Foreign Research Reactor Spent Nuclear Fuel
MTR MODEL MASS INVENTORY SENSITIVITY
This appendix examines the sensitivity of MTR-type fuel assemblies to the number of fuel plates in the assembly as well as the fuel element specifications for the fuel, clad and coolant. An examination of many MTR-type fuel assemblies shows that the ratio of the coolant channel thickness to the fuel meat thickness, times the number of fuel plates, is nearly a constant. This constant is also proportional to the H/U-235 atom ratio which can be used to characterize the neutron spectrum in MTR-type fuel assemblies.
Figure A1 shows the H/U-235 atom ratio as a function of the U-235 mass. The upper curve are for 19-plate (0.51mm fuel, 0.38mm clad, 2.95mm coolant) elements and the lower curve are for 23-plate (0.51mm fuel, 0.38mm clad, 2.19mm coolant) elements. Most all MTR-type fuel assemblies as a function of the fuel element specifications are within the range (6%) of the average H/U-235 ratio.
Tables A1-A3 show the mass inventory results for MTR fuel assembly types with 300g U-235 and 93, 45 and 19.75% U-235 enrichment. The difference between the upper and lower bound results indicate only small differences in the isotopic masses as a function of fuel element specification.