Motivated by efficient assessment of the in-vessel retention (IVR) strategy employed in some designs of light water reactors, a lumped-parameter code named as transIVR was developed in the present study, which has the features as (i) quick estimates of transient one-and two-layer melt pool heat transfer with reasonable accuracies; and (ii) two-dimensional representation of heat conduction in the reactor pressure vessel wall, facilitating the coupled thermo-mechanical analysis of the reactor pressure vessel under thermal loads. The transIVR code was first benchmarked against the UCSB FIBS case of two-layer configuration and then validated against the LIVE-7V experiment. Both cases showed good predictions in the IVR-interested parameters like the heat flux profile, residual wall thickness and energy split. It indicates the code's capability in predicting two-layer and transient melt pool heat transfers, respectively. Finally, the code was coupled with ANSYS Mechanical for the efficient assessment of vessel integrity. Good results were achieved in the validation against the vessel failure experiment FOREVER-EC2. Hence, the transIVR code is an efficient tool suitable to address related safety concerns of light water reactors, including IVR efficacy and vessel integrity (with its coupling to mechanical solvers).