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 standard 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 concentration, the mean neutron generation time, and the effective delayed neutron fraction has been investigated. Increasing the MF ratio to values larger than three gives a moderator void coefficient sufficiently large to recycle the plutonium at least four times. Also, the values of other parameters like the boron reactivity worth, the fuel temperature coefficient, the moderator 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 response of a MOX-fueled reactor with an MF ratio of four to a moderator temperature 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 water reactors reduces the plutonium production by a factor of three compared with a reference once-through scenario, but the americium and curium production triples. If the plutonium remaining after recycling four times is disposed of, the radiotoxicity reduces by only a factor of two. This factor increases to a maximum of five if the plutonium can be eliminated in special burner reactors. Recycling of americium and curium is needed to reduce the radiotoxicity of the spent fuel to lower values. In general, the plutonium mass reduction increases and the minor actinide production decreases with increasing MF ratio of the MOX fuel.
Enlarging the MF ratio can be achieved by increasing the rod pitch or by reducing the fuel pin diameter. In both cases, the economic penalty is about the same and is quite large.