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Multirecycling of plutonium in advanced PWRs
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ECN-R--98-007 Overig
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The influence of the moderator-to-fuel ratio of MOX fueled PWRs on themoderator 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 assessed. Increasing the moderator-to-fuel 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 moderator-to-fuel ratio. The effective delayed neutron fraction is rather independent of the moderator-to-fuel ratio. According to a point-kinetics model, a MOX fueled reactor with a moderator-to-fuel ratio of four responds similarly to a moderator temperature decrease as an UO2 fueled reactor with a moderator-to-fuel ratio of two. This conclusion holds for both the instantaneous power increase and for the deposited energy during a specific time interval. Scenario studies show that four times recycling of plutonium in PWRs reduces the plutonium production with a factor of three compared with a reference once-through scenario, but that the americium and curium production triple. If the remaining plutonium after four times recycling is disposed of, the reduction of the radiotoxicity reaches only a factor of two. This factor increases to five at the maximum when the plutonium is further recycled. 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 actinides production decreases with increasing moderator-to-fuel ratio of the MOX fuel. Enlarging the moderator-to-fuel 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. 11 refs.

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