Investigation of core degradation (COBE)

The COBE project started in February 1996 and finished at the end of Januar y 1999. The main objective was to improve understanding of core degradation behaviour during severe accidents through the development of computer code s, the carrying out of experiments and the assessment of the computer codes ' ability to reproduce experimental behaviour, A major effort was devoted t o quenching behaviour and a substantial achievement of the project was the design and commissioning of a new facility for the simulation of quenching of intact fuel rods. Two tests, carefully scaled to represent realistic rea ctor conditions, were carried out in this facility and the hydrogen generat ed during the quenching process was measured using two independent measurin g systems. The codes were able to reproduce the results in the first test, where little hydrogen was generated but not the second test, where the extr a steam produced during quenching caused an invigorated Zircaloy oxidation and a substantial hydrogen generation. A number of smaller parametric exper iments allowed detailed models to be developed for the absorption of hydrog en and the cracking of cladding during quenching. COBE also investigated ot her areas concerned with late-phase phenomena. There was no experimental ac tivity - the work included code development and the analysis of experimenta l data available to the project partners - either from open literature or f rom other projects such as Phebus-FP. Substantial improvement was made in t he codes' ability to simulate heat transfer in debris beds and molten pools and increased understanding was reached of control rod material interactio ns, the swelling of irradiated fuel and the movement of molten material to the lower head. (C) 2001 Elsevier Science B.V. All rights reserved.