Analysis of chemical failure of coated UO$sub 2$ and other oxide fuels in the high-temperature gas-cooled reactor

Abstract

This report was prepared at an account of work sponsored by the Unitad Stares Government. Neither the Unitad States nor aha United States Atomic Enanjy Cornmiaeion. nor any of tfteir amployaai. nor any of dwir contractors. fubcontractors, or their ampioyaas. makes any warranty, e«prees or implied, or aaiumai any legal haMify or tesponeifcility for the accuracy, completeness or uaafutnass of any information, apparatus, product or process discioead. or represents thet its use would not infrmaa privately owned n(hts. CRNL-4926 UC-77 i,t* Cooled Reactor Technology Contract No. V-7405-eng-2b CHEMICAL TECHNOLOGY DIVISION Chemical Development Section A Ail ANALYSIS OF CHEMICAL FAILURE OF COATED 1*0, A»D OIHER OXIDE FUE1S IN THE HIGH-TOffERAlUKt GAS-COGLED RSACTOR T. B. Lln4emer H. J. d* ffsrdimll

Progress in understanding fission-product behaviour in coated uranium-dioxide fuel particles

“…At À380 kJ mol À1 (maximum value in [20] for 6% FIMA), the sum of CO and CO 2 pressures could reach a maximum of 6 and 23 MPa at 1450 and 1873 K, respectively (Fig. 1).…”

“…Oxygen-potential measurements in irradiated TRI-SO particle are very scarce. Lindemer and de Nordwall [20] reported calculated oxygen potentials from measurements of CO released from irradiated UO 2 -coated particles (6% FIMA) between À480 and À380 kJ mol À1 at 1450 K and between À500 and À380 kJ mol À1 at 1873 K. The highest value (À380 kJ mol À1 ), located below the equilibrium oxygen potential of Mo/MoO 2 in the temperature interval 1450-1873 K, is consistent with the microanalyses available in [9] which indicated that, for a slightly lower burn-up (<5.2% FIMA), Mo was mostly present in the metallic precipitates.…”

“…These values are in agreement with the calculations previously performed by Schram et al [10] or with the evaluations of Minato et al [9] for the same particle geometry and U, O, C inventories. This pressurization, acting on the silicon carbon layer has to be compared to the maximum pressure (about 100 MPa [20]) equivalent to the ultimate tensile strength, / SiC , of silicon carbide (SiC). For such a moderate burn-up (6% FIMA), the total pressure CO + CO 2 within the coated particle does not attain this critical value either in nominal operations (1450 K) or in accidental conditions (1873 K).…”

“…5). It is interesting to underscore that it is not foreseen formation of oxide forms of caesium (uranate or zirconate) for practically all the interval range of oxygen potentials given in [20], i.e., between À380 and À500 kJ mol À1 . At temperatures higher than 1500 K, the caesium carbide compounds are assumed to be no more stable in MEPHISTA.…”

Modeling Fission Product Sorption in Graphite Structures

“…As a first approximation to air ingress conditions, O2 was included in the system, along with Cs2O molecules, which is expected to be the main oxide with Cs that is formed [40--42]. Starting from this system, fission product sorption behavior was calculated for PO2=10 --20 atm and PO2=0.2 atm, which provides an approximate range of pressures that can be expected between operation and accident conditions [48]. The temperature range of interest for this work is between 600 and 1900 K. This temperature range covers normal and expected accident conditions for the HTR reactor [49] Table 6.…”