NUCLEAR MASS INVENTORY

The mass inventory of the heavy metals in research reactor fuels has been calculated using the WIMS code¹ for unit-cell models of MTR, TRIGA and DIDO fuel assembly types. Models of each fuel assembly type were neutronically burned for a length of time corresponding to typical fuel-cycle lengths and U-235 burnup². Table 1 summarizes the fuel assembly models for which mass inventory calculations were made.

Table 1. Fuel Assembly Models

Assembly Type	U-235 Burnup, %	U-235 Enrichment, %	U-235 Mass, g
MTR (19 fuel plates)	5, 10, 20, 30, 40, 50, 60, 70, 80	93 45 19.75	100 200 300 400 200 300 400 100 200 300 400 500
TRIGA (single rod)	5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60	70 (8.5wt% U) 20 (20wt% U) 20 (12wt% U) 20 (8.5wt% U)	133 98 54 38
TRIGA (25 rod cluster)	10, 20, 30, 40, 50, 60	93.1 (10wt% U) 19.7 (45wt% U)	41.4 53.6
DIDO (4 fuel tubes)	10, 20, 30, 40, 50, 60	93 80 60 20	150 150 150 200

Mass inventory calculations for MTR models were made for assemblies with up to 80% U-235 burnup, for 93, 45 and 19.75% U-235 enrichments, and for initial U-235 masses of 100 to 500 g. The specific MTR model was for a 19-fuel plate assembly. (Supplemental mass inventory calculations, shown in Appendix A, indicate that the MTR model in not a strong function of the number of fuel plates or the specific fuel-clad-coolant geometry.)

Similar calculations were made for two TRIGA assembly types - a single rod model and a 25-rod cluster model. The maximum U-235 burnup in these models was 60%. There were four fuel types for the single rod model and two fuel types for the cluster model.

For DIDO fuel assembly types, mass inventory calculations were made for a 4-fuel tube model with up to 60% U-235 burnup, and for four fuel enrichments and assembly masses.

The results of the mass inventory calculations are shown in the following tables:

• Table 2 - MTR Fuel 93% Enrichment
  
• Table 3 - MTR Fuel 45% Enrichment
  
• Table 4 - MTR Fuel 19.75% Enrichment
  
• Table 5 - TRIGA Fuel Single-Rod Model
  
• Table 6 - TRIGA Fuel 25-Rod Cluster Model
  
• Table 7 - DIDO Fuel

The tables show the isotopic masses of U, Np, Pu and Am that are present in spent fuel as functions of the fuel assembly U-235 burnup and initial U-235 mass. As will be noted in the tables for most fuel assembly types, the uranium fuel compositions have excluded initial enrichments of U-234 and U-236. In order to account for initial enrichments of U-234 and/or U-236 in the tables, initial U-234 and U-236 masses can be simply added to the spent fuel mass inventory. (See Appendix B for an assessment of the effect of initial enrichments of U-234 and U-236 upon the overall mass inventory of U, Np, Pu and Am in spent fuel.) Within the uncertainty of the calculations, the results in Tables 2-7 can be used to estimate the spent fuel mass inventory in most MTR, TRIGA and DIDO fuel assembly types. (See Appendix C for a comparison of calculational techniques.)

The mass inventories given in Tables 2-7 are at the time of reactor discharge and therefore do not account for decay of Pu-241 to Am-241 for times after discharge. When necessary to estimate mass inventories after discharge, the Pu-241 mass is decreased and the Am-241 mass is increased by an amount where is the Pu-241 mass at discharge, d^-1 (Pu-241 half-life, 14.4 y), and is the time in days after discharge. No mass inventories are given for U-239 (half-life, 23.5 m) and Np-239 (half-life, 2.355 d) as they are assumed to decay instantaneously to Pu-239.