Yes, you are partially right. Modern US submarine reactors use uranium enriched to at least 93%.But in addition to that they have an module called 'burnable neutron poison'. This apparently slows down the degradation of the fuel bundle assembly as it counters the harmful radiation and poisonous radioactive elements released as a byproduct of nuclear fission.I think that is because of difference in Uranium enrichment levels. Americans use 90% enrcihed Uranium whereas we use 40%, I think. There may be other factors also, but this seems to be a major one.
From information available on the appropriate US websites.
Though reading it makes it look simple, the technology may be complex. Probably a lot of research was conducted to realise the technology.