Other miscellaneous damage

Over the last decade, a significant number of Westinghouse fuel assemblies have been affected by fracturing of the hold-down spring screw on the top nozzle. These failures have sometimes caused assembly handling difficulties. The cause of the failures was stress corrosion cracking.

In several cases, repair of fuel assemblies was required. The top affected nozzle in the Inconel 718 model was replaced by a new top nozzle equipped with screws less sensitive to stress corrosion cracking.

6.5.2. Resonance vibration, Darlington 2

In November 1990 [6.44], a routine fuelling operation for channel N12 of Darlington Unit 2 was aborted owing to difficulties encountered in the insertion of a pair of fuel bundles recycled from another channel. A follow-up investigation showed that the centre seven elements of the downstream bundle had broken loose, and had interfered with normal refuelling operations. These elements had been carried past the fuel latch by coolant flow through the channel, and had obstructed other bundles from being completely inserted into the channel. The bundle was extensively damaged during the attempted refuelling operation. The damage increased the activity levels of most fission products in the primary circuit. The increased activity levels were difficult to quantify at the time, since the GFP system was not fully commissioned.

Inspections of other outlet bundles in Darlington fuel bays revealed the presence of end plate cracks. PIE of the damaged fuel revealed that the end plate cracks were the result of high cycle/low amplitude fatigue. Subsequent investigations demonstrated that the five vane impellers of the primary circuit pumps introduced pressure pulsations which were acoustically amplified within certain channels. The pulsation frequency of 150 Hz coincided with the resonant frequency of the inner seven fuel elements of the 37 element bundle. With fuel column latch support, which is unique to the Darlington and Bruce reactors, the non-outer fuel elements are unrestrained and free to vibrate in an axial direction. Axial vibration at the resonant frequency led to end cap cracking.

To eliminate the acoustic amplification of pressure pulsations in the fuel channels and to decouple the axial resonant response of the fuel, five vane pump impellers were replaced with seven vane impellers. This change shifted the pressure pulsation frequency from 150 to 210 Hz, which eliminated the end plate cracking problem at the Darlington reactor.

6.5.3. Lower end cap fretting, Oskarshamn 1

During the May 1988 refuelling outage at Oskarshamn 1 reactor, foreign objects resembling ‘nails’ were found in the reactor vessel [6.45]. These objects were seated on the fuel inlet orifice plate located under each fuel element. Subsequent checks revealed that these objects were the remains of lower end cap shanks from spacer capture rods (SCRs) of Advanced Nuclear Fuels Corporation (ANF) supplied fuel. An inspection of all ANF fuel assemblies in the core revealed that of 338 irradiated assemblies from five different reloads, 127 had varying degrees of fretting damage to the SCR lower end cap. In 55 cases, the end cap had completely worn off.

The shape of the SCR end cap wear pattern led to the strong suspicion that the problem stemmed from flow induced vibrations. In 1986, the reactor underwent some modifications to improve fuel cycle economics. These modifications resulted in the maximum average core flow changing from 6950 to 7300 kg/s. Fuel assemblies loaded after 1983 had incorporated a longer SCR lower end cap with a 4 mm projection below the lower tie plate.

To evaluate the effect of changes to fuel design and reactor operating conditions on lower end cap fretting, a large number of tests were performed. The results of the testing programme and the evaluation showed that the root cause of the problem was excessive pressure fluctuation caused by lateral oscillations of a water jet at the inlet orifice directly below the lower tie plate of the fuel assembly. This jet was found to oscillate laterally, producing lateral cross-flows which, in turn, subjected fuel rods to oscillating lateral forces, causing the rods to vibrate. The magnitude of forces driving the vibrations was found to increase with flow rate.

In order to stabilize flow in the conical diffuse zone, a 10 hole orifice was developed. This orifice was tested and installed, as results demonstrated that rod vibration amplitude was small enough. Further steps in design were taken to prevent recurrence in any reactor.

6.5.4. Hydrogenation of Zry guide thimbles, Ringhals 2

In 1990, Zry guide thimbles in two fuel assemblies were damaged during handling in Ringhals 2. Hot cell examinations showed that the damage was due to material embrittlement in the cold state by excessive hydrogenation, i.e. external hydriding. More detailed investigations and laboratory tests revealed that the effect was caused by a combination of specific water chemistry parameters during initial startup after refuelling, and a particular susceptibility to hydrogen uptake of the fresh grit blasted inner surfaces of guide thimbles under these conditions. The main water chemistry factor was a combination of high initial nickel concentration plus the early addition of hydrogen. As coolant heats up, nickel can precipitate as a metal on the fresh surface and, probably assisted by specific impurities embedded in the surface, can act as a catalyst to intensify hydrogen uptake [6.46].

The plant startup procedures in place had been used before, and guide thimbles subject to the applied process of grit blasting had been used before in other plants. The incidental combination of these two proven practices created the problem. A manufacturing process improvement to reduce sensitivity of the fresh surface was introduced and has solved the problem.

6.5.5. Deformation of upper grid rim, WWER-440

Axial clearance for fuel rod irradiation growth in WWER assemblies is sufficient for elongations up to burnups beyond the design value. Nevertheless, in some cases, severe deformation of the upper grid and upper grid rim was observed in wrapped assemblies. The cause was found (and confirmed by testing) to be the jamming of growing fuel rods in the upper spacer grid, which was firmly fixed in the top nozzle. Design changes were made to avoid this effect [6.47].

6.5.6. Displacement of spacer grids in WWER-1000 FAs with Zr based alloy spacer grids and guide tubes Advanced WWER-1000 FAs with Zr based alloy E-110 spacer grids (SGs) and guide tubes (GTs) were first loaded into reactors in 1995, and with E-110 SGs and Zr based alloy E-635 GTs starting in 1996. During inspections of FAs with SGs and GTs made of Zr based alloys operated during 2nd and 3rd cycles in Zaprozhe-3 and Rovno-3 reactors, vertical displacement of SGs was found. After two cycles (burnup ≥ 27 MWd/kgU), movement was observed for SGs from the 10th to the 12th positions (from the bottom, altogether 15 SGs). After three cycles (burnup ≥ 27 MWd/kgU), movement was observed for SGs from the 4th to the 13th positions, thus practically involving 2/3 of the FA volume [6.23] in the SG displacement process. This took place because the SGs were not structurally reliably attached to GTs (either mechanically or through welding). After design debugging, no SG movement was observed.

REFERENCES TO SECTION 6

[6.1] JACOBSON, S., FRANCILLON, E. “Incomplete control rod insertion due to fuel element bow”, Nuclear Fuel and Control rod:

Operating Experience, Design Evolution and Safety Aspects, Madrid, 1996.

[6.2] JADOT, J. et al., “Incomplete rod insertion at DOEL 4 and TIHANGE 3”, International LWR Fuel Performance Meeting, TopFuel 1999.

[6.3] BALEON, J.P, DANGOULEME, D., VRIGNAUD, E., “Traitement des anomalies de chute des grappes de commande rencontrées dans les réacteurs EDF”, Fontrevaud, 1998.

[6.4] FRANCILLON, E., “Remedies to F.A. bowing and incomplete RCCA insertion in PWR”, TOPFUEL 97, Manchester, 1997.

[6.5] ANDERSON, T. “A decade of assembly bow management at Ringhals”, Int. Mtg on LWR Fuel Performance, Orlando, FL, 2004.

[6.6] KEE, E., “Experience with incomplete control rod insertion in fuel with burnup exceeding approximately 40 [6.7]

GWD/MTU”, 24th Water Reactor Safety Info. Mtg, Brookhaven, National Laboratory, 1996.

[6.7] WILSON, H.W., et al., “Westinghouse fuel performance in today’s aggressive plant operating environment”, Int. Topical Mtg on Light Water Reactor Fuel Performance, Portland, 1997.

[6.8] SALAUM, H., BALEON, J.P., FRANCILLON, E., “Analytical approach to PWR fuel assembly distortions”, SMIRT 97, Lyon, 1997.

[6.9] BOSSELUT, D., et al., “Insertion and drop of control rod in assembly: simulation and parametric analysis”, Structural Behaviour of Fuel Assemblies for Water Cooled Reactors, IAEA-TECDOC-1454, IAEA, Vienna (2005).

[6.10] AULLO, M., RAENSTEIN, W.D., “European Fuel Group experience on control rod insertion and grid to rod fretting ”, ibid.

[6.11] KNOTT, R.P., et al., “Advance PWR fuel designs for high duty operation”, TopFuel 2003, Würzburg, 2003.

[6.12] CHAPLIN, D., et al., “EFG Fuel designs and experience in EDF reactors”, TopFuel 2006, Salamanca, 2006.

[6.13] GOTTUSO, D., CANAT, J.N., MOLLARD, P., “A family of upgraded fuel assemblies for PWR”, TopFuel 2006, Salamanca, 2006.

[6.14] PROVOST, J.L., DEBES, M., “EDF PWR Fuel operating experience and high burnup performances”, ibid.

[6.15] REPARAZ, G., et al. “Advanced BWR channels ” Int. Topical Mtg on Light Water Reactor Fuel Performance, Portland, 1997.

[6.16] KARVE, T., et al. “A methodology for calculating and a process for mitigate channel distortion and cell friction”, TopFuel 2006, Salamanca, 2006.

[6.17] MAHMOOD, R., et al. “Channel bow in boiling water reactors — hot cell examination results and correlation to measured bow”, 2007 Int. LWR Fuel Performance Mtg, San Francisco, CA, 2007.

[6.18] VASILCHENKO, I., DEMIN, E., “Operational indices of VVER-1000 fuel assemblies and their improvements”, Proc. 1st Int.

Conf. on VVER Reactor Fuel Performance, Modelling and Experimental Support”, Varna, BAS, INRNE, Sofia (1995) 49.

[6.19] AFANASYEV, A., “The summary of WWER-1000 fuel utilization in Ukraine”, Proc. Int. Top. Mtg on LWR Fuel Performance, Portland, 1997, ANS (1997) 96.

[6.20] BEKIREV, D., NIKOLOV, A., “Kozloduy NPP nuclear fuel cycle experience”, Proc. IAEA TM on Structural Behaviour of Fuel Assemblies for Water Cooled Reactors, Cadarache, 2004, IAEA-TECDOC-1454, IAEA, Vienna (2005) 165.

[6.21] BIBILASHVILI, Y., et al., “Operation experience of WWER fuel, including analysis of abnormal condition”, Proc. 2nd Int.

Sem. on WWER Reactor Fuel Performance, Modelling and Experimental Status, Sandanski, 1997, BAS, INRNE, Sofia (1997) 11.

[6.22] VON JAN, R., WEIDINGER, H.G., “PWR and WWER fuel assembly bow”, Workshop in Rez/Prague, 1998, Summary (Document HGW-02-98).

[6.23] AFANASYEV, A., “Fuel cycle activity and the summary of VVER-1000 fuel utilization in Ukraine”, Proc. TCM, 15th Plenary Mtg, of the Int. Working Group on Water Reactor Fuel Performance and Technology, Vienna, 1999, IWGFPT/49, IAEA, Vienna (1999).

[6.24] VASILCHENKO, I.N., et. al., “Design measures for providing geometrical stability of WWER reactor cores”, Proc. IAEA TM on Structural Behaviour of Fuel Assemblies for Water Cooled Reactors, Cadarache, 2004, IAEA-TECDOC-1454, IAEA, Vienna (2005) 169.

[6.25] VASILCHENKO, I., DRAGUNOV, Y., MIKHALCHUK, A., “Results of trial operation of the WWER advanced fuel assemblies”, Proc. 4th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support”, Albena Resort, 2001, BAS, INRNE, Sofia (2002) 83.

[6.26] TROYANOV, V., et. al., “Lessons learned from the computational simulation of thermo mechanical behaviour of the WWER-1000 reactor cores: FA development and its implantation into the Balakovo NPP Unit 1”, Proc. 5th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2003, BAS, INRNE, Sofia (2004) 200.

[6.27] LIKHATCHEV, Y., et. al., “Theoretical approach to the WWER core thermo mechanical modelling”, Proc. 5th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2003, BAS, INRNE, Sofia (2004) 209.

[6.28] KISELEV, A.S., et. al., “Substantiation of thermal-mechanic stability and strength of TVSA-T type fuel assembly in four-year fuel cycle”, Proc. 7th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2007, BAS, INRNE, Sofia (2008).

[6.29] TROYANOV, V.M., et. al., “Numerical and analytical investigation of WWER-1000 fuel assembly and reactor core thermal mechanics”, Proc. IAEA TM on Structural Behaviour of Fuel Assemblies for Water Cooled Reactors”, Cadarache, 2004, IAEA-TECDOC-1454, IAEA, Vienna (2005) 113.

[6.30] ROZKOV, V., ENIN, A., BEZBORODOV, Y., PETROV, V., “Fuel improvement and WWER-1000 FA main operational results”, Proc. 5th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2003, BAS, INRNE, Sofia (2004) 177.

[6.31] SOLONIN, M., et al., “WWER fuel performance and material development for extended burnup in Russia”, Proc.2nd Int.

Seminar on WWER Reactor Fuel Performance, Modelling and Experimental Support, Sandanski, 1997, BAS, INRNE, Sofia (1998) 48.

[6.32] BIBILASHVILI, Y., et al., “Development of alternate fuel assembly for WWER-1000 Reactor”, ibid., p. 123.

[6.33] MOLCHANOV, V., et. al., “The results of AFA Operation at Kalinin NPP and the trends of further perfection of the fuel based on AFA”, Proc 5th Int. Conf on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2003, BAS, INRNE, Sofia (2004) 182.

[6.34] LAVRENYUK, P., MOLCHANOV, V., TROYANOV, V., IONOV, V., “Nuclear fuel for VVER reactors. Current status and prospects”, Proc. 7th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support, Albena, 2007, BAS, INRNE, Sofia (2008).

[6.35] MARKOV, D.V., et. al., “Basic results of post-irradiation examination of advanced VVER fuel”, ibid.

[6.36] TYUKIN, V., “Operational experience of fuel assembly TVS-2 at Balakovo NPP”, ibid.

[6.37] VASILCHENKO, I.N., et. al., “Choosing the governing solutions for FA of AES-2006”, ibid.

[6.38] SMIRNOV, V.P., “VVER fuel results of post irradiation examination”, TopFuel, Kyoto, 2005.

[6.39] MECIR, V., “Temelin NPP fuel experience”, Proc. 7th Int. Conf. on WWER Reactor Fuel Performance, Modelling and Experimental Support”, Albena, 2007, BAS, INRNE, Sofia (2008).

[6.40] DENNIER, D., MANZER, A.M., RYZ, M.A., KOHN, E., “Element bow from new and irradiated CANDU fuel bundles”, 17th Annual Conf. of the Canadian Nuclear Society, Fredericton, 1996.

[6.41] MANZER, A.M., personal communication.

[6.42] TARIN, F., DONCEL, N. “ Experimental verification of pH and nickel influences on AOA ”, TopFuel 2003, Würzburg, 2003.

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[6.44] JUDAH, J., “Overview of fuel inspections at the Darlington Nuclear Generating Station”, 3rd Int. Conf. on CANDU Fuel, Chalk River, 1992.

[6.45] REPARAZ, A., NORDLOF, S., “An innovative solution to an end cap fretting problem in Oskarshamn Unit 1”, Proc. Int. Top.

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[6.46] SIEMENS, Siemens Nuclear Fuel Report, No. 2 March (1992) 42.

[6.47] SMIRNOV, A.V., et al., “WWER-1000 and WWER-440 fuel operation experience”, Proc. Int. Top. Mtg on Light Water Reactor Fuel Performance, West Palm Beach, 1994, ANS, LaGrange Park, IL (1994) 31.