### RERTR Publications:

Foreign Research Reactor Spent Nuclear Fuel

#### ANL/RERTR/TM-26

##### NUCLEAR MASS INVENTORY

The mass inventory of the heavy metals in research reactor fuels has been
calculated using the WIMS code^{1} 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 burnup^{2}. 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.