Research on Defects of Pressure Resistance Welding for HWR Fuel Element
3. Potential defect analysis and verification 1 Potential leaking defects analysis
Even the inspection items are all acceptable, potential welding defects are still possible, because all the test items are sampled from the production run, any element may have leakage. According to Canada ZPI and AECL operating experience, damage of fuel in the reactor are mainly concentrated in the position of weld due to the manufacturing reasons. On condition of end plug bar used in BNFP had been ultrasonic detected twice, the possibility of holes in the plug remains very low, that means if the element has leakage in the reactor, leaking points may occurs mainly in the welds. And QS–3 HWR has also caught some leakage successfully, see Figure 9 and Figure 10:
FIG. 7. Burst test sample
FIG. 10. Photographs of fuel leakage in welding joint in the reactor
FIG. 8. Corrosion test sample
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FIG. 9. Photographs of fuel leakage in welding joint in the reactor
So the abnormal phenomenon in the manufacturing process should be strictly considered, through visual inspection and destructive testing methods for abnormal joint, main defects are: joint discoloration, irregular shapes of upset, materials splash, low joint combination intensity (inner welding line existing), internal defects (foreign material inclusion) and superficial defects etc.
3.2 Production defect data analysis
Starting from 2003 to 2009, the qualified parameters are strictly implemented, BNFP adopted a tightened sampling plan, and BNFP has established more strict internal control criterion. But under all these efforts, welding defects are still frequently detected in production under the condition of sampling; therefore, we have to continue doing a lot of simulation experiments and sample more fuel element to detect the root cause of defects. Here one point should be emphasized, it is seldom to found failure for the burst and tensile test during the production and it is also rare to find the defects for splash and upset with irregular profile with the welding electrodes are well prepared. The main defects are welding joint discoloration (see Figure 11) and internal welding line (see Figure 12, 13, 14).
3.2.1 Welding discoloration test and analysis
At the first stage of production, the quality of the discolored welds are suspected, we took 345 accumulated samples and classified them into 3 groups with different level of discoloration, then made statistical analysis for different test (detailed as Figure15,16,17). The sample test distribution see Table 1.
FIG. 11. Welding joint discoloration FIG. 12. Discontinuous welding lines in the welding joint
FIG. 13. A continuous thin welding line in the welding joint
FIG. 14. A continuous thick welding line in the welding joint
TABLE 1. THE SAMPLE TEST DISTRIBUTION Inspection
items
Discoloration line (long)
Discoloration line (middle)
Discoloration line
(short) Total Test result
metallographic 68 122 110 300 acceptable
tensile 5 5 10 acceptable
burst 5 5 10 acceptable
corrosion 10 15 25 acceptable
total 88 257 345 acceptable
All the samples listed in Table 1 are accepted and meet the inspection requirement, so the fuel element with discoloration weld should not cause rejection to the product. But by analyzing all the metallographic section, we found it is common that an indentation exists on the inner upset opposite to the discoloration position, it is lack of material in this area compared with elsewhere on the inner upset, and the longer of the discoloration line, the probability for this appearance is increasing. See Table 1I and Figure 18 for details.
TABLE 2. THE RELATION OF DISCOLORATION LINE AND INNER UPSET INDENTATION
Discoloration line Long Middle Short
Sample quantity with discoloration line 68 122 110
Quantity of sample with indentation 56 95 67
Percent 82% 78% 61%
FIG. 15. Discol. lines long FIG. 16. Discolocation line middle FIG. 17. Discoloration lines short
61 %
7 8 % 8 2%
0 % 1 0 % 2 0 % 3 0 % 4 0 % 5 0 % 6 0 % 7 0 % 8 0 % 9 0 %
t he pr o ba b ili ty f o r a pp ea r a nc e o f ind en ta tio n( % )
s h o r t m i d d l e l o n g
L e n g th o f d i sc ol o r a ti o n l in e
Analyzed reason for discoloration line: the heat source of upset welding is resistance heat; the change of resistance heat affects the form of welds. If there are small particles on the surface of the tube, or the chamfer of electrode collets are worn, the resistance of this area is increased, and the current intensity is decreased here, local heat decreases relatively, the material is not well heated up and then cause the lack of material in the position when welding; the small particles of this part or the wearing out of the collets impedes the heat spreading, then the heat generated has accumulation effect, oxidation of the surface and discoloration line are formed. Judging from the required inspection results, the phenomenon has no evidently influence to the performance and strength of the welds. But anyway the discoloration line is not a good phenomenon, and it should be kept as low as possible.
3.2.2 Internal welding line simulation test and analysis
The internal welding line displayed in Figure 12, 13, 14 are a kind of defect, we think when the fuel is in high temperature and high pressure and after long-time service in the reactor, the defect is able to spread, if it becomes large enough, it will low the welding joint strength, then potential leakage passage in the fuel element welds is formed. And under normal production condition, it is hard to reproduce this defect, therefore in order to find the root cause of the defect, artificial simulation process samples are used and intend to find the expected defect. Detailed test plan is as follows:
FIG. 18. Photo of the inner upset indentation
FIG. 19. Discoloration line and inner upset indentation relationship
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Cleanliness conditions of welding preparationCondition A: Graphite contamination samples: 20 PCS (Process Control Sample). Contamination sources: the graphite slurry used for graphite coating.
Condition B: UO2 contamination samples: 20 PCS. Contamination sources: UO2 pellets.
Condition C: Oil contamination: 20 PCS. Contamination sources: mechanical oil.
Condition D: Oxidation film: 10 PCS. Contamination sources: tube surface with slight oxidation film.
Condition E: Oxidation film: 10 PCS. Contamination sources: tube surface with serious oxidation film.
Condition F: Normal samples: normal samples used for process control.
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Welding chamfer damagedCondition G: Slight damage of chamfer: 10 PCS. Use a knife to cut slight groove;
Condition H: Serious damage of chamfer: 10 PCS. Use stainless steel edge to perform serious damage to the chamfer.
The test results see Table 3 below, the percentage of reject to test samples see Figure 20.