The mass fabricator can take up to HV power, but it providing all the power is all at that rating then it will take as much as it's given. If you use a HV-transformer, that will split up to 2048 EU/t down into 4 HV packets that the mass fabricator would accept without exploding.
And would you be so kind and tell me how much a MOX nuclear reactor outputs? (Voltage wise) Or does that depend on the heat/number of cells?