jeffb
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Interesting read:
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Nuclear-Powered Ships - World Nuclear Association
Over 160 ships are powered by more than 200 small nuclear reactors. Most are submarines, but they range from icebreakers to aircraft carriers. In future, constraints on fossil fuel use in transport may bring marine nuclear propulsion into more widespread use.
world-nuclear.org
Naval reactors (with the exception of the ill-fated Russian Alfa class described below) have been pressurised water types, which differ from commercial reactors producing electricity in that:
* An IAEA Tecdoc reports discharge assay of early submarine used fuel reprocessed at Mayak being 17% U-235.
- They deliver a lot of power from a very small volume and therefore most run on highly-enriched uranium (>20% U-235, originally c 97% but apparently now 93% in latest US submarines, c 20-25% in some western vessels, 20% in the first and second generation Russian reactors (1957-81)*, then 21% to 45% in 3rd generation Russian units (40% in India's Arihant). Newer French reactors run on low-enriched fuel.
- The fuel is not UO2 but a uranium-zirconium or uranium-aluminium alloy (c15%U with 93% enrichment, or more U with less – eg 20% – U-235) or a metal-ceramic (Kursk: U-Al zoned 20-45% enriched, clad in zircaloy, with c 200kg U-235 in each 200 MW core).
- They have long core lives, so that refuelling is needed only after 10 or more years, and new cores are designed to last 50 years in carriers and 30-40 years (over 1.5 million kilometres) in most submarines, albeit with much lower capacity factors than a nuclear power plant (<30%).
- The design allows for a compact pressure vessel with internal neutron and gamma shield. The Sevmorput pressure vessel for a relatively large marine reactor is 4.6 m high and 1.8 m diameter, enclosing a core 1 m high and 1.2 m diameter.
- Thermal efficiency is less than in civil nuclear power plants due to the need for flexible power output, and space constraints for the steam system.
- There is no soluble boron used in naval reactors (at least US ones) but boron may be a burnable neutron poison in the fuel.
- A submarine reactor is required to withstand the shock and vibration experienced by all warships in active service due to ocean turbulence and enemy action.
The long core life is enabled by the relatively high enrichment of the uranium and by incorporating a 'burnable poison' such as gadolinium – which is progressively depleted as fission products and actinides accumulate and fissile material is used up. These accumulating poisons and fissile reduction would normally cause reduced fuel efficiency, but the two effects cancel one another out.
However, the enrichment level for newer French naval fuel has been dropped to 7.5% U-235, the fuel being known as 'Caramel', originally developed for research reactors and providing the possibility for greater fuel density, so helping to minimize the increased size of an LEU-fuelled core. It needs to be changed every ten years or so, but avoids the need for a specific military enrichment line, and some reactors will be smaller versions of those on the Charles de Gaulle. In 2006 the Defence Ministry announced that Barracuda class submarines would use fuel with "civilian enrichment, identical to that of EdF power plants," about 5% enriched, and certainly marks a major change there.
Long-term integrity of the compact reactor pressure vessel is maintained by providing an internal neutron shield. (This is in contrast to early Soviet civil PWR designs where embrittlement occurs due to neutron bombardment of a very narrow pressure vessel.)
The Russian, US, and British navies rely on steam turbine propulsion, the French and Chinese in submarines use the turbine to generate electricity for propulsion.
Russian ballistic missile submarines as well as all surface ships since the Enterprise are powered by two reactors. Other submarines (except some Russian attack subs) are powered by one. A new Russian test-bed submarine is diesel-powered but has a very small nuclear reactor for auxiliary power.
The smaller US Virginia-class SSN submarine first commissioned in 2004 has has a S9G reactor of about 210 MW driving a 30 MW pump-jet propulsion system built by BAE Systems (originally for the Royal Navy). The reactor does not need refuelling for the 33-year service life and can operate with convection circulation without pumps. The vessels are about 7900 dwt submerged, and 19 were in operation by mid-2021, with more being built – a total of 28 from initial contracts. In 2019 ten larger Block V versions (25 m longer, 10,800 dwt) were ordered for delivery 2025-29, costing $22.2 billion for the first nine. These are effectively a new class.
The 14 US Ohio-class SSBNs (and four converted to SSGNs for guided missiles) have a single S8G nuclear reactor of 220 MWt delivering 45 MW shaft power. These require mid-life refuelling at about 25 years. The 12 slightly larger Columbia class to replace these will require no refuelling, hence shorter mid-life maintenance (2 years instead of 4). They will have an S1B nuclear reactor with electric drive (without reduction gears) and pump jet propulsion. They have been developed in collaboration with the UK, which will deploy them as Dreadnought-class SSBNs.
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The Rolls-Royce PWR1 of about 78 MWt was used to power the first 23 British nuclear submarines. It was based on the Westinghouse S5W reactor, one of which was provided by the US Navy in 1958 under a mutual defence agreement. The PWR1 with high-enriched fuel required refuelling every ten years or so. British Vanguard-class ballistic missile submarines (SSBNs) of 15,900 dwt submerged have a single PWR2 reactor with two steam turbines driving a single pump jet of 20.5 MW, implying a reactor power of about 145 MWt.
UK Astute-class attack submarines of 7400 dwt submerged have a modified (smaller) PWR2 reactor driving two steam turbines and a single pump jet reported as 11.5 MW. The first of seven vessels was commissioned in 2010, and five were delivered by mid-2021 at a cost of £1.65 billion each. New versions of this with 'Core H' will require no refuelling over the life of the vessel, about 25 years*. In March 2011 a safety assessment of the PWR2 design was released showing the need for improvement, though they have capacity for passive cooling to effect decay heat removal.
* Rolls-Royce claims that the Core H PWR2 has six times the (undisclosed) power of its original PWR1 and runs four times as long. The Core H is Rolls-Royce's sixth-generation submarine reactor core.
The PWR3 for the Vanguard replacement Dreadnought-class SSBNs will be largely a US design – presumably based on S9G in the Virginia-class – but using UK technology. It will be more expensive to build but cheaper to maintain than the PWR2. All UK submarine reactors use highly-enriched fuel, obtained from the USA.
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