Hafnium and dysprosium titanate based control rods for thermal water-cooled reactors

Abstract

operation is limited to a burnup of up to 45-50% t~ This is due to the large amount of radiation damage to the absorbing materials as a result of the formation of gaseous products of nuclear reactions, which results in strong swelling of the materials and destruction of the control rods. The maximum burnup l~ hi boron ~arbide Dr the absorbing eleraeats of a BBER-1000 reactor is reached within three years of operation of the automatic tonal rod~.. Control rods based on n, ~, absorbers are promising for thermal reactors. Gaseous pmdtm,'~ of-tmc~.m' ,re, actions are not formed in such rods, and the rods are characterized by a higher radiation resistance. Of,a, -~, ,absorbers, ~aly europium oxide and europium oxide in a metallic matrix are used in reactors built in this country. It has'been confu'med erperimentally that the operating time of control rods based on europium oxide absorbers can exceed 20 yr. :But, despite the high radiation resistance and neutron absorption efficiency with prolonged operation, europium based materials cannot be recommended for widespread use in reactor construction because of the high induced "r activity and the slow decrease :in this activity, which creates serious problems during storage and utilization of spent parts and can result in catastrophic con~quences in the ease of an accident. Therefore a search is being made for other radiation resistant n, 7r absorbers. These iinclude dysprosium titanate and hafaium. This paper reports the results of investigations of the radiation resistance of hafnium amd dyslmasium titanate a~ well as the main criteria which the absorbing materials must satisfy in order to ensure that the comical rode.can,operate for :~ long tinle. Choice of Criteria. Allowing for the increasing requirements on :safety ard x, eliabiI'm/,of almxation~ ~ arollo~ing criteria for the working capacity of the control rods were adopted: at least 8-10% reactivity margin; not more than 10% decrease in the efficiency of control rods at the .end of,~ration; shape and integrity preservation during operation; not more than 0.5 % increase in the diameter of the claddings of the control rods; strucatrally stable material for the absorbing core, not subjected to phase transitions with a large change in volume (not more than 10-15%); ultimate strength of carrying structural elements of at least 200 MPa, and relative elongation not exceeding 0.5%; rate of interaction of the absorbing and structural materials of not more than 0.001 mm/yr; and, rate of corrosion of absorbing and structural materials in the coolant medium of not less than 0.01 ram/yr. The first four criteria are the main criteria. If one of these criteria is not satisfied, then in order to show that the control rod is operable, specific features of the construction and characteristics of the reactor, in which it is used, must be specially substantiated. The next four are also extremely important. They make it possible to judge the state of the control rod materials at intermediate stages and also indicate any trends in the further behavior of the rods under irradiation in the reactor. The proposed set of criteria is necessary and, as experience over many years shows, these criteria are sufficient to give a reliable description of the behavior of control rods during operation. Materials and Procedures Employed. Samples of GFI-1 and GFI~-I hafnium were investigated. The hydrogen, nitrogen, oxygen, and metallic impurity content in GFI-1 hafnium did not exceed 1.10 -3, 3-10 -4, 1-10 -3, 2-10-2%,