PIPER-ONE loop: simulation of heat transfer between fluid and structures during a SBLOCA in a BWR" (in Italian)

The document (in Italian) is part of a series of reports dealing with the design, the construction and the operation of the PIPER-ONE loop installed at the Scalbatraio Laboratory managed by DCMN (Dipartimento di Costruzioni Meccaniche e Nucleari – previously IIN, Istituto di Impianti Nucleari) of University of Pisa. PIPER-ONE was built in Italy as a consequence of the Three-Mile accident (1979): it aimed at simulating the Boiling Water Reactor (BWR) performance following Small Break Loss of Coolant Accidents (SBLOCA). The natural circulation which occurs between down-comer and core into the vessel constitutes the reference thermal-hydraulic phenomenon which needed to be simulated. The concerned document allowed the finalization of the thermal-hydraulic design of a key peculiarity of the PIPER-ONE loop: the concerned peculiarity is the passive structure cooling/heating system. Any experimental loop (namely those characterized by full height scaling and full pressure) suffer of the scaling distortion caused by the high value of the ratio ‘passive structure area / volume of fluid’ which ends up (unavoidably) to be 10 – 50 times the ideal value which characterizes the prototype. As a consequence of this timing of accident is distorted too. Unavoidable (again) heat losses to environment contribute further to distortion in performance between prototype and model. It might be noted that during some period of an assigned transient scenario (e.g. during the depressurization time) passive structure transfer too much thermal power to the fluid (e.g. related to the ideal value or the value expected in the prototype); otherwise during long lasting transient heat losses cause too much thermal power removal from the fluid also distorting the natural circulation. . Cooling coils (e.g. a pipe welded around the entire loop) and electrical heating wire were wrapped around the passive structure of PIPER-ONE to optimize the scaling performance. The current report describes the design features of both the electrical coils (heaters) and the cooling coils (coolers). The PIPER-ONE loop was used for an International Standard Problem (ISP) by Organization for Economic Cooperation and Development (OECD) / Nuclear Energy Agency (NEA) / Committee on the Safety of Nuclear Installations (CSNI) during the middle of 80’s. The design and the operation of the loop constituted a milestone achievement in the area of nuclear thermal-hydraulics with main reference to the scaling.