A REVIEW AND UPDATE OF THE BNFL CEMENT FORMULATION DEVELOPMENT PROGRAMMEFOR THE IMMOBILISATION OF INTERMEDIATE LEVEL WASTES FROM MAGNOX POWERSTATIONS

摘要

For several years BNFL has carried out a cement formulation development programme for the immobilisation ofrnintermediate leve l wastes from decommissioning Magnox power stations. This has been supported by programmes ofrnwaste characterisation, simulant development and characterisation, retrieval and de-watering technology, and latterlyrnthe predicted behaviour of containers of cemented waste under conditions of interim storage and deep geologicalrndisposal. This paper discusses the above strategy in the context of a wide range of waste types, and updates previouslyrnpublished information now that the dates for initial retrieval and immobilisation are approaching.rnThe paper identifies some of the challenges met during the simulant and formulation development programme, and inrnparticular the storage and disposal sensitive issues, which have evolved as wasteform specifications have developed.rnExamples are given with reference to particular wastes showing how the available information is discussed andrnassessed, and then applied and extrapolated to similar waste streams. This is particularly important where mixtures ofrnwastes arise in the same storage facility, and a fully developed methodology will be explained for the successfulrntreatment of such waste streams. The question of changes in raw materials sources and properties is also discussedrnwhere it influences the detailed cement fo rmulation and encapsulation plant operation.rnDue to the age of some of the accumulated Magnox wastes, the question of physical and chemical ageing is discussedrnfor resins, sludges and metallic wastes. Sludge characteristics are particularly important and examples are given of howrnwaste ageing influences the retrieval and encapsulation strategies.rnSpecifically, the use and limitations of long-term monitoring of samples is discussed, giving example of the wastesrnunder test. Overall, it is considered that the results from this part of the programme underwrite the current referencernformulations for all the waste streams considered.rnThe container corrosion work is important as the time for retrieval, packaging and interim storage is reached.rnContainer corrosion related data is discussed, including effects of wasteform degradation and gas generationrnmechanisms, which can be influenced by interim storage environments and eventual transport and disposal strategies.rnPredicted container lifetimes are given and the uncertainties discussed in terms of non-uniform corrosion mechanisms.rnThe situation for the related mechanism of Galvanic corrosion is updated in the context of the high pH cementrnenvironment.rnThere is a brief account of the development process for two fairly intractable wastes, namely borated ion exchangernresins and phenolformaldehyde based ion exchange resins. The reference formulations are identified and the process isrnthen extended to mixtures of wastes which if separate would require two different formulations.rnFinally a number of issues are discussed, including gas generation, migration of actinides, materials specifications,rnretrieval of wastes, simulants, monitoring of waste packages in interim storage and the tolerances expected on thernoperation of an encapsulation plant. All these issue (and others) need to be considered when the reference formulationrnis being developed and when it is finally optimised just prior to operations.rnThe overall conclusions are that the BNFL programme of cement formulation development for a wide range ofrnintermediate level waste types has been successful in leading to tailored stabilization formulae for diverse waste types.rnAs the time approaches when some retrieval and immobilisation is to begin there is confid ence that the wasteformrnspecifications can be met. It is acknowledged that there will always be the need and incentive for final optimisation ofrnthe processes of retrieval and immobilisation and to assist in this formulations have been developed to be as robust as isrnpracticable. In the UK context, a careful analysis of the response of immobilised radioactive waste in stainless steelrnvented containers to the minimum 50 year envisaged interim storage regime, is being carried out with emphasis onrnthose with the greatest potential for internal corrosion.