Management of Commercially Generated Radioactive Waste - U.S. ...

7.7
By combining the ORIGEN to match the annual operating status of all plants in the sys-
tem and the amount of uranium and plutonium available for recycle, the spent fuel composi-
tion with or without recycle in any year can be determined. This method of using a
relatively small number of fuel irradiation (burnup) calculations to characterize a large
number of spent fuel combinations provides an efficient and reasonably accurate representa-
tion of spent fuel compositions each year for the entire system.
The number of recycles for both uranium and plutonium was limited to three. The amount
of third-recycle uranium and plutonium is small and the accumulation of 242Pu in the third-
recycle plutonium discharge reduces its value substantially. For these reasons and to sim-
plify the calculation, the discharge from third-recycle fuel was discarded. In a real sys-
tem whether or not the plutonium from the third recycle would be recycled would most likely
be an economic decision. It could continue to be recycled and ultimately either be fis-
sioned or transmuted to higher actinides and be discarded in the waste.
The computer code ENFORM (Heeb et al. 1979) was used to develop fuel cycle logistics
and isotopic compositions of the fuel cycle streams. ENFORM was originally developed to
evaluate environmental impacts of the entire nuclear fuel cycle. However, only its fuel
cycle logistics capabilities were used here to provide fuel cycle source data for the
WASTRAC module, which determined waste management logistics.
ENFORM input requirements include:
- a nuclear power growth projection
- a life-cycle operating schedule for the nuclear power plants
- recycle assumptions, i.e., once-through or recycle
- a fuel reprocessing schedule if recycle is selected
- inventory and timing assumptions for the entire fuel cycle
- spent fuel compositions as calculated by ORIGEN.
The output of the logistics calculation is a year-by-year mass flow and isotopic compo-
sition for each operation in the fuel cycle.
The computer code WASTRAC, developed for this analysis, models the storage, treatment,
packaging, shipment and disposal operations for each waste stream. Figure 7.2.2 illustrates
the waste management steps and the items calculated in a typical WASTRAC subsystem. Waste
management steps can be added or deleted as required to model a specific subsystem. Each
waste stream was tracked through a series of steps similar to that displayed in
Figure 7.2.2.
WASTRAC computes waste volume and waste composition as a function of year, waste type
and waste management step. The entire radionuclide content of the spent fuel is accounted
for by allocating it either to a product stream, i.e., uranium or plutonium in a reproces-
sing case, or to one of the waste streams. Radionuclide inventories are corrected at each
step for decay or buildup during the time interval since reactor discharge and/or reproces-
sing. Radionuclide inventories are also calculated for times up to one million years after
placement in a final repository.