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Determination of Acceptance Criteria for V VER Steam Generator Tubes Based on Application of Advanced Eddy Current Techniques Damir Stanic, M.Sc.M.E. INETEC - Institute for Nuclear Technology Koturaska 51, Zagreb 10000, CROATIA tel: 385 - 1 - 6171 - 355, 385 - 1 - 6171 - 325 e-mail: damir.stanic@inetec.hr Contact

Structural integrity of steam generator (SG) tubes is of prime importance because tube wall is only boundary between nuclear and "classic" part of power plant. In accordance with experience on NPP's with VVER type of steam generators, several degradation mechanisms cause tube degradation. Considering damage morphology, these degradation mechanisms form different types of damages, depending of theirs nature. Good characterization of damages may be achieved through the development and application of advanced eddy current methods. Due to inconvenient geometry, these techniques were not applied for VVER steam generator examination. Up to now, VVER steam generators were examined only with standard bobbin probe technique which gives limited information about defects and implies application of conservative acceptance criteria based only on flaw depth. Recently, INETEC developed and applied special probes which provide application of advanced eddy current techniques for VVER steam generator examination. Application of these techniques provides much more information about detected damage such as length, width, orientation and others. Understanding of degradation mechanisms nature in conjunction with results of advanced eddy current techniques provides determination of acceptance criteria which take into account morphology of defect. Assessment of defect influence on tube integrity may be achieved through the experiments or by performance of extensive numerical calculations. Application of so-called "alternative" acceptance criteria usually decreases the number of tubes which requires performance of corrective actions (e.g. tube plugging). Beside the significant economical effects which may be achieved, these acceptance criteria increase the safety and reliability of steam generator operation.

1. INTRODUCTION

The steam generator (SG) tube wall is the only boundary between primary and secondary circuit in nuclear power plants. In order to assure reliable steam generator operation without shutdowns caused by primary-to-secondary leakage, it is of prime importance proper definition of acceptance criteria for damages on steam generator tubes. Since the plugging of damaged tubes is the only corrective action on VVER SG tubes, these criteria are usually called plugging criteria. Definition of plugging criteria considers determination of critical value of certain flaw parameter for given loading (normal or accidental operating conditions). Choice of flaw parameter has to take in account the nature of degradation mechanism in order to evaluate the parameter which is most representative for particular mechanism of degradation. This parameter has to be measurable by eddy current (EC) examination technique and measurement uncertainty has to be determined as well. The flaw parameters may be: depth, length, volume or any other. Since flaws tend to grow, the progression rate for inspection interval has to be included in plugging criteria definition, also. Finally, the formula for plugging criteria has a form as presented on the Figure 1.

Fig 1: Definition of Acceptance Criteria


The first step in plugging criteria definition is evaluation of the influence of different flaw morphology on the tube structural integrity. Considering different nature of degradation mechanisms and specific locations of their appearance it is obvious necessity of intensive integrity analysis of damaged tubes. Basis for analysis is good knowledge of degradation mechanisms acting gained through metalography studies on pulled steam generator tubes, as well as, development and application of advanced examination methods capable to provide good information about damage morphology.

2. DEGRADATION MECHANISMS ON VVER SG TUBES

According to INETEC experience, VVER steam generators tubes are affected by several degradation mechanisms. On the basis of various eddy current data (bobbin probe data, rotating probe data), as well as, tubes cut from the first row the present active degradation mechanisms are:
1. IGA/SCC Inter Granular Attack associated with Stress Corrosion Cracking from the secondary side. This corrosion process is characterized by deep thin axial cracks on the places with sludge from the secondary side (under tube support structures, in the free span parts of tubes covered with sludge, low circulation areas of steam generator, etc.). It is the most often and most dangerous type of corrosion found on VVER steam generators, especially because of its high progression rate (Figure 2a).
2. Pitting corrosion was found on the same places as IGA/SCC, but it is characterized with the higher volume of degradation and not so deep as first one. The progression rate of pitting is slower than IGA/SCC, and it is easy to detect in early phases (Figure 2b).
3. Thinning is a general type of surface corrosion in the regions of sludge. It is present on all places in VVER steam generators, but it is very slow and not so dangerous as first two degradation mechanisms.
4. Wear is the type of degradation mechanism which has mechanical cause. The wear appears when tube support structures make contact with the tube because of vibration, degrading its surface. Wear was found but in negligible extent.

Fig 2a: IGA/SCC on VVER SG tube

Fig 2b: Pitting on VVER SG tube

Depending of nature of degradation mechanisms, different shapes of damages appear on steam generator tubes. Considering morphology, the defect may be classified as: cracks (IGA/SCC), volumetric loss of material (wear, thinning) or hole-like damage (pitting). This classification is important from the analytical point of view, because depending of type of the defect, the most suitable method for evaluation of tube structural integrity have to be used.

3. EDDY CURRENT EXAMINATION OF STEAM GENERATOR TUBES

SG tubing condition has to be monitored through the periodical performance of eddy current inspections, which is the most appropriate method in respect of time and quality. Standard examination considers application of bobbin probe technique which gives data about volume and percentage of tube wall loss (Figure 3a). However, variety of degradation mechanisms requires advanced EC techniques to be applied in order to assure reliable detecting and sizing of defects. To obtain more data about damage, such as shape, length and width of defects, indications found by bobbin probe are examined by rotating probe (Figure 3b). Due to inconvenient tube geometry advanced techniques were not applied on VVER SG's. Recently, INETEC started to apply rotating probe technique for VVER tube inspection. This provides better understanding of degradation mechanisms, as well as, for performance of structural integrity analysis on the basis of flaw morphology.

Fig 3a: measurement of depth and volume	Fig 3b: measurement of shape, length and width
Fig 3: Bobbin probe signal (a) and rotating probe signal (b)

4. METHODS FOR STRUCTURAL INTEGRITY EVALUATION

4.1 Analytical methods
These methods are usually aplicable only for simpler flaw morphology. For some type of flaws, determination of proper analytical expression is very complex and time consuming because of flaw geometry and high material ductility. In accordance with applicable regulative [1] for pipes in VVER type of NPP's the following criteria have to be satisfied:

s _eq < [s] , where s _eq - equivalent stress according to maximum shear stress criterion;



Application of safety factors [ 1] (n_m=2.6; and n_0.2=1.5.) leads to the allowable stress intensities as follows:


4.2 Experimental Methods
The most common experimental method for tube integrity evaluation considers performance of burst pressure test. Beside confirmation of analytical solutions, experimental results may provide definition of empirical formula or determination of some unknown coefficients in formula. Test results give the values of burst pressure for different damage parameters (depth, length, shape or any other). Allowable operating pressure is then obtained by dividing obtained value by safety factors. According to US regulative, safety factors are 3 for operating conditions and Ö2 for accidental conditions.

4.3 Numerical methods
Various flaw types and different operating conditions may be very effectively evaluated by numerical methods, such as finite element method (FEM). It provides relatively simple and fast modeling of real circumstances, as well as good ability of visual presentation of results.

5. STRUCTURAL INTEGRITY EVALUATION

5.1 VVER-440 Steam Generator Data
VVER-440 SG consists of 5536 tubes where outer radius is 8mm and thickness is 1.4mm. Tubes are made of austenitic steel 08X18H10T (GOST standard) which material characteristics [ 1] , are given in Table 5.1 for 50^o C and 300^o C (near operating temperature). It should be noted that specified values are minimal values for this material. Tensile tests performed on real tubes showed much larger values than those given in Table. In spite of greater values obtained by testing, the calculations were made assuming minimal values of material characteristics. Further investigations have to confirm that all tubes in particular SG have characteristics as those which were tested. Since tube material exhibits property of extensive yielding, material behavior in plastic area is presented by bilinear diagram.

Temperature[oC]	E[MPa]	n	Rp0,2[MPa]	Rm[MPa]
50	202000	0.3	206	471
300	180000	0.3	177	412
Table 5.2: Tube Material Characteristics

Three types of loading [2] has been taken in consideration: operating conditions (OC), upset conditions (UC) and hydro test (HT). Values of internal and outer pressure for specified conditions are given in Table 5.2. In addition to these pressures, SG tubes are exposed to bending stresses due to flow, vibrations, and influence of residual stresses (on the location of tube bending and expansion). All those influences are neglected in this analysis. Normal operating conditions considers temperature difference between coolant and feedwater temperature which influence on stress level through the tube thickness may be neglected. It may be assumed that tube is exposed to operating temperature of ~ 300^oC, so all material characteristics correspond to that value.

	pi[MPa]	po[MPa]	Dp[MPa]
Operating condition	12.5	4.32	8.18
Hydro test	17.2	0.0981	12.4
Upset condition	14.7	4.32	10.38
Table 5.3 Loadings of VVER-440 steam generator tubes

5.2 Uniform Thinning
From analytical point of view it is the easiest type of problem for evaluation and it may be evaluated by analytical methods. Calculations are made for all loading combination defined in Table 5.3 and the results are given in Figure 4. Allowable thinning for particular condition is defined by intersection with corresponding value of allowable stress intensity.

Fig 4: Value of s eq for tube wall thinning

Structural integrity for this damage type may be assessed by the burst pressure tests, also. It has been shown that the most suitable formula for uniform thinning is Svenson formula:

where value of burst pressure is given in relation to flow stress (s _f), tube radius (r) and actual tube thickness (t). Because of lack of appropriate data for VVER tube material, factor k has been assumed as 0.5, although experiments showed that it is usually greater (3). Values obtained by Svenson are the values of critical pressure which cause tube burst, so they should be divided by safety factor in order to get allowable internal pressure (or pressure difference where internal pressure is larger than outer). In order to compare burst pressure value per two different approaches, recalculation has been made for analytical approach where pressure had been expressed as pressure difference acting only on inner tube surface. Comparison of results is presented in Figure 5. As may be seen, for operating conditions results match very well. Critical tube thinning per different criteria is defined by crossing the appropriate curve with the values of pressure difference for corresponding operating conditions (P_OC - pressure difference for normal OC, P_HT - pressure difference for hydro test, P_UC - pressure difference for UC).

Fig 5: Allowable pressures per different approach

5.3 Pitting


Fig 6: FEM model with hole-like flaw simulating pitting damage

Damages formed due to pitting have a hole-like form. Influence of damages formed by pitting on tube integrity, was simulated by circular flaw with the spherical bottom. Calculations were performed by finite element method assuming elasto-plastic material behavior (FEM model presented in Figure 6). Von-Mises stresses at the bottom of flaw are presented in Figure 7 in relation to internal pressure. Also, diagram contains stresses in undefected tube (th-0%) and tube with uniform thinning of 60% (th-60%). It may be concluded that increase of pressure on tube with certain flaw depth will cause higher stress level around the flaw because of stress concentration. After reaching the yield point, the local plastic zone starts to grow with the very slow rate because surrounding material is still in elastic state. Stresses will grow much faster again, when whole tube wall would starts to yield.

Fig 7: s eq for tube with hole-like flaw of 60%, tube with thinning of 60% and undefected tube

Comparison of stress levels for thinning and pitting damages reveals that stresses are greater for pitting damages what lead to more conservative criteria. However, despite of earlier forming of plastic zone in vicinity of flaw, the tube with pitting damage may withstand greater pressure. This is confirmed by testing of different flaw types where burst pressures were greater than those obtained for uniform thinning. It can be concluded that local plastic zone formed around flaw, does not increase the burst pressure. As have been presented, FEM calculations may be used to asses critical pressures for different flaw parameters. Additional confirmation by burst test results would provide determination of appropriate criteria for damages formed because of pitting.

5.4 IGA/SCC
Damages formed due to action of intergranular attack stress/corrosion cracking have a form of a crack. Usual approach for this flaw type is based on pipe theory which assumes that existence of crack on tube will cause local increase of stress level defined by, so-called, "bulging factor". There are several approximations for this factor (Folias, Erdogen, Pruitt, ...), but it may be calculated by finite element method, too. Burst pressure for tube having axial crack is defined by assuming plastic collapse as failure criterion because of high material ductility. For 100% cracks (through-wall cracks), value of critical pressure in relation of flaw length may be derived from the following formula:



where	m = f(c,r,t) - bulging factor	2c flaw length
	t wall thickness	sj circumferential stress
	sy yield stress	su ultimate strength
	p internal pressure	k material coefficient

As may be seen in Figure 8, burst pressure values calculated per different approximations match very well.

Fig 8: Comparison of burst pressure values calculated per different approximations

Beside the length, another interesting parameter of crack is its depth. The influence of crack depth on critical pressure for different flaw lengths is presented in Figure 9. As expected, the longer cracks are more influenced by flaw depth than shorter cracks.

Fig 9: Burst pressure for different crack length

The formula for structural integrity evaluation of tubes having axial crack which takes in account flaw length and depth is the one proposed by Kurtz:

Comparison of experimental results with the Kurtz formula and FEM based calculation, are given in Figure 10.

Fig 10: Burst pressure for corrosion flaws in comparison with FEM and Kurtz results

It may be concluded that both evaluations satisfactorily match with experimental data. The importance of this conclusion is that FEM based evaluation may be effectively applied for structural integrity analysis of tubes having cracks. The benefit of using FEM is the possibility of simple modeling and evaluation of various effects such as: influences of tube support, tubesheet, elbows and others.

6. CONCLUSION

In majority of countries with VVER type of steam generators, plugging criteria is based on flaw depth, which was mostly defined on technical judgment. One of the reasons was lack of information about degradation mechanisms. However, information gained through the metalographical analysis of pulled tubes, as well as, application of rotating technique on VVER tubes provides possibility of application of alternative plugging criteria which may be based on some other parameters than flaw depth (such as flaw length, voltage of indication or any other). Establishment of such criteria requires comprehensive research work in order to evaluate behavior of damaged tube in different operating and accidental conditions. Structural integrity evaluation may be performed by analytical, experimental or numerical methods. It has been shown that FEM may be very effectively applied for structural integrity evaluation of different damage types. This provides relatively simple evaluation of various effects, as well as, different operating conditions. In conjunction with experimental results these evaluations are the base for determination of tube structural limit. Additional investigations of measurement accuracy and growth rate for particular degradation mechanism will provide determination of appropriate plugging criteria which will reduce the number of plugged tubes without negative influences on SG operation safety level.

References

1. Normi rashcheta na prochnost oborudovania i turboprovodov atomnih energeticheskih ustanovok PNAE G-7-002-86,1998g.
2. "Jakostni presmjatanija na toploobemnna trubichka 16´ 1.4 ot PG na bl. 1 na AEC Kozloduy", ENERGOPROEKT, Sofia, 1999.
3. M. Tvrdy, "Preventive Measures to Reduce the Rate of Ageing and Degradation of Primary Circuit Components Other than RPV", IAEA Course: Trnava, Slovakia, 29 May-17 June 1995.
4. J-P. Hutin, and R. Serres, "Steam generator surveillance and maintenance principles", Proceedings of the International ENS/ANS Conference on Thermal Reactor Safety, Avignon,1988.
5. G. Frederick, J. Mathonet , P. Hernalsteen, D. Dobbeni, "Development and Justification of new plugging criteria applicable to the cracking phenomena in the tubing of steam generators" - Part I&II, Belgatom, Brissel, 1989.

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