Development of the Nirex Generic Transport Safety Assessment to Assist in the Provision Advice of Waste Packaging

Abstract

The current Nirex mission is to provide the United Kingdom with safe, environmentally sound and publicly acceptable options for the long-term management of radioactive materials. As part of this role, Nirex has developed a phased deep geological disposal concept which is defined by six ‘generic documents’ that describe systems, processes and safety assessments that are not specific to anyone location or geology. These generic documents give access to detailed information about the ideas and approaches that underpin the phased disposal concept, and have been published with an invitation to enter into dialogue with Nirex regarding these issues. The generic documents identify the requirements for an integrated transport system that would be necessary for the management of the intermediate-level (ILW) and low-level (LLW) wastes within Nirex's remit — the so-called reference case volume. This has involved Nirex in the development of transport hardware and associated safety reports and modelling and assessment tools for transport system logistics and system safety. Although the phased disposal concept is only one option for the long-term management of waste, the integrated transport system and associated modelling tools are likely to be of equal relevance to other options. The safety assessment of the generic transport operation for the movement of ILW and LLW waste from waste producers' sites to a future radioactive waste disposal facility is described in one of the generic documents — the generic transport safety assessment (GTSA). The GTSA demonstrates that the transport operation is compliant with Nirex safety principles, and that the nuclear and non-nuclear risks to the public and workers from routine transport and from accidents are acceptable. This paper describes the types of risk that are calculated, and discusses the data requirements and calculation methodology. The verification and validation methodology is outlined, together with a discussion of the results and a comparison of the risks with the Nirex dose and risk targets. In addition, this paper also describes how the methodology of the GTSA has been developed into an innovative software tool, TranSAT, which is routinely used as part of the packaging waste advice service offered by Nirex.     

NIREX Transport Activities since Rejection of the Rock Characterisation Facility Planning Application

Abstract

The application by Nirex to build an underground Rock Characterisation Facility, was rejected by the Secretary of State on 17 March 1997. This decision has caused Nirex to reconsider its forward programme for the development of an underground disposal facility for intermediate level waste. This paper describes the transport related activities being undertaken by Nirex since that date.     

Testing of the Sealing Arrangements for the Nirex Re-Usable Shielded Transport Containers

Abstract

As part of its responsibility for the development of a deep repository for intermediate and low level radioactive waste, UK Nirex Ltd is developing a range of Type B re-usable shielded transport containers (RSTCs). A testing programme has been carried out on two alternative concepts for the RSTC sealing arrangements over the temperature range ?40^°C to 200^°C. For each sealing system, a test rig was developed to measure the performance under simulated normal and accident conditions of transport. The elastomer O-rings used for some of the tests had been irradiated to the maximum dose they might receive in normal transport. The performance of both sealing systems was good and it is concluded that either concept would meet the specified leakage criteria over the full temperature range under both normal and accident conditions of transport. However, further testing is required to confirm the performance of Concept N under accident conditions.     

The Planned Integrated Transport System for the Deep Repository in the UK

Abstract

UK Nirex Ltd is responsible for developing a deep repository for the safe disposal of intermediate and low level radioactive wastes (ILW and LLW), and is concentrating its investigations on Sellafield as a potential location. A key part of the repository development programme is a transport system to deliver packaged wastes from sites elsewhere in the UK. The transport system must be able to handle a range of standard waste packages, and all transport through the public domain must comply with the IAEA Transport Regulations. Two design concepts have been developed for re-usable shielded transport containers for ILW, which are predicted to withstand accidents at least as severe as the IAEA Type B test conditions. Assessment, testing and further development of both concepts continues, with a view to selecting one for quantity production. Nirex is working closely with various organisations to establish the optimum transport routes for a potential repository at Sellafield. The current policy is that rail transport shall be used wherever practical for the transport of waste to the repository, although some road transport may also be required; the company has assessed a range of options. A Probabilistic Safety Assessment of the proposed transport operations has predicted that the radiological risks are expected to be extremely low, reflecting the adequacy of the packaging concepts. In addition, Nirex has identified a suitable transport emergency plan to deal with any unforeseen events.     

Application of IAEA TECDOC 717 to the Assessment of Brittle Fracture in Transport Containers with Plastic Flow Shock Absorbers

Abstract

UK Nirex is developing re-usable shielded transport containers (RSTCs) in a range of shielding thicknesses (from 70 nun to 285 nun) to transport immobilised intermediate level radioactive waste (ILW) to a deep repository. The RSTCs are being designed to meet the requirements of the IAEA Transport Regulations for Type B packages, including the requirement to maintain shielding and containment following a drop of 9 m onto an unyielding surface. The RSTCs are essentially monolithic in construction and the heaviest version weighs up to 65 tonnes when loaded with contents. They rely principally on plastic flow of the material of construction to absorb the high energies involved in impact events. Specific features of the designs, such as the solid metal comer shock absorbers and side ribs have been optimised for this purpose. Nirex has investigated the feasibility of manufacturing the RSTCs from ductile cast iron (DCI) or cast steel instead of from forgings, since this would bring advantages of reduced manufacturing time and costs. In this paper the methodology set out in IAEA-TECDOC-717 is applied to the Nirex RSTC, including the application of elastic plastic fracture mechanics methods.     

Probabilistic Safety Assessment for the Transport of Radioactive Waste to a UK Repository at Sellafield

Abstract

A study has been undertaken to provide a detailed understanding of the radiological and non-radiological risks associated with the transpott of radioactive waste from the sites at which waste is produced in the UK to a proposed deep repository at Sellafield, and to ensure that these risks meet the design targets specified by Nirex. The routine transport collective dose to members of the public was assessed to be 0.2 man.Sv per year, which is only about 0.004% of the natural background dose. Accident frequencies were calculated using event tree methodology. The radiological consequences of accidents were assessed using the probablistic computer code CONDOR. The risk expectation value was calculated to be 1.5 × 10^?5 ? 8.6 × 10^?6 latent cancer fatalities per year (depending on the transport mode scenario). These values are significantly lower than the corresponding prediciions for non-radiological accident fatality rates, 0.05 ? 0.035 fatalities per year. The radiological accident risk for the most exposed individual member of the public was assessed to be 5 × 10^?11 ? 1.7 × 10^?11 per year, very much less than the Nirex target of 5 × 10^?7 per year. Plots of societal risk were shown to lie in the region of ‘negligible risk’, as defined by the UK Health and Safety Commission for non-radioactive dangerous goods transport.     

Mechanical Properties of Ductile Cast Iron and Cast Steel for Intermediate Level Waste Transport Containers

Abstract

UK Nirex Ltd is developing Type B re-usable shielded transport containers (RSTCs) in a range of shielding thicknesses to transport intermediate level radioactive waste (ILW) to a deep repository. The designs are of an essentially monolithic construction and rely principally on the plastic flow of their material to absorb the energies involved in impact events. Nirex has investigated the feasibility of manufacturing the RSTCs from ductile cast iron (DCI) or cast steel instead of from forgings, since this would bring advantages of reduced manufacturing time and costs. However, cast materials are perceived to lack toughness and ductility and it is necessary to show that sufficient fracture toughness can be obtained to preclude brittle failure modes, particularly at low temperatures. The mechanical testing carried out as part of that programme is described. It shows how the measured properties have been used to demonstrate avoidance of brittle fracture and provide input to computer modelling of the drop tests.     

Conceptual Design of Large Boxes for the Packaging of Intermediate and Low Level Radioactive Waste

Abstract

United Kingdom Nirex Limited (Nirex) is developing standard containers for the packaging of radioactive waste for disposal in a deep underground repository. Waste Package Specifications have been produced for each standard package to provide the essential link between waste package design and the design of the deep repository. Previous studies carried out by Nirex identified the dimensions and key features of standard boxes for decommissioning intermediate level waste and for low level waste: the 4 m ILW box, 4 m LLW box·and 2 m LLW box. Nirex has now produced conceptual designs for these boxes.     

Boxes for the Transport and Disposal of Low Level and Decommissioning Intermediate Level Radioactive Wastes

Abstract

UK Nirex Ltd is developing specifications for standard containers for the packaging of low and intermediate level radioactive wastes for disposal in a deep underground repository. The methodology used for selection of the dimensions and payloads of boxes for LLW and decommissioning ILW are described, and design features for handling the boxes are also briefly discussed. It must be emphasised that the results of this study are as yet preliminary, and may change in the light of the evolving system design for waste packaging, storage, transport and disposal.     

Performance of the IAEA transport regulations in controlling doses and risks from a large-scale radioactive waste transport system

Abstract

The role of United Kingdom Nirex Ltd is to provide the UK with safe, environmentally sound and publicly acceptable options for the long-term management of radioactive materials generated by the UK's commercial, medical, research and defence activities. An important part of this role is to set standards and specifications for waste packaging. Waste producers in the UK are currently developing processes for packaging many different types of intermediate-level waste (ILW), and also those forms of low-level waste that will require similar management to ILW. When packaging processes are at the proposal stage, the waste producers consult Nirex about the suitability of the resulting packages for all future aspects of waste management. The response that Nirex provides is based on detailed assessments of the proposed packages, their compliance with Nirex standards and specifications, and their predicted performance through the successive phases of waste management. One of those phases is transport through the public domain. This paper draws on experience gained from more than 200 separate transport safety assessments which have cumulatively covered a wide range of waste types, waste packages and transport packages.     

Nirex—the Complete Package

Abstract

Through continuous interaction with its customers, Nirex identified the need for a comprehensive range of waste containers, reflecting the variety of wastes and operational undertakings. The current range consists of five standard containers. Standardisation is introduced across all waste packages to enable the safe and efficient operation of future waste management facilities. The practical lessons learned during the development of standard containers are in turn reflected in the container design work that Nirex has undertaken. They are also fed into the advice given to customers during evaluation of waste packaging proposals.     

The Development of a ‘Large liner’ For the Processing and Solidification Of Intermediate Level Wastes

Abstract

Magnox Electric have developed a ‘Large Liner’ for the processing and solidification of the majority of the slurry form intermediate level wastes which arise on its power station sites (operational or undergoing decommissioning). The Liner comprises a cylindrical vessel of approximate dimensions 1.7 m diameter by 1.2 m high and it has been developed to comply with the requirements of the current Nirex Package Specifications. It is manufactured from stainless steel, being of all welded construction, with a bolted lid, incorporating an ‘O’ ring elastomer seal. The Liner can be fitted with a paddle for the ‘in drum’ solidification of slurry from wastes. This paper describes the development of the Liner, outlining the main stages from initial concepts to the final testing and receipt of approval from Nirex, who have adopted the Liner as one of its ‘Standard’ waste containers.     

The Transport System Approval Concept

Abstract

The needs for, and merits of, a new concept for the safety assessment and approval of shipments of radioactive materials is introduced and discussed. The purpose of the new concept is to enable and encourage integration of analysis and review of transport safety with similar safety analysis and review of the handling operations involving the radioactive material at the despatching and receiving ends of a shipment. Safety contributing elements or functions of the means of transport (the Transport System) can thus readily be taken into account in the assessment. The objective is to avoid constraints — experienced or potential — introduced by the package functional provisions contained in the transport regulations, whilst maintaining safety during transport, as well as during facility handling operations, at least at the level currently established.     

Application of the Transport System Concept to the Transport of LSA Waste

Abstract

Transport Regulations rely essentially on the packaging and do not take into account the contribution to safety which may be made by other features of the operation. In some situations, mainly for routine transports not fully complying with the Regulations, it would be beneficial to envisage the possibility of using a package which does not meet all the Type B requirements, complemented by additional safety measures put in place to compensate for these shortfalls. The ‘Transport System’ concept will take into account the contributions to safety from these additional measures. It will ensure that the proposed system is at least as safe as a reference operation complying fully with the Regulations. If this equivalent safety level can be properly demonstrated, the Competent Authority will provide a ‘Transport System Approval’ for well defined shipments over a specific period. Two examples are presented. In the first case, a thermally insulated ISO container is envisaged for the transport of drums containing combustible LSA material having a total activity per conveyance up to 600 A₂. In the second one, two dedicated trucks transporting conditioned waste in drums has been shielded so as to comply with the regulatory dose rate limits. These examples show the benefits of the TS concept. Nevertheless, the full requirements of the Regulations should be implemented as far as reasonably practicable, and the TS concept should be applied only to particular difficulties and is not suitable to all situations. Therefore, some general restrictions (applicable to every TS) have to be set by IAEA. Depending on the case, complementary ones may be required by the CA. Bearing in mind possible restrictions presented in this paper, the TS concept will be useful in solving some of the current problems of the transport of waste without needing a fundamental change in the Regulations.     

UK Transport Events and International Cooperation

Abstract

In the UK, an accident and incident database has been established for transport events from 1964 onwards. The CEC is promoting a Community transport event databank and the IAEA has established an international system EVTRAM. The objectives of these systems are different and each has merit provided an adequate level of response is established and maintained. Such systems provide information on the effectiveness of regulatory requirements and enable lessons learnt to be made widely available. The reporting procedures in the UK and the results of an analysis of transport events are presented.     

Packaging for the Transport and Storage of Spent Sealed Sources in Brazil

Abstract

Since mid-1994, the Centro de Desenvolvimento da Tecnologia Nuclear has been assigned the task of receiving and safely conditioning spent sealed sources until a federal disposal site is available. At the moment (October 1995) there are approximately 1300 sources in the CDTN interim storage hall. As part of the measures taken to accomplish this task, the transport group has developed a simple and low cost packaging, which consists of an outer 200 litre drum surrounding a cylindrical lead shielding, the intermediate space being filled with concrete. In the primary concept a concrete internal lid allowed the sources to be retrieved for future re-encapsulation. In view of a failure in the drop test, a modification was introduced to gather additional information about the ultimate packaging strength, although the resulting concept does not allow future recovery of the contents. The next improvement to be introduced will be the use of a shell-type shock absorber to protect the packaging closing system.     

Radioactive Waste Transport in the Russian Federation

Abstract

According to the national radwaste management programme, low and intermediate level wastes originating from nuclear power plant (NPP) operation, from the nuclear fuel cycle and from the application of radioisotopes in medicine, industry and research are centrally collected and transported for disposal at 16 special repositories. Packagings and conveyances for these purposes are discussed. A concept of regional repositories and transport of wastes from NPPs and the Navy is also considered together with transport packagings and transport methods.     

LLW Transport by IP-2 Packaging

Abstract

As the Japanese nuclear power plants are located on the sea coast, the optimal system for LLW transport consists of sea and land modes. A special ship ‘Seiei Mam’ was built to transport the LLW from nuclear power plants to the LLW Burial Centre in Rokkasho-mura, Aomori Prefecture, and dedicated trucks were prepared to transfer the LLW from the receiving wharf to the Burial Centre. Containers were developed to transport LLW drums efficiently and were designed and tested to meet the IP-2 packaging requirements. Of such containers 3000 have been used since 1992 and the safe transport of LLW has been demonstrated by means of the IP-2 packagings.     

Plutonium Transport: Past,Present and Future

Abstract

The transport of radioactive materials dates back to the beginning of the nuclear industry. The development of nuclear plants and the international trade in fuel cycle services has led to a transport infrastructure to service the industry. Advances in radioactive material package design and technology have been led by increasing emphasis on safety assurance and compliance with transport regulations which in many cases exceed those applied to other dangerous goods. In the case of certain materials security during transport has equal emphasis with safety, and plutoniwn. in its many forms, attracts the most onerous security requirements during transport. BNFL has safely and efficiently transported plutonium both nationally and internationally for 30 years. The Company is committed to the continuation of maintaining such transports in a safe, secure and cost effective manner.     

The Cost Implications of Large Shielded Transport Packages for Radioactive Waste

Abstract

Total costs have been estimated, at 1990 prices, for the packaging, transport and disposal of intermediate level radioactive waste. The study covered three generic types of large package with several scenarios of interim storage and rock type at the disposal site. The concept which uses returnable shielding is calculated to be the cheapest, although the choice between self-shielded and returnable shielded concepts has less influence on cost than the repository rock type. The high projected cost of disposal is likely to have a profound effect on the volume of intermediate waste, encouraging the development of volume reduction processes. Such changes will narrow the cost differences between packages. However, the returnable shielded design is sufficiently adaptable to enable enhancement in performance for very little extra cost. It is also relatively insensitive to variations in disposal cost.