A study to characterize the steam waterhammer phenomena of a low pressure cooling injection (LPCI) system for a Mark 1 boiling water reactor (BWR) has been performed using RELAP5 and GOTHIC during a transient event. The scenario of particular interest was a manual switchover from shutdown cooling mode 3 to low pressure injection due to a loss of coolant accident (LOCA). This transient was initiated by opening the isolation valves of the two trains on a LPCI system into the torus. The torus was considered to be at atmospheric pressure and 20°C. The initial condition of the problem was set up such that the liquid was stagnant in the system. The initial temperature and pressure of the liquid, which was between the torus and isolation valves, was considered to be the same as the torus conditions. On the other hand, the initial condition of the liquid upstream of the isolation valves was chosen to be at 1 MPa and near saturation temperature. The analysis showed that the liquid in the system flashed into steam and discharged into the torus after the isolation valves started to open. Discharge of steam continued until the pressure in the LPCI system reached to a hydrostatic equilibrium with the torus. Following this, the cold liquid from the torus began to reflod the LPCI piping while condensing the steam at the liquid-steam interphase. This caused a mild steam waterhammer event when all of the steam condensed in the piping segments with closed ends. A sensitivity analysis showed that, the magnitude of the steam waterhammer predicted by both codes was sensitive to the number of nodes selected to model the piping, where the steam waterhammer phenomena occurred. Technical basis was obtained from the available literature and used as a guide to choose the number of nodes for the models in both codes. Once the steam waterhammer and the hydrodynamic properties associated with this transient were predicted by both codes, the forces exerted on critical pipe components were calculated. Also, selected thermal-hydraulic properties and hydrodynamic loads were compared between both code calculations. Comparisons showed reasonable agreements.