Toughness requirements

Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) 

This ASME Code Appendix, which became mandatory by reference in Appendix G to 10 CFR Part 50, also defined the allowable plant operating heat-up and cool-down P-T limits. The combination of the safety factor of 2 on pressure, conservative reference flaw size and lower bound toughness has the effect of maintaining a safety level equivalent to the nominal safety used in Section III, and provided significant conservatism in the safety and reliability of nuclear RPVs both during design and plant operation. 

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. 

With the establishment of the Km curve as the lower bound reference toughness for pressure RPV steels, the RT m for all RPV materials was 

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Nontransparent containers (e.g., dairy cups) account for the remaining 40% of the European market. If toughness requirements are not met by PEA, Ecovio compounds can be used for the extmsion of rigid containers. They have an adequate viscosity and can be recycled if the moisture problem is solved by drying the recycled material (like virgin PEA, to a moisture level below 200 ppm). 

These toughness requirements are in addition to tests required by the material specification. 

Part AM This part lists permitted individual construction materials, applicable specifications, special requirements, design stress-intensity values, and other property information. Of particular importance are the ultrasonic-test and toughness requirements. Among the properties for which data are included are thermal conductivity and diffusivity, coefficient of thermal expansion, modulus of elasticity, and yield strength. The design stress-intensity values include a safety factor of 3 on ultimate strength at temperature or 1.5 on yield strength at temperature. 

These requirements are, of course, subject to change. In addition to the United States and Canada, many other countries have regulations containing specific requirements. Most users specify more than the minimum toughness required by the governing codes or regulations. 

If a CRM lined pressure vessel is to be designed for below -20°F (-29°C) for reasons other than seasonal atmospheric temperature. Section VIII, Division 1 of the ASME Code requires that the materials and fabrication practices meet minimum notch toughness requirements (Paragraph UG-84). Paragraphs UCS-65, 66 and 67 cover the requirements for vessels operating below -20°F. The use of SA-36 or SA-283 is not permitted. Impact tests are required except for seasonai temperature excursions beiow -20°F (-29°C) or when exempted by paragraph UCS-66 (c) which states that "no impact test is required for materials used for metal temperatures below -20°F (-29°C) when the minimum thickness is the greater of those determined under the most severe conditions of coincident pressure (external or internal) and temperature in accordance with UG-21 for temperatures of (a) -20°F (-29°C) and above and (b) below -20°F (-29°C), in which case the coincident pressure (internal if above atmospheric pressure and external if below atmospheric pressure) shall be multiplied by 2%."... 

The minimum fracture toughness required would depend on the yield strength of the material, and the expected crack size, which is to be set by the efficacy and fidelity of regularly scheduled nondestructive inspections. 

Table 5.1. Minimum fracture toughness requirement as a function of yield strength and thickness based on Irwin s leak-before-break criterion...

In the case of polymer-based materials, composites are often preferred because the mechanical properties of the pure polymer phase are inadequate for the proposed application [4]. To overcome this problan, polymeric materials are reinforced in some way, typically by incorporating a substantial amount of rigid filler. For some polymers, the problem may be that they lack the toughness required for a particular application, and for these materials, elastomeric fillers are used. These fillers have the effect of increasing toughness and the concomitant effect of reducing brittleness. However, this approach is not used for restorative dental materials. 

Code of Federal Regulations, Title 10, Part 50, Section 50.60, "Acceptance Criteria for Fracture Prevention Measures for Normal Operation" and Section 50.61, "Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock."... 

Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock... 

Fracture toughness requirements for protection against pressurized thermal shock are defined in 10 CFR Part 50,61, These requirements should be reviewed for applicability if applicable, they provide the basis for ensuring the integrity of the reactor vessel for pressurized thermal shock conditions,... 

Fed Reg (1983) Fracture toughness requirements for protection against thermal shock events , USA Code of Federal Regulations, 10/50.61. 

In courses on materials engineering, we learn almost from day one that toughness requires a fine microstructure, with no mention of hierarchy. Here we consider whether a hierarchical microstructure confers any toughening benefits additional to those associated with a fine microstructure. 

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