Foreign Research Reactor Spent Nuclear Fuel
EXAMPLE CALCULATION: NUCLEAR MASS INVENTORY,
PHOTON DOSE RATE AND THERMAL DECAY HEAT
In this example, a 280 g235U MTR-type fuel assembly has been irradiated
at an average fuel assembly power ()
of 25 kW over an elapsed time ()
of 3584 days. The irradiation history of this fuel assembly is such that it
can not be described simply, using a constant power ()
and a continuous irradiation time ().
It is assumed, however, that
where the sum of () traces
the fuel assembly irradiation history over all irradiation segments when the
fuel assembly power was constant and the irradiation time was continuous. The
elapsed time is the calendar time from the first through the last irradiation
segment. Assuming 1.25 g235U burned per MWd, this fuel assembly has
112 g235U burned and 40% 235U burnup. The fission product
decay time () or cooling time
for this fuel assembly is assumed to be 3 years.
Nuclear Mass Inventory
If the fuel assembly enrichment is 93%, then 300 g235U, 40% 235U
burnup data of Table 2 can be prorated to 280 g235U. For enrichments
of 45 or 19.75%, similar prorated data from Table 3 or 4, respectively, should
be used. Table D1 summarize the spent fuel mass inventory of 280 g235U
fuel assemblies which have 40% 235U burnup.
Table D1. Mass Inventory of Spent HEU, MEU and LEU Fuel Assemblies
These 280 g235U spent fuel inventory masses could also have been
estimated using linear interpolation of the 200 and 300 g235U, 40%
235U burnup data tabulated in Tables 2, 3 and 4. Note, inventory
masses for non-tabulated fuel assembly burnup should also use linear interpolation
of tabulated data (e.g. 45% 235U burnup, interpolate between 40 and
50% tabulated data).
Photon Dose Rate
The photon dose rate of this fuel assembly is calculated from data presented
in Table 8. The assembly power density is 0.089 MW/kg235U (25 kW
/ 280 g235U), the 235U burnup is 40%, and the decay time
is 3 years. With these data, Table 8 estimates that the photon dose rate is
1.02 rem/h per g235U burned. With 112 g235U burned, the
dose rate is 114 rem/h at 1 meter from the fuel assembly.
For fuel with 40% burnup and with 112 g235U burned, Fig. 1 estimates
that this fuel assembly will be self-protecting (dose rate greater than 100
rem/h) for about 4 years.
The photon dose rate for non-tabulated assembly power densities, 235U
burnup and/or decay times can be estimated using linear interpolation of the
data in Table 8. Linear interpolation to determine the photon dose rate would
be necessary, for example, for a fuel assembly with the following parameters:
3.5 year decay time, 50% 235U burnup and 0.134 MW/kg235U
assembly power density. A simple table which interpolates each parameter separately
is a useful aid. Table D2 is constructed to determine the photon dose rate for
these non-tabulated fuel assembly parameters.
Table D2. Fuel Assembly Parameter Linear Interpolation
|Decay Time, y||Burnup, % 235U||Assembly Power Density, MW/kg235U||Photon Dose Rate, rem/h per g235U burned|
The bottom line, estimated photon dose rate is 1.10 rem/h per g235U
Thermal Decay Heat
The thermal decay heat calculated with the ORIGEN code for this example is
about 4.2 Watts.
Integrated Emission Rate Equation
The thermal decay heat of this fuel assembly using the conservative heat load
equation based upon Eq. -1
is about 10.6 W. This result is based upon an average fuel assembly power
() of 25,000 Watts, a cooling
or decay time () of 1095 days
(3 y) and an elapsed time ()
of 3584 days.
The thermal decay heat with these same data and the heat load equation based
upon Eq. -2
is about 5.1 W.
Untermyer and Weills Equation
Similarly, using the heat load equation based upon Eq. -3 with a decay time
of 9.46·107 seconds (1095 d) and an elapsed time of 3.10·108
seconds (3584 d)
is about 3.8 W.