Gamma radiation fields grow with time on the piping and other components in the primary circuit of all water-cooled nuclear reactors. Corrosion products that have been released from the out-reactor surfaces, made radioactive in the core, and then deposited on out-reactor surfaces cause the fields to grow. Corrosion product deposits on fuel sheaths exposed to nonboiling water in an in-reactor loop test are used to show the importance of high-temperature pH in the process of corrosion product transport. A possible mechanism for radiation field growth in light water reactors is then deduced from this test.