Many people have wondered why the Queen Elizabeth Class aircraft carriers do not have nuclear propulsion like the US Navy’s Nimitz class ships. Here we consider the many good reasons why a conventional, although innovative propulsion system was selected instead.
Range and Replenishment
The primary advantage of a marine nuclear power plant is the unlimited range and available power it provides the ship. This range and power would be desirable for the QEC, but the costs and other factors largely outweigh these benefit. USN carriers are a few knots faster than the QEC, the speed of the ship can generate more wind over the deck to help heavily laden aircraft take off. This wind is less critical for QEC’s ski-ramp launched VSTOL aircraft.
When on operations, the ship’s air group will consume considerable amounts of aviation fuel. Even if the ship is nuclear-powered, she must be accompanied by a tanker to conduct RAS (Replenishment At Sea) at frequent intervals. If you have to conduct RAS with an auxiliary tanker anyway, it is not a big effort to refuel the ship at the same time. The escorts ships that will nearly always accompany the carrier are also conventionally-powered so nuclear powered carrier does not eliminate the need for RAS. The 4 Tide class tankers that will soon be joining the RFA can replenish the QEC with aviation fuel and diesel simultaneously, using rigs plugged into receiving points on the carrier’s port side. The US Navy has to operate in the Pacific where distances can be huge. Nuclear propulsion may make more sense in the vast Pacific but how frequently will the QEC be deployed over huge distances where there are no refuelling opportunities?
Nuclear reactors cannot be quickly re-started from cold. A careful sequence of procedures is required to start the reactor and the steam plant to prevent heat stress from damaging the system. Since the 1970s when the RN began to move away from steam propulsion, it has enjoyed the benefit of gas turbine and diesel ships that can be started and shut down very quickly. This is a useful tactical advantage allowing quick departure from harbour when needed. It also has a lower manpower requirement as nuclear plants require constant monitoring, even when shut down.
Cats and steam
Except for the newly commissioned USS Gerald R Ford, US Navy carriers use steam-powered catapults to launch aircraft. Nuclear propulsion has the advantage of providing plenty of steam for the catapults. The older generation of RN carriers had steam turbine propulsion and their boilers also provided steam for the catapults. If the QEC had been fitted with catapults and traps, the intention was to adopt the Electro-Magnetic Aircraft Launch System (EMALS) that has been developed in the US for the Ford class carriers. Considerable electrical power is needed but this system does not require steam, another reason that nuclear power is less critical. (Although EMALS offers a great leap in aircraft launching capability, the US Navy is struggling with developmental issues and the USS Ford may not be ready for combat operations until 2022 at the earliest). It is possible the QEC may one day be retrofitted with EMALS (Probably not in the next 20 years) but there is sufficient spare electrical generation capacity already available.
Building capacity and capability
The only facility in the UK building nuclear-powered vessels is BAE Systems yard at Barrow. (The reactor cores are built by Rolls Royce in Derby) The Barrow site has been running at full capacity for the last 10 years constructing the Astute class attack submarines and that will continue into the future as work starts on the 4 Dreadnought class ballistic missile submarines. Barrow is probably the only UK site with the skills and experience to build an aircraft carrier’s nuclear propulsion section, although their workforce is very much focused on submarines. Even if Barrow could have fitted such work into its schedule, the hull sections containing nuclear power plants would have had to be transported by barge to Rosyth for the assembly – a potentially hazardous journey.
The UK has considerable experience building nuclear submarines but has never constructed a nuclear-powered surface ship. Theoretically, the PWR2 nuclear power plant fitted to the Vanguard and Astute class submarines could have been up-rated and adapted for use in the carrier. It would have required at least three PWR2 plants per ship, each of which generate around 27,500 shaft horse power. (The propulsion system of the QEC, as built today, can generate 100,000 shp). There would still have been considerable cost and complications in adapting the submarine plant and associated gearing and shaft arrangements for the ship. Alternatively, at great expense, an entirely new and more powerful reactor could have been developed specifically for the carriers.
Avoiding the French experience
France completed their single nuclear-powered carrier, the Charles de Gaulle in 2001 but she took more than 11 years to build (QE took 8 years) The de Gaulle was delivered 5 years late, expensive and beset by technical problems with her propulsion. The K15 nuclear reactor design, derived from existing submarines, proved underpowered and inadequate shielding exposed crew members to doses of radiation that exceed regulations. France is already studying options for replacing the de Gaulle, a ship that has spent more time in dock than operational.
Nuclear engineers in short supply
France struggled to build the de Gaulle, despite having a much larger nuclear industry than the UK, more nuclear scientists, engineers and technicians. The design and construction of two British nuclear carriers would probably have required expensively imported nuclear expertise from France or the US. The RN is already hard-pressed to find sufficient qualified personnel to man its existing fleet of 10 nuclear-powered submarines. In the current climate, there would be another struggle to recruit and retain more nuclear watch-keepers for two aircraft carriers.
Attractive conventional options
Despite the unhappy experience with the Type 45 destroyers propulsion, advances in marine engine technology make conventional power attractive. Accumulated experience, extensive testing and the selection of proven low-risk engines will ensure that QEC is very unlikely to have similar problems. The CODLAG (combined diesel-electric and gas turbine) arrangement adopted by the QEC is both efficient, reliable and allows great design flexibility. By using electric motors to drive the propellers, the diesel and gas turbine generators can be placed where convenient, rather than having to sit on the shaft line, as in traditional designs. The gas turbines are actually sited inside the sponsons on the starboard side of the ship with their exhaust uptakes going through the two island structures immediately above. The saves internal volume, allowing for bigger hangars. Nuclear power obviously removes the need for uptakes and funnels entirely but modern gas turbines and electric motors have a very much higher power-to-weight ratio than a heavy nuclear plant with lead shielding and reduction gears. Servicing simple diesel engines is an entirely different prospect to maintaining a nuclear plant. The QE’s gas turbines are also easily accessible and can be replaced with new units in a matter of days if required.
The PWR2 reactor was designed not to need refuelling and to have a life of around 30 years. Unfortunately, HMS Vanguard’s reactor has required refuelling after 22 years service at considerable expense because it appears her PWR2 core H may not last as long as expected. Since the QEC have a design life of up to 50 years, if nuclear-powered they would need a mid-life reactor refuelling refit. Even if a reactor can be made to last for that long, refuelling is a colossally expensive process and could take the carrier out of commission for at 3 years.
The lifetime cost of a nuclear-powered vessel is much higher than that of a conventionally powered vessel. Initial construction outlay is also greater because of the physical complexity and regulatory framework that the builders would have to work within. The bill for filling up QE with diesel fuel runs into hundreds of thousands of pounds each time, but the total cost of installing, maintaining and disposing of a nuclear power plant over the life of the ship, would far exceed that of the fuel. Disposal of nuclear vessels also presents a significant problem. The US Navy benefits from a dedicated nuclear vessel disposal facility in Puget Sound and the waste is stored away from populated areas in Idaho, deep in the vast land mass of America. Rather embarrassingly, the UK has yet to completely dispose of a single decommissioned nuclear submarine, although tentative steps to start this process have finally been made. Disposal is slow, costly and the storage of nuclear waste is controversial. A nuclear-powered carrier would one day create another expensive decommissioning headache.
Some nations will not allow a nuclear-powered or nuclear-armed vessel within their territorial waters. This would not be a particular problem but highlights the additional political baggage and sensitivities that come with nuclear vessels. Aircraft carriers are high profile ships and are intended as a tool for trade and diplomacy in a way that secretive nuclear submarines actively avoid. There is a strong anti-nuclear movement and a nuclear-powered QEC would inevitably attract unwanted controversy and protest. In the very unlikely event the ship was sunk or damaged by enemy action or in a serious collision, the presence of a nuclear reactor presents a long-term environmental hazard that could leak radiation into the sea. While this calculated risk makes sense for submarines where there is no alternative source of air-independent propulsion that can provide the required performance, for a surface ship this is an avoidable risk.
Think of the paperwork…
Sensibly, all British nuclear submarines and facilities are subject to strict regulation and inspection. There is no avoiding this complication that would add further manpower, cost and security overheads to an already complicated aircraft carrier programme. Nuclear-powered submarines have to make use of specially prepared “Z berths” when alongside in the UK. These berths have to be certified as safe and secure and the local authority is required to have plans in place in the very unlikely case of a nuclear accident. Although her size prevents her from entering many ports, a nuclear-powered QEC would be further restricted as to where she could dock, both in the UK and abroad. Maintenance of nuclear vessels in the UK can only be conducted at certified “X berths”. Currently, they are only available in Devonport and Faslane. As Portsmouth-based ships, there would be further considerable investment required for nuclear-certified infrastructure at Portsmouth.
The selection of conventional propulsion for the QEC will undoubtedly prove to be the correct choice over the lifetime of the ships and maybe a big factor in ensuring they are affordable to operate and remain in service for many decades. It is difficult to imagine a scenario where a future aircraft carrier CO will be wishing his vessel was nuclear-powered.
- Powering the Queen Elizabeth class (Rolls Royce PDF)
- How to Control 50,000 Shaft Horsepower (G Captain)