You know from experiment that a certain solution of fissile material dissolved in liquid is almost exactly critical when placed in a cubical "tank" 40 cm in width. the mixture has thermal diffusion length lth- 5 cm and thermal diffusion coefficient dah 1 cm. you need to pump lots of this solution through a long, straight, thin-walled pipe, but you want to make sure that it remains subcritical in the pipe. what is the largest pipe diameter you can use if you want to make sure that the multiplication factor remains smaller than 0.95 no matter how long the pipe is? [for purposes of generating this estimate, assume that pfnl, p, and e remain the same in the pipe as in the cubical tank.]