the one-speed diffusion estimate is remarkably close, given the crudeness of the model. For ${}^{239}\text{Pu}$ ($\rho = 19.8\,\text{g/cm}^3$, larger $\sigma_f \approx 1.7\,\text{b}$, $\bar{\nu} \approx 2.9$), the bare critical mass is only about **10 kg**.