The Breeding Blanket (BB) is one of the key components of the European Union Demonstration (EU-DEMO) fusion reactor. It accomplishes several functions, such as cooling device, tritium breeder (ensuring the reactor self-sufficiency) and neutron shield. Different BB concepts were selected to be investigated in the DEMO R&D strategy. Among them, there is the Water-Cooled Lithium-Lead (WCLL), that is the option of interest for the work planned in the current proposal. The WCLL blanket design relies on lithium-lead as breeder and neutron multiplier, pressurized water as coolant and EUROFER as structural material. It is divided in two main subsystems: the breeder zone (BZ) and the first wall (FW). Each one is provided with an independent cooling system, consisting of FW channels and Double-Wall Tubes (DWT), respectively. The two cooling circuits are strongly coupled within the Breeding Cell (BRC), that is the BB elementary unit. The power sources are the heat flux incident on the FW component and the nuclear heating, mostly produced by interactions between liquid breeder and plasma neutrons. The resulting BRC thermal field is quite complex. The aim of the proposed simulation activity is developing a detailed model of the WCLL-BB elementary cell by using RELAP5/Mod3.3 thermal-hydraulic (TH) system code. Calculations involving the BRC were already performed with more refined codes (CFD) to predict with a high level of detail its TH behavior during DEMO normal operations. The idea is to tune the RELAP5 model by using the outcomes of these simulations and then study the phenomena occurring in the elementary cell during accidental conditions. The advantages of this approach is saving computational time and the possibility to simulate two-phase flow (not easily modelled by CFD codes in transient condition). In this way, it is possible to evaluate if, in abnormal conditions, thermal crisis occurs within FW channels or DWTs, compromising the elementary cell structural integrity.
In nuclear engineering, accidental analysis is fundamental to determine design principles leading to component layout improvements and to adoption of preventive and mitigative actions. So far, evaluating the system performances in accidental scenarios and demonstrating the respect of safety requirements in such conditions plays a crucial role in the licensing of fission reactors. This approach will be followed also for DEMO nuclear fusion reactor. As a key reactor component, a detailed analysis shall be focused on the breeding blanket. Its thermal-hydraulic behavior during abnormal conditions shall be evaluated in search of issues to be addressed in the next design phases. The layout of the COB equatorial cell is the one at a more mature stage and represents an optimum starting point for this kind of analysis. Elementary cell TH performances can be investigated with both CFD and system codes. The main limit associated to the former is the difficulties in simulating two-phase flow for long transients and with an extended calculation domain. During DEMO normal operations primary coolant (water) is exercised at typical PWR conditions, i.e. a significant margin from saturation is maintained. Instead, in accidental transients, above all the ones following a loss of primary flow initiating event, water temperatures can experience a sharp increase leading to two-phase conditions. Similar transients cannot be studied with CFD codes, requiring simulations with system codes. Although, CFD codes are able to investigate with a high degree of detail the local behavior within the BRC, highlighting hot spots or criticalities present in the 3D thermal field. This is of sum importance to correctly evaluate the blanket thermal inertia, that provides the component temperature feedback during any transient condition. Such outcomes can be used to tune models developed with less-detailed, more transient-oriented codes. Within this framework, the usage of RELAP5/Mod3.3 represents a promising solution. In fact, referring to PWR operative conditions (i.e. BB water coolant conditions), RELAP5 series of codes have been validated for decades against experimental data coming from dozens of facilities. Hence, this code is strongly recommended for the transient scenarios selected for this simulation activity (¿Decrease in Reactor Coolant System Flow Rate¿). The main innovation related to the activity proposed relies on the approach adopted. CFD simulation outcomes, referred to DEMO normal operations, produce guidelines for the RELAP5 mesh and reference values for comparison. RELAP5 model, once tuned in the local effects, allows transient calculations involving accidental scenarios and the evaluation of complex two-phase phenomena occurring within the elementary cell during such conditions. These assessments lead to important design improvements, such as DWTs disposition, variation of FW channels number, etc. In this way, the best features of both kind of codes, CFD and system ones, are properly exploited. This approach also increases the confidence on the capability of design solutions developed for the blanket component in withstanding accidental conditions considered.