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