SRE FUEL ELEMENT DAMAGE. Interim Report

During the course of power run 14 on the Sodium Reactor Experiment (SRE) at low power, the temperature difference among various fuel channels was found to be undesirably high Normal operating practices did not succeed in reducing this temperature difference to acceptable values and on July 26, 1959, the run was terminated. A series of fuel element inspections was begun to ascertain the cause of these circumstances, and several fuel elements were discovered to have suffered substantial damage. On July 29, 1959, an Ad Hoc Committee was appointed by Atomics Intennational to assist in the analysis of the existing situation in the reactor and the determination of its origin. During the three-month period since the termination of power run 14, there has been a very active program of investigation. The data accumulated duning the operation of the SRE have been re- examined and evaluated. Metallurgical examination was made of a few samples of the fuel and other components of the reactor where possible. Some chemical analysis was made of the coolant and its contaminants. Radiochemical analyses have been made of the coolant and gaseous activity. Reactivity effects were investigated. Scme experimental programs were initiated to examine mechanisms of damage and potential deleterious effects on the reactor system. Tentative conclusions, based on data obtained to date by the current investigation into the causes of the fuel element damage, may be summarized as follows: (1) The fuel cladding failed as a result of the formation of low-melting iron-uranium alloy which was produced because of partial blockage of some of the coolant passages and local overheating of the fuel elements. Coolant channel blockage was initiated by accumulation of the decomposition products of tetralin which had leaked into the primary system Sodium oxide and sodium hydride may have contributcd to this situation. (2) The high temperature runs on SRE bear no relation to the cladding failure of run 14. (3) While several possible explanations have been suggested for the reactivity changes incurred during run 14, no definitive conclusions are as yet available. (4) The extent of possible surface damage to the components of the primary system by carburization, nitriding or hydniding is not yet known. It is anticipated, however, that damage has been sustained by fuel cladding (known), moderator cans (probable), and the intermediate heat exchanger (just possible). (5) In spite of the cladding failure to at least 11 of the fuel elements, no radiological hazard was present on the reactor environs. (auth