Feasibility Study of an Emission Tomography Monitoring System for Dry-Stored Spent Nuclear Fuel. Annex I

This research project is in accordance of the IAEA Coordinated Research Project (CRP) on "Demonstrating Performance of Spent Fuel and Related Storage System Components during Very Long Term Storage", and fits with its specific Research Objective 6 "Evaluate the confinement capability of concrete cask systems, including enabling examination of existing actual casks" and its Specific Activity 6 "Survey/develop techniques for remote inspection of concrete cask systems, checking for temperature, salt, corrosion, and integrity". The research is a probe of principles of an emission tomography scanner to monitor the containments of spent fuels, using several high-performance gamma detectors to perform scanning periodically and collecting an image or map of the spatial distribution of the radiation collected at the external surface of the concrete cask, as well as to detect and locate radiation hot spots, in order to determine its potential ability to detect cracks, voids and eventually the rebar corrosion. The main objective of the present research was to determine the possibility to perform tomographic studies in spent fuel concrete casks; the case of study selected was huge concrete silos 2.4 m in diameter and 6 m in high, located at the Embalse NPP facility; the attention was focused on Silo No 1 loaded in 1993, it was the worst case from the point of view of radiation emission at the surface of the silo. It was found that there are enough direct gamma rays at the silo surface mainly from 137Cs content in spent fuel. A low-resolution image was reconstructed from measurements performed with a detector attached to a specially designed collimator. From on-site measurement and the image reconstruction, a simplified model was derived in a way that makes simulations possible using a reasonable amount of computer time, based on GATE (Montecarlo code platform), to determine construction parameters of an ideal scanner. In this sense the global objective was fulfilled and also intermediate objectives like the selection of the energy window with the best chance for image acquisition. Besides the evaluation of the minimum necessary shielding to isolate the detectors from the influence of surrounding silos and the design by simulation of the collimator that solves the compromise between efficiency and spatial resolution were milestones of the present research. Also, a model was developed to be applied in an iterative algorithm: Maximum Likelihood Expectation Maximization (ML-EM). This reconstruction process considers effects like the collimator cone of view, attenuation in the path of a gamma beam, scatter correction and spatial effects of the mechanical structure of the scanner. Based on the results of this preliminary research it is possible to distinguish two promising lines that deserve further study: one devoted to design a tomographic scanner for tracking the integrity of concrete casks by gamma pattern with limited resolution; the other devoted to developing a technique for a scintigraphy of the external surface using scattered radiation as a backlight to track the surface of the cask exposed to environmental ageing.