ORNL-1816 - the Molten Salt Energy Technologies Web Site

ANP QUARTERLY PROGRESS REPORT
In calculating the activity of the fission frag-
ments, use is made of Fig. 2 in ORNL-53,6 which
is a curve of the dose from a uranium slug exposed
for three years plotted against time after exposure.
The dose is given in mrhr at 1 meter per watt of
exposure,
Calculations now show that 1 curie of 2-Mev
gammos gives a dose of approximately 1 rhr at
1 meter. If it is assumed that the average energy
of the decay gammas from the fission fragments
is 2 Mev (an overestimate), the curve can be used
to obtain an estimate of the curies of activity for
c?y time after exposure. However, it is first
necessary to calculate the watts of exposure in
the regions of the reactor close enough to the
lnconel core shells so that the fragments due to
fissions will strike the Inconel. This is done by
finding the range, R, of the fission fragments in
the fuel and again approximating the fuel annulus
by a spherical shell of 25 cm OD and 15 cm ID
to find the total power generated in the regions
adjacent to the Inconel. This power is divided
by 2 to account for the fact that approximately
one-half the fission fragments generated in this
region will impinge on the shells. Since the curve
was plotted for a slug exposed for three years,
any values obtained by using this curve will be
overestimates of the fission fragment activity of
a 60-Mw reflector-moderated reactor,
The range of fission fragments is estimated to
be 0.001 cm, and the
1
watts of exposure = 6 x lo7 w x -
2
4n (252 + 152) x
X x 2
4
- r7 (253 - 153)
3
= 1.3 io4 .
6G. Ascoli and 0. Sisman, Absorption o Radiation
from an "X" Slug by Lead, Fig. 2, p 7, ORAL-53 (May
1948).
28
The last factor of 2 in the above equation is due
to the flux at the surface of the lnconel being
twice that of the average flux. Some values of
the total activation of the lnconel core shells
obtained by using the above methods are given in
Table 2.1. One conclusion to be drawn from
Table 2.1 is that little would be gained by trying
to obtain special lnconel with especially low
cobalt content; the activity could not be reduced
below that caused by the fission fragments.
TABLE 2.1. ACTIVATION OF INCONEL
CORE SHELLS
Activity (curies)
Days After
Fission
Exposure Cobalt
Fragments
Total
10 480 390 870
20 480 260 740
100 480 80 560
Mathematical Models for Reflector-Moderated
Reactors
L. T. Anderson, Consultant
The thermal absorption rate has been calculated
for an idealized reactor consisting of a nonmoder-
ating spherical-shell fuel region surrounding a
moderator and surrounded by an infinite moderating
reflector with a fission source of 1 neutrodsec
in the fuel region. The thermal capture cross
section of the fuel region was taken to be infinite.
The nonthermal capture cross section of the fuel
region was assumed to be zero, and the thermal
and nonthermal absorption cross sections of the
moderator were likewise assumed to be infinitely
small. The neutrons slowed down according to age
theory, and the thermal neutrons diffused according
to diffusion theory.
The absorption rate is, for the source located at
the inner
(2
radius of the fuel region,
1 + - 1) x cotx