Australia Navy Australia SSN Program

Nilgiri

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To add to what Paro said,

The reactor is designed for 33 years (sometimes quoted as 30+ for rounding down with margin of safety etc) and does not need refuelling within that period as:

A) It is highly enriched Uranium (HEU) that provides this longer cycle duration to begin with

(LEU like what France uses needs refuelling every 10 years or so, can read more here: https://defencehub.live/threads/indian-nuclear-submarine-programs-ssbn-ssn.8844/page-5#post-254341). France more or less did that option deliberately for reasons explained there.


B) It is run at much lower capacity factor than say a nuclear power plant (or the nuclear reactors on say a ship, where say mid life refuelling is not as big a penalty). This ties in with the "overdesign" of point C.

Thus this helps the design of extending the refuelling cycle duration to what you want when that becomes a dominant design driver.


C) Importantly, the reactor (fuel amount, moderator, geometry etc) is "overdesigned" with the final years (neutron) flux supply in mind and backworked to what is then needed at the start.

The overdesign that results from this is balanced by inclusion of burnable neutron poison at the start in the reactor (to keep neutron flux at the optimal design level for the power output)

These neutron poisons (essentially long term absorbers similar to control rods which are for more transient control) are used up over time and the two effects (over neutron production and extra neutron absorption) essentially always cancel each other during whole lifetime to produce a steady net flux, i.e they both reduce at same rate (that stems from the logarithmic equation from how half lives etc. work).

I made a little graphic to accompany this:
neutronsproduction.jpg
 

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