Evaluation of WWER fuel ‘leakers’

3.7.1. WWER fuel failure rates

This section reviews fuel failure data for WWER-1000s and 440s. WWER-1000s operate in Bulgaria (2 units), the Russian Federation (9 units) and the Ukraine (13 units), adding up to a total of 24 units. Two Temelin units in the Czech Republic with WWER-1000 reactors possessing some Westinghouse modified safety systems and fuel are not included and will be considered separately. WWER-440s are divided into two groups: 18 units of the newer 213 design, abbreviated WWER-440/213, (Czech Republic, 4 units; Finland, 2 units; Hungary,4 units;

the Russian Federation, 2 units; Slovakia, 4 units; and Ukraine, 2 units) and 11 units of the ‘older’ 230/179/270 designs, abbreviated WWER-440/230, (Armenia, 1 unit; Bulgaria, 4 units, now all shutdown; the Russian Federation, 4 units; and Slovakia, 2 units).

Figure 3.17 presents the number of leaking FAs per 1000 discharged FAs observed during outages in WWER reactors. The average number was: Bulgaria, 45; the Russian Federation, 27.5; and Ukraine, 33.9 The average for all WWER-1000s operated between 1994–2006 is 32.1. The maximum FA failure rate was observed in Bulgaria and the Russian Federation in 2001 and in Ukraine in 2004. It is worth noticing that fuel reliability significantly improved for units in the Russian Federation between 2003–2006, dropping to 12.3 failed FAs per 1000 discharged FAs. Progress was also obviously seen for Ukrainian units in 2005–2006 (with an average rate of 17 failed FAs per 1000 discharged FAs).

Two Temelin units use Westinghouse fuel of Vantage 6 design with Zry-4 claddings, intermediate grids and guide tubes, and Inconel top and bottom grids. Grid to rod fretting was a major cause of leaking in one FA in Temelin 1 in 2004, 5 and 3 FAs in 2005 and 6 and 10 FAs in 2006, in Units 1 and 2 respectively (see Fig. 3.17 and answers to the questionnaire).

The average annual FA failure rate in WWER-440/213s has been kept rather low annually for the entire 1994–2006 period (4.7 failed FAs per 1000 discharged FAs per year). Increases in the FA failure rate in 1995 (12 failed FAs per 1000 discharged FAs) and in 2001–2002 (7 failed FAs per 1000 discharged FAs annually) were due to 9 failed FAs in Loviisa-2 in 1995 and 6 and 4 failed FAs in Kola-3 in 2001 and 2003 respectively.

0

FIG. 3.16. Percentage of BWR units with zero fuel ‘leakers’.

The average annual FA failure rate in WWER-440/230s for 1994–2006 was 17 failed FAs per 1000 discharged FAs. This failure rate was steady throughout the period. However, fuel failure statistics are rather poor for 2003–2006 — from the nine reactors operating in this period, data were only available for three (Kola-1 and Bohunice-1 & 2).

Figure 3.18 shows the WWER fuel rod leaker rate for reload batches (using ‘new’ methodology).

A maximum fuel rod leaker rate was observed in 2001 (286 ppm for WWER-1000s and 149 ppm for all WWERs).

The rate increase in 2001 was due to massive fuel failures at WWER-1000s: Balakovo-1 and 2 (9 FAs each) and Rovno-3 (11 FAs), where failure was suspected to be due to debris. The average values for 1994–2006 are: 134 ppm for WWER-1000 fuel, 34.5 for WWER-440/213, with an overall average for all WWERs of 94 ppm. An evaluation of the WWER fuel rod leak rate using ‘core inventory’ (the ‘old’ methodology used in Technical Reports Series No. 388) was also made (Fig. 3.19). The average values for 1994–2006 are: 39.8 ppm for WWER-1000 fuel, 8.7 ppm for WWER-440/213, with an overall average for WWERs of 25.2 ppm. Maximum fuel rod failure rates were observed in 2001 and were recorded to be 84.5 ppm for WWER-1000s and 40.2 ppm for all WWERs.

As mentioned above, significant decreases in FR failure rates were observed for all WWER fuel types over the last 2–3 years.

3.7.2. Distribution of failure causes for WWER fuel

According to an official paper by the Russian Federation representative to the IAEA Technical Working Group on Water Reactor Fuel Performance and Technology (TWGFPT), cladding leaktightness tests and visual inspection at NPP spent fuel storage pools and PIE in RIAR hot cells revealed the following major causes of leak failure for WWER-1000 fuel:

— Debris damage to FR claddings;

— Fretting wear on FR plugs in the bottom support grids;

— Displacement of FRs during transportation.

Grid to rod fretting, crud/corrosion, PCI/SCC and manufacturing issues were not identified as causes of failure. As noted in this paper [3.11], debris type failures in WWER-1000s accounted for 14.2% over the 2002–2006 time period, while mechanical damage of lower plugs reached 5.6%, and the remaining 80.2% of failures stemmed from undetermined causes. The Ukrainian representative to the IAEA TWGFPT, reported that debris fretting was a suspected cause of failure for WWER-1000 fuel in Ukrainian WWER-1000s.

The Russian Federation’s report to the IAEA 2007 TWGFPT Plenary Meeting identified the following major causes for leaking of WWER-440/230 fuel: Number of failed FAs per 1000 discharged FAs

FIG. 3.17. Annual FA failure rate for WWERs.

— Fretting wear (claddings, spacer grids, FR plugs);

— Debris induced damage to FR claddings;

— Deposits in FR bundles.

WWER/213s fuel reliability is quite high — the FR failure rate, calculated using Technical Reports Series No. 388 methodology, is lower than 10 ppm. Only one significant fuel failure in these reactors is known; 9 FAs failed in Loviisa-2 in 1995 due to deposits in the fuel bundle; details are available in Section 5. A dismountable WWER-440 FA design was developed and implemented in the beginning of 2000, initially to serve Loviisa NPP, Finland. The Finnish Safety Authority, STUK, required a pool side inspection of failed FAs. Failure causes at Loviisa NPP Units 1 and 2 were well investigated, however no failures were observed starting in 2000, thus Finnish fuel failure experience covers only the years prior to 2000 [3.23]. Earlier causes of failure are listed in the following order: grid to rod fretting — 39%, crud/corrosion — 23%, manufacturing — 3%, unknown — 35%.

The difference in fuel failure rates between WWER reactors with 213 and 230 designs can be explained by several factors. For example, there are four units with WWER-440 reactors at Bohunice NPP in Slovakia — Units 1 & 2 (230 design) and Units 3 & 4 (213 design). Between 1986–2001, there were 47 leaking FAs in Units 1 (15) & 2 (32) and 2 leaking FAs in Units 3 & 4 loaded with the same fuel. The difference between the two 230 units is also noticeable (15 and 32 leaking FAs). Analysis shows [3.24] that:

0

FIG. 3.18. WWER fuel rod ‘leaker’ rate calculated for reload batches (‘new’ methodology).

0

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload

Fuel rod leak rate, ppm

WWER1000 WWER440/213 WWER All

FIG. 3.19. WWER fuel rod ‘leaker’ rate calculated for core inventories (‘old’ methodology).

— In-core component vibrations are more intensive in the 230 units compared to the 213 units;

— The evaluation of analysis confirmed more significant fuel vibrations with 230 reactors than 213 reactors;

— Long term monitoring of 230 units and comparison between both units confirmed higher vibration amplitudes of the basket at Unit 1, while Unit 2 monitoring drew attention to more intensive frequency vibrations of in-core components. An explanation of the differing failure rates between the two units could be that Unit 1 has a basket rigidly connected with its bottom, while at Unit 2, there is no such connection.

According to the Russian Federation contribution, in 2000–2002, an increased FA vibration load was also seen at Kola-2 with the WWER-440/230 reactor, which resulted in an FR failure increase greater than 50 ppm (using the ‘old’ methodology). After implementation of a vibration resistant FA design, the failure rate decreased.

Units 3 & 4 of the Novo-Voronez NPP, which has WWER-440 reactors of the ‘old’ 179 design, operated in 1996–2002 with a fuel failure rate of ~ 50 ppm (using the ‘old’ methodology); this rate has since increased due to deposit accumulation on fuel bundles. At present, a programme on FA cleaning is underway to enhance further use of these FAs.

3.7.3. Number of reactors free of defect evolution

Figure 3.20 presents the percentage of all WWERs units which ran with zero fuel ‘leakers’. The average values for all WWER units with zero ‘leakers’ between 1994–2006 were 43.4% (WWER-1000s), 79.4%

(WWER-440s/213) and 57.6% (all WWERs). Distribution by country for WWER-1000s during 1994–2006: 31.9%

for Bulgaria, 39.6% for Ukraine and 52.7% for the Russian Federation.

3.7.4. Major observations regarding WWER fuel failures

— The world average (1994–2006) fuel failure rate is 15.1 leaking FAs per 1000 discharged FAs;

— For all WWERs, the range is from 6 to 18 leaking FAs for the whole time span with the exception of 2001 (27);

— A rate increase in 2001 was due to debris suspected massive fuel failures at WWER-1000: Balakovo-1 and 2 (9 FAs each) and Rovno-3 (11 FAs);

— Failure causes for WWER-1000s were attributed to debris damage, fretting wear of FR plugs in the bottom support grids, and displacement of FRs during transportation. For WWER-440s, fretting wear (claddings, spacer grids, FR plugs), debris induced damage to FR claddings and deposits in the FR bundle were mentioned as causes of failure.

0 20 40 60 80 100 120

1994 1996 1998 2000 2002 2004 2006

Year of fuel reload Percentage of WWERs free of fuel leak

3.8. EVALUATION OF FUEL ‘LEAKERS’ IN CANDUs/PHWRs