Evaluation of PWR fuel ‘leakers’

3.5.1. PWR fuel failure rates

FA failure rates for the United States of America (69 units), France (58 units), and the worldwide average are presented in Fig. 3.3, and those for Europe, with the exception of France (31 units), Japan (23 units) and the Republic of Korea (16 units) are presented in Fig. 3.4. The world curve in Fig. 3.3 shows the average result for countries and region (Europe–France), including Brazil and one PWR from China, thus summarizing fuel performance for 95% of PWR units worldwide.

Figures 3.3 and 3.4 show that there is an overall downward trend number of fuel leaks, with the exception of two increases. These are observed in 1995 and 2001 for PWRs in Europe, including France, the Republic of Korea, the United States of America and worldwide. Only Japan has a leak level of practically zero for the entire period 1994–2006. Average FA failure rates (number of failed FAs per 1000 discharged FAs) for 1994–2006 are:

— World average — 13.8

— United States of America — 20.9

— France — 8.8

— Europe-France (Belgium, Germany, Spain, Sweden, Switzerland, UK) — 16.0

— Japan — 0.5

FIG. 3.1. Ratio of FR/FA leaks in PWRs.

0

1994 1996 1998 2000 2002 2004 2006

Year FFAS FFRs

Number of leak FAS and FRs

FIG. 3.2. Ratio of FR/FA leaks in BWRs.

TABLE 3.2. CALCULATION OF FUEL FAILURE RATES IN JAPAN IN 2006 (EXAMPLE)

Type PWR BWR

Units 23 32

Number of FAs in all cores 3579 20828

Number of FAs in reloads 816 1952

Number of FRs in all cores 834 316 1 479 602

Number of FRs in reloads 197 394 144 112

Number of failed FAs 2 1

Number of failed FAs per 1000 discharged FAs (FA failure rate) 2.4 0.5

Number of leaking FRs, new ‘reload’ model 3.2 1.1

Number of leaking FRs, old ‘core’ model 2.6 1.1

FR failure rate, ppm, new ‘reload’ model 16.2 7.6

FR failure rate, ppm, old ‘core’ model 3.1 0.7

0 5 10 15 20 25 30 35 40

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload World France USA Number of leak FAs per 1000 discharged FAs

FIG. 3.3. PWR FA leak rate for France, the United States of America and worldwide.

0 5 10 15 20 25 30 35 40

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload Number of leak FAs per 1000 discharged FAs

The world average PWR fuel rod failure rates calculated for reload batches (using the ‘new’ methodology proposed for present study) and for core inventories (using the ‘old’ methodology in Technical Reports Series No. 388) are presented in Fig. 3.5.

By definition, the curves in Fig. 3.5 look similar, with the same tendency in rises and declines. However, the fuel rod leak rate is lower for the last three years reviewed (2004–2006) in comparison to the whole time period 1994–2006. Average fuel rod leaker rate values for 1994–2006 are given in Table 3.3. This table indicates that rates calculated for reloads are around five times higher than those calculated for core inventories. This fivefold increase in the fuel rod failure rate derives from the average reload batch size, with a factor of approximately four, a rise due to the increased average number of failed fuel rods in an assembly (from 1.3 to 1.6) and further small changes due to variation in the average number of rods per assembly.

3.5.2. Distribution of failure causes in PWRs

The causes of PWR fuel leaks in the United States of America, France and other European countries (with exception of France) are presented in Figures 3.6–3.8, respectively. A worldwide summary of fuel leak causes for the period 1994–2006 can be seen in Fig. 3.9.

Grid to rod fretting is the dominant fuel rod leaker mechanism in PWRs worldwide, reaching up to 65% in the United States of America, 39% in France and 37% in Europe (excluding France), which is in agreement with Refs [3.18, 3.19]. The second most common cause is debris related failures, which add up to 6%, 11% and 18%, respectively, for the three regions. The third greatest cause is fabrication related failures (~5%), which are more or less uniformly distributed throughout regions and time. Crud/corrosion related failures are not typical for PWRs.

Currently, the world average for this type of failure is 4%, primarily due to a large number of such failures in the Republic of Korea between 1994–2006, where 40 of a total of 90 rods failed due to Zry-4 cladding corrosion (thus Republic of Korea’s figures add up to 44% of the total) [3.20]. Some random corrosion related failures were also observed in the United States of America, though no failures of this type were observed in other countries. Two isolated cases of PCI–SCC type failures were observed in US PWRs, and there were five handling related failures worldwide.

TABLE 3.3. AVERAGE PWR FUEL ROD LEAKER RATES FOR 1994–2006

World United States of America France Europe—France Japan Korea, Rep. of

Rate (‘new reload’), ppm 86.8 131.6 56.9 108.1 3.7 40.5

Rate (‘old core’), ppm 18.2 25.8 11.4 22.2 0.7 8.7

0 20 40 60 80 100 120 140

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

FR failure rate, ppm

FRFRNew FRFROld

FIG. 3.5. PWR world average fuel rod leak rate calculated using new ‘reload’ and old ‘core’ methodologies.

0 20 40 60 80 100

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Crud/Corr Debris Fabricat Gr-R Fret

Handling PCI-SCC Unknown

Number of failed FAs

FIG. 3.6. PWR fuel leak causes in the United States of America.

0 5 10 15 20 25 30 35 40 45

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Number of failed FAs

Debris Fabricat Gr-R Fret Unknown FIG. 3.7. PWR fuel leak causes in France.

0 5 10 15 20 25 30 35 40

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Number of failed FAs

3.5.3. Number of reactors free of defect evolution

Another way to present the fuel reliability trend is to evaluate the percentage of units experiencing an outage during each year and by reporting zero fuel failures. The percentage of ‘reactors free of defect’ for a specific year is the number of reactors free of defect with an outage in that year divided by the total number of reactors with an outage in the same year. Again, calculations were done using TWGFPT and other data sources for France, Europe (excluding France) and Japan (Fig. 3.10). EPRI only reported data on plants free of defects in the United States of America for the period 2000–2006 (EPRI’s FRED database began in 2000); thus the US data only cover the period 2000–2006. For 1994–1999, the total curve reflects the percentage of defect free PWR units worldwide, with the exception of the United States of America. Average values are: Total — 76.6%, United States of America — 62.7%, France — 75.6%, Europe (excluding France) — 68.6 and Japan — 98.0%.

Figure 3.10 shows a tendency toward fuel reliability improvement in 1994–2000 for Europe as well as worldwide. However, no significant changes in fuel reliability have been observed between the years 2001–2006.

3.5.4. Major observations regarding PWR fuel failures

— The world average (1994–2006) fuel failure rate is 13.8 leaking FAs per 1000 discharged FAs;

— Failures range from 17 in 1994–1996 to 9.5 in 2004–2006, rising in 1995 and 2001 to 20.5:

— the rate increase in 1995 was due to grid to rod fretting fuel failures in several plants in Europe, and a massive fabrication fuel failure in one plant in the United States of America;

— the increase in 2001 was also due to massive fuel failures (grid to rod fretting in one plant in the United States of America and specific fretting at the bottom grid level in one plant in France);

— Grid to rod fretting failures were highest (~75%) in 1999–2002.

3.6 11

5

54.8 0.4

0.1

25.1

Crud/Corr Debris Fabricat Gr-R Fret Handling PCI-SCC Unknown FIG. 3.9. PWR fuel leak causes worldwide in 1994–2006.

0 20 40 60 80 100 120

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Total Europe-France France USA Japan Percentage of PWRs free of fuel leak

FIG. 3.10. Percentage of PWR units with zero fuel leakers.