Since the early days of the UK nuclear industry, low-level radioactive waste (LLW) has been disposed of in waste disposal vaults at the low-level waste repository (LLWR) located near Drigg in Cumbria. Until 1988, the LLW was ‘tumble tipped’ into special vaults consisting of a shallow trench lined with clay; after these trenches were full, they were covered in clay to prevent ingress of rainwater. The LLWR used to be managed by BNFL but now comes under the ownership of the UK government’s Nuclear Decommissioning Agency, and is managed on their behalf by LLW Repository Ltd.
In 1987 BNFL decided to improve the efficiency of the disposal area available and improve the long term environmental safety associated with the trenches. This was achieved by the introduction of a number of measures that included using concrete-lined vaults, packaging waste in primary disposal containers, and pre-sorting waste to reduce waste volumes.
The packaging (primary disposal containers) originally proposed were 200 litre drums in 12 pack stillages (3x2x2 high array) and steel boxes with volumes of 4.5m3 and 9m3. As an alternative Croft Associates proposed the use of special IP-2 ISO freight containers of volumes 17 m3 and 37 m3 , these being the half-height IP-2 ISO freight container (2.4 m wide, 6 m long and 1.2 m high) and the full-height IP-2 ISO freight container (2.4 m wide, 6 m long and 2.6 m high). These special IP-2 ISO freight containers were manufactured using conventional technologies, but modified to provide improved containment (not required in the conventional freight container industry), to meet the requirements for an IP-2 package. Cost estimates had shown that there would be substantial savings by using these special IP-2 ISO freight containers instead of using the much smaller containers proposed by BNFL. The government-owned Central Electricity Generating Board (now mainly reformed as British Energy and Magnox), who were major shippers of LLW from their power stations to LLWR, were very interested in Croft’s proposals. Croft gained acceptance from the UK Department of Transport (the competent authority) of the design principles and the approaches proposed to demonstrate compliance with the IAEA transport regulations relevant at that time.
Design parameters
In the 1985 edition of the IAEA Transport Regulations, a performance standard was introduced for LLW classified as low-specific activity (LSA) material or as surface-contaminated objects (SCO); the requirement added was that there be no loss or dispersal of the radioactive contents and no loss of shielding under routine and normal conditions of transport. A 1990 Amendment to the IAEA Transport Regulations stipulated that IP-2 ISO freight containers should meet ISO 1496/1-1978 [1], among other requirements.
The objective of the containment standard of “no loss or dispersal”, which has never been defined quantitatively in the regulations, is to ensure that under normal conditions of transport the radioactive contents of the package cannot escape in sufficient quantities to create a radiological hazard.
The containment standard adopted for IP-2 ISO Freight Containers in the UK following a test programme and consultations with the Department of Transport, is that containment be verified at the appropriate stages by a gas leak test with a pass criteria as follows [2]:
- Containment body: 0.1 bar cm3 s1 SLR (individual leaks detected)
- Closure seals: 1.0 bar cm3 s1 SLR (gross leaks detected)
The UK Department of Transport issued a guide to the approval of IP-2 ISO freight containers as Type IP-2 and Type IP-3 Packages in 1999 that was reissued as the current document in 2005 [3].
Croft developed the following generic design specification for IP-2 ISO freight containers that covers the key requirements necessary to meet the regulatory requirements and interpretations developed in consultation with the Department of Transport and the UK nuclear industry.
- Designed to meet ISO 1496/1-1978
- Fabrication in steel with continuous, externally-accessible seal welding
- For end-opening containers, single-door closure (double doors in ISO freight containers cannot be shown to provide adequate containment of particulates)
- For top-opening containers, removable lid
- Closure (lid or door) also fabricated from steel with continuous seal welds
- Elastomeric seals used in closure system
- Closure seal leak-testable by method such as the use of double seals with an intervening space, and leak testing by the gas pressure drop method
- Containment vessel (including lid or door) leak-testable by method such as soap bubble
- For design verification, the containment standard for the containment vessel and seals tested before and after ISO 1496/1. Containment of the closure seals should be verified during the ISO racking tests
- Annual verification of seal containment standard for reusable containers
- Provision of tie-down arrangements
- Provision of HEPA-filtered vents when necessary to ensure equalisation of changes in pressure due to changes in ambient temperature and pressure.
After due consideration, BNFL issued its approval for use of these special IP-2 ISO freight containers by the CEGB and other waste producers, and in due course the Croft half-height and full-height IP-2 ISO freight containers were adopted as the de facto standard containers for disposal of LLW, including BNFL’s own LLW, at the LLWR. BNFL went on to improve the efficiency of waste disposal by the introduction of supercompaction for some of the wastes (that is, compactable wastes) designated for disposal in the half-height IP-2 ISO freight containers.
At the end of 1987, Croft had designed, tested and manufactured the first two batches of full- and half-height IP-2 ISO freight containers. These were supplied to the UK nuclear industry in early 1988 through a novel UK industry procurement group.
The ISO Procurement Club
The ISO Procurement Club was a UK nuclear group formed specifically to co-ordinate the needs of the UK nuclear industry for IP-2 ISO freight containers for LLW. From 1988, the club coordinated the commercial requirements of the principal producers of LLW in the UK. At the club’s monthly meetings, decisions were taken on technical and commercial requirements: this included managing the development of new designs and determining optimal procurement by bulk purchase for the containers through Croft. The design ownership was agreed to be Croft and all members of the club – this was deemed to be equitable as all members contributed to the cost of new design work, and Croft contributed the original designs. The club ceased in 1995 when BNFL declared that it would become the supplier to club members. By that time, it had developed its supercompaction plant at Sellafield and had further developed its methodology for efficient use of Drigg.
After dissolution of the club, Croft continued to provide design services in the development of IP-2 ISO freight containers to BNFL and continues to provide design services for new design of IP-2 ISO freight containers to LLWR.
Bob Vaughan and Ron Hows, Croft Associates Ltd, Building F4, Culham Science Centre, Culham, Abingdon, Oxfordshire OX14 3DB, GB www.croftltd.com. The authors would like to thank the following parties for their contribution to the development of IP-2 ISO Freight Containers in the UK: AWE Aldermaston; BNFL; CEGB, Barnwood; DoT, RMTD; LLWR (Drigg); Nirex, later RWMD, NDA; UKAEA.
Paper originally published at PATRAM 2010, London, UK 3-8 October 2010.
References
1. ISO 1496-1:1990, (BS 3951-2.1:1991), Freight containers. Specification and testing of series 1 freight containers. General cargo containers for general purposes
2. ISO 12807:1996, Safe transport of radioactive materials – Leakage testing on packages
3. DfT/RMTD/0002 (Freight Containers) Issue 2, July 2005, A DfT Guide to the Approval of Freight Containers as Type IP-2 and Type IP-3 Packages
Also:
IAEA Safety Standards, Regulations for the Safe Transport of Radioactive Material, 2005 Edition, TS-R-1
M C Janicki and R A Vaughan, The Development of ISO Freight Containers as IP-2 Packagings, PATRAM 92, Paper 20-4
