PSI - Issue 52

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

764

3

σ ref Reference stress (used for plastic collapse assessment)

Fusion reactors are being considered as an alternative to Fission reactors although they are not yet su ffi ciently developed to produce electricity on a useful scale. Nevertheless, fusion reactors present the possibility of providing approximately four times more energy than that produced from fission without producing high activity, long lived isotopes i.e. they produce no high level waste. Further research on fusion technology is therefore important. Prototype fusion reactors around the world are being developed to support this research e ff ort but the low level waste that is generated still requires transportation, treatment and / or storage. The International Atomic Energy Agency (IAEA) Regulations for the Safe Transport of Radioactive Material (SSR-6 (2018)), herein referred to as the “regulations”, require that packaging is developed to satisfy onerous safety criteria. For type B packages (high activity and / or fissile) these criteria include a 9m drop test, a 1m drop onto a mild steel punch and a subsequent all engulfing pool fire for 30 minutes at 800 ◦ C. For type A packages (low activity) the criteria are relaxed but still includes assessment in fault conditions - such as a smaller drop height at sub-zero temperatures. NTS develops new packages for all stages of the fuel cycle. It also re-purposes older packages to fulfil important roles within the industry, for example transporting low level waste or medical isotopes. One package, the 1647B or “thin white” package, originally developed by Magnox in 1992-93 to transport intermediate level waste, is being considered for transporting waste from a prototype fusion reactor within the UK. The 1647B is designated a type A (low activity) transport package and as such does not have to satisfy the accident conditions of transport described for type B packages. Due to its age, the original basis of the 1647B design and manufacture predates modern requirements of the regu lations to demonstrate the avoidance of brittle fracture. However, the current regulations suggest that a demonstration of the avoidance of brittle fracture may be required in a handling drop from 1.2m at -40 ◦ C, see para 639 of SSR-6 (2018). Appendix V of the regulations advisory material (SSG-26 (2018)) provides three alternative methods to demonstrate the avoidance of brittle fracture. The first method is by exclusion of austenitic stainless steels which are not prone to brittle fracture at -40 ◦ C. The second method describes approaches for correlating nil ductility transition temperature (NDTT) or Charpy impact energy to fracture resistance. This method has been explored for the 1647B and is later discounted as a viable option for demonstrating regulatory compliance. The third method is based on a fracture mechanics approach which provides the benefit of considering flaw size(s) in the assessment. The third method is adopted in this paper. To perform a method 3 assessment SSG-26 (2018) suggest several codes of practice or standards that can be applied. One recommended standard in SSG-26 (2018) is BS PD6493 (1991). This code has now been superceded by BS7910 (2019) which is a suitable, recognised alternative. The ASME boiler and pressure vessel code ASME Section III (1992) is also cited in the advisory material - this has also been updated to ASME Section III (2023). An additional publication, excluded for equipment design to ASME Section III (2023), but widely used by structural integrity engineers and specifically for engineering critical assessments is ASME FFS-1 / API579-1 (2016). Method E of the Level 3 assessment using ASME FFS-1 / API579-1 (2016) permit the use of alternative, recognised procedures such as R6 (2001); BS7910 (2019). In the assessment described in this article the procedure described in BS7910 (2019) has been followed, supplemented by a particular stress linearization technique from ASME FFS-1 / API579-1 (2016). The application of up-to-date standards provides enhanced techniques for demonstrating greater margins of safety. Proposals for additional requirements within the IAEA regulations for the demonstration of the avoidance of brittle fracture began in earnest in the early 1990s. Warnke (1997) described an outline of the proposals for safety demon strations, focussing on research conducted in Germany on ductile cast iron packages such as the CASTOR packages. It was widely recognised at the time that insu ffi cient advisory material on the subject was available Price (1994). As a consequence an international e ff ort was made to revise the regulations which were subsequently updated in 2002 with a much more comprehensive appendix to the Advisory Material TSG-26 (2002) and older revision of SSG-26 (2018). Research programmes in the UK during the period up to 2002 focussed on manufacturing aspects of the materials used in transport packaging. For example Cory (2004) expanded the existing fleet of NTL 11 spent fuel transport pack ages in the late 1990s by developing four new packages. One of his major focusses was on improving the metallurgy