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Federal Register / Vol. 86, No. 20 / Tuesday, February 2, 2021 / Proposed Rules
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requirements in N869 are the same as in Code Case N5133.
In 2018, the NRC found an instance where a licensee misinterpreted the provisions in Section 5 of N5133 and did not perform the required augmented examinations to disposition a throughwall leak in a service water system pipe.
Other licensees have similarly misinterpreted the augmented examination provisions in Section 5 of N5133. The NRC found that the issue stems from the definition of the terms flaw and significant flaw in Sections 5b and 5c of N5133, respectively. The NRC, therefore, proposes two conditions to define flaw and significant flaw as those terms are used in Section 5 of N869.
Licensees would be required to apply these definitions to Section 5 when using the Code Case.
The first proposed condition defines a flaw as a non-through-wall planar or nonplanar flaw with a wall thickness less than 87.5 percent of the nominal wall thickness of the pipe or the design minimum wall thickness. The NRC
notes that the pipe wall thickness at the time of the plant construction may deviate from the nominal pipe wall thickness slightly as part of manufacturing process. The generally accepted deviation is 12.5 percent of the nominal pipe wall thickness or the design minimum wall thickness.
The second proposed condition defines significant flaw as any pipe location that does not satisfy the provisions of Section 3 of N869 or if any detected flaw that has a depth greater than 75 percent of the pipe wall thickness. The NRC staff notes that the criterion of the 75 percent wall thickness criterion originates from the provisions of IWC/IWD3643 of the ASME Code, Section XI, which prohibit a flaw that exceeds 75 percent of the pipe wall thickness to remain in service.
Under Section 5 of N869, a planar flaw that exceeds 75 percent of the pipe wall thickness may remain in service;
however, the licensee needs to perform an augmented examination.
Code Case N876 Supplement 2, 2017
Edition Type: New.
Title: Austenitic Stainless Steel Cladding and Nickel Base Cladding Using Ambient Temperature Automatic or Machine Dry Underwater Laser Beam Welding ULBW Temper Bead Technique, Section XI, Division 1.
Some irradiated stainless steel reactor vessel internal components are susceptible to experiencing irradiation assisted stress corrosion cracking. Code Case N876 provides guidelines for
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repair welding the irradiated stainless steel components inside the reactor vessel. Code Case N876 provides an alternative to the cladding temper bead repair rules of Section XI, IWA4400, which requires preheat and postweld heat treatment. This alternative establishes new rules governing ambient temperature temper bead cladding repairs using the ULBW process.
The NRC is proposing two conditions on this Code Case. The first proposed condition that must be applied when performing ULBW on irradiated materials provides guidance on what level of neutron irradiation and/or helium content would require review and approval by the NRC because of the impact of neutron fluence on weldability. The second proposed condition limits the depth of the cladding repair due to concerns with the fracture toughness of the base metal.
The technical basis for imposing conditions on the welding of irradiated materials are that neutrons can generate helium atoms within the metal lattice through transmutation of various isotopes of boron and/or nickel. At high temperatures, such as occurs during welding, these helium atoms rapidly diffuse though the metal lattice, coalescing and forming helium bubbles at the grain boundaries. In sufficient concentration, these helium bubbles can cause grain boundary cracking that occurs in the fusion zones and heat affected zones during the heat-up/
cooldown cycle.
The first proposed condition applies conditions already applicable to Code Case N5165 Underwater Welding Section XI, Division 1, that the provisions of 50.55ab2xiiA and B must be met. This regulation provides limits on specific levels of neutron irradiation and/or helium content, above which welding is prohibited without prior NRC review and approval. The NRC is proposing to apply the same condition to uses of Code Case N876.
The second proposed condition is necessary because the Code Case does not require impact testing of the base metal heat affected zone HAZ to verify adequate fracture toughness. The Code Case allows the depth of the repair cavity into the ferritic base metal to be up to 14. This would allow welding directly to the base metal, thus it will affect the fracture toughness of the base metal in the HAZ. Therefore, the NRC
is proposing a condition restricting the use of the Code Case to repairs where at least 18 of cladding remains. The basis for the 18 limit is that this amount of austenitic material between the ferritic base metal and the first weld layer has
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generally been considered to sufficiently limit the heat input to the base metal such that deleterious effects on the fracture toughness will not occur;
therefore, impact testing of the base metal is not necessary. The NRC notes that Code Case N803, which is approved without conditions, allows repair of ferritic base material using nonferritic weld filler material based on welding procedure qualifications performed using tensile tests, side bends, and impact tests, and could be used to perform a cladding repair in which excavation into the base metal is required.
Code Case N878 Supplement 1, 2017
Edition Type: New.
Title: Alternative to QA Program Requirements of IWA4142, Section XI, Division 1.
Code Case N878 provides alternatives to the quality assurance requirements in IWA4142 for procurement of Class 1, 2, or 3 nonwelded fittings. This Code Case addresses the testing and certification of material used in the manufacture of non-welded fittings, but does not address how the licensee must ensure that the procured non-welded fittings meet the design and testing requirements of the ASME Code, Section III, NB/NC/ND3671.7 for Class 1, 2, or 3 applications. Verification that the Section III requirements for the design and testing of these non-welded fittings have been met prior to use is essential in ensuring the structural integrity of these Class 1, 2 and 3
systems is maintained. Therefore, the NRC is proposing conditions for the licensee to verify the design and testing activities associated with qualification of non-welded fittings required by Section III, NB/NC/ND3671.7 that are performed by the fabricator.
Code Case N880 Supplement 2, 2017
Edition Type: New.
Title: Alternative to Procurement Requirements of IWA4143 for Small Nonstandard Welded Fittings, Section XI, Division 1.
Code Case N880 provides alternatives to the material procurement requirements of IWA4142 and IWA
4143 for small nonstandard welded fittings. This Code Case does not address how the licensee must ensure the procured welded fittings meet the design and testing requirements of the ASME Code, Section III, NB/NC/ND
3671.7 for Class 1, 2, or 3 applications.
Verification that the Section III
requirements for the design and testing
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