VERIFICATION OF ACTIVITY RELEASE COMPLIANCE WITH REGULATORY LIMITS WITHIN SPENT FUEL TRANSPORT CASKS

Admissible limits for activity release from Type B packages for spent fuel transport specified in the IAEA regulations (10-6 A2 per hour for normal and A2 per week for accident 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 standardized leakage rates. Applicable procedure and calculation methods are summarized in the International Standard ISO 12807 /2/. 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 accident 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 accident conditions of transport 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.