Leakage estimation of the high-pressure and high-temperature natural circulation helium loop František Világi a,⇑ , Branislav Knízˇat a , Marek Mlkvik a , Róbert Olšiak a , Peter Mlynár a , František Ridzonˇ a , František Urban a a STU in Bratislava, Faculty of Mechanical Engineering, Námestie slobody 17, 812 31 Bratislava, Slovak Republic article info Article history: Received 23 July 2019 Received in revised form 29 April 2020 Accepted 1 May 2020 Available online 16 June 2020 Keywords: Natural circulation Gas Fast Reactor (GFR) ALLEGRO Decay heat removal Helium loop abstract Natural circulation loop systems are promising a reasonable option to provide the emergency decay heat removal from a nuclear reactor during an electrical power outage. The present paper deals with an exper- imental study on the influence of leakages on the behaviour of a single-phase natural circulation loop. The research shows the thermodynamic and hydraulic processes in a large scale natural circulation loop during several measurements. The experimental device was designed to provide heating and cooling power of 250 kW with the maximum operating temperature and pressure 520  C and 7 MPa, respectively. The continuous loss of coolant is very dangerous because it can result in coolant overheating. Ó 2020 Elsevier Ltd. All rights reserved. 1. Introduction One of the reactor concepts selected in the frame of the Gener- ation IV International Forum is the helium cooled Gas Fast Reactor (GFR). ALLEGRO, the medium-power GFR demonstrator, will be the first GFR ever built (Mayer and Bentivoglio, 2015). The first com- mercial application of Gen. 4 is expected no sooner than 2030 (Locatelli et al., 2013). Until then, the massive research of GFRs, as well as their support subsystems is being conducted on model devices (Miletic et al., 2014). An important and often discussed topic, the safety of the nuclear reactors, is a feature closely related to the cooling systems. Natural circulation loops seem to be a rea- sonable option to provide emergency decay heat removal when the electric supply of a facility is compromised (Pioro et al., 2013). The idea of a natural circulation loop (NCL) transmitting heat between the hot and cold points was introduced by Keller (1966), who provided experimental results as well as the mathe- matical description of such a device. Later works were oriented to NCL stability Welander (1967) in the meaning of flow-rate and pressure oscillations when working with a constant heating power. The research of NCL was further developed by the theoretical work of Zvirin (1982). The theoretical results, mostly obtained from the small scale NCL model, could not be adapted to industrial-scale NCLs without proper scaling criteria, provided first by Zuber (1980) and Heisler (1982). Scaling laws, proposed by Zuber, are also known as the philosophy of power/volume ratio. This philos- ophy has some inherent flaws which surpass some specific phe- nomena in the NCL, such as flow instability Nayak et al. (1998). Furthermore, the main problems with the scaling laws was that they contained a number of dimensionless factors and conversion to an NCL with another geometry layout was complicated. The problems were solved in the work of Vijayan et al. (2000), who pro- posed new scaling laws which described the instability nature of NCLs. Later the model was expanded Vijayan et al. (2001) to the state as it is used to date. The mathematical model is capable of calculating the Reynolds number at one specific point in the loop during turbulent or laminar flow. The results were verified by sev- eral experiments (Misale, 2001; Lakshumu et al., 2016; Saha et al., 2015). These experiments were conducted on small scale experi- mental NCLs and the results were in conformity with the calcula- tions. There are no records of experiments on large-scale experimental loops. The presented research was conducted on a model of the natu- ral circulation helium loop built by the Faculty of Mechanical Engi- neering STU in Bratislava (Fig. 1). The test device is the model of the cooling system, to remove the GFR decay heat. It was designed to provide cooling power of 250 kW with maximum operating temperature and pressure 520  C and 7 MPa. Experiments on the large-scale model provided valuable data. However, we observed disagreement between our experimental results and theoretical models. The NCL did not achieve the desired operating pressure calculated from the initial conditions-pressure and temperature https://doi.org/10.1016/j.anucene.2020.107584 0306-4549/Ó 2020 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Annals of Nuclear Energy 146 (2020) 107584 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene