Modeling of coolant channel of CNA I for evaluation of accidental transients using ICARE2 core

The ICARE2 computer code, developed and validated since 1988 at the Institut de Protection et de Surete Nucleaire (IPSN) of the French Commisariat a l'Energie Atomique (CEA), calculates in a mechanistic way the physical and chemical phenomena involved in the core degradation process during possible severe accidents in LWR's. Using the input data definition language of ICARE2 V2 mod2.3 code, the fuel element and coolant channel assembly of CNA I type was described. This input data description was utilized to analyze the system behavior during a hypothetical accidental transient at CNA I. The thermohydraulic data of the accidental transient analyzed indicate that the uncovering of the fuel element active zone begins four hours after the start of the accident, if there are not control actions. It was analyzed the sequence of system degradation under these conditions and from the beginning of the active zone uncovering up to total uncovering of the active zone. The other analyzed scenario is the partial uncovering up to 1/3 of the active zone. In both cases the thermal evolution of the components, the chemical reactions in the system materials, the Zircaloy oxidation, hydrogen production, the release of fission products coming from the gap between the fuel and the cladding, the material relocated and the sequence of system degradation were determined. In the first case was observed fission product release from the gap due to cladding rupture from 2.25 104 s of calculation. At 2.3 104 s the temperatures in the upper part, ∼ 1300 K, present a localized increment because of the start of Zircaloy oxidation of claddings. It was observed instantaneous hydrogen production accompanying the energy generation in the Zircaloy oxidation with 13 kW as maximum contribution at 2.35 104 s. In the second case was observed fission product release from the gap due to cladding rupture from 2.05 104 s (2.0 103 s earlier than in the other). The hydrogen production confirms the start of fast Zr oxidation from 2.0 104 s (3000 s before than the previous case). The instantaneous hydrogen production and energy contribution came up to 60 kW at 2.2 104 s (4.5 times greater than in the first analyzed transient). (author)