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Safety during whole life time: important aspect in safety assessment of sealed radioactive sources
(2008)
Many sealed sources with long halflife isotopes commonly used in industry or medicine have a long working life, up to several decades. Source integrity must be guaranteed in transport and use at any time. On the one hand, safety during the working life has to be ensured by the source design. Its strain has to be tested. On the other hand, source durability depends on the specific operating conditions. BAM as the competent authority in Germany has to assess the suitability of a source design for safe transport and use also for a longer service life for: (a) sources approved as special form radioactive material according to the regulations for the safe transport of radioactive material, (b) sources in approved devices for licence free use according to the Radiation Protection Ordinance, Para 25, and (c) sources with an extended leak test period according to Radiation Protection Ordinance, Para 66. In all these domains BAM has to assess if design and additional arrangements are qualified and guaranteed to prevent a release of radioactive content under the mechanical, chemical and physical operating conditions of the specified working life of a sealed radioactive source. As a result, limits for the duration of validity of the special form status of a source or a type approval of a device are specified and, in many cases, special additional responsibilities for users, such as periodical control and test measurements, have to be specified in approval certificates as binding conditions to satisfy the required safety standards in regulations. This paper presents BAM's experiences and shows which aspects should be considered in assessment of a lifetime limit of sealed sources.
When storage of spent nuclear fuel or high level waste is carried out in dual purpose casks (DPC), the effects of aging on safety relevant DPC functions and properties have to be managed in a way that a safe transport after the storage period of several decades is capable and can be justified and certified permanently throughout that period. The effects of aging mechanisms (e.g. radiation, different corrosion mechanisms, stress relaxation, creep, structural changes and degradation) on the transport package design safety assessment features have to be evaluated. Consideration of these issues in the DPC transport safety case will be addressed. Special attention is given to all cask components that cannot be directly inspected or changed without opening the cask cavity, like the inner parts of the closure system and the cask internals, like baskets or spent fuel assemblies. The design criteria of that transport safety case have to consider the operational impacts during storage. Aging is not the subject of technical aspects only but also of intellectual aspects, like changing standards, scientific/technical knowledge development and personal as well as institutional alterations. Those aspects are to be considered in the management system of license holders and in appropriate design approval update processes. The paper addresses issues that are subject of an actual International Atomic Energy Agency TECDOC draft 'Preparation of a safety case for a dual purpose cask containing spent nuclear fuel'.