Update on German research work on LSA provisions
Meeting of the European Association of Competent Authorities for transport of radioctive material (EACA), June 26-27, 2012, Brugg, Switzerland
Frank Nitsche
Bundesamt fuer Strahlenschutz (BfS), Salzgitter, Germany
Uwe Büttner
Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS)mbH, Cologne, Germany
Florentin Lange
Consultant, Mataré Str. 3, 40667 Meerbusch, Germany
Introduction
The aim of the research work was mainly to question the justification of the leaching test for LSA-III material as currently required by the IAEA Transport Regulations with respect to its relevance to transport safety based on current experience and knowledge and to develop solutions for suitable regulation changes without decreasing the level of transport safety.
This document is an updated version of the short report for the European Competent Authorities Association meeting on May 19-20, 2011 [1] about the same project. It summarizes the main research results after completion of the comprehensive research work which was performed in recent years in Germany [2], [3] and which includes results of collaboration with other European research institutes in this field [4], [5], [6], [7]. It contains also a first proposal for changes in TS-R-1 and TS-G-1.1.
Background
In accordance with the safety concept of the IAEA Transport Regulations TSR1 the package as combination of packaging and radioactive contents has to provide required safety functions in all conditions of transport. Since Type IP-2 packages and Type IP-3 packages only have to protect against loss or dispersal of their contents under normal conditions of transport, IP packagings for LSA-II and LSA-III materials may have only limited accident resistance. Consequently, the Transport Regulations specify material properties required for LSA-II and LSA-III materials being mainly based on accident considerations and limitation of potential radiological consequences.
Among other required material properties the Transport Regulations comprise a limitation of the specific activity of LSA-II material to 10-4 A2/g (10-5 A2/g for liquids) and to 2×103 A2/g for LSA-III. In addition, the following more restrictive requirements were introduced for LSA-III material in order to justify the higher specific activity limit for LSA-III material compared to solid LSA-II material:
· a limitation of LSA-III material to solid material only excluding powders ,
· a higher degree of homogeneity of the activity distribution in LSA-III material (“essentially uniformly distributed”) when contained within a solid compact binding agent compared to solid LSA-II material (“distributed throughout”) and
· a limitation of the solubility of the material.
The latter requirement has to be demonstrated by a leaching test of a material sample representing the entire contents of the package in water for one week and a following analysis of the dissolved activity. As argument for this leaching test the associated Advisory Material TSG1.1 considers a scenario of sufficient rain intake into an IP package during transport to surround the package content for one week followed by a handling accident which leads to a partial loss of this water and to the intake of a specified fraction of the released activity by a nearby person.
Objective
The justification of the assumptions of the leaching test and its relevance with respect to transport safety have often been questioned. On behalf of the German competent authorities Federal Ministry of Environment, Nature Conservation and Nuclear Safety (BMU) and Federal Office for Radiation Protection (BfS) GRS has analysed the consistency and adequacy of TS-R-1 requirements for LSA-II und LSA-III material and of the leaching test requirement in particular with respect to transport safety.
In the case of LSA-II material it is quite apparent by reviewing the historical development and current Advisory Material (TS-G-1.1) that the specific activity limit of 10-4 A2/g was introduced regarding transport and handling accidents connected with airborne release of radioactive material. In contrast, the corresponding reasoning for the justification of the 20-fold higher specific activity limit of 2×10-3 A2/g for LSA-III materials is not that evident since the leaching test introduced into the Regulations in the early 70s of last century is focussed on a material property which has hardly any connection with the majority of accident sequences with airborne release.
Consequently, a major objective of the current project was to analyse potential accident scenarios of LSA-III material with respect to radiological consequences in order to judge whether
· accident scenarios following a prior leaching process or with a subsequent leaching due to immersion may lead to an unacceptable exposure
· the remaining requirements for LSA-III (only non-powder solids, required activity distribution) are sufficient to guarantee a high level of transport safety
· alternative regulatory requirements are needed to replace the leaching test.
Compared to the accident scenarios given in the Advisory Material TS-G-1.1 as a justification for the LSA-III requirements more detailed analyses have been carried out applying current knowledge on airborne release behaviour of powders, brittle materials and liquids for different types of accident scenarios. These analyses and assessments have been performed in a comprehensive and conservative way and do also include considerations of various material types used as LSA-III material in practice to achieve representative results.
Summary of results of accident scenario analyses and potential radiological consequences
Potential radiological consequences from accident conditions involving LSA-II or LSA-III material may result from inhalation of airborne release. In the case of LSA-III material for which a leaching test is required also accident scenarios resulting in release of water with enhanced activity levels from leaching may contribute to exposure. Therefore the analysed transport accident scenarios performed in the research project were chosen with the following intentions:
1. To examine potential radiation exposure by inhalation of persons being nearby to a severe accident either outdoor or indoor involving LSA-II and LSA-III packages with respect to the 50 mSv effective dose criterion of the Transport Regulations. For such kind of land-based transport accidents airborne release of respirable particulate is dominated by material properties which are independent of a postulated leaching behaviour of the LSA material
2. To examine potential radiation exposure resulting from leaching scenarios under accident configurations. In each case it was assumed that the leaching behaviour of the material would not fulfil the leaching test requirement but would have a 100-fold higher leach rate. The aim was to check whether a scientifically well based consequence analysis may give any support to the current leaching requirement.
Analysed accident scenarios and boundary conditions
Area 1: Transport and handling accidents with mechanical impact of IP packagings containing LSA-II or LSA-III material (no water involved)
As package contents representative LSA-II or LSA-III materials with comparatively high airborne release fractions following mechanical impact have been chosen. As average specific activities of the radioactive contents the respective limits of 10-4 A2/g for LSA-II and 2 10-3 A2/g for LSA-III have been assumed. In addition, the possible effect of locally increased specific activities in conformance with the homogeneity requirements of 226.13 to 226.17 of TS-G-1.1 have been investigated by assuming that this very volume part of the package is suffering the impact forces.
As LSA-II material a highly dispersible powdery material was assumed. Such kind of material, e.g. ashes from incineration of radioactive wastes, would conform to the material requirements for LSA-II but is explicitly excluded for LSA-III materials. As representative and frequently used LSA-III material a cement/concrete matrix material has been chosen on the basis of experimental investigations which included a large spectrum of other brittle materials. Data from extensive experimental programs to investigate the release behaviour of such materials are available [2], [3], [4] and have been applied to estimate airborne release fractions as function of the severity of impact, e.g. equivalent drop height, and the mass of representative LSAII or LSAIII materials in a Type IP2 or IP3 packaging. The experiments included the drop of sheet steel packages of increasing size up to 200 l drums from drop heights up to 27 m to establish the effect of the cladding and of the volume of the LSA material on the airborne release.
The analysed accident scenarios are
· A road or rail accident involving an IP-2 or IP-3 package with either LSA-II or LSA-III material resulting in a mechanical impact equivalent to the severity of the 9 m drop test for Type B packages. To represent typical package sizes, e.g. for the shipment of radioactive wastes, three internal volumes of 200 l, 1 m³ and 10 m³ were considered.
· As handling accident the drop of an LSA-II or LSA-III package when lifted and moved by crane in a storage hall. To simulate varying boundary conditions a small hall with 300 m³ free volume (assumed drop height 3 m) and a large all with 3000 m³ free volume (drop height 6 m) was considered.
The potential exposure via inhalation of airborne particulate radioactivity was determined for persons close to the accident location (outdoor scenario) and for personnel within the storage hall and compared with the 50 mSv effective dose criterion (10-6 A2 intake) of the Transport Regulations.
Area 2: Transport and handling accidents of LSA-III packages involving the release of water after penetrating into the package and a consecutive leaching period of 1 week
· A scenario is analysed which considers the postulated sequence of events as given in 601.2 of TS-G-1.1 as explanation for the required leaching test and its pass criterion of 0.1 A2 activity in the leaching water after seven days (heavy rain fall during normal transport operation è water penetrating into the packaging è leaching attack during 7 days è then accident which leads to partial loss of penetrated water è resulting radiation exposure of a person nearby). In contrast to the modelling approach of the Advisory Material much more severe conditions are postulated by assuming a 100-fold higher activity content in the leaching water (10 A2), complete loss of penetrated water following the handling accident (assumed drop from 3 m in small storage hall and from 6 m in large storage hall). The potential exposure of personnel in the hall is assessed by applying a well founded modelling approach and data regarding resulting airborne activity either after loss of water and evaporation from a resulting pool on the floor or by a spray release from the impacting package.
· An accident of an inland waterway craft on a small navigable river is considered which results in the submersion of an LSA-III package. Before recovery activity is released from the damaged package by leaching during 7 days. A 100-fold higher leaching rate of 10 A2 in 7 days is assumed compared to the pass criterion of the leaching test (0.1 A2/week). Neglecting retention and dilution effects from the usual bank filtration of drinking water a direct consumption of river water by a person downstream from the accident location is assumed and the resulting effective dose determined.
In summary it can be stated that conservative and covering scenarios have been taken to achieve reliable results.
Potential radiation exposure from analysed scenarios
In each of the analysed scenarios a number of conservative assumptions are involved when assessing the potential radiation exposure of persons close to the accident location. The scenarios were selected to cover conceivable accidents involving LSA-II and LSA-III packages with very adverse circumstances. In all cases calculated effective doses remain below the 50 mSv effective dose criterion of the Transport Regulation, often even substantially below. It should be emphasised that the selected scenarios and associated boundary conditions can all be seen as very low probability events.
In the case of the scenario of an accident involving the drowning of a damaged LSA-III package in a small navigable river for one week combined with an unrealistically high leaching rate the exposure of an individual directly consuming river water downstream would remain well below 1 mSv effective dose. This is reassuring because for such circumstances of public exposure a 1 mSv criterion could also be of relevance.
Main findings can be summarized as follows:
· The current requirements of the Transport Regulations for LSA-II packages are sufficient and effective to meet the high transport safety level as provided by TS-R-1.
· The 20-fold higher specific activity limit for LSA-III packages is fully justified based on required material properties which do not include the leaching requirement.
· .Potential radiological consequences of transport and handling accidents which additionally involve a hypothetical leaching scenario but do not take credit of the current solubility restriction for LSA-III material remain well below the 50 mSv criterion of the Transport Regulations.
· In summary, from the point of accident consequences and accordingly of transport safety no significant contribution is associated with the current leaching requirement for LSA-III material.
Suggested changes to the Transport Regulations
Currently the required LSA-II and LSA-III material properties are specified in para. 409 of TS-R-1 (2009) and are essentially:
LSA-II (solids): The activity is distributed throughout and the estimated average activity does not exceed 10-4 A2/g
LSA-III:
1. Solids excluding powders
2. The radioactive material is distributed throughout a solid or a collection of solid objects, or is essentially uniformly distributed in a solid compact binding agent (such as concrete bitumen, ceramic, etc.)
3. The radioactive material is relatively insoluble, or is intrinsically contained in a relatively insoluble matrix, so that even under loss of packaging, the loss of radioactive material per package by leaching when placed in water for seven days would not exceed 0.1 A2
4. The estimated average specific activity of the solid, excluding any shielding material, does not exceed 2 x 10-3 A2/g.
In short, for solid LSA-II material requirements concern the activity distribution and the specific activity of the material. For LSA-III powdery material is excluded, the specific activity may be 20-fold higher than for LSA-II material and a solubility limit is introduced by limiting a leaching rate.
Regarding further requirements for shipments of LSA-II and LSA-III packages TS-R-1 also contains conveyance activity limits which distinguish between combustible and non-combustible solids as well as contents restrictions due to dose rate limits at 3 m distance from the unshielded LSA material.