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Robert, the problem is decay heat. A lot of that 3 months is spent waiting for decay heat generation to decline to levels where it is safe to depressurise the plant and begin fuel handling. This is why refuels tend to be included as part of a general overhaul period for the ship. The maintenance crew won't be sitting on their hands for those three months. They will be maintaining other areas of the ship.
It is possible to refuel at high decay heat, but risky. Heat removal will be heavily reliant on pumped flow using systems like ECCS. Any loss of power for pumped flow or loss of heat sink and the water covering the fuel will start to boil off. The higher the decay heat, the less grace time you have for water makeup. The safety case ends up being difficult and the more redundancy you need for things like power supply, the more expensive the operation becomes. This risk is probably why the Russians do what they do at sea. I am not familiar with their operations. But you may find they also have to wait for decay heat levels to decline, especially if they are removing fuel from water to some sort of dry storage. The refuel itself may take only 3 days, but the cooldown period preceding it will probably be measured in months.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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The other option to consider is avoiding refuel altogether. This means building a whole-life reactor core. Design the core to last for the intended operational life of the ship and then scrap the whole ship when the core is depleted. At least one nuclear navy does this already. It means that maintenance periods are shorter and easier. When time comes to scrap the ship, there is no problem just laying up in a basin for a decade, letting decay heat decline to low levels before removing the fuel.
There are pros and cons to this arrangement. It is tricky to do. The problems with this approach are: (1) It tends to necessitate high burnup, which can cause fuel swelling or even bowing of fuel elements. This is less of a problem if using oxide fuels. (2) To avoid excessive burnup of the central core, it is necessary to zone fuel enrichment and include burnable poisons, so that power generation is flat across the core. That makes the core more expensive. (3) Core chemistry control is more important. The coolant will carry more soluble neutron absorbers to counteract the reactivity introduced by more heavily enriched fuel. The fuel cladding will be exposed to neutron flux and oxidative attack from radiolytic oxygen for much longer. So tight control of dissolved oxygen is necessary. (4) A whole life core is safer in some ways as there are fewer intrusive operations that can potentially disrupt decay heat removal. But the core also carries a greater liad of fission products making the consequences of an accident worse.
On balance, this is probably the best approach for your ice breakers. Their mode of operation exposes the hull to a lot of flexural and shearing stresses. So replacing the boat every 20 years with a new one isn't a bad idea. It allows your navy to avoud the costs of refuelling and reduces the need for heavy maintenance periods. Although 20 year lifespan means higher capital costs, a more regular build schedule achieves economies of scale and better quality control.
There are a number of ways that the Canadian Navy could do this. Canada never invested in enrichment capability, because the CANDU can burn natural uranium. A CANDU is a less desirable option for a ship because core volume is large and online refuelling would be difficult at sea. The uranium market is well established now, so Canada could simply buy enriched U from the US, Russia or Europe. Or it can develop its own enrichment capability. Not a huge problem, its just time and money. The other option would be to reprocess spent CANDU fuel and make MOX for your ship reactors. That would cost a bit more, but it also reduces the spent fuel stockpile and puts in place a technology that could support wider Canadian nuclear industry. If Canada has reprocessing, it is a service that US nuclear operators will probably be interested in buying. Especially if they start building things like the natrium fast neutron reactor. Nuclear engineering is an area where there could be a lot of cross border trade.
Last edited by Calliban (Today 06:19:12)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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