistance by specially trained teams, including deep sea divers, in the event of a nuclear accident wherever it occurs”.

It is quite astounding to read this memo and realise that the UK was genuinely concerned at the risks of an elderly armaments vessel carrying a Polaris warhead being washed ashore on the coast of Ireland in the 1960s, potentially causing a nuclear accident. Yet despite this, the use of THROSK was deemed an acceptable risk given the mitigations put in place, involving clear requirements to the Master of the vessel to put into harbour if weather changed. It was generated through a special weather forecast for the THROSK from Northwood, sent every 3 hours – a good reminder of the critical importance of the Royal Navy Met Branch in providing information of strategic importance to nuclear operations. With these precautions in place, the RN director of transport felt obliged to state that:

“DGST(N) wishes to use sea transport regularly in future since he regards road conditions as increasingly difficult. The risks involved in sea transport with an RM guard but without escorting vessel are slight,  but it does involve, which road transport does not, the remote risk of serious embarrassment in our relations with the Irish Republic should an accident occur. However this eventuality has already been implicitly accepted in the authorisation for occasional use of THROSK”

It was clear though that not everyone in the Royal Navy agreed with the decision to use THROSK for this role. In a stinging rebuke to the memo setting out the case for using her over road transport, the Director of Naval Warfare recommended warning the Committee on Nuclear Weapons safety that:

“Despite DNOTs minute… DNW considers that the following should be brought to the attention of the Board since it represents a change of circumstances since the Board approved the arrangements for the transportation of Polaris warheads in December 1967:

“The small and elderly THROSK, originally the third choice backup to RFA FORT LANGLEY and Road transportation is now to be the main method of transport without an escort ship.”

In DNWs view this materially increases the danger of a collision or serious fire with consequent loss of warheads, which should be brought to the Boards attention”.

FPGRM at sea in 2020 - Crown Copyright

Despite these concerns, THROSK was pressed into use in this role and seemed to carry it out to reasonable effect. On Tue 09 February 1971 THROSK embarked on passage, with an RM party onboard, from Portsmouth to Coulport, carrying a cargo of Polaris warheads. Her orders were to report in every two hours, reporting her position to provide assurance of her location. Unfortunately she did not check in at either 0400 or 0600 hours, leading to Flag Officer Plymouths staff to raise serious concerns that an incident had occurred onboard.

This resulted in orders being given to both HMS MONCKTON and helicopters from Culdrose to carry out a search for her, and at 0824 a ‘Nuclear Weapons Accident’ signal was released from FO Plymouth, which would have put in chain a series of response measures. Thankfully before the operation ramped up too quickly, helicopters from Culdrose located THROSK and she was escorted into Plymouth naval base by HMS MONCKTON.

Further investigation highlighted that there had been a communications failure onboard and that the ships Master had misunderstood his instructions on what to do when this occurred. The vessel was repaired and found to be in no danger and allowed to proceed on passage to Coulport.  Thus ended the ‘accident that never was’ scenario, but it highlighted the risks and challenges of using an elderly vessel at sea in this way, as well as the challenges of communications.

The THROSK continued in this role for several more years, although it is not possible to work out how many journeys she made to Coulport during this time. She was paid off in 1977, being sent for scrap and eventually being scrapped in the early 1980s. She appears to have been replaced in this role by a purpose built series of ammunition vessels operated by the Royal Maritime Auxiliary Service (RMAS), including RMAS THROSK and KINTERBURY, which were according to internet reports, used to occasionally transport nuclear warheads on the same routes during the 1980s and into the 1990s.  The KINTERBURY was a much larger vessel, displacing over 2000 tonnes and remained in service until around 2006. Its often forgotten just how many extremely capable RMAS vessels existed up until the mid 2000s, and which were used in a huge variety of ways from harbour tugs to deep sea rescue vessels like TYPHOON or the more specialist trials vessels like NEWTON. Although Serco Denholm continues to carry out this work, it is far reduced in scale and size from previous decades.

Its not clear when the RMAS ceased to carry out coastal moves of nuclear weapons – it was probably around the time that Polaris left service. Its successor Trident does not have missiles based in the UK, instead they are maintained in the USA through a joint UK/US agreement, and loaded onto the SSBN in the US prior to having the warheads loaded in the UK at Coulport, in the unique floating dock on site. What is likely is that this function ceased around 30 years ago, with all nuclear warhead moves instead defaulting to road convoys. The modern nuclear weapons convoy is a very different beast to its 1960s predecessor – the author drove past one on the M4 a few weeks prior to writing this, and the sheer scale of the move, including recovery trucks, fire engines, multiple heavily armed police vehicles and other transporters made clear that the vehicles would not be separated by traffic in the same way as the 1960s.

Although the incident is well over 50 years ago, it is worth remembering the importance of the THROSK and the FORT LANGLEY in the wider nuclear story. They pioneered transport of nuclear warheads and missiles at sea, and helped establish the nascent role of the Royal Marines in protecting nuclear warheads at sea. This work continues to this day, through the work of 43 Commando Fleet Protection Group, a critical node in the Royal Marines force, and which plays a vital role in securing British nuclear weapons from harm.

The final part of this story is that it also played host to a novel by the renowned journalist Chapman Pincher. He became aware of the story around the THROSK incident at some point in the early 1970s and used it to form the basis of a novel called ‘The Eye of the Tornado’ around a terrorist movement stealing a nuclear weapon from a ship very much like THROSK to establish a Soviet backed government in the UK.  Given the concerns raised by MI5 about security and the likely risks to THROSK during this period, its not only a surprisingly plausible scenario but also one that feels incredibly pertinent still given the wider use of sabotage in France to coincide with the Olympics. Sometimes truth is even stranger than fiction.

The THROSK incident marks a nearly forgotten part of Royal Navy and RFA history, but it is still important to this day. It’s a reminder of the challenges faced in getting a credible nuclear deterrent to work, and in turn the many steps required to ensure that warheads can be in the right place at the right time. Understanding the risk balance is challenging and at times decisions needed to be taken that with hindsight could sound mildly reckless to some people, but which made sense at the time. It’s important that these lessons are not forgotten and that they are remembered as work begins on the next generation of nuclear warheads to continue CASD into the 2050s and beyond…