The use of narrow gap for thick component welding as applied in nuclear industries and especially by AREVA NP, requires the mastering of several parameters and especially shrinkage. The prediction through to numerical simulation is very helpful for welding procedure definition. This paper describes an approach used to determine a 3D heat source dedicated to a new industrial welding process configuration (deep narrow groove multipass lowcarbon steel gas metal arc (GMA) welding, two passes per layer) to assess the groove shrinkage which occurs during welding by numerical simulation. Parameters of this 3D heat source are identified by solving an inverse heat conduction problem by a least square method. A multiobjective optimization is performed with a new proposed metric (Hausdorff distance) in the objective function (sum of square) in order to simulate relevant bead shape and temperatures in the solid zone. Finally, the identified 3D heat source model is a combination of two volumetric heat sources containing five parameters each. It can be used as thermal loading for subsequent thermal metallurgical mechanical calculations.