Reactor Vessel Flaw Evaluation
SIB95120, Rev. 0

In accordance with the requirements of Section XI of the ASME Boiler and Pressure Vessel Code, reactor vessel shell and nozzle welds must be inspected once in each inspection interval (e.g., 10 years). With changes in inspection technology and an increasing awareness of the importance of reactor vessel embrittlement, the economic risk associated with identifying flaws during reactor vessel welds inspection is significant.

Structural Integrity Associates (SI) is a leader in applications of fracture mechanics technologies, especially with respect to Section XI applications. SI personnel are involved in Section XI Code committee activities on a regular basis and are aware of the latest Code and regulatory issues related to reactor pressure vessel integrity issues. SI is also well-known in the regulatory environment. Over the last several years, SI has provided cost-effective service in this area to several utilities, providing services normally thought to be available only from the reactor vessel or nuclear steam supply system vendor. This is advantageous to utilities in that competition tends to keep operating costs low.

SI proposes a two-stage approach in preparation for evaluation of potential flaws during RPV inspection. Considering the relatively low probability of finding flaws during inspections, SI recommends that the front-end activities before the outage be kept relatively small. As such we recommend a flaw evaluation handbook for the shell welds only, which can be accomplished in a very economical way. This flaw evaluation handbook helps to make quick decisions during the inspections when flaws are encountered. Flaws that are found during inspection of the shell welds and in other components of the vessel such as nozzles can be evaluated in a very timely manner if all the design inputs are made available prior to the inspections. As such, flaws in these components can be dispositioned very rapidly once the inspection results are made available.

Materials Evaluation
One of the key pieces of data required in reactor pressure vessel flaw evaluations is the fracture toughness. Recent regulatory changes related to Regulatory Guide 1.99 Rev. 2 and Generic Letter 92-01 have increased the requirements with respect to uncertainties, how to determine the initial RTNDT and determination of irradiation shift. This is especially important for relatively older reactors whose material testing may not meet current Code requirements.

SI has recently performed detailed materials evaluations for four BWR and one PWR reactor vessels. This involved a complete review of vessel fabrication records, translating this into the appropriate values of initial RTNDT, chemistry factors and uncertainties. Where necessary, available generic data is used to supplement actual data, especially where plant-specific data are not available. Results of surveillance testing is included as appropriate. The end product of this effort is a reactor pressure vessel materials data handbook that summarizes all materials properties necessary for vessel flaw, PTS or pressure/temperature curve evaluations.

Flaw Acceptance Handbooks
The figure to the right shows the steps in a flaw evaluation. Most relatively smaller indications can be evaluated per the requirements of Section XI, IWB-3500. If the "evaluation standards" contained therein are met,then no further consideration of the indications is needed. However, it the standards are exceeded, detailed flaw evaluations meeting the requirements of IWB-3600 are required.

During vessel examination, it would be desirable to have a handbook that would allow pre-evaluation of any flaw that might be found. Unfortunately, this would be very cost prohibitive due to the essentially unlimited combination of locations, geometries and stresses that would have to be considered. On the other hand, the work associated with pre-evaluation of the reactor pressure vessel shell welds is not such a large undertaking and can be accomplished very economically. This includes the majority of the volume of inspection. SI has assisted in developing flaw acceptance handbooks for three BWR and one PWR plants. These handbooks take the data from the materials evaluation report and present flaw acceptance charts for 8-12 typical locations. For each location, both axial and circumferential flaws are evaluated (with separate charts for each). Separate flaw acceptance charts are provided for inside surface and outside surfaces, as well as eight distributed subsurface depths. Figures 2 and 3 show two typical flaws acceptance charts. These acceptance charts can provide an immediate assessment for any indication found at the shell welds during the inspections. The ability to perform an immediate assessment will allow the inspections to focus on any problem area (to assure that the inspection results are not too conservative) and to allow continued inspection of other locations (when significant margin exists as determined by the charts).

Rapid Flaw Evaluations/IWB-3600 Evaluations
For reactor vessel nozzles, pre-evaluation of all potential flaws is generally not economical. For planning for the potential evaluation of these flaws, SI recommends the "rapid flaw evaluation" approach. This involves collecting all drawings, stress reports, etc. in our files prior to an outage inspection. We will typically provide preliminary assessment of any indications in 24 to 48 hours, based on judgment, hand-calculated stresses, and results of similar evaluations. The amount of data available in stress reports is not sufficient to perform a very thorough evaluation, so detailed finite element analysis is almost always required to perform the final evaluations. Thus, we follow up with a detailed IWB-3600 evaluation report, and any licensing support if required.

SI will also be available to perform additional evaluation of any significant vessel shell welds. With location-specific analysis, taking into account actual material properties, fluence, stresses, etc., larger flaws than determined by the "handbook" approach can be justified.

RPV and SG Integrity Consulting
SI provides a wide variety of services related to reactor vessel (and steam generator) integrity. These have included:

• Re-evaluation of fatigue usage and flaw tolerance for BWR feedwater nozzles as affected by sparger (thermal sleeve) leakage,
• Evaluation of clad cracking and potential underclad cracking for a BWR vessel,
• Determination of pressure temperature limit curves · Fatigue evaluation and monitoring, and
• Flaw evaluation and replacement of feedwater nozzle safe-ends on PWR steam generators.

Software And Training
SI has developed several special-purpose programs for assisting with reactor pressure vessel flaw evaluation. SI uses these software programs in the evaluations described above, and also offers to license these programs to our clients, including the necessary training for their use.

pc-CRACK is an industry standard program for solving fracture mechanic problems in the power industry. It contains a wide range of models for evaluating various types of flaws in vessels and piping, by use of linear elastic fracture mechanics (LEFM) and simplified elastic-plastic fracture mechanics (EPFM) evaluations.

APPENDA is a program that determines allowable flaw sizes for a reactor vessel flaw acceptance handbook. As its name suggests, it performs analysis in accordance with Appendix A of Section XI. It includes all methodology per Appendix A, and also includes the methodology of Reg. Guide 1.99 Rev. 2 for handling vessel fluence and material property uncertainties. Special models are included for handling surface and cladding-induced stress intensity factor that are not well defined in Appendix A. An accompanying program MAPPA (standing for Multiple APPENDA Analyses) creates a bounding flaw acceptance chart, determined from a number of different loading conditions or assumed stress conditions. If you are interested in these services or the associated software, please contact SI