CONFRONTATION OF THE MODEL AGAINST OBSERVATIONS

The model predictions are presented for the major parameters in figures 5 to 9. First, figure 5 is a representation of the evolution of the fuel fraction restructured versus burnup Most of the restructuring is operating between 50 GWd/tU and 80 GWd/tU with a value of 80 % at 70 GWd/tU This is in accordance with the observations reported in reference [8] where a modification of the porosity is already observed for local burnup about 50 GWd/tU.

Figure 6 is a representation of the pore volumic concentration with a first singular point when restructuring starts and a second when the energy threshold is overpassed. The number of bubbles stabilize and their growing starts.

The average bubble radius (figure 7) evolves differently, depending on the local temperature, with a competition between resolution and bubble feeding. That explains why the 700 °C curve is above the 500 °C curve while the 800°C and 900°C curves are below.

The average bubble radius is evaluated somewhere between 0.42 and 0.50 /tm around 100 GWd/tU ( 0.84 and 1.00 /um diameter). These values are to be compared with the observations reported in reference [8], giving a pore distribution centered around 1.2

100

go

80

s-\ »-«-» ,.'

* /

Q 7Q _________ L_

/ U

so

50

ffl

£ 40

O

6 30

Ul

2 10 0

0 2 4 6 8 1 0 1 2 1 4 1 6 xLO+4

LOCAL BURNUP (MWd/tU)

FIG. 5. Simulation of the fuel fraction restructured versus local Burnup

i I *

o r>

AVERAGE BUBBLE RADIUS (urn) 10 W °° * vo

i§

o n g

n

1

S- 3.

PORE VOLUMIC CONCENTRATION (bubbles/ m3 ) oa

w t

io in on

GAS FRACTION IN PORES (%)

I &

3

N w

o.

n n

§

Oo 0

5

o

p

1 B to

POROSITY VOLUME FRACTION (%)

-s

n° n

S 01 •a 1' i "8 o n

S j? I

Another characteristic parameter is the porosity volume fraction. Various values are brought in the open literature. An accurate work has been conducted in TUI [8] giving an evolution of the pore volume fraction versus local burnup and for pellet average burnup ranging from 40.3 to 66.6 GWd/tU. The main conclusion was an apparent saturation of the porosity volume about 14 % . Figure 8 gives the prediction of the modelling for various temperatures. The values reported by SPINO for the 66.6 GWd/tU sample, for which the rim can be considered as almost formed, have been plot on the same graphic. The calculation gives an overestimation of the experimental data. Nevertheless, the modelling has been applied, assuming no fission gas release in the rim. Accounting for a local release, the porosity volume tend to decrease as shown on figure 10 and is more in accordance with the experimental values.

Figure 9 gives a the evolution of the fraction of gas in the pores. At 500 °C, a temperature representative of the pellet edge, the fraction of gas in the porosity is evaluated above 25 % at 100 GWd/tU. This amount, added to the gases released in the rim (about 15 to 18 % evaluated by our models) explains a lack of about 40 % of Xenon created in the pellet edge. This amount is compatible with the Xenon depletion observed on radial EPMA.

40

800 °C

• from Spino works [8]

32

:' / 700 °C

28 ——————————————————— '- — '- ——

500°c

0 2 4 6 B 10 12 14 If3 xlO⁺⁴

LOCAL BURNUP (MWd/tlT)

FIG. 10. Porosity volume fraction versus local burnup assuming an athermal fission gas release

5. CONCLUSIONS

The "rim effect" was identified fifteen years ago with the examination of the first rods irradiated over 45 GWd/tU (pellet average burnup) in test reactors as well as in power plants.

Detailed PIE have shown a progressive subdivision of the original grains at the pellet edge, in

relation with a strong increase of the porosity. Despite the low temperatures in this region, cavities about 1 mm size are formed, surrounded by subgrains of 100 to 300 nanometers. Many observations have since been collected for a better understanding of the rim formation. Some programs are still undergoing such as the High Burnup Rim Project.

The fuel thermomechanical codes are obviously concerned and must be provided with models able to simulate the evolution of the materials properties. Many interpretations are already brought in the open literature. In this paper, a scenario has been retained for the rim buildup, based on a saturation process of the lattice by the irradiation defects at low temperature. In this approach, restructuring propagates around stabilized porosities able to trap gas atoms and vacancies, which diffusion is made easier by the local disorder of the matrix. Assumption is then made on the existence of short cuts, increasing the average Xenon diffusion rate at very short distances in the fuel zones in restructuring.

The formulation of a phenomenological modelling has been described. This model as been compared against experimental data. It allows to evaluate properly the pore volume fraction in the fuel, the mean size of this pores and the fission gas volume repartition within the bubbles and the fuel matrix.

e+

The model presented herein was issued in the framework of a PhD, held on May 31 1996 in the Marseille II University by Dr. Bruna HERMITTE [22].

REFERENCES

[1] BARON D., HOUDAILLE B, TROTABAS M., RAYBAUD A., Experience on Fission Gas Release in High Burnup Fuel Rods Operating in Power Plants, Fuel Rod IAEA TCM on Internal Chemistry and Fission Products Behaviour, KARLSRUHE,

11-15 November 1985.

[2] NOGITA K., UNE K., Radiation-induced Microstructural Change in High Burnup UO2 Fuel Pellets, Nuclear Instruments and Methods in Physics Research B 91 (1994) 301-306.

[3] NOGITA K., UNE K., Irradiation-induced Recrystallization in High Burnup UO2 Fuel, JNM 226 (1995) 302-310.

[4] UNE K., NOGITA K. ,KASHIBE S., IMAMURA M., Microstructural Change and its Influence on Fission Gas Release in High Burnup Fuel, 188, (1992) pp 65-72.

[5] CUNNINGHAM M.E., FRESHLEY M., LANNING D.D., Development and Characteristics of the Rim Region in High Burnup UO2 Fuel Pellets, JNM, 188 (1992) pp 80-89.

[6] EARNER J.O., CUNNINGHAM M.E., FRESHLEY M.D., LANNING D.D., Procedures International Topical Meeting on LWR Fuel Performance, Fuel for the 90's AVIGNON, France, Vol II(ANS/ENS,1991), pp 538.

[7] C BAGGER, M.MOGENSEN and C.T.WALKER, Temperature Measurements in High Burnup UO2 Nuclear Fuel : Implications for Thermal Conductivity, Grain Growth and Gas Release, JNM 211 (1994), pp 11-29.

[8] SPINO J., BARON D., Microstructure and Fracture Toughness Characterisation of Irradiated PWR Fuels in the Burnup Range 40-67 GWd/tU, Same IAEA TCM, TOKYO, Japan, 29 October to 1 November 1996;

[9] THOMAS L.E., BEYER C.E., CHARLOT L. A., Microstructural Analysis of LWR Fuels at High Burnup, 188, (1992) pp 80-89

[10] PATT S.R., GARDE A.M., CLINK L.J., Contribution of Pellet rim Porosity to low Temperature Fission Gas Release at extended Burnups, ANS Topical Meeting on LWR Fuel Performance, WILLIAMSBURG (Virginia), 1988, PP 204-215.

[11] WALKER C.T., TAMEYAMA T., KTTAJIMA S., KONOSHTTA M., Concerning the Microstructure Changes that occur at the Surface of UO2 Pellets on Irradiation to High Burnup, JNM 188 (1992) pp73-79.

[12] PIRON J.P., GEOFFROY G., MAUNIER C., BORDIN B., BARON D., Fuel Microstructure and Rim Effect at High Burnup, ANS Topical Meeting on LWR Fuel Performance, Palm Beach, April 1994.

[13] MANZEL R., EBERLE R., Fission Gas Release at High Burnup of the Pellet Rim, International ENS/ANS Topical Meeting on LWR Fuel Performance, AVIGNON, France, 21 au 24 Avril 1991.

[14] MATZKE Hj., On the Rim Effect in High Burnup UO2 LWR Fuels, JNM 189 (1992) pp 141-148.

[15] GUEDENEY P. ,TROTABAS M., BOSCHIERO M., FORAT C., Fragema Fuel Rod Behaviour Characterization at High Bumup, International ENS/ANS Topical Meeting on LWR Fuel Performance, AVIGNON, France, 21 au 24 Avril 1991.

[16] NOGITA K., UNE K., Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated Fuels, Journal of Nuclear Science and Technology, 30(9),pp900-910, September 1993.

[17] SPINO J., VENNIX K., COQUERELLE M., Detailed Characterisation of the Rim Microstructure in PWR Fuels in the Burnup range 40-67 GWd/tM, JNM (1996).

[18] HAWKINS R.J., ALCOCK C.B., A Study of Cation Diffusion in UO^2+X and ThO² using a Spectrometry, Journal of Nuclear Materials, 1968, vol 26, pp 112-122.

[19] TURNBULL J. A. ,FRESHLEY M.D., LANNING D.D., The Diffusion Coefficients of Gaseous and Volatile Species during the Irradiation of UO2, Journal of Nuclear Materials, 1982, vol 107, pp 168-184.

[20] REST J., HOFMAN G.L., Fundamental Aspects of Inert Gases in Solids, Eds S.E.Donelly and H.H.Evans, Plenum, New York, 1991, p 443.

[21] HARDING J.H.and MARTIN D.G., A Recommendation for the Thermal Conductivity of UO², Journal of Nuclear Materials, 1989, vol 166, pp 223-226.

[22] HERMTTTE B., Etude et modelisation du rim dans le combustible des crayons REP, PhD held in Marseille II University on May 31st. 1996.

[23] OLANDER D.R., Fundamental Aspect of Reactor Fuel Elements, TID-26711-P1, Department of Nuclear Engineering, Berkeley, CA, 1976.

DISCUSSION

(Questions are given in italics)

Why is the energy accumulation E*proportional to the "short cut" diffusion coefficient?

In grain, the amount of local disturbance could be approximated to be proportional to the diffusion coefficient.

How can you explain a temperature drop of200°C in the rim region? Porosity is not enough to explain such a degradation? It has been shown by Mr. Bakker that only 20-30 C can be explained by porosity buildup.

Our model does not consider measured porosity saturation at about 15% and thus might over-predict the effect of porosity. But based on the reported temperature drop of about 200°C by RISO, 20-3 0°C maximum temperature drop by porosity seems to be too small for rim.

DEVELOPMENT OF A THERMAL CONDUCTIVITY CORRELATION

Dans le document Advances in Fuel Pellet Technology for Improved Performance at High Burnup | IAEA (Page 194-200)