In this paper the problem of system integration and dynamic modeling of a hydraulically actuated manipulator with seven degrees of freedom, i.e. HydroLek HLK-7W is investigated. The arm is fitted on Multi-Arm mobile Robot System for Nuclear Decommissioning (MARS-ND) applications purposes. This is a step forward with respect to the previous works where only kinematics of the robot was taking into account. As the decommissioning robot has to perform precise and complex tasks autonomously using effective model-based nonlinear control algorithms having an accurate dynamic model of the arm which is reliable enough to predict the behavior of the manipulator under different operating conditions would be crucial. To this end the symbolic, and numerical model of the dynamic of robot is developed and a first attempt for model validation and tuning the parameters of the model is taken forward.