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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963

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LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any
person acting on behalf of the Commission:
A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
report may not infringe privately owned
rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
method, or process disclosed in this report.
As used in the above, “person acting on
behalf of the Commission” includes any em-
ployee or contractor of the Commission, or
employee of such contractor, to the extent
that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
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SEP 1 6 1965
Conf.651101-16
PREDICTED AND OBSERVED NUCLEAR CHARACTERISTICS
of
THE MOLTEN SALT REACTOR EXPERIMENT*
VUN
$
J. R. Engel, P. N. Reubenreich, and B. E. Prince
Oak Ridge National Laboratory
.
The MSRE is a unique reactor incorporating features of
proposed molten-salt thermal breeders. Its fuel is a mixture
of fluoride salts which circulates at 1200 gpm through a core
of channeled, unclad graphite. Although the mechanical, chemi-

T. IT
cal and metallurgical aspects of the MSRE rest on a broad base
of technology reaching back to the Aircraft Reactor Experiment..
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paratus,
plogna or contractor of the Commission, or
tes, or pronous mocow to adoration permat to Waploss of contract
mogna a contacter of the Commission, or aplogue of met woordtot, to the extent that
na wod ta dhe above, pesma notte a hall at the counterton" module nga
ong wedoration, appunta, method, or prosous droloond na
, or wetlas of the detonado contatood ta
modo o proceedinoloned to this report
progaredu a noun o Governo
A. Makes mytuty of mortados, presund or implied, more poct to the moor-
tan, nor do Condeston, nor my person methog a boat the Coundation
LEGAL NOTICE
ce to the wool, omni tempus rudthung trom the
plogu o mal
de reports
de roparts of t
sponsored worth Wetter to Dettol
tractor proper
....................... io...
to tetrting
he wo
(1954); the predicted nuclear characteristics were untested by
experiment before the reactor began nuclear operation on June 1,
1965. Experiments carried out in June thus tested the ability
of today's calculational methods and neutron data to predict
.
the characteristics of an unusual new system. Results are
RELEASED POR ANNOUNCEMENT
IN NUCLEAR SCIENCE ABSTRACI'S
gratifying.
.
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.
In the first nuclear experiment, the 2350 was brought up
E.
to the minimum critical concentration at 1200°F by the addition
of 235UF 4 - LlF concentrate to carrier salt. (For experimental
purposes the carrier salt contained 0.5 mole % 238UF4.) The
critical 2350 concentration had been predicted by calculations
i
.
using a multigroup, one-dimensional diffusion code, MODRIC,
with thermal group cross sections obtained from cell calculations
-
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by the THERMOS code and fast group cross sections calculated
U
by GAM-2.
The geometrical approximations in MODRIC were checked
:
Research sponsored by the U. S. Atomic Energy Commission
under contract with the Union Carbide Corporation.
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by using a two-dimensional, two-group code, Equipoise-3, with
group cone tants for each region from MODRIC. The calculations
predicted that 65.1 kg 2350 in the 1980-liter circulating
system would make the reactor critical at 1200°F, with fuel
not circulating and with rods withdrawn. Criticality was
first attained with 61.4 kg 2350 at 1181°F and one rod slightly
inserted. Corrected to the reference conditions, the critical
loading would have been 61.5 kg, 5.5% below the prediction
(equivalent to 1.4% 8k/k error).
Subsequently 79 capsules of concentrate, each containing
858 235U, were added to the circulating fuel. At the same
time measurements were made of concentration coefficient of
reactivity, temperature coefficient, pressure coefficient and
cortrol rod worth.
Total worth of the three control rods was predicted by
Exterminator, a multigroup, two-dimensional code, using four
groups: a thermal group, a fast group and two epithermal groups
covering the important gadolinium resonances. (The poison
elements are G4203 - Al2O3.) The ratio of the worth of one rod
to the worth of three, and the fractional worth of partially
inserted rods had been computed by Equipoise-3. These ratios
were used with the total worth from Exterminator to give a
predicted worth over their operating range of 5.16% 8k/k for
.
three rods and 2.18% $k/k for one rod with the others withdrawn.
The identical value, 2.18% 6k/k, was obtained for one rod by
integration of rod sensitivities measured during the 2350
additions.
m askintetani di natura
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ndecske
a
mutl
cara
neko merkatar anda hana humana
The addition of the capsules showed a 235U concentration
coefficient of reactivity of 0.25 (8k/k)/8c/c); 0.248 had been
calculated by the MODRIC criticality searches.
The temperature coefficients of reactivity were computed
by multigroup calculations in which GAM-1 was used to calculate
slowing down spectra and THEROS was used in cell calculat:lons :
which separated the effects of the salt and the graphite on the
thermal spectrum. Results were -3.7 x 10-5 °F-2 for the graphite,
-3.3 x 10-5 °Fºl for the fuel salt, or -7.0 x 10-5 °F~1 total.
The observed coefficient was -7.6 x 10-5 °F~1.
A spray device in the fuel pump bowl (used to strip xenon)
results in gas entrainment in the circulating fuel when the pump
bowl level is abnormally low. This gives rise to a pressure
coefficient of reactivity. In a special experiment with 2.5
vol % gas in the core, the pressure coefficient was found to be
l1 x 10–5 psi-?, consistent with a value of 6 x 10-5 ps1-4 pre-
dicted for a bubble fraction of 1.2%. At the normal operating
level in the pump, a gamma-ray densitometer and other evidence
indicated no bubbles in the salt and there was no measurable
effect of pressure on reactivity.
Circulation of the salt reduces reactivity because many of
the delayed neutron precursors decay outside the core. (Transit
times are 9.4 sec in the core, 15.8 sec in the rest of the
loop.) . The predicted effect was -0.30% 5k/k; the observed
change with circulation is 0.23% Sk/k. The difference is
attributed to inadequacies in the calculational treatment of
tretman.
na
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delayed neutrons emitted in the upper and lower heads of the
reactor vessel.
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10/ 20 / 65
DATE FILMED
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