uranium 238: the commonest isotope of uranium; it is not fissionable but when irradiated with neutrons it produces fissionable plutonium 239
uranium: a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
vaquero: local names for a cowboy (`vaquero' is used especially in southwestern and central Texas and `buckaroo' is used especially in California)