CFD Pre-Test Analysis and Design of the NACIE-UP BFPS Fuel Pin Bundle Simulator

Abstract

In the context of GEN-IV heavy liquid metal-cooled reactors safety studies, the flow blockage in a fuel sub-assembly is considered one of the main issues to be addressed and one of the most important and realistic accident for Lead Fast Reactors (LFR) fuel assembly. The blockage in a fast reactor Fuel Assembly (FA) may have serious effects on the safety of the plant leading to the FA damaging or melting. The temperature of the coolant leaving the FA is considered an important indicator of the health of the FA (i.e. the effective heat removal) and is usually monitored via a dedicated, safety-related system (e.g. thermocouple). The external or internal blockage of the FA may impair the correct cooling of the fuel pins, be the root cause of anomalous heating of the cladding and of the wrapper and potentially impact also fuel pins not directly located above or around the blocked area. In order to model the temperature and velocity field inside a wrapped FA under unblocked and blocked conditions, detailed experimental campaign as well as 3D thermal hydraulic analyses of the FA is required. The present paper is focused on the CFD pre-test analysis and design of the new experimental facility ‘Blocked’ Fuel Pin bundle Simulator (BFPS) that will be installed into the NACIE-UP (NAtural CIrculation Experiment-UPgrade) facility located at the ENEA Brasimone Research Center (Italy). The BFPS test section will be installed into the NACIE-UP loop facility aiming to carry out suitable experiments to fully investigate different flow blockage regimes in a 19 fuel pin bundle providing experimental data in support of the development of the ALFRED (Advanced Lead-cooled Fast Reactor European Demonstrator) LFR DEMO. In particular, the ‘Blocked’ Fuel Pin bundle Simulator (BFPS) cooled by lead bismuth eutectic (LBE), was conceived with a thermal power of about 250 kW and a uniform wall heat flux up to 0.7 MW/m2, relevant values for a LFR. It consists of 19 electrical pins placed on a hexagonal lattice with a pitch to diameter ratio of 1.4 and a diameter of 10 mm. The geometrical domain of the fuel pin bundle simulator was designed to reproduce the geometrical features of ALFRED, e.g. the external wrapper in the active region and the spacer grids. Pre-tests calculations were carried out by applying accurate boundary conditions; the conjugate heat transfer in the clad is also considered. The numerical simulation test matrix covered the envisioned experimental range in terms of mass flow rate; the wall heat flux was imposed in order to have a fixed temperature difference across the BFPS in unblocked conditions. The blockages investigated are internal blockages of different extensions and in different locations (central subchannel blockage, corner sub-channel blockage, edge subchannel blockage, one sector blockage, two sector blockage). High resolution RANS simulations were carried out adopting the ANSYS CFX V15 commercial code with the laminar sublayer resolved by the mesh resolution. The loci of the peak temperatures and their width as predicted by the CFD simulations are used for determining the location of the pin bundle instrumentation. The CFD pre-test analysis allowed also investigating the temperature distribution in the clad to operate the test section safely.

GEN-IV LFR development: Status & perspectives

“…The safety analysis is a key aspect for the GEN-IV reactors development, for this reason several research activities aims to assess all the most important and probable accidents which can occur in a LFR, evaluating the consequence for the integrity and the safety of the plant, and developing solutions for the plant protection. An important issues which has been investigated in this frame is the flow blockage in a fuel sub-assembly which leads to serious impacts on the safety of the plant, provoking the fuel assembly damaging or melting [R. Marinari et al, 2016]. In order to carry out the experimental campaign, the NACIE-UP experimental facility (described above) at the ENEA Brasimone Research Centre will be equipped with a new test section consisting in a Blocked Fuel Pin Simulator (BFPS), composed by 19 electrical pins arranged in an hexagonal wrapper for a total power of 250 kW.…”

“…The BFPS will be used to simulate different flow blockage regimes, as shown in Figure 15, providing experimental data in support of the development of ALFRED. A preliminary CFD analysis of the BFPS experiment has been performed with ANSYS CFX 15 code and it has been presented in R. Marinari et al, 2016. Figure 16 shows the clad temperature field in a blocked region of the Fuel Pin Simulator, obtained by CFD calculations.…”

“…Figure 15: different blockage regimes(R. Marinari et al, 2016) Figure 16: example of CFD calculation -clad temperature field in the region behind the blockage (type 1)( R. Marinari et al, 2016) Progress in Nuclear Energy 105 (2018) 318-331, https://doi.org/10.1016/j.pnucene.2018.02.005…”