In India, ThO2 has been extensively used in PHWRs for neutron flux flattening of the initial core during start-up. During the last few years, ThO2 bundles have been utilized for this purpose in seven units of PHWR 220 including the two units each at Kakrapar Atomic Power Station (KAPS 1&2), Kaiga Atomic Power Station (KGS 1&2), Rajasthan Atomic Power Station (RAPS 3&4), and in RAPS 2 after mass coolant channel replacement. So far, some 232 thoria bundles have been successfully irradiated in the operating PHWR up to a maximum power of 408 kW and burnup of 13,000 MWd/Te HM without any failure [134]. Details of irradiation of ThO2 bundles in India are shown in Table 14. In-pile irradiation of Zircaloy-clad (Th, Pu)O2 fuel pins have been successfully carried out in the pressurized water loop of CIRUS research reactor. A six-pin cluster of free­standing Zircaloy-2 cladded ThO2-4 % PuO2 was successfully irradiated up to a burnup of 18,400 MWd/Te. Subsequently, two additional six pin clusters of col­lapsible Zircaloy-2 cladded pins containing high density ThO2 and ThO2-6.75 % PuO2 were successfully irradiated up to a burnup of 10,300 MWd/Te without failure. The peak pin-power rating was 40 kW/m [135]. Details of ThO2-PuO2 pin irradiation are shown in Table 15.

Although several isolated irradiation experiments on ThO2 and ThO2-UO2 materials have been conducted, the bulk of the irradiation data generated has come US from four goal-oriented programs [8, 12, 39, 136]:

1. Boiling water reactor (BWR) Program, which culminated in the irradiations performed in the BORAX-IV and Elk River reactors.

2. Thorium Utilization Program, which included the irradiation of vibratory compacted and pelletized fuels.

3. Babcock and Wilcox developmental work on pressurized water reactor (PWR) fuels.

4. Light water breeder reactor (LWBR) Program at Bettis Atomic Laboratory which resulted in the core loading of the Shippingport Reactor.

The majority of experiments were performed on fuels with less than 10 wt% UO2. From the above experiments and other experiments carried out in India, Russia, Canada, Japan, and Korea the following conclusions can be drawn about thermophysical properties.