[en] For the cooling of water forced circulation through the RBMK-1500 reactor at the Ignalina Nuclear Power Plant eight Main Circulation Pumps (MCPs) are employed. These pumps are joined into groups of four pumps each (three for normal operation and one on standby). There were few events when one or few MCPs were inadvertently tripped. On 14 May, 1996 one MCP at Ignalina Unit 2 was inadvertently tripped. The similar event took place on 23 January, 1998. During this event, the MCP check valve failed to close, causing a recirculation loop to develop by means of a reversed flow through tripped pump. On 31 July, 2000 three MCPs at Ignalina Unit 2 were tripped one after another, due to inadvertent activation of fire protection system. Simultaneous trip of all MCPs occurred on 26 March, 1986. In the case of one MCP trip the throughput of the rest running pumps in the affected Main Circulation Circuit loop increased, however, the total coolant flow through the affected loop decreased. The main question arises whether this coolant flow rate is sufficient for adequate core cooling. In the case of all MCPs trip, the coolant during the first few seconds is supplied to the reactor by pumps coastdown. Later the reactor is cooled by natural circulation. This paper presents investigation of one and all MCP trip events at the Ignalina NPP by employing best estimate code RELAP5. For single tripped MCP and simultaneous trip of all MCPs cases uncertainty and sensitivity analysis was performed. For that purpose the GRS (Germany) developed System for Uncertainty and Sensitivity Analysis package was used. Uncertainty analysis of flow energy loss in different parts of the Main Circulation Circuit, initial conditions and code-selected models was performed. On the basis of these analyses recommendations for the improvement of the Ignalina NPP RELAP5 model have been developed. (author)