Irradiated IFA-558 results

For all the IFA-558 samples studied, the nature of the fuel dictated that only small fragments of dimension ~2x2mm could be obtained. Samples were therefore studied using sample holders with 1.5 mm diameter holes. Fig. 2 shows the form of the temperature history used in the thermal diffusivity measurements of all three irradiated IFA-558 samples.

The thermal diffusivity data obtained for the unannealed sample are shown in Fig. 4. The data show the temperature cycle to 500°C to have little effect on thermal diffusivity values, with data on the up and down legs of the cycle being almost coincident (apart from a small dip at 400°C on the heating ramp). However, during the second heating cycle the effect of heating to above 500°C is to cause the thermal diffusivity to increase with temperature up to 700°C, with a large net recovery (~40%) then being apparent at 200°C compared with the starting value. Heating in the next cycle to 900°C shows a more ‘normal’ temperature dependence, with a further (smaller) recovery apparent on the return to 200°C. The behaviour over the first

two cycles is attributed to the low temperature at which the oxidation was performed.

Subsequent heating to a temperature above the sample preparation temperature appears to have led to a more uniform redistribution of the oxygen, with a significant increase in thermal diffusivity occurring in the temperature range 500–700°C (on the second cycle).

The thermal diffusivity data are shown in Fig. 5 for the sample that had been annealed at 700°C prior to the measurements. Qualitatively the data show the expected trends when compared with the data from the unannealed sample shown in Fig. 4. The step change seen for the unannealed sample on the second cycle is now absent, and the first and second cycle results are virtually identical.

The observation of higher values for the 900°C annealed thermal diffusivity (i.e. taken on the cool down from 900°C) for the unannealed sample compared with the annealed sample is consistent with their respective O/M ratios of 2.09 and 2.11.

A final test was performed in order to investigate the effect of fission gas re-distribution at these temperatures by subjecting an unoxidised IFA-558 sample to the same measurement cycle as was applied to all the oxidised samples. The data are shown in Figure 6, and data are fully consistent with data previously reported for this fuel [4].

It can be seen that a small amount of recovery is obtained on thermal cycling up to temperatures of 900°C, but the main feature is that of a significant degradation in thermal diffusivity of the oxidised fuel compared to the unoxidised values (almost a factor of two). It is interesting to note that in absolute terms the amount of recovery at 200°C achieved in the third cycle (up to 900°C) is comparable for each of the irradiated samples. However, in percentage terms the increases are much more significant for the oxidised samples as the absolute values are so much lower.

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

200 300 400 500 600 700 800 900

Temperature (°C)

Thermal Diffusivity (mm²/s)

900°C cycle 700°C cycle 500°C cycle

Figure 4. Results for unannealed oxidised IFA-558 sample.

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

200 300 400 500 600 700 800 900

Temperature (°C)

Thermal Diffusivity (mm²/s)

900°C cycle 700°C cycle 500°C cycle

FIG. 5. Results for annealed oxidised IFA-558 sample.

0.6 0.7 0.8 0.9 1 1.1 1.2

200 300 400 500 600 700 800 900

Temperature (°C)

Thermal Diffusivity (mm²/s)

900°C cycle 700°C cycle 500°C cycle

FIG. 6. Results for unoxidised IFA-558 sample.

COMPARISON OF 900°C ANNEALED RESULTS WITH DATA FROM OTHER SOURCES

Thermal diffusivity measurements have been reported on four samples. Three originated from IFA-558 (40 GWd/tU) and comprised one as-irradiated sample and two samples oxidised to O/M ratios of 2.09 and 2.11. A fourth sample of unirradiated UO2, oxidised to an O/M ratio of 2.096 was also subjected to a similar measurement programme. For all samples a progressive recovery in measured thermal diffusivity was observed by cycling to higher temperatures, with 900°C being the peak temperature attained. A comparison of the 900°C annealed results is plotted in Fig. 7. A striking feature of this plot is that the unirradiated fuel diffusivities are lower than the values for both the IFA-558 samples, despite the fact that their O/M ratios are all comparable at around 2.10.

The results in Fig. 7 have been analysed in terms of a standard phonon conductivity model of the form:

BT k A

= +1

(1)

where T is the absolute temperature (K). Knowing the fuel density and taking the specific heat to be that of unirradiated UO2 the parameters A and B can easily be determined from linear regression fits. Care has to be exercised for the unirradiated fuel due to the UO2+x/U4O9-y

phase transition that occurs at around 500°C, and as a consequence only data in the range 600–900°C have been included in the fitting procedure for this sample (corresponding to UO2+x).

0 0.2 0.4 0.6 0.8 1 1.2

200 300 400 500 600 700 800 900

Temperature (°C)

Thermal Diffusivity (mm²/s)

IFA-558 (x = 0) IFA-558 (x = 0.09) IFA-558 (x = 0.11) Unirradiated (x = 0.096)

FIG. 7. Comparison of 900°C annealed results.

In Fig. 8 the effect of stoichiometry (represented as the O/M ratio of the material) on A is shown, based on information derived from a variety of sources. This is discussed in detail in reference [11], but the data shown in Fig. 8 can be grouped into three distinct sets:

· the AEAT data points shown correspond to the A values derived from equation (1) for the measurements presented in this paper. These samples were oxidised at low temperature.

· the Goldsmith and Douglas[1], Howard and Gulvin[2] and Lucuta et al.[3] data all refer to out-of-pile measurements made on fuel that had been oxidised at high temperatures (>

1000°C) in a controlled oxygen potential.

· the CIM data refer to values inferred from Conductivity Integral to Melt (CIM) measurements. These are in-pile measurements and provide useful information on the behaviour of the conductivity at very high temperatures. In reference [11] an unpublished compilation of data due to Martin[12] was used, which draws on the work of Marchandise[13], Christensen[14], Ridal[15] and Hawkings[16] was used in the present study. The CIM value is defined as:

ò

=

Tm

dT K CIM

773

(2)

where K is the thermal conductivity Tm is the melt temperature (K)

0 200 400 600 800 1000

2 2.02 2.04 2.06 2.08 2.1 2.12 2.14 2.16 2.18 2.2

Stoichiometry (O/M ratio)

A Value (mK/kW)

AEAT (Windscale) Goldsmith & Douglas Howard & Gulvin Lucuta et al.

Inferred from CIM

Figure 8. Variation of A with Stoichiometry (O/M ratio).

This question could be addressed by taking the AEAT samples to higher temperatures to determine whether more recovery can be generated. In addition, laser-flash measurements could be performed in situ while the samples are undergoing oxidation at high temperatures.

The aim would be to investigate the variation of the thermal properties with the degree of oxidation for samples that are maintained at the temperatures at which the oxidation occurred, and that are not brought down to low temperatures prior to the measurement. The feasibility of performing such measurements is currently being considered.

CONCLUSIONS

Thermal diffusivity measurements have been reported on four samples. Three originated from IFA-558 (40 GWd/tU) and comprised one as-irradiated sample and two samples oxidised to an O/M ratios of 2.09 and 2.11. A fourth sample of unirradiated UO2, oxidised to an O/M ratio of 2.096, was also studied.

In order to avoid the restructuring processes that occur above ~900°C in irradiated UO2 a low temperature anneal in air at 460°C for 5 minutes was used to oxidise the fragments of UO2

used in this work. A Netzsch DSC-404 was used which ensured a high degree of reproducibility in the temperature history between the oxidation runs on the different samples.

The level of oxidation for the unirradiated UO2 sample was determined by the position of the characteristic peak in the specific heat curve, which corresponds to the UO2+x/U4O9-y phase transition. This peak was only seen after the sample had been annealed to 700°C, implying that heating to a temperature above the sample preparation temperature leads to a more uniform redistribution of the oxygen. This view is supported by the thermal diffusivity results for the unannealed IFA-558 oxidised sample for which a significant increase in thermal diffusivity occurred between 500 and 700°C.

For all three oxidised samples and the unoxidised IFA-558 sample the thermal diffusivity was observed to improve on progressively heating the sample to temperatures higher than previously attained. The unirradiated UO2 diffusivities were markedly lower than the values for both the IFA-558 samples, despite the fact that their O/M ratios are all comparable at around 2.10.

There was a significant degradation in thermal diffusivity for the oxidised IFA-558 fuel compared to the unoxidised values. The observation of higher values for the 900°C annealed thermal diffusivities for one of the two IFA-558 samples is consistent with a lower O/M ratio of 2.09, compared with 2.11 for the other sample.

For out-of-pile experiments markedly different values of A are obtained for oxidised samples manufactured in different ways, implying an effect of the manufacturing process on the effectiveness of the phonon scattering centres in the material. The inconsistencies between the A values inferred from out-of-pile data and in-pile data could also imply that the present approach to the manufacture of samples for laboratory measurements results in materials which are not representative of in-pile conditions. This could be investigated by taking the AEAT manufactured samples to higher temperatures to determine whether more recovery can be generated.

Laser-flash type measurements could also be made while samples are undergoing oxidation at high temperatures, so as to avoid cooling the samples down to low temperatures prior to the measurement. The feasibility of performing such measurements is currently being considered.

REFERENCES

[1] GOLDSHMITH, L.A., DOUGLAS, J.A.M., Measurements of the thermal conductivity of uranium dioxide at 670-1270K, J. Nucl. Mater. 47 (1973) 31-42.

[2] HOWARD, V.C, GULVIN, T.F, UKAEA Report-IG Report 51 (RD/C) (1961).

[3] LUCUTA, P.G., VERALL, R.A., MATZKE, Hj., Thermal conductivity of hyperstoichiometric SIMFUEL, J. Nucl. Mater. 223 (1995) 51-60.

[4] SHAW, T.L., CARROL, J.C., GOMME, R.A., Thermal conductivity determinations for irradiated urania fuel, High Temperatures - High Pressures 30 (1998) 135-140.

[5] MATZKE, Hj., LUCUTA, P.G., VERALL, R.A., HENDERSON, G., Specific heat of UO2-based SIMFUEL, J. Nucl. Mater. 247 (1997) 121-126.

[6] De FRANCO, M., GATESOUPE, J.P., Plutonium and Other Actinides, North Holland Publishing Co. (1976).

[7] GOMME, R.A., “Thermal properties measurements on irradiated fuel: An overview of capabilities and developments at AEA Technology Windscale”, Thermal Performance of High Burn-up LWR Fuel (Proc. CEA-OECD/NEA-IAEA Seminar Cadarache, 1988), OECD, Paris (1988) 31-41.

[8] COWAN, R.D., Pulse method of measuring thermal diffusivity at high temperatures, J.

Appl. Phys. 34 (1963) 926-927.

[9] JAMES, H.M., Some extensions of the flash method for measuring thermal diffusivity, J. Appl. Phys. 51 (1980) 4666-4672.

[10] SHAW, T.L., ELLIS, W.E., Heat loss corrections applied to the measurement of thermal diffusivity of small samples by the laser flash technique, High Temperatures - High Pressures 30 (1998) 127-133.

[11] ELLIS, W.E., PORTER, J.D., SHAW, T.L., Effect of oxidation, burnup and poisoning on the thermal conductivity of UO2: A comparison of data with theory, Light Water Reactor Fuel Performance (Proc. International Topical Mtg Park City, Utah, USA, 2000), ANS (2000) 565.

[12] MARTIN, D.G., UKAEA Internal Document (1990).

[13] MARCHANDISE, H., Commission of the European Communities Report EUR-4568F, 1970.

[14] CHRISTENSEN, J.A., Report BNWL-536 (1967).

[15] RIDAL, A., BAIN, A.S., ROBERTSON, J.A.L., Report CRFD-994 (AECL-1199), Canada (1961).

[16] HAWKINGS, R.C., BAIN, A.S., Report CRDC-1153 (AECL-1790), Canada (1963).

THERMAL BEHAVIOUR OF HIGH BURNUP PWR FUEL