Evaluation of BWR fuel ‘leakers’

3.6.1. BWR fuel failure rates

Data presented in this section describe fuel leaker rates in the United States of America (34 units), Japan (32 units), Europe (Finland — 2, Germany — 6, Spain — 2, Sweden — 9 and Switzerland — 2, total — 21 units) and India (2 units) for 1994–2006. Only two units located in Mexico are not included in this section. Figure 3.11 shows BWR FA leak rates for 1994–2006 in the United States of America, Japan and the European region.

In the last decade, BWR fuel failure rates have been three times lower than those for PWR fuel. Similar to an effect observed with PWRs, Fig. 3.10 shows a tendency for recurrent peaks in the leaker rate. The maximums are observed in 1994, 2000 and 2002 for Europe, and in 1998 and 2003 for the United States of America, with a leaker level of practically zero for Japan between 1994–2006. Average FA failure rates (number of failed FAs per 1000 discharged FAs) for the entire period 1994–2006 are:

• World average — 4.4;

• United States of America — 5.4 (4.3 if a massive fuel failure at the BF-2 in cycle 12 is excluded; see below);

• Japan — 0.4;

• Europe (Finland, Germany, Spain, Sweden, Switzerland) — 6.8.

BWR fuel rod failure rates calculated for reload batches (using the methodology proposed for the present study) and for core inventories (using the methodology from Technical Reports Series No. 388) are presented in Fig. 3.12.

Analysis of initial data shows that a sharp increase in the US BWR failure rate in 1998 and 2003 contributed to the increased world average fuel failure rate observed in these years. Average rod leaker rate values for 1994–2006 are shown in Table 3.4. Surprisingly, the average fuel rod failure rate for reloads is slightly higher for Europe than the United States of America, but the rate calculated for core inventories is lower. This is explained by differences in fuel reload schemes for BWRs in the United States of America and Europe. In the United States of America, the BWR fuel cycle length is 17–21 months (data provided by EPRI for this study, though in published American papers PWR and BWR cycles are often indicated to be two year cycles), European BWRs generally operate on 11–12 month cycles (except Spain, which has a 15–24 month long cycle). Due to the relatively smaller sizes of reload batches in European BWRs, the fuel failure rate calculated for core inventories is smaller than the reload related fuel failure rate for the same number of failed FAs. In the old ‘core’ calculation scheme, the share of failures in the whole core inventory depended on reload batch size, approaching the real failure rate only when the whole core was reloaded, while in the new ‘reload’ scheme, calculations will provide statistically equivalent results irrespective of batch size.

0 5 10 15 20

1994 1996 1998 2000 2002 2004 2006

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

A comparison of Tables 3.3 and 3.4 shows that the BWR fuel rod leaker rate in 1994–2006 is lower than that for PWRs. The same tendency was observed in previous years (1987–1994) as indicated in Technical Reports Series No. 388.

3.6.2. Distribution of failure causes in BWRs

The distribution of failure causes in United States of America BWRs is shown in Fig. 3.13. Accelerated cladding corrosion and tenacious crud deposition were recognized as being behind massive failures in 1998 (46 failures were due to crud induced localized corrosion-CILC [3.21]) and in 2003 (63 crud/corrosion induced failures in the Browns Ferry Unit 2 during cycle 12 [3.22]). If BF-2 failures are excluded from the statistics, corrosion/crud and debris related fuel failures in US BWRs are at the same level.

Debris related failures dominate in European BWRs with a significant amount of PCI-SCC failure types before 2002; see Fig. 3.14.

Figure 3.15 reveals an estimate of BWR fuel failure causes worldwide. It is seen that crud/corrosion and debris related failures occur at the same frequency. PCI-SCC related failures are more or less uniformly distributed throughout the period 1994–2006, practically independent of the implementation of barrier cladding. Fabrication related failures were not observed during the last three years (2004–2006).

3.6.3. Number of reactors free of defect evolution

Figure 3.16 shows the percentage of leaker free BWR units worldwide and in Japan for the period 1994–2006, and in the United States of America for the 2000–2006 time frame, much like Fig. 3.10 for PWRs. The two figures show very similar trends. Notably, the average value for Japan throughout 1994–2006 was 96.0%. The world total percentage with the exception of the United States of America was 73.7%, while the United States of America and worldwide averages for 2000–2006 were, respectively, 63.3% and 77.6%.

TABLE 3.4. AVERAGE BWR FUEL ROD LEAKER RATES FOR 1994–2006

World average United States of America Japan Europe

Rate (new, ‘reload’), ppm 64.7 78.9 6.9 101.4

Rate (old, ‘core’), ppm 10.7 14.2 1.3 14.0

0 20 40 60 80 100 120 140 160

Year of fuel reload

FR leak rate, ppm

FRFRNew FRFROld

1994 1996 1998 2000 2002 2004 2006

FIG. 3.12. BWR world average fuel rod leaker rates calculated using ‘new’ and ‘old’ methodologies.

0 10 20 30 40 50 60 70 80

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Number of failed FAs

Crud/Corr Debris Fabricat PCI-SCC Unknown

FIG. 3.13. BWR fuel failure causes in the United States of America.

0 2 4 6 8 10 12 14

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Crud/Corr Debris Fabricat PCI-SCC Unknown

Number of failed FAs

FIG. 3.14. BWR fuel failure causes in Europe.

32%

31%

4%

9%

24%

Crud/Corr Debris Fabricat PCI-SCC Unknown FIG. 3.15. Estimated world distribution of BWR fuel failure causes.

3.6.4. Major observations regarding BWR fuel failures

— The worldwide average (1994–2006) fuel failure rate is 4.4 leaking FAs per 1000 discharged FAs;

— This value is lower by a factor of about three than that for PWR fuels;

— The span of the failure rate ranges from 2.5 to 5, with two spikes: 7.9 in 1998 and 10 in 2003;

— These two increases were due to massive crud/corrosion failures in one plant in 1998 and in a second plant in 2003 in the United States of America.