ORNL-4191 - the Molten Salt Energy Technologies Web Site

23. Steady-State Fission Product Concentrations and
Heat Generation in an MSBR and Processing Plant
Concentrations of fission products and the heat
generation associated with their decay are re-
quired for design of systems €or processing the
fuel and fertile streams of an MSBR. A method
for calculating the steady-state concentration of
fission products in an MSBR will be described,
and heat-generation rates in the fuel stream will
be given. Changes in heat-generation rate re-
sulting from removal of fission products in both
the reactor and the fuel-stream processing system
will be shown.
Several simplifying assumptions were made in
calculating the concentration of fission products
in the fuel stream of an MSBR. The differential
equation used to define the concentration of a
given isotope was
where
J. S. Watson L. E. McNeese W. L. Carte1
Nz,A = number of atoms of an isotope with
atomic number 2 and mass number A, . .. .. . .-
Vc/V, = core volume/total fuel volume,
T - processing cycle time €or isotopes
z with atomic number 2 (sec),
t : time (sec).
The reactor was assumed to be at steady state
with respect to all fission products. At steady
state, dN /dt -: 0; so the differential equation
reduces to
Z,A
which defines the steady-state Concentration of
each isotope. This equation was solved for each
isotope using an IHM 7090. The calculations used
235U yields given by Blomeke and Todd;' yields
for 233U were obtained by distributing "mass
yields" reported by England' among the isotopes
in each mass chain so that each isotope had the
same fraction of the mass yield as reported for
235U. Decay chains were simplified and approxi-
mated by "straight" chains considering only beta
decay. Where isomeric transitions or other
complications were noted, personal judgment was
used to approximate the res1 process with straight
beta decay.
P - power (w),
Y = primary yield of isotope of atomic
number and number A (atoms/
' 1. 0. Rlomeke . and Marv ,- F. Todd. Umninm-235
~
Fission-Product Production as a Function of Thermal
Neutron I'lux, Irradiation Time, and Decay Time. 1.
Atomic Coricentrafions and Gross Totals, ORNL-2127
f iss i on),
+ = flux (neutrons sec-' cm-'),
(part I.), vol. 1.
'T. R. England, Time-Dependent Fission-Product
(2 7 cross section of isotope with atomic
%,A
number 2 and mass number A (cm'),
'~liemia? andKesonance Absorption Cross Sections
matil Revisions azld Colculationa1 Extensions), WAPD-
TM-333, addendum No. 1.
245