[en] The Molten Salt Fast Reactor is a reactor concept developed by the European Union based on a liquid fuel salt circulating through the reactor core. A peculiar emergency system, which takes advantage of the liquid fuel state, is represented by a tank located underneath the core, where the fuel can be passively drained and cooled; its geometry ensures that the fuel remains in subcritical conditions. In the framework of the SAMOFAR project, a design for the Emergency Draining Tank has been proposed: the tank shall be equipped with vertical cooling elements, arranged in a hexagonal grid; the liquid fuel salt, which heats up due to decay heat, will fill the gaps between the elements. In this work, analytical methods (Green’s functions and orthogonal decomposition) are employed to study the transient heat transfer associated with the proposed design and to perform a preliminary dimensioning of the system, such that overheating is avoided in any moment of the transient and the fuel salt is kept in a liquid state and in safe conditions for a long time. The models are constituted by multilayer monodimensional slabs and cylinders, with a pure heat conduction model. The assessment of the available grace time and preliminary considerations about fuel salt freezing and its influence on the system effectiveness are also included.

[en] Highlights: • A genetic algorithm for the automatic cross-section energy grouping is proposed. • The algorithm effectively finds appropriate energy structures for each test system. • Results show a strong relation between meshing and system spectrum. • Energy structure effect on the results is larger for heterogeneous systems. • Usage of inappropriate energy structures may lead to large results discrepancies. - Abstract: The generation of multigroup neutron cross-section libraries is a key issue of the multigroup transport calculations in reactor physics. The correct choice of the boundaries of the energy groups, in particular, is decisive for obtaining reliable results. Knowledge of the reactor physics, general and specific of the studied reactor, along with long and refined analyses are required for finding out a reasonable energy structure, which is specific for the considered reactor and might be unsuitable for other systems. The genetic algorithm presented in this work aims to choose the most appropriate energy structure for the considered system to collapse a fine multigroup library into a few-groups one, usable for transient transport calculations. The user is free to choose the number of energy groups of the final library, which is in direct relation with the precision required and the time available for the simulation. The methodology is coupled with SIMMER-III code and applied to 3 reactor systems: ESNII+ ASTRID, ESFR and MSFR. The results show that the algorithm can find representative energy structures, providing accurate results on the multiplication factor. The results of each test are analyzed, showing how different compositions, geometries and neutron spectra guide the algorithm choices, so demonstrating the effectiveness of the method.

[en] The multigroup transport theory is the basis for many neutronics modules. A significant point of the cross-section (XS) generation procedure is the choice of the energy groups' boundaries in the XS libraries, which must be carefully selected as an unsuitable energy meshing can easily lead to inaccurate results. This decision can require considerable effort and is particularly difficult for the common user, especially if not well-versed in reactor physics. This work investigates a genetic algorithm-based tool which selects an appropriate XS energy structure (ES) specific for the considered problem, to be used for the condensation of a fine multigroup library. The procedure is accelerated by results storage and fitness calculation speed-up and can be easily parallelized. The extension is applied to the coupled code SIMMER and tested on the European Sustainable Nuclear Industrial Initiative (ESNII+) Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID)-like reactor system with different fitness functions. The results show that, when the libraries are condensed based on the ESs suggested by the algorithm, the code actually returns the correct multiplication factor, in both reference and voided conditions. The computational effort reduction obtained by using the condensed library rather than the fine one is assessed and is much higher than the time required for the ES search

[en] Highlights: • A new passive decay heat removal system has been invented for molten salt reactor emergency draining tank system. • This system is based on the loop heat pipe concept. • It includes an evaporating tank (emergency draining tank), and where the liquid fuel is mixed with a working fluid (sodium) and to vaporizes the working fluid. • The resulting vapor pressure drives the vapor through the pipes to the condensers, where the vapor condenses, releasing its latent heat of vaporization to a provided heat sink. • The condensers are located above the emergency draining tank at a higher elevation than the vaporization tank, so the condensed working fluid can flow back to the emergency draining tank by gravitational force. • Therefore, the system can continuously and passively transport the latent heat of vaporization from the emergency draining tank to the condenser section. - Abstract: A passive decay heat removal system is proposed that is actuated by the phase change heat transfer method (latent heat of fusion and evaporation) for an emergency draining tank system of molten salt reactors. The emergency draining tank serves an evaporating tank, where the liquid fuel is mixed with an immiscible working fluid and later to vaporize this working fluid. The top header of the vaporizing tank is connected by pipes to a condenser. The resulting vapor pressure of the working fluid drives the vapor through these pipes to condensers, The condensers are located above the reactor, where the vapor condenses, releasing its latent heat of vaporization to a provided heat sink (air heat exchanger). The condensed working fluid flows at first in a Pythagorean cup type collector, which is located just below the condensers, the collected working fluid flows back, in batch, to the emergency draining tank by gravitational force. In this batch manner the working fluid can be strongly mixed with the liquid fuel to enhance the heat transfer between liquid fuel and working fluid without any mechanical or electrical assistance, i.e. a completely passive heat removal process. Therefore the system can passively transport the latent heat of vaporization of the working fluid from the emergency draining tank to the condenser section. This proposed decay heat removal system is described in more detail in this paper. The passive cooling of the liquid fuel to extract the residual heat has been studied.

[en] The integrated ICE (Ingress-of-Coolant Event) facility, scaled 1/1600 with respect to the ITER-FEAT design, was built at JAERI with the aim of reproducing the phenomenology occurring in an ICE accident. An ICE occurs when a rupture in the coolant pipes causes the pressurized coolant to enter into the Plasma Chamber, which is held under high vacuum condition. A suppression system is used to mitigate the overpressurization and to prevent mechanical damages to the structures. The CPA module of the ASTEC severe accident code (Study carried out with ASTEC V2, IRSN all rights reserved, [2020]), has been adopted for the modelling and the simulation of a test conducted in the ICE facility. The experimental results of the main thermal-hydraulic parameters have been compared to the code results to characterize the ASTEC capability to predict the phenomenology of a low-pressure two-phase flow transient occurring in a fusion reactor. By coupling the ASTEC code with the uncertainty tool RAVEN, developed by Idaho National Laboratory, an uncertainty analysis has been conducted on the transient. The aim of the present activity is to investigate the dispersion and the sensitivity of the code response to the variation of selected uncertain input parameters, which could influence the simulation of an ICE. The activity also provides a first application of uncertainty analysis through the RAVEN-ASTEC coupling.