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Assessment of different mechanisms of C-14 production in irradiated graphite of RBMK-1500 reactors

Einschätzung verschiedener Mechanismen der C-14-Produktion im bestrahlten Graphit von RBMK-1500-Reaktoren
E. Narkunas, A. Smaizys, P. Poskas and R. Kilda
From the journal Kerntechnik


Two RBMK-1500 water-cooled graphite-moderated channel-type power reactors at the Ignalina Nuclear Power Plant (INPP) are under decommissioning now. The total mass of irradiated graphite in the cores of both units is more than 3600 tons. The main source of uncertainty in the numerical assessment of graphite activity is the uncertainty of the initial impurities content in graphite. Nitrogen is one of the most important impurities, having a large neutron capture cross-section. This impurity may become the dominant source of C-14 production. RBMK reactors graphite stacks operate in the cooling mixture of helium-nitrogen gases and this may additionally increase the quantity of the nitrogen impurity. In this paper the results of the numerical modelling of graphite activation for the INPP Unit 1 reactor are presented. In order to evaluate the C-14 activity dependence on the nitrogen impurity content, several cases with different nitrogen content were modelled taking into account initial nitrogen impurity quantities in the graphite matrix and possible nitrogen quantities entrapped in the graphite pores from cooling gases.


Zwei wassergekühlte, graphitmoderierte RBMK-1500-Leistungsreaktoren des KKW Ignalina (INPP) befinden sich zur Zeit im Rückbau. Die gesamte Masse des bestrahlten Graphits in den Kernen beider Blöcke beträgt mehr als 3600 Tonnen. Die Hauptunsicherheitsquelle bei der numerischen Bestimmung der Graphitaktivität ist die Unsicherheit über die anfänglichen Verunreinigungen des Graphits. Stickstoff ist eine der wesentlichen Verunreinigungen mit einem großen Wirkungsquerschnitt für Neutronenabsorption. Diese Verunreinigung kann zur dominierenden Quelle für die C-14-Produktion werden. Die Graphitblöcke der RBMK-Reaktoren werden durch ein Helium-Stickstoff-Gemisch gekühlt und dies kann zusätzlich die Menge an Stickstoffverunreinigung erhöhen. In diesem Beitrag werden die Ergebnisse der numerischen Modellierung der Graphitaktivierung für den Reaktor des Blockes 1 im KKW Ignalina vorgestellt. Um die Abhängigkeit der C-14-Aktivität von der Menge der Stickstoffverunreinigung zu bewerten, wurden verschiedene Fälle mit unterschiedlichem Stickstoffgehalt unter Berücksichtigung der Menge an anfänglicher Stickstoffverunreinigung in der Garphitmatrix und möglichen in den Graphitporen des Kühlgases eingeschlossenen Stickstoffmengen modelliert.

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Received: 2010-2-8
Published Online: 2013-04-05
Published in Print: 2010-08-01

© 2010, Carl Hanser Verlag, München