Licensing of Atucha-II: the LBLOCA

The paper deals with the safety evaluation and the embedded licensing process of the 9 Atucha-II 800 Mwe Pressurized Heavy Water Reactor (PHWR) in Argentina. Atucha-II was 10 designed by Kraftwerk Union (KWU) during the 60’s, the related construction was started/stopped 11 in the 90’s and restarted on 2006, and was connected to the electrical grid in 2014. Because of market 12 policies, the KWU designer could not be directly involved in the licensing process during the first 13 decade of the 2000 millennium: a licensing suited safety evaluation was performed by the 14 Nucleoeléctrica Argentina (NA-SA), utility owner of the Atucha-II, with support of external experts 15 group. A designer-independent assessment was performed having access to the installed systems 16 and components other than the relevant design documents. Large core size (related to Pressurized 17 Water Reactor – PWR – producing the same thermal power), presence in the core of natural 18 uranium and heavy water fluids, i.e. the coolant and the moderator driven by two circulation loops 19 with different average temperatures, characterize the system design. In those conditions, during 20 the early phase of a depressurization transient, the ‘hot’ coolant vaporizes and the colder 21 moderator remains in the liquid phase: a positive void coefficient is created. The relevance of the 22 Large Break Loss of Coolant Accident (LBLOCA) in safety assessment is discussed with emphasis 23 given to the system design features, the approach pursued in the analysis and the key results. Break 24 opening time and time of occurrence of the safety boron injection affect, during the early period of 25 the transient, the propagation of (negative) pressure wave, the fluid flashing, the heat transfer and 26 the neutron flux: a fission power excursion is expected to occur. The analysis of a complex 27 three-dimensional situation, considering the unavoidable uncertainties associated with the 28 computation, demonstrates that safety limits are preserved. 29