[en] In-reactor densification of ThO₂ and ThO₂--UO₂ fuel of low burnup and low power operation (hence low temperature) was assessed by measuring fuel pellet diameter changes. Pellet diameter changes ranged from nil in a large grain, low temperature thoria pellet (98.9 percent theoretical density) to -1.06 percent in a small grain, moderate temperature ThO₂-30 w/o UO₂ pellet (93.8 percent theoretical density). A correlation was established between quantity of small pores (<2.3 μm diameter) and as-fabricated fuel grain size. An empirical equation, based on densification (pore closure) plus fuel swelling, was formulated for pellet diameter change as a function of initial grain size and fuel burnup