ASME Code Developments

Many other codes, standards, specifications, and recommended practices have been developed by various organizations (26). Some apply to specialized piping systems others, particularly those covering materials and dimensions, are referenced in the ANSI/ASME Code for Pressure Piping. 

Nickel Steel Low-carbon 9 percent nickel steel is a ferritic alloy developed for use in cryogenic equipment operating as low as —I95°C (—320°F). ASTM specifications A 300 and A 353 cover low-carbon 9 percent nickel steel (A 300 is the basic specification for low-temperature ferritic steels). Refinements in welding and (ASME code-approved) ehmination of postweld thermal treatments make 9 percent steel competitive with many low-cost materials used at low temperatures. 

In the case of the ASME codes for nuclear pressurised components, the questions of fatigue design and of flaw evaluation are dealt with separately in ASME Section III and Section XI Appendix A, respectively. The design S-A curve for machined butt welds typical of thick section pressurised components is set at a factor of two on stress range or twenty on cyclic life, whichever is more conservative, below the mean of S-N data developed on smooth cylindrical specimens in air. (A somewhat similar design curve obtained by a different method from experimental S-A data for machined butt welds is given in British Standard 5500.) These safety factors are intended to encompass any adverse influence of minor weld defects, size effects, data scatter and environment. As far as environmental effects are... 

Today the ASME codes are still mandatory in the United States and Canada. Both ASME and API are applied worldwide. Many European countries also developed their own national rules for the protection against overpressure of process equipment and these remained in force well into the twentieth century. Most were based on the ASME code, but they were sometimes also developed to protect national trade (see also Appendix M). 

Construction Codes Rules for Construction of Pressure Vessels, Division 1, which is part of Section VIII of the ASME Boiler and Pressure Vessel Code (American Society of Mechanical Engineers), serves as a construction code by providing minimum standards. New editions of the code are usually issued every 3 years. Interim revisions are made semiannually in the form of addenda. Compliance with ASME Code requirements is mandatory in much of the United States and Canada. Originally these rules were not prepared for heat exchangers. However, the welded joint between tube sheet and shell of the fixed-tube-sheet heat exchanger is now included. A nonmandatory appendix on tube-to-tube-sheet joints is also included. Additional rules for heat exchangers are being developed. 

When a vessel is subject to repeated loading that could cause failure by the development of a progressive fracture, the vessel is in cyclic service. ASME Code, Section VIII, Division 2, has established specific criteria for determining when a vessel must be designed for fatigue. 

For AS ME Code vessels the allowable compressive stress is Factor B. The ASME Code, factor B. considers radius and length but does not consider length unless external pressure is involved. This procedure illustrates other methods of defining critical stress and the allowable buckling stress for vessels during transport and erection as well as equipment not designed to the ASME Code. For example, shell compressive stresses are developed in tall silos and bins due to the side wall friction of the contents on the bin wall. 

Differences in design practices have in the past handicapped developments that could have reduced costs. For example, some difficulties with past design practices are evident in examining application of the ASME Code and Code Case N-47 to pool type LMFRs, such as Super Phenix. The design of Super Phenix resulted in considerably thinner components. This required different buckling rules. Super Phenix design creep effects were negligible... 

This section describes the design of mechanical systems and components for the Standard MHTGR. Mechanical systems and components required to fulfill lOCFRlOO-related radionuclide control functions under design basis conditions to meet the Top-Level Regulatory Criteria are designed to the applicable sections of ASME III (Ref. 1) as described below. Should validated methods consistent with the basic philosophy of the Integrated Approach be developed and found to be capable of providing results in which adequate confidence can be placed, the ASME Code may be modified or alternative methods applied and submitted for NRC concurrence. 

On privacy, see the Association for Computing Machinery (ACM) Code of Ethics, Section 1.7. On sustainable development, see the ASCE Code of Ethics, Canon 1, parts e and f, and the ASME Code of Ethics of Engineers, Fundamental Canon 8. 

With the reference toughness curve approach, the RT m index first came into use as the reference nil-ductility temperature which is determined in accordance with the ASME Code, Section III, Subsection NB-2331. The reference toughness Km curve and the Ki curve for static crack initiation later came into use as part of Section XI where the Km curve was called the crack arrest Xia curve. Thus, the RTndt reference temperature index has become the key material parameter in determining the allowable (P-T) limits for plant operation and for evaluating RPV integrity as the result of extreme transients such as PTS. Note that several years ago, the concept of a different, directly measured fracture toughness Master Curve approach was accepted in the ASME Code based on the index parameter RTjq. This development is covered in detail in Chapter 10. 

Materials for nuclear RPVs developed to meet the advances in RPV technology and attain the safety and reliability are discussed in this section. The designation of the materials has been standardized in the pressure vessel codes and regulations of many countries. The evolution of the ASME Code is described as a typical example. 

Table 6.3 illustrates the various definitions of these features as used in the different calculative approaches. The fracture mechanics approach in the USA is contained in the ASME Code, Section XI (ASME, 2010b), Appendix G. Recently there have been risk-informed probabihstic analyses performed (Gamble et al., 2009) that have been reduced to the same form as shown in Eq. 6.7 with values of = 1, j8 = 61 °C and 7=1. These risk-informed values have been included in the 2011 edition of the ASME Code as an alternative to the traditional deterministic method. The risk-informed approach evolved out of the risk-informed development of the US alternative PTS Rule. 

Because the Ki, and Ki curves in the ASME Code are normalized to REndt, potential application of the Master Curve procedure within the ASME Code structure is not straightforward. Figure 10.7 from Sokolov and Nanstad (1999) shows the relationship between REndt and the Master Curve Eo for the steels used to construct the A curve shown in Fig. 10.2. To overcome the lack of correlation between Eo and REndt, the Pressure Vessel Research Council and the Electric Power Research Institute (EPRI) in the USA developed a separate reference temperature based on Eo (Server et al, 1998 Van Der Sluys et al., 2000, 2001a, b). This reference temperature, RTj, was developed by determination of a temperature offset to Eo that would bound the fracture toughness data similar to that of the Ric curve in the ASME Code and has been incorporated into the ASME Code by Code Cases N-629 (ASME, 2013b) and N-631 (ASME, 2013c). Thus, in this scheme, Eq. 10.11 is contained in both Code Cases ... 

This section presents some example calculations and results from licensing criteria and ASME Code guidelines that have been developed using risk-based safety goals and probabilistic fracture mechanics computational... 

---