Advanced Test Reactor (ATR) Cycle 153B-1 was a 14 day, typical operating cycle for the ATR and did not result in any unusual plant transients.
However, when the fuel elements were removed from the core and inspected post-operation, several thousand flow-assisted erosion pits and “horseshoeing” defects were readily observed on the surface of the several YA-type fuel elements (these contain aluminum “dummy” plates which contain no fuel).
In order to understand these erosion phenomena, a thermal-hydraulic model of a coolant channel on a YA-M fuel element adjacent to the beryllium reflector block was created.
The flow, which was expected to vary linearly with gradual heating of the coolant as it passed through the channel, was extremely turbulent. The temperature rise, which was expected to be a smooth “S” curve, was represented by a series temperature rise “humps,” which occurred at each horizontal saw cut in the beryllium reflector block. Each of the 13 saw cuts, placed to arrest the propagation of radiation-induced embrittlement cracks, had a chamfered edge which resulted in the coolant flow being redirected as a jet across the coolant channel into the surface of the Al plate.
This led us to believe that the principal feature responsible for the appearance of these defects was indeed the coolant flow redirection due to the horizontal cuts in the beryllium reflector block. This assumption was fully confirmed by the results of thermohydraulic simulations.