The thermal neutron flux distribution at various points of the active eore and irradiation channels of the UAR water-water reactor, was measured by the neutron activation method of copper and gold foils. Ah increase in the thermal neutron flux in the active core is achieved by creating a water eavity of 7 cm 2 sectiom The thermal neutron flux in the cavity is increased 2.3 and 2.6 times compared with that in the active core and the irradiation channels, respeetively.