Reactor Vessel Materials and Flaw Evaluation
SIB-96-149, Rev. 2

The Issue: RPV In-Service Inspection
In accordance with the requirements of Section XI of the ASME Boiler and Pressure Vessel Code, reactor pressure vessel (RPV) shell and nozzle welds must be inspected once during each inspection interval (e.g., 10 years).

The Problem: Disposition of Indications
During in-service inspections (ISI), any indication which is discovered must be evaluated under Section XI requirements, as depicted below:

The probability of finding indications during ISI is relatively low. However, with changes in inspection technology and an increasing awareness of the importance of reactor vessel embrittlement, the economic risk associated with finding flaws during inspections is significant.

The Solution: Flaw Handbook
With this in mind, Structural Integrity Associates (SI) has developed a cost effective two-step approach for the pre-evaluation of indications that may potentially be found during ISI:

Step 1: Materials Evaluation
Reactor vessel flaw evaluations require the determination of fracture toughness for the location under question. SI can perform a detailed compilation of all key material properties for reactor vessel weld, plate and nozzle locations resulting in an easily accessible materials handbook. This information can be used not only for vessel flaw evaluations, but can also be used for pressurized thermal shock evaluations or pressure/temperature curve evaluations.

Step 2: Flaw Acceptance Handbook
Using SI's computer programs, APPENDA and MAPPA, flaw evaluations for 8 to 12 grouped locations in the reactor vessel are performed according to the requirements of ASME Code, Section XI. These grouped locations are based upon the materials evaluation (Step 1), stress analysis, and collected plant-specific documents. Typical grouped locations for a BWR RPV are shown in the following figure:

As part of any flaw evaluation, the vessel stresses associated with operating conditions, welding and cladding must be determined. Vessel operating stresses in many cases are not easily obtained or are outdated. Thus, many utilities have utilized SI's expertise in finite element stress analysis to obtain updated and detailed stress results, as shown below in the finite element model developed for an upper flange region.

The end-product is a handbook which provides acceptance charts (as shown below), for every evaluated grouped location, for both axial and circumferential flaws at inside surfaces, outside surfaces, and subsurface locations. Flaw acceptance charts can also be generated for key nozzle locations. The handbook includes the complete basis upon which the allowable flaw sizes were determined and clear instructions for its use. The flaw acceptance diagrams may also be provided in electronic format.

SI has developed flaw acceptance handbooks for both pressurized water reactor (PWR) and boiling water reactor (BWR) plants. In addition, SI can also assist utilities during ISI by providing rapid flaw evaluations (24 hours) for flaws which exceed IWB-3500 evaluation standards, or on-site support can be provided to assist in flaw evaluations and dispositions. If you would like more information on flaw acceptance handbooks and/or ISI support, please contact SI.