how long the fuel stays in the reactor. Low-burnup fuel ($< 1\,\text{GWd/tHM}$) produces plutonium with $> 93\%$ ${}^{239}\text{Pu}$ (weapons-grade). High-burnup fuel from power reactors ($\sim 33$–$60\,\text{GWd/tHM}$) produces reactor-grade plutonium with significant fractions of ${}^{240}\text{Pu