Failure of a TRIGA low enriched uranium (LEU) fuel during long term burnup testing in the Oak Ridge Reactor (ORR)

The development of higher loaded LEU uranium zirconium-hydride fuel culminated in long term burn-up testing in the Oak Ridge Research Reactor (ORR). A sixteen rod cluster was designed as the test article containing ER-U-ZrH1.6 fuel pins clad with Incoloy 800. The nominal fuel pin geometry is 0.542 in. (13.77 mm) outside diameter, 22.0 in (559 mm) active length, with a cladding thickness of 0.016 in. (0.406 mm). Irradiation of the TRIGA-LEU cluster in the ORR began on 13 December 1979 and is still in progress. A total of 19 test rods have been included in the irradiation campaign; nine with 45 wt-% uranium (3.7 gms/cc), six with 30 wt-% and four with 20 wt-% - all are enriched to 19.7 percent in uranium 235. The irradiation of the 20 and 30 wt-% rods was terminated in May 1982 after the targeted burn-up levels of 35 and 40% respectively were successfully attained. One of the 45 wt-% rods (not the highest burnup rod) failed in November 1983 as a result of internal pressure causing the clad to split in a typical ductile failure. Of the two possible modes of failure - excess steam pressure or excess hydrogen pressure - steam pressure is the only really probable means of failure. Fuel temperatures would have had to exceed 1150 deg C to produce the necessary hydrogen pressure to cause failure and temperatures that high are extremely improbable without film blanketing at the clad surface. There was no evidence of film blanketing which would have caused obvious clad discoloration. The best explanation for the failure is that a very small clad leak developed in the region where the major failure subsequently occurred. The clad leak was likely the result of a clad imperfection manifesting itself after four years of irradiation. Fission gas release from the small leak resulted in a decision to shut down the ORR. The test fuel rod became water-logged during this shutdown period and during the subsequent startup of the reactor, steam pressure built up. As the power was increased, fuel temperatures in the failure area continued to increase, the clad continued to yield, thinned down appreciably, and at a reactor power level of about 12 MW the pressure was sufficient to cause cladding rupture. Extensive post-irradiation examination of the failed rod confirms that this was not a generic failure and the experiment was reinserted and irradiation of the 45 wt-% uranium rods is continuing. (orig.)