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, Explosion phase

, Second Pre-mixing

C. Figure, RNR-Na), une défaillance dans le système de refroidissement du coeur ou de protection du réacteur peut conduire à un accident grave. Dans un tel scenario, les matériaux du coeur (combustible et acier) fondent pour former un mélange appelé corium susceptible d'interagir avec le réfrigérant (sodium), X-Ray images of different phases of the ALISA test. Appendix D Résumé Dans les réacteurs à neutrons rapides refroidis au sodium, vol.2

, En raison de lacunes dans les connaissances de la physique de l'interaction, le CEA s'intéresse à la FCI d'un point de vue de la modélisation mais aussi d'un

, Pour les aspects expérimentaux, la plateforme PLINIUS-2 est en cours de conception au CEA, l'objectif étant de réaliser des essais où interviennent des masses importantes de corium prototypique et du sodium dont la plage de température initiale pourra varier de la température moyenne de fonctionnement d'un RNR-Na (? 400K) à la saturation. De plus, des essais à petite échelle sont aussi en cours de définition afin de comprendre les phénomènes fondamentaux tels que l'ébullition en film du sodium autour d'un fragment chaud fait le cas en interaction corium-sodium, surtout si le sodium est sous-refroidi. Les différentes étapes existent toujours mais les échelles de temps sont beaucoup plus courtes et l'étape de prémélange peut être extrêmement rapide. De plus, même si un film de vapeur de sodium se forme autour des fragments de corium, ce film est, par nature, beaucoup plus instable et se déstabilise spontanément comme l'ont montré les programmes expérimentaux passés où il n, Pour la modélisation de la fragmentation du corium et des transferts de chaleur entre corium et sodium, le CEA s'est engagé dans le développement du logiciel SCONE, capable de décrire les écoulements 3D, multiphasiques et multiconstituants

, En effet, les expériences du passé ont montré que la phénoménologie globale de l'interaction corium-sodium évoluait avec la température du sodium. Lorsque celui-ci s'approche de la saturation, le comportement se rapproche de celui observé en eau, en termes de temps de montée en pression (plus rapide) et d'énergie mécanique dégagée (plus grande). Précisons que nous ne savons pas encore quantifier

, De plus, il a été observé qu'une masse cohérente de corium ne pouvait pénétrer dans du sodium sous-refroidi. Elle se fragmente très rapidement et la vaporisation soudaine engendrée bloque l'écoulement de corium mais contribue aussi à réchauffer le sodium. L'écoulement de corium se fait par à-coups, donnant lieu à de multiples interactions de faible amplitude, jusqu'à ce que le sodium soit suffisamment chaud pour que l'on puisse parler d'un jet de corium qui se, L'une des explications possibles de la différence des comportements pourrait être que, lorsque le sodium est froid (? 400K), l'établissement d'un film de vapeur est rapidement bloqué par la solidification en surface des particules de corium

, En effet, pour l'interaction corium-sodium, les expériences du passé ont montré que le matériau fondu subissait toujours une fragmentation très intense lors de son contact avec le sodium ce qui conduisait à des débris sub-millimétriques, que le dégagement d'énergie soit important ou pas c'est-à-dire qu'il y ait réelle explosion ou pas, La taille des débris obtenus constitue une différence supplémentaire entre l'interaction corium-sodium et l'interaction corium-eau

. Dans-le, les expériences d'intérêt mettant en jeu des matériaux prototypiques ont ensuite été étudiées sous l'angle de la forme et de la taille des débris produits. Notre étude des résultats expérimentaux montre que, dans la plupart des cas, la distribution en taille des particules suit une courbe bimodale que nous interprétons comme un signe de l'existence d'au moins deux régimes de fragmentation