The problem of measuring continuously the thermal-neutron flux intensity in high-power nuclear reactors at temperatures up to 500°C considered, and it is concluded that the mean-current ionization chamber provides the best solution.The development of a suitable chamber is described, which is made from stainless steel and has a gas filling of xenon. When the electrodes are coated with uranium oxide (U3O8) containing 0.48 mg of uranium 235, the current sensitivity is 7 × 10−17A/n/cm2/sec, and at a neutron flux intensity of 1013n/cm2/sec the equilibrium residual current measured in a graphite-moderated reactor is 0.6% of the total output. Life tests have been carried out successfully to a total neutron dose of 3 × 1020n/cm2.At a flux intensity of 1014n/cm2/sec a chamber with uncoated titanium electrodes has the same fraction of residual current as one containing uranium 235 and is preferred as it is not subject to depletion effects.