ASME Code is organized in 12 sections, with sections relevant to nuclear power plants (NPP) are Section II (Section on Materials), Section III (Section on Nuclear Power), and Section XI (Section on Inservice Inspection of NPP structures, systems, and components). Some other sections, such as Section V (Non-Destructive Examination) and Section VIII (Pressure Vessels), have been used for older NPPs, when the nuclear sections were not available.
Each section is divided in subsections. Section XI (ISI) is divided in the following subsections:
- • IWA — General Requirements
- • IWB — Class 1 Components
- • IWC — Class 2 Components
- • IWD — Class 3 Components
- • IWE — Class MC and CC (metal containments and liner of CC Containments)
- • IWF — Classes 1, 2, 3, and MC Component Supports
- • IWG — Core Internal Structures
- • IWL — Class CC — Concrete and Post-tensioning
Requirements of Subsection IWE.
Subsection IWE consists of the following Articles:
- • IWE-1000: Scope & Responsibility
- • IWE-2000: Examination & Inspections
- • IWE-3000: Acceptance Standards
- • IWE-4000: Repair Procedures (Delineated to IWA-4000 since 1998 Edition)
- • IWE-5000: System Pressure Test
- • IWE-7000: Replacements (merged with IWA-4000)
The essential portions of these articles are summarized in the following paragraphs:
The reader should note the following ASME Section XI definitions provided in Article IWA-7000, for “Inservice Examination,” and “Inservice Inspection:”
Inservice Examination: the process of visual, surface, or volumetric examination performed in accordance with the rules and requirements of Section XI.
Inservice Inspection: methods and actions for assuring the structural and pressure-retaining integrity of safety-related nuclear power plant components in accordance with the rules of Section XI.
Subsection IWE provides requirements for inservice inspection of Class MC (ASME Subsection NE of Section III, Division 1) pressure retaining components and their integral attachments, and of metallic shell and penetration liners of Class CC (ASME Section III, Division 2) pressure retaining components and their integral attachments in light-water cooled plants.
Article IWE-1000 addresses the components to be examined, components exempted from examination, required accessibility for examination, inaccessible areas requiring examinations, and surface areas requiring augmented examinations. Article IWE-2000 addresses requirements for pre-service examinations, visual examination personnel qualifications, required qualifications for “Responsible Individual,” inspection schedule, successive examinations. Table IWE-2500 provides the detailed examination requirements for various situations encountered in practice. For example, the required examination of wetted or submerged areas encountered in suppression chambers (or sometimes termed as wet well areas) of BWR Mk I, Mk II, and Mk III containments. Please see Chapter 1 of this book for description of theses containments. This Article also incorporates the visual examination requirements of IWA-2210. The visual examinations, method of performing these examinations, and qualifying personnel for these examinations, are described in IWA-2210. The Code defines three types of visual examination methods: termed as VT-1, VT-2, and VT-3 examinations.
VT-1 examination is conducted to detect discontinuities and imperfections on the surface of components, including such conditions as cracks, wear, corrosion, or erosion.
VT-2 examination is conducted to detect evidence of leakage from pressure retaining components, as required during the conduct of system pressure test.
VT-3 examination is conducted to determine the general mechanical and structural condition of components and their supports by verifying parameters such as clearances, settings, and physical displacements and to detect discontinuities and imperfections, such as loss of integrity at bolted or welded connections, loose or missing parts, debris, corrosion, wear, or erosion. VT-3 includes examination for conditions that could affect operability or functional adequacy of constant load and spring-type supports.
If it is necessary to identify the magnitude of a crack or a flaw identified during visual examinations, or to identify any degradation in inaccessible areas, IWA-2220 and IWA-2230 requires the use of surface and/or volumetric examinations. Detailed procedures for conducting these examinations, as well as qualifying the examination personnel, are provided in IWA-2200 and IWA-2300.
Article IWE-3000 describes acceptance standards when the examinations have shown degraded conditions. The article describes three methods of accepting a degraded component: (1) Accept, because, the degradation is within the acceptable limits, and the component could perform its intended function(s); (2) Accept, when appropriate repairs or replacement of the degraded area of the component are made so that its functionality is retained; and (3) Accept, if the engineering evaluation indicate that the component could perform its intended function. In the later Editions (i.e., 2001 thru 2010) of SS IWE, Article IWE-4000 on “Repair and Replacement,” has been omitted from this SS, and the rules established in Article IWA-4000 (Repair and Replacement) are applicable. IWA-4000 of SS IWA (General Requirements) is also applicable to SS IWB (Class 1 Components), SS IWC (Class 2 Components), and SS IWD (Class 3 Components), unless specifically called out otherwise in respective subsections. Article IWE-5000 requires system pressure tests to verify integrity of pressure retaining components after repair or replacement of the components.
As Subsection IWE has been incorporated by reference in regulation 10 CFR 50.55a, NRC expanded or modified certain requirements of the subsection in the regulation. They are discussed in the following paragraphs.
- (A) For steel shell or steel liner of concrete containment, the licensee is required to evaluate the acceptability of inaccessible areas when conditions exist in accessible areas that could indicate the presence of or result in degradation to such inaccessible areas. For each inaccessible area identified, the licensee is required to provide the following in the ISI Summary Report as required by Article IWA- 6000 (Reporting Requirements). The details of this item is not discussed here, as the requirement has been incorporated in 2001 and later Edition of SS IWE.
- (B) When performing remotely the visual examinations required by Subsection IWE, the maximum direct examination distance specified in Table IWA-2210-1 may be extended and the minimum illumination requirements specified in Table IWA-2210-1 may be decreased provided that the conditions or indications for which the visual examination is performed can be detected at the chosen distance and illumination. This NRC position is a relaxation of the code requirement, evolved as a result of recognizing the large size of the containment structure.
- (C) The examinations specified in Examination Category E-B, Pressure Retaining Welds, and Examination Category E-F, Pressure Retaining Dissimilar Metal Welds, is optional. In the 1998 Edition of SS IWE of the Code, these requirements were deleted, as experiences with the steel containments indicated no concern for metal welds. However, the requirement for examination of dissimilar metal welds was added in the guidelines for license renewal. Please see Chapter 5 of the book for more information on license renewal guidelines.
- (D) The following examination is required to be used as an alternative to the requirements of successive inspections of IWE-2430.
- (1) If the examinations reveal flaws or areas of degradation exceeding the acceptance standards of Table IWE-3410-1, an evaluation is required to be performed to determine whether additional component examinations are required. For each flaw or area of degradation identified which exceeds acceptance standards, the licensee is required to provide the following in the ISI Summary Report required by IWA-6000:
- (i) a description of each flaw or area, including the extent of degradation, and the conditions that led to the degradation;
- (ii) the acceptability of each flaw or area, and the need for additional examinations to verify that similar degradation does not exist in similar components, and;
- (iii) a description of necessary corrective actions.
- (2) The number and type of additional examinations to ensure detection of similar degradation in similar components.
This requirement has been incorporated in Section IWE-2420 of 2010 Edition to an extent. However, NRC keeps the above requirements as an alternative to the Code requirements.
- (E) A general visual examination as required by Subsection IWE must be performed once each period. This requirement has been incorporated in the Code since the 2001 Edition, in Table IWE-2500-1 of SS IWE.
- (F) The 1998 Edition of Subsection IWE deleted the requirements for VT-1 and VT-3, and substituted them with Owner defined general and detailed examinations. NRC did not accept the substitution, and in 2001 requirements of the Regulation. SS IWE was revised. In the 2004 Edition of Subsection IWE, the Code incorporated the original requirements for examinations, and for qualifications of the personnel.
- (G) The VT-3 examination method must be used to conduct the examinations in Items E1.12 and E1.20 of Table IWE-2500-1, and the VT-1 examination method must be used to conduct the examination in Item E4.11 of Table IWE-2500-1. An examination of the pressure-retaining bolted connections in Item E1.11 of Table IWE-2500-1 using the VT-3 examination method must be conducted once each interval. The “owner-defined” visual examination provisions in IWE-2310(a) are not approved for use for VT-1 and VT-3 examinations. In the 2007 Edition of IWE, the Table was revised to incorporate the NRC position.
- (H) Containment bolted connections that are disassembled during the scheduled performance of the examinations, as required in Item E1.11 of Table IWE-2500-1, must be examined using the VT-3 examination method. Flaws or degradation identified during the performance of a VT-3 examination must be examined in accordance with the VT-1 examination method. The criteria in the material specification or IWB-3517.1 must be used to evaluate containment bolting flaws or degradation. As an alternative to performing VT-3 examinations of containment bolted connections that are disassembled during the scheduled performance of Item E1.11, VT-3 examinations of containment bolted connections may be conducted whenever containment bolted connections are disassembled for any reason. A Table 2500-1, Examination Category E-G was added in the 2007 Edition of Subsection IWE to incorporate the NRC position.
- (I) The ultrasonic examination acceptance standard specified in IWE-3511.3 for Class MC pressure- retaining components must also be applied to metallic liners of Class CC pressure-retaining components. This NRC addition to the SS IWE is incorporated in IWE-3511.3 in 2007 Edition of the Subsection.