**4. Comparison oxidation correlation to experimental data on Zircaloy-3 samples**

Zircaloy-3 samples approximately 1 in. square were oxidized in air for a range of temperatures for various times. The thickness of the oxide layer has been measured by an eddy current instrument. A color picture of the resulting sample set was taken and included in this chapter. The measured oxide layer is compared to the conservative equation (Equation 3) developed earlier. A best estimate correlation for cladding thickness was developed from the results. The sample pictures are placed on an excel spread sheet as a function of temperature and oxide thickness. The metal thickness remaining is estimated for those samples.

#### **4.1. Comparison of conservative oxide calculation and measured data**

**Figure 8** shows a palette of 25 mil thick, 1 in. square samples of legacy Zircaloy-3 oxidized at specific temperatures for fixed periods of time (time listed is in hours(h)) starting at 500°C up to 1100°C. Two samples of the un-oxidized metal are also included in the top row. According to personnel observations, these samples, (photo, 2014) have the same appearance as when originally placed on the chart (circa, 1983). This provides evidence that the TREAT Zircaloy-3 cladding does not oxidize while in storage or in the inactive reactor.

**Figure 8.** Photograph of Zircaloy-3 oxidation chart (Photo taken 10/1/2014).

Wachs [24] recently measured the oxide thickness of these 1 in. square coupons. The oxide thickness was also calculated using the conservative technique (Equation 3). Both the meas‐ ured and the conservative values are included in **Figure 8**. The reaction products of Zircaloy-2 are adherent and tough, while those of zirconium are fragile and flaky [11]. The samples of Zircaloy-3 also are tough and none of the oxide has scaled off.

The conservative equation for calculating the oxide thickness discussed earlier is:

$$\overline{M}\_{\text{gain}} = \text{t}^\* \, \text{85000000}^\* \, \text{c}^{\left(-\frac{\pi \cdot 1000}{1.9872^\*(\text{T} + 273)}\right)}\tag{17}$$

The deposition in mg/cm2 of each sample is converted to mils (Equation 4) as

instrument. A color picture of the resulting sample set was taken and included in this chapter. The measured oxide layer is compared to the conservative equation (Equation 3) developed earlier. A best estimate correlation for cladding thickness was developed from the results. The sample pictures are placed on an excel spread sheet as a function of temperature and oxide

**Figure 8** shows a palette of 25 mil thick, 1 in. square samples of legacy Zircaloy-3 oxidized at specific temperatures for fixed periods of time (time listed is in hours(h)) starting at 500°C up to 1100°C. Two samples of the un-oxidized metal are also included in the top row. According to personnel observations, these samples, (photo, 2014) have the same appearance as when originally placed on the chart (circa, 1983). This provides evidence that the TREAT Zircaloy-3

thickness. The metal thickness remaining is estimated for those samples.

72 Nuclear Material Performance

**4.1. Comparison of conservative oxide calculation and measured data**

cladding does not oxidize while in storage or in the inactive reactor.

**Figure 8.** Photograph of Zircaloy-3 oxidation chart (Photo taken 10/1/2014).

Zircaloy-3 also are tough and none of the oxide has scaled off.

Wachs [24] recently measured the oxide thickness of these 1 in. square coupons. The oxide thickness was also calculated using the conservative technique (Equation 3). Both the meas‐ ured and the conservative values are included in **Figure 8**. The reaction products of Zircaloy-2 are adherent and tough, while those of zirconium are fragile and flaky [11]. The samples of

The conservative equation for calculating the oxide thickness discussed earlier is:

$$T\_{\text{ZnO}\_2} = 0.253 \, ^\circ M\_{\text{gain}} \tag{18}$$

For a few cases (very thin oxide layers), the measured values appear larger than the calculated values, but surface roughness and imperfections cause measured values to be at least 0.1 mils. In those cases, the conservative calculation is more accurate than the measured and the conservative values would be larger than the actual. A correlation which describes the actual thickness will be derived using the measurements and the conservative equation. For the highest temperature 1100°C, the measured values were slightly greater than the calculated values, which is probably due to the sample distortion.

All samples with measured oxide thicknesses less than 5 mils show color change due to oxidation and the samples are flat and undeformed. The largest oxide thickness measured on samples held at 700°C and lower was 4.9 mils (equivalent to 3.33 mils of metal loss) for 30 hours at 700°C. The oxidation occurs on both sides of these 25 mil thick samples, but for this subset of samples the backside oxidation has not affected the top side. Due to the TREAT fuel cladding cans being evacuated during fabrication, these samples with a metal loss of 3.33 mils or less on a side are representative of the TREAT cladding which only is exposed to air on one side.

This sample chart is reproduced in black and white, so colors have not been preserved. The five samples that appear in 500, 530, and 600°C are grey. The dots on the samples are light pink. All of the colors for samples 700 and 800°C that appear grey are actually a light pink. The grey colors in 1000 and 1100°C are really grey.

**Figure 9** is a more detailed look gradual transition from a black oxide through pink dots to all grey for the samples at 530°C illustrating the color change as oxide thickness increase to 0.5 mils.

**Figure 9.** Expanded view of the 530°C gradual transition from black to grey oxide.

#### **4.2. Correlation between the oxide measured and calculated thicknesses**

A plot of the measured data versus calculated values from **Figure 8** is shown in **Figure 10**. An identity line is also shown. Data points below this line indicate that the calculated values are larger than the measured values. Points above are non-conservative. Three points are above the line which means they are non-conservative, one is for 1000°C and the other two are for 1100°C. Using all the data points to fit a straight line which goes through the origin gives a relation between measured and calculated of

$$T^{M}\_{\
u\text{O}\_2} = 0.4794 \,\text{\*}\, T^{C}\_{\text{ZnO}\_2} \tag{19}$$

**Figure 10.** Measured versus conservative oxide values.

Combining this with the previous equation gives a correlation which predicts the actual oxide thickness.

$$T^{M}\_{\
u\_{\rm Z0\_2}} = t^{\ast}1030949.7^{\ast} \text{EXP} \left( \frac{-31000}{1.9872^{\ast} \left(T + 273\right)} \right) \tag{20}$$

#### **4.3. Oxide color as a tool for estimating oxide thickness**

A qualitative evaluation may be made of oxide thickness by correlating color to oxide thick‐ ness. The surface color change to black occurs almost immediately with the start of oxidation (0.1 mils), but then remains the same for a certain amount of oxidation until it gets spots of pink. This occurs at slightly different amounts of oxidation for different temperatures. To see this more clearly, the samples have been placed on a grid in order of ascending oxidation as shown in **Figure 11**. The oxidation values used for plotting are the smoothed values from the above correlation, which are 0.4794 of the conservatively calculated values in **Figure 8**.

Oxidation, Embrittlement, and Growth of TREAT Zircaloy-3 Cladding http://dx.doi.org/10.5772/62708 75

**Figure 11.** Samples placed according to oxide thickness and temperature.

larger than the measured values. Points above are non-conservative. Three points are above the line which means they are non-conservative, one is for 1000°C and the other two are for 1100°C. Using all the data points to fit a straight line which goes through the origin gives a

Combining this with the previous equation gives a correlation which predicts the actual oxide

æ ö - <sup>=</sup> ç ÷

A qualitative evaluation may be made of oxide thickness by correlating color to oxide thick‐ ness. The surface color change to black occurs almost immediately with the start of oxidation (0.1 mils), but then remains the same for a certain amount of oxidation until it gets spots of pink. This occurs at slightly different amounts of oxidation for different temperatures. To see this more clearly, the samples have been placed on a grid in order of ascending oxidation as shown in **Figure 11**. The oxidation values used for plotting are the smoothed values from the above correlation, which are 0.4794 of the conservatively calculated values in **Figure 8**.

( ) ZrO2 <sup>31000</sup> \*1030949.7 \* EXP

1.9872 \* 273

+ è ø

*<sup>T</sup>* (20)

ZrO2 2 ZrO 0.4794 \* *M C T T* = (19)

relation between measured and calculated of

74 Nuclear Material Performance

**Figure 10.** Measured versus conservative oxide values.

*<sup>M</sup> T t*

**4.3. Oxide color as a tool for estimating oxide thickness**

thickness.

Pink dots start to appear with about 0.2 mils of oxide for 500, 530, and 600°C. But this does not represent higher temperature oxidation, since the 5 hours 800°C is still black with an oxide thickness of 3.2 mils, and the 1 hour 1000°C is still black with an oxide thickness of 6.3 mils. The 500, 530, and 600°C start to turn grey at a thickness of 0.45 mils, although the grey in the 600°C samples appear to start at slightly larger thicknesses.

As a result of the above observation, it may be concluded that the oxide thickness transition between black oxide and grey oxide increases with temperature, so that a single color does not indicate a unique thickness of the oxide, but it does if the temperature history is known as it is in TREAT.

**Figure 12** reproduces the 500, 530, and 600°C data, which is the expected region of interest in oxidation of the TREAT cladding since none of past transients have exceeded 600°C. As long as TREAT reactor excursions remain in the temperature range of 600°C and below, then the

**Figure 12.** Color chart of concern to TREAT operation.

start of a grey color oxide layer would be evidence of an oxide between 0.35 and 0.75 mils thick. Black oxide would indicate an oxide thickness of 0.18 or less.

Since Equation 7 yields an estimate of the actual oxide thickness, the metal remaining in each sample is calculated by subtracting the metal loss from the original 25 mils. The metal loss is obtained from the ratio of oxide growth to metal loss, which is shown below as 0.680.

$$T\_{2x} = 0.172 \, ^\circ \overline{M}\_{\text{gain}} T\_{2x0\_2} = 0.253 \, ^\circ \overline{M}\_{\text{gain}}, \text{ so } \frac{\text{Zinc Loss}}{\text{Oxide growth}} = \frac{T\_{2x}}{T\_{2x0\_2}} = \frac{0.172}{0.253} = 0.680 \tag{21}$$

For the 500, 530, and 600°C samples, the oxidation on the back side of the samples has no effect on the remaining metal, so the oxidation characteristics would be the same as that which would occur on the TREAT cladding which only oxidizes on one side. Therefore, the remaining cladding thickness on TREAT cladding which looks like the color of these samples can be calculated by subtracting the metal loss from 25 mils as in the following

$$T\_{\text{Zr}} \stackrel{\text{remaining}}{=} 2\text{S} - 0.680 \, ^\circ T\_{\text{ZrO}\_1} \tag{22}$$

These numbers have been included in **Figure 12** as the second number under some of the samples.

#### **4.4. Determination of the minimum cladding thickness remaining**

Although the TREAT cladding will probably never oxidize more than 2 mils, regulations require that a minimum undamaged metal (no oxygen incursion into the metal grain boun‐ daries) remaining thickness limit be specified. This section determines the thickness of undamaged metal that is required for sufficient mechanical strength to be able to remove, insert, or handle a fuel assembly. This is then followed by a determination of the amount of oxidation that is acceptable to have the required thickness of the metal remaining.

The minimum undamaged cladding thickness needed during handling operations is based on the strength of the cladding. The handling forces,FH, that the assembly must withstand are primarily in the axial direction being the weight of the fuel being suspended, friction between assemblies during insertion or removal, and sticking forces between the assemblies and the bottom grid plate. These forces are assumed to be borne by the cladding horizontal cross section,*A*c. This undamaged cladding cross section is modelled as a uniform layer of metal around the circumference of the fuel times the perimeter of the fuel cross section. This assumes that oxidation leaves a uniform thickness of undamaged metal. The stress,σ , which occurs during fuel handling, is then estimated by the equation:

$$
\sigma = \mathbf{F}\_{\mathrm{H}} \;/\; \mathbf{A}\_{\mathrm{e}} \tag{23}
$$

The minimum area, and hence the minimum thickness allowed, would be that area able to support the maximum stress which the metal can support without incurring damage. This stress is taken to be the yield stress of Zircaloy-3. From the Alloy Digest, the yield strength of Zircaloy-3 at room temperature is 44.2 ksi and 16.7 ksi at 500°F (260°C) for undamaged metal. The yield strength of 16.7 ksi will be used to calculate minimum cladding thickness. Using the yield strength at 260°C is conservative in two ways. First, the yield strength is the stress where proportional elongation ends and where plastic deformation begins. The ultimate yield strength is where the material breaks. The yield strength is approximately 58% lower than the ultimate yield strength. Second, the use of the yield strength at 260°C is conservative because fuel handling operations are normally done at room temperature, with no reason to attempt to remove fuel from the reactor at higher temperature.

The force used to calculate the minimum cladding thickness is 300 lbs. This includes the weight of the fuel assembly (95 lbs) and any friction forces associated with assembly removal. The value of 300 lbs has historically been used as the limit during fuel handling operations. The area of the cladding, *A*c, is obtained by solving Equation 12 for the area, by using the values of 300 lbs, and by requiring a safety factor of five times the area to account for non-uniformity in the oxide layer. The thickness of the cladding, *T*, is calculated by dividing the area required by the approximate circumference of the cladding.

$$A\_c = 5 \text{\* } \frac{300}{16700} = 0.090 \text{ in}^2 \quad T = 0.090 \frac{\text{in}^2}{16 \text{ in}} = 5.6 \text{ miles} \tag{24}$$

**Figure 13.** High temperature, high oxidation samples.

start of a grey color oxide layer would be evidence of an oxide between 0.35 and 0.75 mils thick.

Since Equation 7 yields an estimate of the actual oxide thickness, the metal remaining in each sample is calculated by subtracting the metal loss from the original 25 mils. The metal loss is

Zirc Lost 0.172 0.172 \* 0.253\* , so 0.680

For the 500, 530, and 600°C samples, the oxidation on the back side of the samples has no effect on the remaining metal, so the oxidation characteristics would be the same as that which would occur on the TREAT cladding which only oxidizes on one side. Therefore, the remaining cladding thickness on TREAT cladding which looks like the color of these samples can be

These numbers have been included in **Figure 12** as the second number under some of the

Although the TREAT cladding will probably never oxidize more than 2 mils, regulations require that a minimum undamaged metal (no oxygen incursion into the metal grain boun‐ daries) remaining thickness limit be specified. This section determines the thickness of undamaged metal that is required for sufficient mechanical strength to be able to remove, insert, or handle a fuel assembly. This is then followed by a determination of the amount of

The minimum undamaged cladding thickness needed during handling operations is based on the strength of the cladding. The handling forces,FH, that the assembly must withstand are primarily in the axial direction being the weight of the fuel being suspended, friction between assemblies during insertion or removal, and sticking forces between the assemblies and the bottom grid plate. These forces are assumed to be borne by the cladding horizontal cross section,*A*c. This undamaged cladding cross section is modelled as a uniform layer of metal around the circumference of the fuel times the perimeter of the fuel cross section. This assumes that oxidation leaves a uniform thickness of undamaged metal. The stress,σ , which occurs

oxidation that is acceptable to have the required thickness of the metal remaining.

s

*<sup>T</sup>* = = == = (21)

2 Zr

ZrO

Zr ZrO *T T* = - 25 0.680 \* (22)

= F /A H c (23)

Oxide growth 0.253

2

obtained from the ratio of oxide growth to metal loss, which is shown below as 0.680.

Black oxide would indicate an oxide thickness of 0.18 or less.

*<sup>T</sup> T MT M*

calculated by subtracting the metal loss from 25 mils as in the following

remaining

**4.4. Determination of the minimum cladding thickness remaining**

during fuel handling, is then estimated by the equation:

2

Zr gain ZrO gain

samples.

76 Nuclear Material Performance

Since data are not available on 25 mils thick, one-sided Zircaloy-3 oxidized on one side, the minimum cladding thickness must be inferred from the data 25 mils samples oxidized on two sides. It is seen in **Figure 12** that the longer oxidation runs at 800, 1000, and 1100°C have experienced oxygen incursion and have become brittle. **Figure 13** shows this high temperature large oxidation part of Figure 12.

If a sample has become brittle, then oxygen incursion must have occurred from both the top and the bottom. Assuming symmetry, this would mean that the oxygen incursion reached half way through the 25 mils or 12.5 mils. A determination of half metal thickness remaining is calculated by the equation

$$T\_{\rm Zn}^{\rm remaining} = 12.5 - 0.680 \, ^\circ T\_{\rm ZnO\_2} \tag{25}$$

Even though the 20 hours 800°C sample is below the 862°C limit temperature mentioned in Section 14a, it has become brittle which is evidenced by the large crack. To be conservative, it is assumed that in addition to the temperature limit of 862°C, there is also a limit on the amount of oxidation which can occur before oxygen penetration of the grain boundaries occurs at temperatures below 862°C. For this sample, the effective thickness of 12.5 mils shows that 10 mils of oxide formed (a loss of 6.8 mils of metal) and that 5.7 mils of metal under it became brittle. Almost the same value is observed in the 1000°C sample. This result is applied to TREAT cladding that is 25 mils thick and protected on one side by realizing that if 10 mils of oxide forms (which is 6.8 mils of metal loss), then an additional 5.7 mils of cladding under it has been damaged (brittle) and could not support a load. This would leave a thickness of 25 − 6.8 − 5.7 = 12.5 mils of undamaged metal left.

This is extended to other oxide thicknesses by assuming the damage thickness (embrittled) is proportional to the cladding thickness as

$$T\_{\text{damaged}} = \\$.7\*T\_{\text{oxide}} / 10\tag{26}$$

The amount of undamaged metal *T*Good remaining is estimated by subtracting the metal loss due to the oxide layer 0.68 \* *T*oxide and the damaged metal layer 0.57 \* *T*oxide from the original metal thickness of 25 mils to obtain

$$T\_{\text{Good}} = 25 \text{ miles} - 0.68 \, ^\circ T\_{\text{oxide}} - 0.57 \, ^\circ T\_{\text{oxide}} \tag{27}$$

Thus, the oxide thickness limit required to leave 5.6 mils of undamaged metal is determined by

$$\text{S.6 miles} = 25 \,\text{miles} - 0.68 \, ^\circ T\_{\text{oxide}} - 0.57 \, ^\circ T\_{\text{oxide}} \tag{28}$$

Oxidation, Embrittlement, and Growth of TREAT Zircaloy-3 Cladding http://dx.doi.org/10.5772/62708 79

$$T\_{\text{oxide}} = \frac{2.5 \text{ miles} - 5.6 \text{ miles}}{0.68 + 0.57} = 15.52 \text{ miles} \tag{29}$$

It must be remembered that the measurement technique measures oxide thickness and does not differentiate damage metal from undamaged metal. The remaining 5.6 mils of metal thickness remaining provides a factor of 5 safety factor to account for experimental uncertain‐ ties and non-uniformities in the oxide layer growth and excess.

The total remaining metal is 5.6 + 0.57 \* 15.52 = 14.4464 mils, which is the minimum allowable metal thickness. These three components, oxide, brittle metal, and undamaged metal add up to the original 25 mils = 0.64 \* 15.52 + 0.57 \* 15.52 + 5.6. In summary, the two limits are:

Maximum oxide thickness allowable = 15.52 mils

Since data are not available on 25 mils thick, one-sided Zircaloy-3 oxidized on one side, the minimum cladding thickness must be inferred from the data 25 mils samples oxidized on two sides. It is seen in **Figure 12** that the longer oxidation runs at 800, 1000, and 1100°C have experienced oxygen incursion and have become brittle. **Figure 13** shows this high temperature

If a sample has become brittle, then oxygen incursion must have occurred from both the top and the bottom. Assuming symmetry, this would mean that the oxygen incursion reached half way through the 25 mils or 12.5 mils. A determination of half metal thickness remaining is

Even though the 20 hours 800°C sample is below the 862°C limit temperature mentioned in Section 14a, it has become brittle which is evidenced by the large crack. To be conservative, it is assumed that in addition to the temperature limit of 862°C, there is also a limit on the amount of oxidation which can occur before oxygen penetration of the grain boundaries occurs at temperatures below 862°C. For this sample, the effective thickness of 12.5 mils shows that 10 mils of oxide formed (a loss of 6.8 mils of metal) and that 5.7 mils of metal under it became brittle. Almost the same value is observed in the 1000°C sample. This result is applied to TREAT cladding that is 25 mils thick and protected on one side by realizing that if 10 mils of oxide forms (which is 6.8 mils of metal loss), then an additional 5.7 mils of cladding under it has been damaged (brittle) and could not support a load. This would leave a thickness of 25 − 6.8 − 5.7

This is extended to other oxide thicknesses by assuming the damage thickness (embrittled) is

The amount of undamaged metal *T*Good remaining is estimated by subtracting the metal loss due to the oxide layer 0.68 \* *T*oxide and the damaged metal layer 0.57 \* *T*oxide from the original

Thus, the oxide thickness limit required to leave 5.6 mils of undamaged metal is determined

remaining

2

Zr ZrO *T T* = - 12.5 0.680 \* (25)

damaged oxide *T T* = 5.7 \* / 10 (26)

Good oxide oxide *T TT* =- - 25 mils 0.68\* 0.57 \* (27)

oxide oxide 5.6 mils 25 mils 0.68\* 0.57 \* =- - *T T* (28)

large oxidation part of Figure 12.

= 12.5 mils of undamaged metal left.

metal thickness of 25 mils to obtain

by

proportional to the cladding thickness as

calculated by the equation

78 Nuclear Material Performance

Minimum metal thickness allowable = 14.45 mils

The oxide thickness which leaves only damaged metal is

$$00 = 25 \text{ miles} - 0.68 \, ^\circ T\_{\text{oxide}} - 0.57 \, ^\circ T\_{\text{oxide}} \\ T\_{\text{oxide}} = \frac{25 \text{ miles}}{0.68 + 0.57} = 20 \text{ miles} \tag{30}$$

The metal converted to oxide 0.68 \* *T*oxide = 13.6 mils. The damaged metal is 0.57 \* *T*oxide = 11.4 mils. This remaining oxygen damaged metal layer would not provide a complete barrier to keep air from the graphite-carbon fuel which might burn if it is at temperature over 700°C.

#### **4.5. Deductions about cladding oxidation**

The calculation of the cladding remaining with the Zircaloy-2 reaction rate from Section 1 compared to the oxidation samples shows that this equation is conservative. The measured data and calculations also show that, after approximately 35 years of operation at recorded temperatures up to approximately 570°C, and an accumulated energy development of over 2.6 × 106 MJ, there is minimal measured cladding oxidation. A continued program of oxide growth tracking for fuel assembly temperatures that exceed 400°C is considered prudent, but is not of sufficient concern to be included as a technical specification surveillance. However, as discussed in the previous subsection, an evaluation of cladding oxidation should be made if fuel assembly temperatures exceed 600°C. Part of this recommended evaluation should be the requirement to remove any fuel element that has less than 14.45 mils of cladding remaining or an oxide layer greater than 15.52 Mils. These are sufficient criteria to ensure fuel elements can be removed from the core.

As long as TREAT reactor excursions remain in the temperature range of 600°C and below, then the start of a grey color oxide layer would be evidence of an oxide between 0.45 and 0.92 mils thick. Black oxide would indicate an oxide thickness of 0.23 mils or less.

## **4.6. Current oxidation of TREAT fuel**

The current oxidation of the TREAT fuel is basically the same as measured in the report of Kramer [20] in 1983. In the report, he determined the average amount of oxidation at that time (0.76 mils) and pointed out that it was less than the average measurement of Mouring [21] in 1977 (0.83 mils). In fact, the accuracy of the measurement method used on the fuel assemblies seems to be about 0.5 mils because the measurement head is affected by surface irregularities [20]. This is about the same magnitude as the most recent oxide measurements. Although Kramer [20] measured the average thickness at 0.76 mils, he also said that the observation of color put it in the range of 0.4 to 0.6 mils. Observations made in 1981 showed that the color was black with a touch of pink. This corresponds to a thickness of about 0.4 mils. Thus, it seems that the oxidation is less than 0.76 mils and may be as small as 0.4 mils.

It should be pointed out that even though the average measurements in Mouring and Kramer are about 0.8 mils, some of the measurements were as high as 2 mils. But Kramer pointed out that several factors caused inaccuracies in the measurements made with the fuel assembly in motion and tended to increase the value so that these higher values are probably misleading. These factors included roughness of the surface due to oxidation, scratches on the surfaces with left ridges, weld seams, etc. Also, the fact that the oxidation did not increase over the time between the two measurements in 1977 and 1983 indicates that the oxidation rate has been small.

A conservative method described in the next section is in place to keep track of an upper limit on how much oxidation has occurred. This tracking method is based on the conservative method of estimating the oxide growth rate. Measurements may be required as part of the evaluation for any fuel assembly that exceeds 600°C. It should be noted that 151 transients were performed since the last measurement and the oxide buildup is estimated at 0.229 mils [26].

The current number of transients on the fuel assemblies over 35 years of operation is 2880. The oxidation of the TREAT fuel has resulted in a loss of less than 1 mil of cladding loss over those transients. No more than an additional 1 mil cladding loss would be expected within the next 35 years of operation if the same type of transients and frequency as the original 35 years resulting in a minimum cladding thickness of 23 mils.

#### **4.7 Predicted cladding losses during 10 years of operation**

The section presents calculated results of temperature predictions in transients which are run in TREAT reactor and the amount of cladding oxidation which can be expected in each. **Figure 14** shows calculated temperatures for fuel and cladding following a transient where the hot spot fuel temperature reaches a maximum of 600°C.

It is assumed in the analysis that the reactor air flow rate is about 6000 cfm, which is the normal mode of operation during a transient. **Figure 14** shows temperatures for the case where the cladding has collapsed on the fuel. The control rods are pulled to their full out programmed location to begin a transient. The graphite carbon uranium fuel matrix heats up very fast (~1 sec). In this case, the cladding and fuel surface temperature equilibrate within 0.5 minutes.

**Figure 14.** Cladding and fuel temperatures for the no cladding-fuel gap case.

**4.6. Current oxidation of TREAT fuel**

80 Nuclear Material Performance

small.

[26].

The current oxidation of the TREAT fuel is basically the same as measured in the report of Kramer [20] in 1983. In the report, he determined the average amount of oxidation at that time (0.76 mils) and pointed out that it was less than the average measurement of Mouring [21] in 1977 (0.83 mils). In fact, the accuracy of the measurement method used on the fuel assemblies seems to be about 0.5 mils because the measurement head is affected by surface irregularities [20]. This is about the same magnitude as the most recent oxide measurements. Although Kramer [20] measured the average thickness at 0.76 mils, he also said that the observation of color put it in the range of 0.4 to 0.6 mils. Observations made in 1981 showed that the color was black with a touch of pink. This corresponds to a thickness of about 0.4 mils. Thus, it seems

It should be pointed out that even though the average measurements in Mouring and Kramer are about 0.8 mils, some of the measurements were as high as 2 mils. But Kramer pointed out that several factors caused inaccuracies in the measurements made with the fuel assembly in motion and tended to increase the value so that these higher values are probably misleading. These factors included roughness of the surface due to oxidation, scratches on the surfaces with left ridges, weld seams, etc. Also, the fact that the oxidation did not increase over the time between the two measurements in 1977 and 1983 indicates that the oxidation rate has been

A conservative method described in the next section is in place to keep track of an upper limit on how much oxidation has occurred. This tracking method is based on the conservative method of estimating the oxide growth rate. Measurements may be required as part of the evaluation for any fuel assembly that exceeds 600°C. It should be noted that 151 transients were performed since the last measurement and the oxide buildup is estimated at 0.229 mils

The current number of transients on the fuel assemblies over 35 years of operation is 2880. The oxidation of the TREAT fuel has resulted in a loss of less than 1 mil of cladding loss over those transients. No more than an additional 1 mil cladding loss would be expected within the next 35 years of operation if the same type of transients and frequency as the original 35 years

The section presents calculated results of temperature predictions in transients which are run in TREAT reactor and the amount of cladding oxidation which can be expected in each. **Figure 14** shows calculated temperatures for fuel and cladding following a transient where

It is assumed in the analysis that the reactor air flow rate is about 6000 cfm, which is the normal mode of operation during a transient. **Figure 14** shows temperatures for the case where the cladding has collapsed on the fuel. The control rods are pulled to their full out programmed location to begin a transient. The graphite carbon uranium fuel matrix heats up very fast (~1 sec). In this case, the cladding and fuel surface temperature equilibrate within 0.5 minutes.

that the oxidation is less than 0.76 mils and may be as small as 0.4 mils.

resulting in a minimum cladding thickness of 23 mils.

**4.7 Predicted cladding losses during 10 years of operation**

the hot spot fuel temperature reaches a maximum of 600°C.

**Figure 15** shows the cladding loss during this transient, which is seen to reach 0.00152 mils. This value is less than 1/10 of one used by TREAT to estimate cladding oxidation, showing that the TREAT method is quite conservative.

**Figure 16** shows the temperatures when it is assumed that the original 55 mil design gap still exists. The lower cladding temperature results in a cladding loss of only 0.000007 mils. The fuel assemblies in the outer regions of the core may retain their original gap, but the gap in assemblies in the inner region where the temperatures are highest have closed due to the weaker cladding at higher temperatures and the vacuum in the cladding can. Visual observa‐ tion of the fuel (by technicians using binoculars) in the past has shown that the cladding has shrunk down on the higher temperature fuel because cladding indents were observed at the interfaces between the 8 in. long fuel blocks.

**Figure 15.** Cladding loss for the no cladding-fuel gap case.

**Figure 16.** Cladding and fuel temperatures with a 55 mil gap.

Since the no-gap model of the fuel shows the most metal loss, it is conservatively used to estimate the cladding loss in other transients. The calculated amount of metal loss at the fuel hot spot for three temperature limited transients with no gap fuel is shown in **Table 6**. This calculation uses the conservative reaction rate described in the previous sections of this chapter.


**Table 6.** Comparison of calculated metal loss to loss used by TREAT.

The TREAT constants used to estimate cladding loss uses values [25, 26] also listed in Table 6. The metal loss for a 600 and 500°C transient that TREAT uses to estimate the total metal loss is seen to be over 10 times larger than the calculated value. Thus, the method used by TREAT is very conservative and estimates a metal loss more than 10 times larger than the conservative calculation presented here.

To be even more conservative, the TREAT method assumes that all the transients which have a maximum temperature between 500 and 600°C use the metal loss for a 600°C transient, and those which had a maximum temperature between 400 and 500°C use the metal loss calculated for 500°C. The metal loss for transients below 400°C is neglected.

As an example, the metal loss calculated with the TREAT method for an experimental program of 100 experiments in the 500 to 600°C range and 300 in the 400°C to 500°C range is

$$\text{Metal loss} = 100 \, ^\circ 0.01693 + 300 \, ^\circ 0.00154 = 2.17 \, \text{mils} \tag{31}$$

which corresponds to a buildup of 3.19 mils of oxide. If 600 additional transients are assumed which reached 400°C, this would increase the calculated metal loss less than 1%. Thus, the metal will still be at least 22 mils thick after the above program assuming this conservative result. In fact, the smaller conservative values calculated after such a program is less than 0.2 mils.

$$\text{Metal loss} = 100 \, ^\circ 0.00152 + \text{300\*} 0.00014 = 0.194 \, \text{mils} \tag{32}$$

Even this later estimate is extremely conservative. Based upon the more realistic estimate of cladding oxidation, it is recommended that the TREAT values could be reduced by a factor of 10.

#### **5. Cladding growth**

**Figure 16.** Cladding and fuel temperatures with a 55 mil gap.

**Metal Thickness Loss Calculated, mils**

**Table 6.** Comparison of calculated metal loss to loss used by TREAT.

for 500°C. The metal loss for transients below 400°C is neglected.

conservative calculation presented here.

600 0.00152 0.01693 500 to 600°C 500 0.00014 0.00153 400 to 500°C 400 0.000007 0 <400°C

chapter.

**of Transient**

**Maximum Temperature**

82 Nuclear Material Performance

Since the no-gap model of the fuel shows the most metal loss, it is conservatively used to estimate the cladding loss in other transients. The calculated amount of metal loss at the fuel hot spot for three temperature limited transients with no gap fuel is shown in **Table 6**. This calculation uses the conservative reaction rate described in the previous sections of this

> **Metal Thickness Loss Used by TREAT, mils**

The TREAT constants used to estimate cladding loss uses values [25, 26] also listed in Table 6. The metal loss for a 600 and 500°C transient that TREAT uses to estimate the total metal loss is seen to be over 10 times larger than the calculated value. Thus, the method used by TREAT is very conservative and estimates a metal loss more than 10 times larger than the

To be even more conservative, the TREAT method assumes that all the transients which have a maximum temperature between 500 and 600°C use the metal loss for a 600°C transient, and those which had a maximum temperature between 400 and 500°C use the metal loss calculated

As an example, the metal loss calculated with the TREAT method for an experimental program

of 100 experiments in the 500 to 600°C range and 300 in the 400°C to 500°C range is

**Temperature Range Where the TREAT**

**Metal Loss is Used.**

Cladding oxidation also causes lateral growth of the cladding due to the increased specific volume of the oxide over that of the cladding. This growth could reduce cooling of the fuel assembly because it decreases the flow area between fuel assemblies. Such a decrease could also hamper fuel handling operations. When Zircaloy is oxidized in air, the oxide film is under lateral compression [11], and the growth of the base metal occurs from the tension the oxide puts on the base metal. Three measurements of growth were made: (1) A Zircaloy-3 fuelelement 48 in. long can exposed to air at 700°C for 48 hours exhibited longitudinal and transverse growth of 7/8 and 1/16 in., respectively [17]. (2) The growth observed in the 1 in. samples at 600°C for 69 hours was 3.2 × 10−3in./in. (3) The sample heated to 800°C for 2 hours grew 2.5 × 10−3in./in. Based on these tests, growth rate constants of 4.6 × 10−5, 3.25 × 10−4, and 1.2 × 10−3in./in./h were obtained for temperatures of 600, 700, and 800°C, respectively.

An Arrhenius expression for the metal growth rate (MGR) has been used to describe the above data which is

$$\text{MGR} = \\$000 \,\text{\*}\,\text{e}^{\left(\frac{-32000}{\text{RT}}\right)}\tag{33}$$

where MGR is in (in./in./hour).

This expression is plotted for a range of temperatures in **Figure 17** and compared to the data. The expression is slightly larger than the above reported data so it is slightly conservative.

**Figure 17.** Growth rate of Zirconium-3.

The lateral growth for the TREAT 4 in. with fuel elements was calculated for 600°C transient presented in the previous section and the growth is shown in **Figure 18**. Similar calculations were done for 500 and 400°C. The resultant growth for each transient is shown in **Table 7**.


**Table 7.** Calculated lateral growth of TREAT fuel elements.

**Figure 18.** Lateral growth of fuel element during a 600°C transient.

Based on the conservative assumptions that the growth rate for transients above 500°C is equal to that at 600°C, growth rates for transients between 400 and 500°C are calculated at the 500°C rate, and transients 400°C or less at 400°C rate. The experimental program analyzed in the last section yields a total lateral increase of 1.15 mils.

Cladding Growth 100 \* 0.011096 300 \* 0.000949 600 \* 0.000041 1.15 mils =++= (34)

This is a value which is too small to measure accurately due to the uncertainty of original width. The highest temperature elements would be the ones with the most growth, but these are the most likely to have the cladding collapsed on the fuel due to high temperatures and the evacuation of the fuel during manufacture. Fission gas production or any leakage into the cans would also change the geometry. The consequences of this possible growth are very small since the core is unclamped when fuel is to be moved and adjacent fuel elements can be moved to take out any elements which have grown.

A fuel element with 48 in. long fuel section could grow in the axial direction; but since the fuel elements are supported on a bottom grid and are free to grow longitudinally, a gradual change in length should not interfere in any way with reactor loading operations.

If temperatures in the fuel assemblies exceed 600°C during an off-normal event, an evaluation of the effect of oxidation on the cladding of a fuel assembly from the region of highest temperature shall be evaluated. If any unexpected oxidation or resulting growth of the cladding is detected during such an inspection, appropriate actions will be taken, such as removing all assemblies that have similar time-at-temperature histories or moving them to cooler areas in the core. Alternatively, reactor fuel temperatures could be limited to 400°C until a complete evaluation is made.
