Nuclear Engineering ETDs

Publication Date

7-15-1975

Abstract

Stainless steel clad uranium dioxide fuel pellets were pulse irradiated in the Annular Core Pulse Reactor to evaluate thermal stress as a contributing mechanism for cladding failure in hypothetical LMFBR transients. The 6.2 mm diameter UO2 fuel pellets, with enrichments up to 25 w/o 235UO2, were helium bonded to 0.381 mm thick Type-316 stainless steel cladding and were irradiated in a helium environment with reactor pulses having initial periods as short as 1.5 milliseconds. In experiments conducted elsewhere with longer initial periods, the primary fuel failure mechanisms observed were differential fuel-cladding thermal expansion, intragranular fission-gas-induced fuel swelling, transient fission gas release, and cladding meltthrough. Analysis indicates that these mechanisms are somewhat insensitive to the heating rate. Thermal stresses, however, exhibit a strong dependence on heating rate and become non-negligible for severe excursions.

Average fission energy deposition in the UO2 pellets initially at room temperature ranged up to 270 cal/gm. An in-core photoelectric optical pyrometer measured the UO2 surface temperature through an aperture in the cladding. Intrinsic 1 mil diameter chromel-alumel thermocouples measured the· inside and outside cladding surface temperatures. CINDA, a multidimensional transient heat transfer computer code, modeled the experiment for comparison with the experimental data.

Measured fuel surface temperatures approached 2800 ° C and temperature differences across the cladding were as high as 180° C. Tangential stresses generated in the cladding were computed from calculated and measured temperature differences across the cladding using quasistatic elastic thermal stress equations. Calculated peak tangential thermal stresses exceeded the yield stress of Type-316 stainless steel. Hence, the cladding would sustain plastic strain prior to melt, reducing the actual stresses.

Transient calculations using the model developed during this investigation were performed on a typical LMFBR fuel pin operating at 15 kw/ft in sodium coolant. Results indicate that stress levels using elastic theory increase rapidly with decreasing reactor periods. It is concluded from this study that thermal stress alone may not cause cladding failure, but when combined with other stress­producing components such as internal gas pressure, it may have a significant effect on the time and threshold of cladding failure.

Document Type

Dissertation

Language

English

Degree Name

Nuclear Engineering

Level of Degree

Doctoral

Department Name

Nuclear Engineering

First Committee Member (Chair)

Robert Leroy Long

Second Committee Member

Theodore R. Schmidt

Third Committee Member

Karl Thomas Feldman Jr.

Fourth Committee Member

Lawrence D. Posey

Fifth Committee Member

David Michael Lucoff

Download

Share

COinS