The critical mass for lower-grade uranium depends strongly on the grade: with 20 % U-235, and surrounded by a 4 cm thick beryllium neutron reflector, it is over 400 kg; with 15 % U-235, it is well over 1000 kg.. . The critical mass is inversely proportional to the square of the density: if the density is 1% more and the mass 2% less than the volume is 3% less and the diameter 1% less. The probability for a neutron per cm travelled to hit a nucleus is proportional to the density, so 1% more, which compensates that the distance travelled before leaving the system is 1% less. This is something that must be taken into consideration when attempting more precise estimates of critical masses of plutonium isotopes than the rough values given above, because plutonium metal has a large number of different crystal phases which can have widely varying densities.