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MECHANISM OF UO2 SELFDISINTEGRATION BY OXIDATION

MECHANISM OF UO2 SELFDISINTEGRATION BY OXIDATION. D. Ohai , I. Furtuna, I. Dumitrescu Institute For Nuclear Research, Mioveni-Pitesti. Experimental conditions. SARNET ( S evere A ccident R esearch Net work)- Contract FI6O-CT-2004-509065 WP-SOURCE TERM-OXIDEN

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MECHANISM OF UO2 SELFDISINTEGRATION BY OXIDATION

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  1. MECHANISM OF UO2 SELFDISINTEGRATION BY OXIDATION D. Ohai , I. Furtuna, I. Dumitrescu Institute For Nuclear Research, Mioveni-Pitesti XIV ICSI Conference Oct 29-31, 2008

  2. Experimental conditions SARNET (Severe Accident Research Network)- Contract FI6O-CT-2004-509065 WP-SOURCE TERM-OXIDEN Fission Products Release from Debris Bed (FIPRED) in air ingress condition The experimental conditions used for UO2 sintered pellets self disintegration by oxidation studies was the following: • Temperature: 673K- 1273K • Atmosphere: 20%, 40%, 60%, 80% air in N2, 4%, 8%, 12%, 16% O2 in N2 respectively; • Flow rate: 250ml/min • Sample: UO2 sintered pellets, density 10,45g/cm3, and grains diameter 4-5 µm; • Samples weight: around 75 g (5 CANDU type pellets); • Equipment: Modified FIPRED-EQ Analyses:disintegration rate, granulometric distribution of resulted powder and microscostructure XIV ICSI Conference Oct 29-31, 2008

  3. Granulometric distributions XIV ICSI Conference Oct 29-31, 2008

  4. Morfological aspects 300grC 400grC 700grC 1000grC XIV ICSI Conference Oct 29-31, 2008

  5. Thermo gravimetric tests XIV ICSI Conference Oct 29-31, 2008

  6. Premises • Crystallographic system modification by oxidation of UO2 (cubic) to U3O8 (hexagonal) – volume growing • Pore and cracks in pellets • Melting point of U3O8 is 1150°C and at 1300°C decomposes to UO2 and oxygen • U3O8 sintering XIV ICSI Conference Oct 29-31, 2008

  7. Self disintegration mechanism (1) • Initially, the attack on surface (preferentially to grain limits) of pellet detaches small fragments, only. • When the first layer is removed, on the surface appear open pores and cracks. • The air (O2) comes into the open pores and cracks, and the UO2 is transformed in U3O8 inducing very strong strengths. • The attack in cracks and pore zones produce dislocations of large fragments with the free surfaces with pores, cracks, corners and edges. • Growing surfaces with pores and cracks, the disintegration rate increase. • The large fragments are broken in fewer fragments and so on. • The UO2 oxidation rate on new appeared corners and edges increase, also. That contributes to acceleration of self disintegration. XIV ICSI Conference Oct 29-31, 2008

  8. Self disintegration mechanism (2) • When the temperature grows, a new phenomenon appears: sintering of U3O8 formed by UO2 oxidation. • The melting point of U3O8 is 1150°C and at 1300°C decomposes to UO2 and oxygen. Temperature better 5000C initiate the sintering process of U3O8. The sintering rate grows with temperature increasing. • Necks and bridges formed by sintering connect the fragments between them and large fragments appear. • During UO2 pellets oxidation both processes disintegration and sintering work opposite. • At low temperature (less 5000C) the disintegration is preponderant and sintering in-significant. When temperature grows, the sintering rate increase and became preponderant. • At temperature better 10000C the disintegration is practically annulated by sintering, the pellets are broken in few pieces, only. • These mechanisms are confirmed by experimental results XIV ICSI Conference Oct 29-31, 2008

  9. Thanks for you attention XIV ICSI Conference Oct 29-31, 2008

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