For in ordinary uranium there is much more of the U238 isotope than there is U235 and, as you know, U238 will not undergo fission easily.
For in ordinary uranium there is much more of the U238 isotope than there is U235 and, as you know, U238 will not undergo fission easily.
因为在普通铀中，铀238同位素要比铀235多得多，而且正如你所知，铀238不易裂变。