Design and FEM modeling of a fire resistant cabinet for fusion environment

Diagnostic systems in fusion environments need to satisfy stringent requirements, often involving radiation and electromagnetic shielding as well as fire protection. A conceptual design of a multilayer cabinet for the RNC (Radial Neutron Camera) diagnostic systems, located in the equatorial port (EP) #01 in ITER, with a required fire rating REI-120 (R -integrity, E -leak tightness to hot gasses and flames, I - thermal) was analyzed from the thermal insulation (I) point of view during external fire scenario (Tmax=805 celcius for 2 h). The modeling of the fire was based on the convective heat transfer represented by a HTC (heat transfer coefficient) of 35 W/m2K and radiative heat transfer with unit emissivity according to ISO-834. Focusing on the thermal criterion prescribed by the REI requirement, the objective was to demonstrate that the temperatures will remain below specified limits for at least 120 min subjected to given fire loads. The adopted strategy was to exploit an advanced microporous incombustible material - Microtherm & REG; overstitched - selected for the fire insulation and being applicable for the ITER environment. A parametric 3D model was developed and solved using the Finite Element Method (FEM) and the ANSYS code, including not only the heat conduction but internal radiation via SURF252 elements and internal convection via LINK34 elements. The relation between the thickness of fire protection and the maximum temperature after 2 h of fire was analyzed for two types of designs: with internal and external fire protection. In addition to the ideal thermal connection between the layers of the cabinet, the effects of imperfect thermal contacts were analyzed by varying the thermal conductance of CONTA174 elements. The effect of the boundary conditions: adiabatic versus specified temperature was analyzed, as well as the need for protecting the supports of the cabinet. The effect of the cabinet size at constant thickness of the walls was analyzed parametrically revealing increasing temperatures for smaller cabinets - more fire insulation needed. An analytical model used to explain the size effect relate to the competing effects of increasing heat transfer area and increasing heat capacity. The developed methodology provides an easy tool for early dimensioning of the necessary fire protection layer often needed before the start of the detailed CAD design. The necessary thickness of external protection layer for the analyzed cabinet resulted in 10 mm.