STUDSVIK/NF(R)-69/83 Studsvik Report FAILURE DEVELOPMENT IN LEAKING LWR FUEL RODS — A LITERATURE tSURVEYJ D O Pickman Studsvik Nuclear CONTENTS Page LIST OF TABLES iii LIST OF ILLUSTRATIONS iv 1. INTRODUCTION 1 2. CAUSES AND CHARACTERISTICS OF DEFECTS 2 3. EFFECT OF DEFECTS ON FUEL PERFORMANCE 6 4. DEFECT DETERIORATION MECHANISMS 8 4.1 Clad Bore Surface Oxidation 10 1.2 U0_ Oxidation and Gap Conductance 11 4.3 Secondary Hydriding Mechanism 14 5. TYPICAL DEFECT BEHAVIOUR 17 5.1 Small PCI Defects 17 5.2 Fretting Defects 22 5.3 Hydride Defects 25 5.4 Other Influences on Defect Behaviour 26 6. DEFECT DETECTION 32 6.1 Measurement Techniques 35 6.2 Activity Release Measurement Requirements 37 6.3 Activity Measurement Systems 37 6.4 Activity Monitoring Systems in Use 38 6.4.1. French PWR Practice 39 6.4.2. FRG Practice 43 6.4.3. U.S. Practice 45 6.4.4. Canadian Practice 48 6.4.5. Swedish Practice 49 6.4.6. U.K. Practice 53 6.4.7. Fiscellaneous Practices 54 Page 6.5 Activity Release levels 56 7. DISCHARGE CRITERIA 61 7.1 U.S. Practice 62 7.2 French Practice 63 7.3 U.K. Practice 6*4 7.4 Japanese Practice 65 7.5 Practice Elsewhere 65 8. DESIGN ASPECTS 65 9. OPERATIONAL ASPECTS 69 10. REMEDIES 72 10.1 Reduced Rating 72 10.2 Gap Atmosphere Control 73 10.3 Hydrogen Getters 74 10.4 Barriers 76 10.5 U02 Pellet Design 77 11. PROP^.ILS FOR FUTURE WORK 78 12. SIP / G 80 13. C<i • -.J3I0NS 88 14. FffSFiNCES 93 ^ 1£S 101 1 .USTRATIONS 104 /. PENDIX A A'.PENDIX B /fl'ENDIX C APPENDIX D BIBLIOGRAPHY ii TABLES Table Title Page 1. Radio-Isotopes of Interest for Activity Release Measurements. 101 2. Cycle Burn-Up and Primary Coolant Activity; Status of EDF's 900 MW (e) PWR Units in Commercial Operation as of June 15th, 1981. 102 3. Behaviour of Some Unusual Defects in the SGHW Reactor 103 ILLUSTRATION'S Figure Title Page 1. Examples of fretting damage: (a) 0.25am deep hole at fixed stop in spring grid design (b) spacer pad wear at grid, (c) smooth clad wall thinning from contact with ring grid. 2. Small PCI crack believed to have re-sealed. 3. Primary hydride sunbursts, (top) early stage but stress effect on hydride orientation already noticeable, (middle) major sunburst with only minor leak, formed in 85 days irradiation, peak LHGR 520W/cm, (bottom) sunburst with large leak showing beginning of hydride re-conversion to Zr. 4. Massive hydride sunburst showing lamination effect resulting from ZrH, , re-conversion to Zr in inner i .o region of cladding. 5. Axial PCI cracks. 6. Bulk off-gas and iodine release data for individual rods with small to moderate defects over first 100 days. 7. Small oxide nodule on bore of cladding opposite radial crack. 8. Deltoid hydriding at mouth of partial penetration PCI crack. Note hydride re-orientation in plastic zone at crack tip. 9. Proposed failure limits (surface heat flux/time to failure) for Zircaloy clad LWF fuel. 10. Bulk off-gas and iodine release data for individual rods with severe defects over first 20 days. IV 11. Transverese clad fracture near end of BWR fuel rod ascribed to fatigue failure in hydrided region. Incipient defects seen at both ends of a pellet in rod 2 below. 12. Axial and 45° cracks in defect rod 999-H2 from Ringhals 1. Probably primary defect, 598mc from bottom end. Original x 6 (See also Figure 14). 13. Spalled bulge in defect rod 999-H2 from Ringhals 1. Probably secondary hydride defect 2100mm from bottom Original x 6 (See also Fig. 15) 14. Section of defect in Ringhals 1 rod 999-H2 at 598mm from bottom. Believed to be primary defect. Original x 53 15. Section of defect in Ringhals 1 rod 999-H at 2100mm from bottom. Probably secondary hydride defect. Original x 53. 16. Tihange 1 parametric study. Correlation of gap escape rate constant with isotope ratios. 17. Correlation between inferred diffusion constants and linear heat generation rate for intact and defect rods. 18. Relationship between fractional release and decay constants for fission gases and iodines. 19. Fission gas and iodine spiking effects on shutdown and start-up. 20. Activity release following a shutdown and during power ramping. 21. H_ flux vs. time to saturate a Zircaloy-2 surface, (calculated) 22. Hydrogen absorption by Zircaloy-2 at 400°c. FAILURE DEVELOPMENT IN LEAKING LWR FUEL RODS D.O. PICKMAN 1. INTRODUCTION Leaking fuel rods, although undesirable, are tolerable in LWR's provided the activity release is within acceptable limits and they are not subject to rapid unpredictable deterioration. This review of failure development is based largely on a literature survey backed up by some personal interpretation and judgement where conflicting evidence appears to exist. In any review of this field it is necessary to recognise that there are several known causes of defects which lead to a leak of fission products in LWR fuel rods. Some are related to rod or system design, some to reactor operational aspects and some to defects in fuel rod or reactor manufacture. The type of defect and how it develops during continued operation is often dependent on the cause. A brief review of known causes of defects in LWF fuel rods is included. Primary defects in fuel rods c-jy not be single, isolated, leak sites, PCI defects being a classical example where very many penetrating cracks can form in a single fvA rod. However, all such leaks formed by the same event are primary leaks. The only secondary defect mechanism is internal i. -Iriding, although knock-on events can follow on the same, or neighbouring rods, from defects that cause an interference with local coolant flow. - 1 - The major part of this survey comprises information on leak rates fro» typical defects, causes and rates of development of primary leak sites, mechanisms and rates of development of secondary hydride defects and analysis of activity release measurements in an attempt to characterise the number and severity of defects present. 2. CAUSES AND CHARACTERISTICS OF DEFECTS The early history of defects in LHRfS was reviewed by Locke. This showed that internal hydriding, crutf deposition, fretting (both by spacers and foreign bodies) and power ramping (PCI) have been the principal causes of defects in BWR fuel rods. In PWR's crud deposition has not been a cause of defects because of lower primary circuit copper content. Fretting experience has been similar, although one source of debris, pieces of stainless steel wire from steam separators, is absent. For various reasons PWR'S have also been freer from PCI defects, but have experienced problems of clad flattening following U0? densification, rod bowing and rod lengthening. These latter two phenomena have not led to defects but to life limitation. Rod lengthening also led to some life linitation in a few BWR's. Except when enhanced by dense crud, waterside corrosion has not caused defects in BWR or PWR fuel, despite some early pessimistic predictions, nor has the associated hydrogen, pick-up. The move in recent years to higher burn-up is now, however, causing concern in relation to clad corrosion as a possible life limiting phenomenon, which could cause defects in both BWR's and PWR's. In looking at the characteristics of defects, some can be forgotten as not relevant to modern BWR's and PWR's operating in Sweden. The causes of relevant defects are fretting by foreign bodies and PCI, with hydride defects also of interest because their mechanism is relevant to secondary - 2 - hydriding. Fretting defects have the characteristic of rapidly enlarging from a small pin-hole leak to a size determined by the object causing the leak. If caused by a spacer for example they can become large leaks with exposed UO very quickly. The PWR baffle jetting defects and some in elements with early wire type sparer g»-ids in BWR*s were of this type (Fig. 1). Fretting i-i caused by a tapping or tapping plus sliding contact between two components. It is probably due tc enhanced corrosion because of the continuous removal of the protective oxide layer which is trying to form, assisted by welding and tearing of asperities or fatigue (Pi of asperities. "' It is believed that a number of recent defects in Swedish BWR's were caused by fretting between foreign bodies trapped in spacer grids and the fuel cladding. The size of penetration produced will depend on the geometry of the foreign body and its freedom of movement. It is a characteristic of fretting damage that, once started, it progresses rapidly. However, fretting caused by foreign bodies trapped in spacer grids will have a random start time, depending on when the foreign body was captured. PCT defects, unlike fretting, form almost instantaneously at the time of a power ramp and may be very minor single cracks or a number of large axial cracks of substantial width. The cause of PCI defects has been exhaustively studied. It is a form of stress corrosion cracking with iodine as the most probable corrosive agent. The stress level at which cracking starts is below the yield stress and reduces with irradiation damage to the cladding, typically reducing with burn up from around 500MPa to 200MPa. Stress concentrations at radial pellet cracks which open during a power increase and bi-axiality at pellet ends are also is-pcrtar.t factors. Some of the minor PCI defects no longer leak fissicr products when the power level is reduced and may be effectively sealed for the remainder of the fuel rod life (Fig.