Failure of a high-power pin in a simulated $3/s TOP accident: Test E6 final report

Abstract

14.16dle . expas r". Failure occurred 70 ms earlier than predicted inthe PSAR. 3.Postfailure fuel motion was principally upward in direction and from the center of the cluster, with no more than 10 g of total fuel loss.4. The outlet blockage was formed in stages. 5.The irradiated fuel is more mobile than fresh fuel. 6.There were no significant fuel-coolant interactions. The lower one-third of the test pin suffered only minor cladding damage and no inlet blockage was formed.It is concluded that the mechanisms that led to initial failure can be reliably applied to the FTR. Characterization of the initial fuel release in amount, location, structure (soid. liquid or foam), and rate were reasonably prototypic. Postfailure coolant dynamics. however, were not very prototypic. Although the amount of fuel swept out is probably realistic, the formation of an outlet blockage and its impact on adjacent channel hydraulics is clearly nonprototypic. 8 mm), and was previously irradiated in EBR-II to a burnup of ~4 at. %, but at a more prototypic fast fluence (4 x 1022 nvt).The major purpose of Test E6 was to identify the sequence of failure events associated with high-power fuel in a hypothetical $3/s transientoverpower (TOP) accident in the FTR. Of particular interest are the timing of events, the magnitude of any fuel or cladding thermal interactions with the coolant, the rate and total amount of fuel that is redistributed, and the propensity for cladding and fuel relocations, including the formation of blockages.A secondary objective of Test E6 was to contribute to the body of data used in verifying failure models built into the large accident-analysis codes. To this end, failure thresholds and coolant-dynamics models in such codes as SAS3A and MELT-III have been partially verified by TREAT experiments. B. Relation to FTRTwo types of accidents are identified in the FTR safety analyses: a loss of sodium-coolant flow due to a pump failure or a pipe rupture (LOF), and a TOP excursion due to the accidental insertion of reactivity. Both have been extensively analyzed by the detailed accident-analysis codes SAS 3 and MELT-III.' 4 Since many of the phenomenological models built into these codes have not been, and indeed, may never be, tested, the loop experiments provide important support to the sequence and magnitudes of postulated events.In the analysis of the TOP accident, it was postulated's that the maximum reactivity available in all the control rods was added at the maximum rate possible. It was further assumed that the plant-protection system (PPS) failed to shut the reactor down. An uncontrolled rod withdrawal in FTR produces a reactivity ramp of 6 /s to a total insertion of as much as $5 reactivity. A ramp rate of about an order of magnitude higher than this (50 /s) was chosen as the base case in the PSAR. A $3/s ramp, corresponding to the design limit of the FFTF PPS, was studied parametrically for rate-dependent effects. A power history corresponding to the $3/s PPS limit was used in Test E6.Predictions of failure ti...

Transient Testing of Nuclear Fuels Performed in the Original Operation of TREAT (2nd ed.)

“…The E-series tests investigated fuel dispersal and fuel thermal energy to coolant work conversion of fresh and preirradiated oxide fuel pins failing into flowing sodium in $3/s FTR overpower transients [70,71,72,73,74,75,76,77,78,79] [80,81,82,83,84,85,86]. A one-page summary of each E-series test is included in Appendix M.…”