Plutonium recycling in pressurized water reactors: influence of the moderator-to-fuel ratio

The reactor physics trends that can be observed when the moderator-to-fuel (MF) ratio of a mixed-oxide (MOX) fuel lattice increases from two (the stan dard value) to four are investigated. The influence of the MF ratio on the moderator void coefficient, the fuel temperature coefficient, the moderator temperature coefficient the boron reactivity worth, the critical boron con centration, the mean neutron generation time, and the effective delayed neu tron fraction has been investigated. Increasing the MF ratio to values larg er than three gives a moderator void coefficient sufficiently large to recy cle the plutonium at least four times. Also, the values of other parameters like the boron reactivity worth, the fuel temperature coefficient, the mod erator temperature coefficient and the mean neutron generation time improve with increasing MF ratio. The effective delayed neutron fraction is almost independent of the MF ratio. According to a point-kinetics model, the resp onse of a MOX-fueled reactor with an MF ratio of four to a moderator temper ature decrease is similar to that of a UO2-fueled reactor with an MF ratio of two. Scenario studies show that recycling plutonium four times in pressurized wa ter reactors reduces the plutonium production by a factor of three compared with a reference once-through scenario, but the americium and curium produ ction triples. If the plutonium remaining after recycling four times is dis posed of the radiotoxicity reduces by only a factor of two. This factor inc reases to a maximum of five if the plutonium can be eliminated in special b urner reactors. Recycling of americium and curium is needed to reduce the r adiotoxicity of the spent fuel to lower values. In general, the plutonium m ass reduction increases and the minor actinide production decreases with in creasing MF ratio of the MOX fuel. Enlarging the MF ratio can be achieved by increasing the rod pitch or by re ducing the fuel pin diameter In both cases, the economic penalty is about t he same and is quite large.