In this study, the criticality and burnup analyses have been performed for full core model of Pebble Bed Modular Reactors, such as PBMR-400, using the computer codes MCNP5.1.4 and MONTEBURNS 2.0. Three different pebble distributions, namely: Body Centered Cubic (BCC) (packing fraction = 68%), Random Packing (RP) (packing fraction = 61%) and Simple Cubic (SC) (packing fraction = 52%) were selected for the analyses. The calculated core effective multiplication factor, k(eff), for BCC, RP and SC came to be 1.2395, 1.2357 and 1.2223, respectively. The core life for these distributions were calculated as similar to 1200, 1000, and 800 Effective Full Power Days (EFPDs), whereas, the corresponding burnups came out to be similar to 99,000, similar to 92,000 and similar to 86,000 MWD/T, respectively, for end of life k(eff) set equal to 1.02. (C) 2010 Elsevier Ltd. All rights reserved.