We present the conceptual core design of a small lead-cooled fast reactor, for which a critical configuration has been achieved with a uranium enrichment of 9.9 wt%. This is a novelty for fast-neutron reactors without incorporating mixed uranium/plutonium fuel. It is shown how a reduction in uranium enrichment by two percentage points from a previously designed small lead-cooled reactor leads to an increase in conversion ratio of 20% and a significantly larger reactivity swing. The lowered enrichment gives a stronger Doppler feedback, which leads to lower temperatures during an overpower transient, despite remaining feedback coefficients being less negative. The new reactor geometry is presented along with a detailed neutronic characterisation, where whole-core reactivity feedback coefficients are derived, and depletion calculations are performed. Thereafter, we use the safety analysis code SAS4A/SASSYS-1 to demonstrate that the proposed design remains safe during enveloping unprotected transients, corresponding to Beyond Design Basis Accidents. We show how the reactor has a >2000 degrees C margin to fuel melting during an Unprotected Overpower transient and that thermally induced creep rupture of the fuel cladding tubes is a non-issue despite conservatively assuming 100% fission gas release.