BNFL Lyslmeter Programme to Investigate the Leaching of Radionuclides from Low-Level Radioactive Waste
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BNFL Lysimeter Proaramme to Investigate the Leachina of Radlonuclides from Low-Level Radioactive Waste
K. Clayton*, R. Clegg*, R.G.G. Holmes* and G.W.A. Newton"5 * Sellafleld Technical Department, British Nuclear Fuels pIc, Sellafleld, United Kingdom. **Newton Systems, Manchester, United Kingdom
ABSTRACT British Nuclear Fuels ple has initiated an experimental programme to measure the leaching behaviour of radionuclides from various low level radioactive waste (LLW) mareials using Lysimeters. The programme commenced in 1986 and to date 10 lysimeters have been commissioned. These have concentrated on simulating shallow trench conditions but a further programme is now planned to study concrete vault environments. The aim of the study is to provide information on leaching processes as part of the ongoing Drigg Near Field Programme, and also to yield input data for radiological assessment purposes. Towards this end, data have been gained from the lysimeters on basic chemistry, gas generation and radionuclide Release Coefficients. This paper concentrates on one of the lysimeters which has recently been decommissioned and for which interim analytical data are available. Some general comments are given on BNFL's experience using lysimeters and their applicability as a rapid and effective technique for studying near field degradation processes.
INTRODUCTION British Nuclear Fuels ple (BNFL) own and operate the Drigg low level radioactive waste (LLW) disposal site in West Cumbria in the U.K.. Historically the waste at the site has been disposed of in excavated shallow earth trenches. The current practice is now based on a purpose built concrete vault in which it is planned to dispose of supercompacted solid LLW grouted into specially modified ISOfreight containers. A programme of technical studies is ongoing for the Drigg site aimed at understanding the important processes governing site perfonance and also developing models and the methodology for radiological assessments. A component of this programme deals with the near field environment and in particular how the physical and chemical conditions in earth trenches and concrete vaults affect radionudlide mobilisation. Towards this end , an experimental programme to provide data on the leaching behaviour of radionuclides from LLW has been initiated using lysimeters. This is aimed at enhancing understanding about the evolution of chemical conditions and the generation of bulk gases, as well as yielding quantitative data on radionuclide Release Coefficients for difcmt waste types.
Mat. Res. Soc. Symp. Proc. Vol. 294.
1993 Materials Research Society
858
A Release Coefficient is a dimensionless quantity and is defined as the amount of a radionuclide released into the leachate divided by its original inventory in the waste. Empirical Release Coefficients have been estimated for the trenches at Drigg based on recorded disposal inventories and analysis of leachate arisings. This method is inherently uncertain and thus the lysimeter experimental programme has been designed to
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