Instrumenting full-scale Boron Injection Test Facility to support Atucha-2 NPP licensing

摘要

The Atucha-2 Pressurized Heavy Water Reactor is equipped with a back-up shutdown system basedudon the fast injection of boron into the moderator tank. Such system had initially been designed toudcope with a 10%-area (0.1A) break Loss Of Coolant Accident (LOCA) scenario, but based on upgradedudlicensing requirements the design had to be revised and possibly improved against a double-endedudguillotine (2A) break LOCA. In particular, the boron injection had to be proven fast enough to allow audtimely shutdown of the reactor, even in the case of a failure of the primary shutdown systemud(control rods).udA full-scale test facility was built for such “design validation” purpose, in the framework of audcooperation program between the University of Pisa – San Piero a Grado Nuclear Research Groupud(GRNSPG) and the utility Nucleoeléctrica Argentina S.A. (NA-SA). A special instrumentation system,udbased on conductivity probes designed on purpose by the Helmholtz Zentrum Dresden-Rossendorfud(HZDR), was adopted for the measurement of the injection delay, as well as for the monitoring ofudpressure at several key locations. Care was taken to reproduce the relevant NPP conditions as closelyudas possible to those expected on the basis of extensive safety analyses performed adopting a BestudEstimate Plus Uncertainty (BEPU) approach. In this respect, not only the test facility is full-scale, butudalso the key components (such as the fast opening air valves, the boric acid tanks, the ruptureuddevice, the injection lance) were directly borrowed from the Atucha-2 NPP.udThe experimental campaign carried out by NA-SA on such test facility allowed to improve the designudof the boron injection system (especially as to some fluid-structure interaction issues) and finally toudachieve the main goal, i.e. the demonstration that the system’s performance is fast enough to assureuda timely and safe shutdown of the reactor. This was a key contribution to the successful completionudof the NPP licensing process.