The effects of applying silicon carbide coating on core reactivity of pebble-bed HTR in water ingress accident

Zuhair; Suwoto; T. Setiadipura; Z. Su’ud

doi:10.3139/124.110628

Article

The effects of applying silicon carbide coating on core reactivity of pebble-bed HTR in water ingress accident

Zuhair , Suwoto , T. Setiadipura and Z. Su’ud

Published/Copyright: April 18, 2017

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From the journal Kerntechnik Volume 82 Issue 1

Abstract

Graphite is used as the moderator, fuel barrier material, and core structure in High Temperature Reactors (HTRs). However, despite its good thermal and mechanical properties below the radiation and high temperatures, it cannot avoid corrosion as a consequence of an accident of water/air ingress. Degradation of graphite as a main HTR material and the formation of dangerous CO gas is a serious problem in HTR safety. One of the several steps that can be adopted to avoid or prevent the corrosion of graphite by the water/air ingress is the application of a thin layer of silicon carbide (SiC) on the surface of the fuel element. This study investigates the effect of applying SiC coating on the fuel surfaces of pebble-bed HTR in water ingress accident from the reactivity points of view. A series of reactivity calculations were done with the Monte Carlo transport code MCNPX and continuous energy nuclear data library ENDF/B-VII at temperature of 1 200 K. Three options of UO₂, PuO₂, and ThO₂/UO₂ fuel kernel were considered to obtain the inter comparison of the core reactivity of pebble-bed HTR in conditions of water/air ingress accident. The calculation results indicated that the UO₂-fueled pebble-bed HTR reactivity was slightly reduced and relatively more decreased when the thickness of the SiC coating increased. The reactivity characteristic of ThO₂/UO₂-fueled pebble-bed HTR showed a similar trend to that of UO₂, but did not show reactivity peak caused by water ingress. In contrast with UO₂- and ThO₂-fueled pebble-bed HTR, although the reactivity of PuO₂-fueled pebble-bed HTR was the lowest, its characteristics showed a very high reactivity peak (0.33 Δk/k) and this introduction of positive reactivity is difficult to control. SiC coating on the surface of the plutonium fuel pebble has no significant impact. From the comparison between reactivity characteristics of uranium, thorium and plutonium cores with 0.10 cm thick SiC coating, it can be concluded that the effect of SiC coating on core reactivity in water ingress accident is more dominant for the pebble-bed HTR fuelled with thorium than those with uranium and plutonium fuels.

Kurzfassung

Graphit wird als Moderator, Barrierematerial und Kernstruktur in Hochtemperaturreaktoren (HTRs) verwendet. Trotz seiner guten thermischen und mechanischen Eigenschaften kann Korrosion als Folge eines Unfalls durch Wassereinbruch nicht vermieden werden. Degradation von Graphit als Hauptmaterial beim HTR und die Bildung von gefährlichem Kohlenmonoxid ist ein ernstes Problem bei der HTR Sicherheit. Zur Vermeidung der Korrosion von Graphit kann eine dünne Schicht von Siliziumkarbid (SiC) auf die Oberfläche der Brennelemente aufgebracht werden. Dieser Beitrag untersucht die Anwendung einer Siliziumkarbidbeschichtung auf die Brennelementoberflächen eines Kugelhaufen HTR bei einem Unfall durch Wassereinbruch im Hinblick auf die Reaktivität. Eine Reihe von Reaktivitätsberechnungen wurden mit Hilfe des Monte Carlo Neutronentransport Codes MCNPX und der Kerndatenbibliothek ENDF/B-VII bei Temperaturen von 1 200 K durchgeführt. Drei Optionen von UO₂, PuO₂ und ThO₂/UO₂ Brennstoffkernen wurden für den Vergleich der Reaktivität von Kugelhaufen HTR bei einem Unfall durch Wassereinbruch betrachtet. Die Rechenergebnisse zeigen, dass die Reaktivität bei UO₂ etwas reduziert und, bei wachsender SiC Beschichtung stärker reduziert wurde. Die Reaktivitätseigenschaften zeigen bei ThO₂/UO₂ einen ähnlichen Trend aber keinen Reaktivitätspeak infolge des Wassereinbruchs. Im Gegensatz dazu zeigt die Reaktivität von PuO₂ Brennstoffkernen, obwohl sie am niedrigsten war, einen sehr hohen Reaktivitätspeak (0.33 Δk/k). SiC Beschichtung auf der Oberfläche der Plutonium Kugelhaufen hat keinen signifikanten Einfluss. Der Vergleich zwischen den Reaktivitätseigenschaften von Uranium-, Thorium- und Plutoniumkernen mit 0.10 cm dicker SiC Beschichtung zeigt, dass der Einfluss der Beschichtung auf die Reaktivität des Kerns bei einem Unfall durch Wassereinbruch bei einem Kugelhaufen HTR mit Thorium entscheidender ist als bei Uran- und Plutonium-Brennstoff.

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Received: 2016-03-11

Published Online: 2017-04-18

Published in Print: 2017-03-16

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https://doi.org/10.3139/124.110628