An assessment of the effect of ISI was carried out for the surge line elbow weld, the defect distribution and density being those generated by RR-PRODIGAL, see section G.4.2. A Probability of Detection (POD) curve was defined by the following equation:

This POD is shown in Figure G.7, and it can be seen that this sets the probability of detection at about 90 percent for defects 70 percent of the way through the wall thickness. This was felt to be

representative of inspections carried out to date, but for future inspections that conform to modern standards, this POD could be much better.

The results are shown in the table below and in Figure G.8 for various ISI intervals.

Table G.5 Reduction Factors Due to ISI

ISI case	Cumulative Probability of Failure at 60 years	Factor for General Use
No ISI	1.3 x 10-4	1
0 years (PSI)	4.2 x 10-5	3
10 years	3.8 x 10-5	3.4
10, 20 years	1.3 x 10-5	10
10, 20, 30 years	6.5 x 10-6	20
10, 20, 30, 40 years	4.8 x 10-6	27
10, 20, 30, 40, 50 years	4.7 x 10-6	28

These results suggest that even with this quite low inspection capability, and for a weld with a high failure probability, reductions of a decade can be achieved with two or three inspections during the life of the plant. It also indicates that going beyond three inspections gives little extra return.

An interesting conclusion from this figure would be that if a fourth inspection is carried out at the end of a forty year period, then, provided this inspection was clear, there would be little gain from an inspection at fifty years for a total life of sixty years! However, at this stage such a conclusion can only be taken as tentative and would require more investigation.

G.6 References

G. 1 NUREG/CR-5505 PNNL-11898 ‘RR-PRODIGAL — A Model for Estimating the Probability of Defects in Reactor Pressure Vessel Welds.

G. 2 NURBIM (Nuclear Risk-Based Inspection Methodology) WP4. Published by the European Commission under the EURATOM programme.

Leak Rate = A*(areaAn)

Source Data Mean

Figure G.1 Leak Rate as a Function of Leakage Area (Data Supplied by USNRC)

Figure G.2 Estimated Defect Separation (COD) Based on Expert Judgment
as a Function of Defect Length Assuming Plastic Deformation

0 5 10 15 20 25 30 35

Full Defect Length (Inches)

Figure G.4 Probability of the Existence of a Defect of a Certain Length for Surge Line Base Case

0.0E+00 -|	Probability of Leak Greater Than ‘X’ (No Leak Detection)
_Q 03 _Q
&_ CL O)
-3.0E+00 —
0	5	1 Log	5 Leak	2 Rate	.5 ‘X’ (gi	3 pm)	5	4	5

Figure G.5 Conditional Probability of a Leak of a Given Size

Figure G.6 Reduction Factors for Leak Detection Based on Expert Judgment

ISI Probability of Detection

Л

Figure G.7 Probability of Detection Curve

ATTACHMENT 1 TO APPENDIX G (ATTACHEMENT G.1) FROM
NOTE ‘SUMMARY OF BASE CASES STRESSES’ APRIL 2003

Summary of Stress Cycles for Surge Line Elbow
(No seismic stresses)

Many of the high stress contributors in Tables 2 and 3 are from rapid excursions of the coolant temperature. The largest stress amplitude (half the peak-to-peak) is 1,310 MPa (190 ksi), so the stresses are large (but localised). These are the stresses at the peak stress location, which is not at weld. The spatial stress gradients (both along the surface and into the pipe wall) are required for a thorough analysis. The radial gradient (into the pipe wall) can be estimated by the following procedure:

1 Cyclic stresses associated with seismic loads were treated as 100 percent uniform stress.

1 Cyclic stresses greater than 310 MPa (45 ksi) were treated as having a uniform component of 310 MPa (45 ksi), and the remainder were assigned to the gradient category.

1 For those transients with more than 1,000 cycles over a 40- year life, it was assumed that 50% of the stress was uniform stress and 50% a through-wall gradient stress. In addition, for these transients, the uniform stress component was not permitted to exceed 70 MPa (10 ksi).

The gradient stress mentioned above is assumed to vary through the thickness as

o(%) = ^0 ^1 — 3£ + 2 j (G.2)

In this equation, c0 is the stress at the inner wall of the pipe, % = x / h, x is the distance into the pipe wall from the inner surface, and h is the wall thickness. The stresses and cycles are high enough that fatigue crack initiation is important, which has been considered in Reference 6. Reference 6 shows a probability of 0.981 of a leak in 40 years for this component. The LOCA probabilities will be less. The use of the gradient along the surface will reduce this.

As mentioned above, the more thorough results that include a better estimate of the radial gradient and also consider the spatial gradient along the surface are available, and could be used for the base case calculations.