The role of nuclear thermal-hydraulics in the licensing of Atucha-II: The LBLOCA

Abstract

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

The technological challenge for current generation nuclear reactors

“…Safety lagging behind research and the connected loss of expertise are perceptions: the provided discussions may be used to enforce the awareness for those issues and the importance of the safety barrier. The ALARA concept and the requirement for Independent Assessment are embedded into the proposal for the new barrier (sections 3.1 to 3.5) Furthermore, the need to consider LBLOCA in NRS as a DBA, and the further justification for the new safety barrier is based upon argumentations (section 2.1) summarized hereafter; see also Mazzantini et al 2019, for insights relate to LBLOCA:…”

An old issue and a new challenge for nuclear reactor safety

“…Until 2020, sophisticate computational tools and procedures have been developed and successfully applied for safety evaluations and licensing in relation to LBLOCA: the best estimate plus uncertainty (BEPU) approach was proposed [5], although outside the scope for discussion here. Fuel weaknesses and connected clad failure mechanisms always posed challenge to the acceptability thresholds for LBLOCA: ballooning and consequential creep type of clad rupture have been the topic of investigation for decades.…”