Study of the thorium cycle in pressurized water reactors

We have been led to consider the PWRs with the present characteristics (reference Bugey 2), an introduction of Th 232-Pu fuel with a view to producing the U 233 in strictly non modified reactors and starting this introduction in 1990 only. Likewise, the Th 232-U 233 cycle in the water reactors would not be launched until the year 2000. The recommended solution consist in starting up the reactor with a Th 232-Pu cycle as from the first core, the plutonium being that which comes from the present PWRs at 33 GWd/t and to abandon during the following cycles the draughtboard loading plans in favour of crown recharging plans. The burnable poisons are no longer necessary and the breakdown of the assembly into several areas with different plutonium levels become pointless. The reactors require much plutonium and only restitute plutonium which is hardly of any worth save in fast reactors. Also, the conversion factor is relatively low; it was retained as such because its improvement would have involved very extensive modifications in the characteristics of the core. Nevertheless this first part of the study showed that there was at least one type of solution to the Th-Pu cycle which complies with the constraints ion the power peak, temperature coefficients, etc. The thorny point concerning the behaviour of the fuel is the temperature coefficient of the moderator. The solution chosen is that which leads to an acceptable temperature coefficient whilst at the same time improving the conversion factor, that is to say the operation in spectrum shift control of the reactor. Having shown the Th 232-Pu and Th 232-U cycles to be feasible, this strategy remained to be compared with other strategies in terms of uranium consumptions