A Westinghouse engineer put out a book way back in 1955 on controlling reactors. My copy was a an ex-library book from the Cranfield College of Aeronautics and I was excited to open it up. Unfortunately it turned out to be too technical for my use. But an interesting introductory remark reveals why reactor engineers and scientists take the greatest care when starting up a reactor. Why? Because instruments are slow when there aren’t many neutrons whizzing around:
In considering the startup of a nuclear reactor the primary fact exists that operation is required over many decades. As we have shown, the range from zero to full power is a range covering nuclear fissions resulting in a few neutrons per second to many billion neutrons per second. Unless adequate control is maintained, particularly in the startup range, an accident is most likely to occur. The primary reason that startup range is the most dangerous one is because here the measuring instruments are poorest. This is not because the instruments are poor in a physical sense, but at low neutron levels the statistics of the instrument circuits usually demand instruments having slow responses. In the event that the neutron level in a reactor starts to rise rapidly in the startup range, the information may not be received by a control or an observer until it is too late to do anything about it.
Schultz, M. A. 1955. Control of Nuclear Reactors and Power Plants. McGraw-Hill, New York, p. 213