> would running a PWR with very high enrichment level (AFAIK some designs use 93% U-235) allow quick spin down and spin up? Well.. there are a lot of factors i

> would running a PWR with very high enrichment level (AFAIK some designs use 93% U-235) allow quick spin down and spin up? Well.. there are a lot of factors in a reactor design affecting the ability to increase or decrease power quickly. Geometry, fuel/moderator ratio, fuel density, burnable poisons (for flattening the reactivity swing over the fuel cycle), amount of control rods etc etc. Fuel enrichment being only one thing, which in turn affects other things as well (e.g. reactors using highly enriched uranium tend to use different fuel designs than low enriched fuels). But yes, military reactors for navy ships obviously have very different demands on them than civilian power reactors, and are designed accordingly. And yes, while US navy reactors use 93% enriched fuel, it's not necessary, e.g. French and apparently Chinese submarines use low enriched fuel (7% for French).