EXPERIMENTAL RESEARCH AT ECN

Recently, we have started research to mitigate the rim effect by fabricating UO2 pellets with an inhomogeneous enrichment distribution [24]. The outer 250 ^m of these pellets has a lower enrichment than the central part of the fuel. Burnup and power computations have shown that this configuration causes a considerable depression of the burnup in the outer ring of the fuel, which might prevent the rim formation. The low enrichment ring should be thin since it increases the fuel temperature compared with homogeneous fuel at the same linear heat

Temperature T

.o03

T Y

Temperature T

FIG. 2 The distribution of the y-component of the thermal flux as obtained from FEM calculations on one of the marked areas in Fig. 1. The left and the right boundary are adiabatic. The lower and the upper boundaries of the areas have the arbitrary temperatures T¹ and T², respectively. The dark regions represent a larger flux than the light regions. The 2D thermal conductivity in the y-direction equals 0.6234 and the porosity equals 0.1585.

A <**,.-' V

F/G. J /4 photograph of fuel in which a relatively large fraction of the grain boundary is covered by fission gas.

rate. Temperature computations show that this temperature difference between both pellet types decreases with increasing burnup. For a pellet average burnup over 40 MWd/kg UO² the temperature of the inhomogeneous fuel can be lower than that of the homogeneous fuel due to the prevention of the formation of the rim region, which acts as a thermal barrier.

ACKNOWLEDEGEMENTS

The authors wish to thank Battelle Pacific Northwest Laboratories for the permission to use the photographs of the fuel pellets and AEA Technology for acting as an intermediary in this.

REFERENCES

[1] LUCUTA, P.O., MATZKE, Hj., VERRALL, R.A., Modelling of UO² -based SIMFUEL thermal conductivity, The effect of the burnup, J. Nucl. Mater. 217 (1994) 279-286.

[2] MARTIN, D.G., A re-appraisal of the thermal conductivity of UO2 and mixed (U,Pu) oxide fuels, J. Nucl. Mater. 110 (1982) 73-85.

[3] FUKUSfflMA, S., OHMICffl, T., HANDA, M., The effect of rare earths on thermal conductivity of uranium, plutonium and their mixed oxide fuels, J. Less Comm. Met.

121 (1986) 631-636.

[4] ISfflMOTO, S., HIRAI, M., ITO, K., KOREI, Y., Effects of soluble fission products on thermal conductivities of nuclear fuel pellets, J. Nucl. Sci. Technol. 31 (1994) 796-802.

[5] LUCUTA, P.O., VERRALL, R.A., MATZKE, Hj., PALMER, B.J., Microstructural features of SIMFUEL - Simulated high-burnup UO2 based nuclear fuel., J. Nucl.

Mater. 178 (1991) 48-60.

[6] BARKER, K., KWAST, H., CORDFUNKE, E.H.P., Determination of a porosity correction factor for the thermal conductivity of irradiated UO² by means of the finite element method, J. Nucl. Mater. 226 (1995) 128-143.

[7] KTTAJIMA, S., KINOSHITA, M., Evaluation of measured high burnup fuel at Ris0 project phase 3. 1992 IAEA report IAEA TECDOC 697 page 205, Proceedings of the IAEA Technical Committee Meeting, Pembroke Canada .

[8] MACDONALD, P.E., SMITH, R.H., An empirical model of the effects of pellet cracking on the thermal conductivity of UO² light water reactor fuel, Nucl. Eng.

Design 61 (1990) 163-177.

[9] MATZKE, Hj., Oxygen potential in the rim region of high burnup UO2 fuel, J. Nucl.

Mater. 208 (1994) 18-26.

[10] MATZKE, Hj., Oxygen potential measurements in high burnup UO2 fuel, J. Nucl.

Mater. 223 (1995) 1-5.

[11] DANEL, J.L., MATOLICH, J., DEEM, H.W., Thermal conductivity of UO², report HW-69945, Hanford Laboratories, Richland, U.S.A. (1962).

[12] CLOUGH, D.J., SAYERS, J.B., The measurement of the thermal conductivity of UO2 under irradiation in the temperature range 150°-1600°C, report AERE-R 4690, U.K. Atomic Energy Authority, Harwell, U.K. (1964).

[13] SHAW, T.L., CARROL, J.C., GOMME, R. A., The Characterisation of the Thermal Properties of High Burn-up Fuel, paper presented at the Annual Meeting of the European Working Group 'Hot Laboratories and Remote Handling', Petten, The Netherlands, May 14-15 1996.

[14] KOLSTAD, E., DEVOLD, H., TEMPEST, P., LOSONEN, P., In-reactor thermo-mechanical measurements on LWR fuel rods in the high burnup range. 1990 IAEA report rWGFPT-36 page 140, Proceedings of the IAEA Techn. Com. Meeting, Studsvik, Sweden.

[15] LANNING, D.D., Irradiation History and Final Postirradiation Data for IFA-432 report NUREG/CR-4717, report PNL-5971, Pacific Northwest Laboratory, Richland, WA (1986).

[16] CLARK, P.A.E., CLOUGH, D.J., SMITH, R.C., Final report on the post irradiation examination of fuel rods irradiated in the Halden IFA 432 experiment, report AERE G 4190, U.K. Atomic Energy Authority, Harwell, U.K. (1986).

[17] HAHN, C.R., BATES, J.L., BRITE, D.W., DANIEL, J.L., DA VIS, N.C., HART, P.E., MARSHALL, R.K., MELLINGER, G.B., WILLIFORD, R.E., Test design, precharacterization, and fuel assembly fabrication for instrumented fuel assemblies IFA-431 and IFA-432, report NUREG/CR-0332, report BNWL-1988, Pacific Northwest Laboratory, Richland, U.S.A. (1977).

[18] SCHULZ, B., Thermal conductivity of porous and highly porous materials, High Temp.-High Pressures 13 (1981) 649-660.

[19] BARKER K.and KONINGS R.J.M., The thermal conductivity of high burnup UO2; the effects of irradiation, to be published in the proceedings of the 14th European conference on thermophysical properties.

[20] MATZKE, Hj., BLANK, H., COQUERELLE, M., LASSMANN, K., RAY, I.L.F., RONCffl, C., and WALKER, C.T., Oxide fuel transients, J. Nucl. Mater. 166 (1989)

165-178.

[21] BAGGER, C., MOGENSEN, M., WALKER C.T.. Temperature measurements in high bumup UO² nuclear fuel: Implications for thermal conductivity, grain growth and gas release, J. Nucl. Mater. 211 (1994) 11-29.

[22] SPINO, J., VENNIX, K., COQUERELLE, M., Detailed characterisation of the rim microstructure in PWR fuels in the bum-up range 40-67 GWd/tM, J. Nucl. Mater. 231 (1996) 179-190.

[23] NOGITA, K., UNE, K., Irradiation-induced recrystallization in high burnup UO² fuel, J. Nucl. Mater. 226 (1995) 302-310.

[24] BAKKER, K., HEIN, H., KONINGS, R.J.M., Using inhomogeneously enriched UO² fuel to reduce the rim effect. To be presented at the 1997 International Topical Meeting on LWR Fuel Performance, Portland, U.S.A.

DISCUSSION

(Questions are given in italics) Comment:

Effect of Xe/Kr should be eliminated from the degradation. SIMFUEL data do not contain the rare gas effect.Effect of grain boundary should be included.

How do your results compare with the prediction of MATPRO for the same amount of possibility?

I do not have the data with me, but I can communicate them to you.

MICROSTRUCTURE AND FRACTURE TOUGHNESS XA9847854