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The UK’s Naval Nuclear Reactors: Ageing Ungracefully?

Commentary, 11 March 2014
Defence Policy, Maritime Forces, Technology, UK Defence
Worrying reactor test results have prompted the Ministry of Defence to schedule an unexpected reactor replacement for the UK’s oldest nuclear-armed submarine, at a cost of £120 million. If forensic tests exacerbate these concerns, the financial costs of reactor replacement may not be the UK’s only worry.

Worrying reactor test results have prompted the Ministry of Defence to schedule an unexpected reactor replacement for the UK’s oldest nuclear-armed submarine, at a cost of £120 million. If forensic tests exacerbate these concerns, the financial costs of reactor replacement may not be the UK’s only worry.


The reactors that power the UK’s fleet of nuclear-armed Vanguard-classsubmarines may be ageing faster than expected. The reactors, known as ‘core H’ pressurised water reactors (PWR-2), were retrofitted onto all four Vanguard submarines between 2002 and 2012 and were set to power each boat until their retirement from 2024 onwards. However, routine tests on the land-based prototype reactor in Dounreay suggest the reactor may not be able to live up to this expectation.

According to a statement from Secretary of State for Defence Philip Hammond, a ‘microscopic breach’ in the prototype’s fuel cladding allowed low-level radiation to leak into its sealed cooling circuit, which transfers heat from the reactor fuel to power-generating turbines. Although the reactor was swiftly deactivated and the radiation was confined to the primary circuit – therefore posing little health risk – a similar occurrence on a patrolling submarine would be far more serious; certainly pulling the submarine off-duty and endangering those confined within it.

In response, when the UK’s oldest submarine HMS Vanguard docks at Devonport for its next thirty-six month maintenance break in late 2015, its reactor will again be replaced, having completed just over half its planned service life. The Defence Secretary emphasised that this was a prudent decision, and that while there was no evidence that the reactor onboard HMS Vanguard would develop a similar fault, there would be little opportunity to fix one if it were to arise after 2015.  

However, it certainly was not a cheap decision. The core replacement will add approximately £120 million to the lifecycle costs of the boat. Furthermore, if this fault cannot be ruled out on the remaining three submarines, the process may have to be repeated again. To allow for this, a further £150 million in capital investment for reactor infrastructure at the Devonport shipyards and Rolls Royce reactor plant has been earmarked, bringing the total cost of this reactor face-lift to at least £270 million.

Old Headaches?

This is not the first time that safety fears have emerged regarding the core H PWR-2, a smaller version of which also powers the conventionally-armed Astute-class submarine. A MoD report released in 2011 revealed that the former head of the Defence Nuclear Safety regulator feared that UK submarine reactors ‘compare poorly’ to safety benchmarks established by civil nuclear power reactors. The report argued that core H PWR-2 sealed cooling circuits were less robust to ‘structural failures’ and therefore to loss of reactor coolant accidents.

Addressing such a systemic weakness would require significant investment and re-design, and is unlikely to have been addressed since 2011. It is possible that the combination of uncertain fuel rod reliability and less-than-ideal resilience to a loss of cooling accident compelled the government to schedule the upcoming core replacement. Indeed, the concerns identified in 2011 were a major factor in the decision later that same year to pursue a major evolution in reactor design (the PWR-3) for the next fleet of ballistic missile submarines.

It is not entirely clear whether the radiation leak detected at Dounreay reflects a similar systemic weakness in the core H PWR-2. The Shore Test Facility (STF) at Dounreay is purposefully run at a greater frequency and intensity to submarine-based reactors to identify any stress-related faults before they emerge at sea. As the Defence Secretary argued, the breach may have been the result of uncharacteristically intense usage, or a combination of factors, rather than simple ageing.

Nevertheless, when the core of HMS Vanguard is removed late next year engineers will be poring over it for any sign of distortion or corrosion of its fuel elements which could have led to a breach. If they are found, the next government may feel pressured to replace the cores of the three remaining submarines, which should have operated until their retirement between 2026 and 2031.

Working to a Tight Schedule

While it is prudent to replace the core of HMS Vanguard at the earliest opportunity, it is telling that this will only come up towards the end of 2015. The UK’s policy of maintaining seamless submarine patrols – known as continuous at-sea deterrence (CASD) – places very rigid restrictions on the maintenance and repair schedules of the fleet, and bringing a submarine into port earlier than anticipated would strain, or likely break, this chain of unbroken patrols. As such, there is a tension between maintaining continuous patrols and addressing faults as and when they emerge that can be seen as conflicting with the Ministry of Defence’s policy of keeping the radiation risk to employees as low as reasonably practicable (ALARP).

The Scottish Campaign for Nuclear Disarmament (CND) has already called for all core H nuclear submarines (including the conventionally-armed HMS Astute) to be pulled off duty until the fuel concerns are ‘properly understood’. This goes far beyond what is ‘reasonably practicable’ inside the government’s CASD policy and will fall on deaf ears, at least until the core of HMS Vanguard is removed for further examination.

However, if forensic analysis of Vanguard’s reactor after 2015 shows that the existing fleet of submarine-borne nuclear reactors are more vulnerable to age-related (or more worryingly, unexplained) disruptions than previously expected, the UK may have to revise what it considers ‘reasonably practicable’. In this case, the next government may be tempted to alter their deployment patterns, either by relaxing the demands placed upon reactors or scheduling repairs earlier than anticipated, before a broken reactor forces them to do so.


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