PSI - Issue 52

A.D. Cummings et al. / Procedia Structural Integrity 52 (2024) 762–784 A. Cummings / Structural Integrity Procedia 00 (2023) 000–000

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2. Regulatory compliance route

SSG-26 (2018) o ff ers three methods for demonstration of the avoidance of brittle fracture. Method 2 describes correlating Charpy energy to fracture toughness without performing a fracture mechanics assessment using PD5500 (2015). This method is applicable to carbon-manganese steels but PD5500 (2015) excludes wall thicknesses greater than 110mm - the 1647B is 112.5mm thick. Instead Annex U of PD5500 (2015) suggests a fracture mechanics approach and advises the use of BS7910 (2019). Furthermore, for wall thickness 110mm or less, to satisfy PD5500 (2015) would require Charpy impact values greater than 40J at -55 ◦ C and this data is not available. A second approach is described in method 2 of SSG-26 (2018) by correlating fracture resistance to the Nil Ductility Transition Temperature (NDTT). This is primarily intended for ferritic steels although it does not exclusively rule out other steels. During manufacture of the 1647B fleet, NDTT was measured by performing Pellini tests ASTM E208 (1985). The average results from the body of the packages were ≈ -30 ◦ C. To correlate this to a Low Service Temperature (LST) of -40 ◦ C the guidance in US NRC 7.12 (1991) was followed. It was concluded that to satisfy a LST -40 ◦ C for this thickness of material a NDTT < -85 ◦ C was required. Therefore method 2 was ruled out as an option and a fracture mechanics approach (method 3) adopted following BS7910 (2019). During the period 1992-93, 6x 1647B packages were manufactured to transport intermediate (and low) level waste from Magnox Chapelcross to Sellafield. It is assumed in the following assessment that no degradation of fracture toughness has occurred since manufacture. The packages (including the lid bolts) are subject to visual inspections and / or corrective procedures, yearly or after 20 - 25 transport cycles, whichever is more frequent. A more detailed inspection is carried out every 3 years where lid seal O-rings are replaced, and the lifting trunnions are removed for inspection of the package body and trunnions. Any evidence of paint defects and / or corrosion is recorded, and paint touch up or full re-painting is carried out as necessary. In 2010 a concession was issued to adjust the maintenance procedure to remove paint from the package body because transports have never been carried out wet or used in a fuel pond facility. Fig. 1(a) shows a 3-D model of the 1647B package design, the package body is forged from a low carbon manganese steel, BS1503 224 430 LT40. The lid is forged from 304S11 (1989) and the lid lifting feature is a welded fabrication, an I-sectioned structure, constructed from plate 304S11. The shock absorber is a welded fabrication of plate Aluminium alloy 5083 ’O’. The 12 x M36 lid bolts are manufactured from a high strength steel grade BS 2S 143C with a waisted shank diameter of φ 32mm. Fig. 1(b) shows a Finite Element Analysis (FEA) model of the package with the basic dimensions of the package and material designations. The package body wall thickness is 112.5mm and the base thickness is 105mm. The wall is thinnest in the lifting trunnion pocket (60mm). BS1503 224 430 LT40 is an old standard and as such the material is no longer readily available. The manufactured package body chemical composition included; Carbon 0.16 wt.%, Sulphur 0.015 wt.%, Manganese 1.23 wt.% and Nickel 0.15 wt.%, see Table. 1. It does not have an equivalent ASTM grade but is most closely matched to ASTM A350 LF1 (C 0.3 wt.%, S 0.04 wt.%, Mn 0.6-1.35 wt.% and Ni 0.4 wt.%). The mechanical and impact properties of the forging are presented in Table 2. The data is taken from test certificates, those values with an asterisk are low temperature values that have been estimated from equations or tables in Chapter 7 of BS7910 (2019). 3. 1647B transport package description

Table 1. Package body - Chemical composition of BS1503 224 430 LT40 [%] C Si Mn P S Cr Mo

Ni

V

Al

Cu

Sn

0.16

0.23

1.23

0.011

0.015

0.16

0.03

0.15

0.01

0.029

0.13

0.014

Charpy V-notch impact energy results were provided for two locations (A) and (B) taken from the package body during manufacture. The exact locations of (A) and (B) are not clear from the records. In the following assessment the lower bound values from location (A) were used in the master curve approach to derive fracture toughness. Applying