Answer
The safeguards to keep the nuclear reactions within check are neutron-absorbing control rods (boron or cadmium), and plenty of nonfissionable U-238 to dilute the U-235.
Work Step by Step
This is discussed on page 642.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.