Year
2010
Abstract
A project has been undertaken working with a customer with the requirement to retrieve Intermediate Level Waste from the de-canning of reactor fuel which was deposited in silos from the mid 1960s until the early 1990s and to place it into passive, safe state in preparation for final disposal. CANBERRA UK Ltd (AREVA Nuclear Measurement Business Unit) was contracted to design a new gamma spectrometry system to accomplish the requirements of this project. The spectrometric system is required for process control to limit the uranium content of the finished waste packages. The calibration with real sources is not possible so it is required that the monitor be designed and calibrated using MCNP modelling, taking into account details of the waste matrix to determine the calibration factors for getting the 60Co and 137Cs activity in the waste and from this, the uranium content. Therefore, the detector choice and position is determined in accordance with geometrical restrictions, the thickness of an attenuator to be fitted in front of the detector is calculated so that the count rate is within the dynamic range of the detection electronics, the collimator is designed to reduce the contribution to the detector from possible surrounding sources, etc. However, some of the imposed geometric restrictions together with a very high count rate in the detector and an important background and scattering component, make the design and modelling of the detector system a more difficult task. This paper describes some of the technical issues that have arisen during this process.