Propuesta de Inserc 1 Prórroga y Res

Thermal ageing is a non-reversible degradation mechanism that is heavily dependent on temperature, material microstructure and time. At high temperatures, the material may lose ductility and become brittle because of microstructural changes in the form of precipitates, whereby certain elements separate from the metal matrix. In the RPV steel, which contains appreciable copper impurities, most precipitates are copper rich. Thermal ageing depends on the time and temperature of exposure, together with the material type and its chemical composition. It occurs even without the presence of radiation related acceleration of element precipitation in the metal microstructure.

Copper in RPV steel is initially trapped in a supersaturated state. With time, at PWR operating temperatures (~290°C), it may be ejected from the metal matrix, as the alloy strives toward a more thermodynamically stable state, thus forming agglomerates of stable precipitates.

Thermal ageing has no hardening effects in RPV steels, as only changes in the transition temperature are observed, without any measurable changes in yield strength. As this degradation is mostly accompanied by the segregation of phosphorus along grain boundaries, some intergranular fracture is usually observed. The effects of long term ageing at temperatures up to 350°C on the ductile to brittle transition temperature of RPV steels have

been studied in several countries (France, Germany, Japan and the USA). No embrittlement has been found to occur in typical RPV steels at these temperatures for operating times as great as 100 000 hours.

The 15Kh2MFAA type steels used to fabricate most WWER-440 pressure vessels also do not appear to be susceptible to thermal ageing, even when they contain relatively high phosphorous impurity levels. Results from thermal ageing surveillance specimens located in the upper plenums of the WWER-440 pressure vessels, removed and tested after 30 operating years at about 300°C, indicate that the shift in the Charpy ductile to brittle transition temperature is small. These results are supported by Charpy ductile to brittle transition temperature measurements from RPV trepans removed from shutdown plants (Novovoronezh 1 and 2 in the Russian Federation), as well as

Flow > 0

Oxygen

<6ppb

6<PH<11

Allowable?

Select the evaluation method on errosion­corrosion

Continuous operation

Exempt evaluation

Measure the  thickness

Allowable? Perform

the AMP Phase II evaluation Carbon/low

alloy

Evaluate

the erosion­corrosion rate

FIG. 24. FAC (erosion corrosion) evaluation at Korea Hydro & Nuclear Power, Republic of Korea. ppb — parts per billion.

The 15Kh2NMFA type steel used to fabricate the WWER-1000 pressure vessels is only slightly susceptible to thermal ageing at operating temperatures, owing to the high nickel and low vanadium content of this material. A shift in the ductile to brittle transition temperature of 10–30°C has been observed, due to some segregation of nickel and phosphorus at the grain boundaries, which results in an increase of susceptibility to intergranular cracking.

Figure 25 shows the thermal ageing evaluation approach for CASS developed by KHNP. CASS is a very ductile material that is resistant to SCC. It is widely used in safety related components, and is particularly suitable for the manufacture of components of complex geometries. However, complex geometries are difficult to inspect.

For static components of CASS with greater than 14% ferrite content, fracture analysis may be conducted.

The input parameters such as the J-integral versus crack growth resistance (J–R) curves for polymeric materials with both elastic and plastic contributions can be obtained directly from load versus displacement records in the literature, without installing an on-line crack monitoring system. Once the input parameters are obtained, the crack

Mo steel?High

Flaw?

Perform the volume inspection

Continued operation

Exempt evaluation

Determine the input  parameters (J­R Curve etc.)

Allowable? Perform

the AMP Phase II evaluation CASS components

Static?

Ferrite

>14%

Centrifugal?

Ferrite

>20%

Fracture analysis?

Evaluate the low Mo

steels

Evaluate the crack  growth rate No

No

No YES

YES

YES

No

Yes No

Yes

No

Yes

No

Yes

Yes

No

FIG. 25. Embrittlement evaluation of CASS at Korea Hydro & Nuclear Power, Republic of Korea.

growth rate can be evaluated. If it is within the allowable limit, the component is allowed to continue operation. If not, an AMP has to be developed before continuing operation.

For centrifugal machinery with casings made of CASS and with a ferrite content greater than 20%, fracture analysis can be conducted in the same manner as for static components. If the ferrite content is below 20%, the component can be exempt from evaluation. The process is well documented in the IAEA IGALL report [29].

5. DEGRADATION AND ITS MITIGATION IN