Loss of coolant in a fast reactor is a crucial, safety-related issue that has two distinctive consequences on a reactor operation: first of all, heat removal ability is lost leading to overheating and possibly to loss of core integrity, standing for one of the main reasons for core melting accidents. Secondly, it can introduce reactivity into the system. Void effect profile for ELSY fast reactor has been evaluated by means of MCNP code, simulating the gradual voiding of the active region, while keeping the lead reflector around. As expected for liquid metal reactors, a positive void reactivity effect (about 5200 pcm) has been calculated; the value corresponds nearly to the amount of reactivity coverable by the twelve B4C control rods designed for the whole core. This fact leads to an important consequence: in case of such unbelievable complete voiding of the core, no more reactivity worth of the absorbers would be available. In order to investigate a possible reduction of positive void feedback in case of hypothetical loss-of-flow (LOF) scenario, an internal blanket of some proper absorbing material has been inserted in the mid-plane of each fuel rod with consequent increase of core height. MCNP criticality calculations have been performed for different blanket thicknesses and for three kinds of materials: natural uranium in oxide form, and natural thorium in both metallic and oxide forms. Results have shown that none of the three materials is able to decrease the void effect for small thickness: more than 300 mm of thorium (and even more than 400 for uranium) are necessary to start reducing the positive reactivity insertion.

A Coolant Void Reactivity Effect Evaluation of ELSY LFR: Optimization of a Mid-Plane Blanket for Void Effect Reduction

GHAZY, RASHA;PADOVANI, ENRICO;RICOTTI, MARCO ENRICO;
2008

Abstract

Loss of coolant in a fast reactor is a crucial, safety-related issue that has two distinctive consequences on a reactor operation: first of all, heat removal ability is lost leading to overheating and possibly to loss of core integrity, standing for one of the main reasons for core melting accidents. Secondly, it can introduce reactivity into the system. Void effect profile for ELSY fast reactor has been evaluated by means of MCNP code, simulating the gradual voiding of the active region, while keeping the lead reflector around. As expected for liquid metal reactors, a positive void reactivity effect (about 5200 pcm) has been calculated; the value corresponds nearly to the amount of reactivity coverable by the twelve B4C control rods designed for the whole core. This fact leads to an important consequence: in case of such unbelievable complete voiding of the core, no more reactivity worth of the absorbers would be available. In order to investigate a possible reduction of positive void feedback in case of hypothetical loss-of-flow (LOF) scenario, an internal blanket of some proper absorbing material has been inserted in the mid-plane of each fuel rod with consequent increase of core height. MCNP criticality calculations have been performed for different blanket thicknesses and for three kinds of materials: natural uranium in oxide form, and natural thorium in both metallic and oxide forms. Results have shown that none of the three materials is able to decrease the void effect for small thickness: more than 300 mm of thorium (and even more than 400 for uranium) are necessary to start reducing the positive reactivity insertion.
TOPSAFE 2008
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11311/544243
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