[en] For several years CIEMAT has been developing for ENRESA knowledge and tools to support the modelling of the migration and accumulation of radionuclides within the biosphere once those radionuclides are released or reach one or more parts of the biosphere (atmosphere, water bodies or soils). The model development also includes evaluation of radiological impacts arising from the resulting distribution of radionuclides in the biosphere. In 1996, a Methodology to analyse the biosphere in this context proposed to ENRESA. The level of development of the different aspects proposed within the Methodology was quite heterogeneous and, while aspects of radionuclide transport modelling were already well developed in theoretical and practical terms, other aspects like the procedure for conceptual model development and the description of biosphere system representatives of the long term needed further developments. At present, the International Atomic Energy Agency (IAEA) Programme on Biosphere Modelling and Assessment (BIOMASS) in collaboration with several national organizations, ENRESA and CIEMAT among them, is working to complete and augment the Reference Biosphere Methodology and to produce some practical descriptions of Reference Systems. The overall purpose of this document is to apply the Methodology, taking account of on-going developments in biosphere modelling, to the last performance assessment (PA) exercise made by ENRESA (ENRESA, 1997), using from it the general and particular information about the assessment context, radionuclide information, geosphere and geobiosphere interface data. There are three particular objectives to this work: (a) to determine the practicability of the Methodology in an application to a realistic assessment situation, (b) To compare and contrast previous biosphere modelling in HLW PA and, (c) to test software development related with data management and modelling. (Author) 42 refs