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BAM, as the competent authority for approval of special form radioactive material, attaches great importance to a detailed audit of the required quality assurance programmes for design, manufacture, testing, documentation, use, maintenance and inspection. Applicants have to submit, together with their application documentation, information on general arrangements for quality assurance, as well as on quality assurance in production and in operation. Fields where BAM has often found deficiencies are leak test methods, weld seam quality and the safety level after use.
BAM, as the competent authority for approval of special form radioactive material, attaches great importance to a detailed audit of the required quality assurance programmes for design, manufacture, testing, documentation, use, maintenance and inspection. Applicants have to submit, together with their application documentation, information on general arrangements for quality assurance, as well as on quality assurance in production and in operation. Fields where BAM has often found deficiencies are leak test methods, weld seam quality and the safety level after use.
Transport packages for spent fuel have to meet the International Atomic Energy Agency requirements for different transport conditions. Physical state of spent fuel and fuel rod cladding as well as geometric configuration of fuel assemblies are important inputs for the evaluation of package capabilities under these conditions. In this paper, the mechanical behaviour of high burn-up spent fuel assemblies under transport conditions is analysed with regard to assumptions to be used in the activity release and criticality safety analysis. In particular the different failure modes of the fuel rods (fine cracks or complete breakage), which can cause release of gas, volatiles, fuel particles or fragments have to be properly considered in these assumptions. In view of the complexity of interactions between the fuel rods as well as between fuel assemblies, basket, and cask containment, the exact mechanical analysis of such phenomena is nearly impossible. The gaps in information concerning the material properties of cladding and pellets, especially for the high burn-up fuel, make the analysis more complicated additionally. In this context some practical approaches based on experiences of BAM Federal Institute for Material Research and Testing within safety assessment of packages for transport of spent fuel are discussed.
Admissible limits for activity release from type B(U) packages for spent fuel transport specified in the International Atomic Energy Agency regulations (10-6 A2 h-1 for normal conditions of transport and A2 per week for accidental conditions of transport) have to be kept by an appropriate function of the cask body and its sealing system. Direct measurements of activity release from the transport casks are not feasible. Therefore, the most common method for the specification of leak tightness is to relate the admissible limits of activity release to equivalent standardised leakage rates. Applicable procedure and calculation methods are summarised in the International Standard ISO 12807 and the US standard ANSI N14·5. BAM as the German competent authority for mechanical, thermal and containment assessment of packages liable for approval verifies the activity release compliance with the regulatory limits. Two fundamental aspects in the assessment are the specification of conservative design leakage rates for normal and accidental conditions of transport and the determination of release fractions of radioactive gases, volatiles and particles from spent fuel rods. Design leakage rates identify the efficiency limits of the sealing system under normal and accidental transport conditions and are deduced from tests with real casks, cask models or components. The releasable radioactive content is primarily determined by the fraction of rods developing cladding breaches and the release fractions of radionuclides due to cladding breaches. The influence of higher burn-ups on the failure probability of the rods and on the release fractions are important questions. This paper gives an overview about methodology of activity release calculation and correlated boundary conditions for assessment.
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.