Analysis of activation product release

The release rates of activated material from SGIs were calculated by applying effective corrosion rates to simplified geometries representing the structure in question. For ihe submarine PWRs and icebreaker, the majority of the activation products came from the thermal shields and RPVs these were modelled using plane geometry for simplicity. 

In the case of the LMRs, the bulk and complex geometry of the thermal shields and ingress routes required a more detailed analysis, and corrosion rates were modelled using a variety of circular corrosion geometries applied to the core, thermal shields and SGs. 

Activity release rates were calculated using a FORTRAN code developed at the Royal Naval College Greenwich, running on a 486DX PC. For each component of the SGI, the actual corrosion rate (and hence activity release rate) was calculated using derived effective corrosion rates. 

The decay from the original inventory for each of the isotopes used was calculated for timesteps into the future. Only the Pu-Am decay scheme was considered for daughter product accumulation. 

The remaining thickness of each component at the timesteps was calculated using the effective corrosion rates, allowing for the fact that these rates will change as levels of containment disappear. For example, the protection provided by the RPV greatly decreases the corrosion rates of the contents. The SS thermal shields for instance will generally outlast the RPVs. When a component has corroded through, it was removed from the calculation and adjustments made in effective corrosion rates from that time onwards. 

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