Transport movements

At the Radioactive Materials Transport - 2002 conference, there were a wide-ranging series of reports on the state-of-the-art in the transport of radioactive material (RAM). Topics worthy of note are:

• The changes in the revision cycle and format of the IAEA transport regulations.

• Advances in the methods of design and mechanical/thermal/nuclear physics assessment of packages as well as increased attention to safety margins.

• Operational and radiological experience in nuclear fuel transport operations. Earlier surface contamination problems had not had radiological implications and the restart of flask movements had gone well, contributing to a feeling of maturity and expertise.

• The increased influence of the World Nuclear Transport Institute (WNTI).

Regulations and revision/review

The IAEA has been operating a ten-year revision/review cycle for the regulations, the latest edition having been published as Regulations for the Safe Transport of Radioactive Material (ST-1) in 1996. ST-1 was later revised and issued with corrections as TS-R-1. In parallel, the United Nations Commission of Experts on the Transport of Dangerous Goods (CETDG) had been transforming the Orange Book into the form of Model Regulations so as to harmonise the content and structure of the regulations covering transport modes for dangerous goods. In addition, alignment with the modal authorities (such as ICAO, IMO) involved conforming to the two-year revision cycle set by CETDG.

These alterations imposed a heavy load on the staff of the IAEA and Member States involved in regulatory matters though the safety value of the changes is less evident. Clive Young of the UK Department for Transport (DfT), who is also chairman of the senior IAEA advisory group for radioactive material transport, and Ron Pope (IAEA) reported on how the transition to the two-yearly review/revision process had worked in practice, whilst Pierre Malesys (Cogema Logistics) outlined initial experience with the modal regulations. Marc-Andre Charette (MDS Nordion) commented that unilateral approval has never been uniformly accepted and that the resulting reviews by the various competent authorities led to wasted effort and inefficiency.

In 1998 the IAEA brought in the TranSAS (transport safety analysis service) system to assist Member States to judge the implementation of the transport regulations at the state level. Attention was drawn to the recent examination of the regulatory situation in the UK (see also the DfT website).

Lorne Green (WNTI) emphasised that RAM transport is integral to the whole industry. By focusing on the collective interests of the industry, WNTI has filled a need and that is underlined by its rapid growth to a membership of 36. As a result it has access to the key international organisations that set the standards for international transport of RAM. Being new and unimpeded by historical 'baggage' it can play an important role in the dialogue between regulators, operators, the media and the public.

Transport by ship - the INF code

P Goodchild (BNFL) explained the history of the international code covering the transport of irradiated material by ship, the INF Code, and the measures taken to comply with it. The INF code was made mandatory by the International Maritime Organization on 1 January 2001 but the need for it is hotly disputed. The IAEA view is that adequate protection is provided by the package and that no credible scientific counterargument has so far been deployed.

Radiological protection & incident analysis

There were three papers from the UK National Radiological Protection Board (NRPB) on this topic, and one from WNTI. A fifth paper by Michael Dean (BNFL) explained new features in the MONK (Monte Carlo) computer code for nuclear criticality safety and reactor physics analysis, which facilitate improved neutron dose assessment in water-filled transport packages.

Ken Shaw (NRPB) predicted that, within the next decade, there would be increased emphasis on individual dose with the introduction of a basic safety standard, envisaging also that the importance attached to collective dose would diminish. Bill Wilkinson (WNTI) estimated the additional radiation dose to the general public and to workers resulting from nuclear fuel cycle transport operations. He concluded that the transport of nuclear fuel cycle materials does not require workplace or individual dose monitoring. The estimated annual collective dose to nuclear fuel cycle transport workers is less than 10% of that estimated for radionuclide transport and is dwarfed by the 3 million times greater annual collective dose to the UK population.

Shaw also reported on a new model for non-fixed surface contamination, which comes primarily from cross-pollution and not from the flask contents. Revised calculations take account of changes in the estimated dose from the types of radionuclide contaminants, their inhalation and the changed annual dose limits for workers. The IAEA has sponsored a coordinated research programme on this subject, the results of which will be published later this year. Currently 49 radionuclides are considered in the model but the aim is to include all radionuclides listed in TS-R-1.

A view on how the RAM transport has operated and on the validity of the IAEA transport regulations is provided by analysis of events during RAM. Sarah Warner-Jones (NRPB) presented an update of the RAMTED (radioactive materials transport event database), which is maintained for the UK regulatory bodies. Numerically the majority of incidents occur during the transport of medical and industrial isotopes. Although there has been a tendency towards more events occurring this is mainly due to the inclusion of those which were previously undetected, such as scrapyard events. Historically, the most significant exposures recorded in the UK were to industrial radiographers, but safety in this area has improved due to training and advances in equipment.

Flask and package design

The conference reports covered a range of diverse design topics as is natural for an industry that has been transporting RAM for over 40 years. Clearly the carriers aim to improve carrying capacity and performance. Thus Thierry Lallemant (Cogema Logistics) described the recent development of two new packagings for fresh PWR and BWR MOX fuels whilst Donald McWilliam (BNFL) gave details of a new 110t spent fuel transport flask - Excellox 8 - the thinner walled design being a consequence of the characteristics of longer cooled fuel.

Bernhard Kühne and Wolfgang Sowa (GNB) described the range of Castor (ductile cast iron) and Constor (steel/ concrete sandwich) casks, some 900 of which have been delivered for transport and storage.

The IAEA transport regulations (TS-R-1) and the associated advisory material (TS-G-1.1) now require that designs are free from brittle fracture. Improved assessment methods using fracture mechanics have facilitated the formulation of new design guidelines and eased the design process for high stress levels to accommodate economic pressures. Frank Norton (BNFL) presented a fracture avoidance system to meet the new requirements by controlling stresses and maximum flaw size, basing his selection of material on the fracture toughness of austenitic and ferritic steels. Work by Uwe Zencker and co-workers at the Federal Institute for Materials Research and Testing (BAM) based on the use of ductile cast iron has shown that the dynamic situation under accident conditions can be approximated using static formulae.

For over ten years the German licensing process for interim storage has required that an aircraft crash be taken into account. In the light of the attack on the World Trade Center, there has been increased attention to studies of safety margins and the effects of missile impact and explosions. Bernhard Droste (BAM) et al cited German missile tests to underline the robustness of flask designs.

Viktor Ballheimer (BAM) et al have shown that because interactions between the contents and the body of a flask during an impact can induce stresses, the gap between flask and contents should be minimised.

Ben Dekker (WNTI-Urenco) described the new requirements in TS-R-1 as they affect the transport of uranium hexafluoride. This has led to the design and development of improved valve protection and further examination of the thermal test situation. Takashi Miyazawa (Mitsubishi) and coworkers reported on studies that aimed at refining the analytical model of the thermal behaviour of enriched UF6 packages under fire test conditions.

A R Cory (BNFL) and colleagues have re-examined the methods of calculating ullage temperatures in wet spent fuel transport flasks using computational fluid dynamics modelling (CFD). This has led to the fuel pin temperatures being predicted to be approximately 150ºC lower than had been calculated previously by other methods.

The task of Nirex is to develop options for the transport and deep geological disposal of intermediate-level waste in the UK. Two papers described the packagings that are proposed and the associated packaging advice. Because a disposal site has not yet been chosen, Nirex is largely restricted to generic studies and providing packaging advice.

In contrast the US government has established a requirement for an underground geological facility to be built at the Yucca Mountain site. Chester Poslusny (US Nuclear Regulatory Commission) explained that the task of the Commission is to ensure public health and safety for this project and, as part of this mission, has interacted with many organs of government at the local level and with citizens. The paper described how the NRC was making the regulatory process more accessible and working to increase public confidence in the process of transporting the spent fuel to the disposal site.

Experience

Comprehensive reports on the resumption of large spent fuel flask transport in Europe were given by Mike Cavanagh (BNFL) and François Harari (Cogema Logistics). Since the restart, transport operations have been more reliable with significantly reduced contamination. Neil Longfellow and D E Haslett (BNFL) described experience at Sellafield where spent gas-cooled reactor fuel and LWR spent fuel has been accepted over the last 40 years, from the UK, Europe and Japan by road, rail and sea. To date over 50,000t has been received in some 30,000 transports.

For cases where operations require a combination of storage and transport, a flexible alternative is to move and/or store the spent fuel assemblies using the same flasks for both types of operations. Rainer Nöring (GNB) and colleagues provided an example of this approach from Italy which is using Castor flasks because a centralised dry storage facility is not yet available. Similarly, Stephane Dudragne (Cogema Logistics) and M Delannay (KKG Switzerland) reported on the introduction of large 135t TN24G flasks for spent fuel management in Switzerland.