Study on effect of void-fraction on characteristics of several thorium fuel cycles in BWR has been carried out. Four fuel cycle scenarios are evaluated and the comparison with once-through-cycle BWR was also conducted. The 1367 nuclides have been employed in the time independent burnup calculation scheme where 129 of them are heavy metals (HMs). The burnup code was facilitated with the calculation method for determining the required ²³³U concentration of the system inherently. The code is coupled with SRAC cell calculation code to compose a time independent-cell burnup code. In cell calculation, 26 HMs, 66 fission products (FPs) and one pseudo FP have been utilized. The JENDL 3.2 library has been employed in this study. The results show that the profile of the required ²³³U concentration for criticality depends on the neutron spectra and the microscopic cross-section. However, the profile of the conversion ratio likely depends on the number density of heavy nuclides.