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NOV 2 9 1966
CRITICAL EXPERIMENTS WITH A MOCK-UP OF THE
REPETITIVELY PULSED REACTOR SORA*
1
G. Kistner** and J. T. Mihalczo
Oak Ridge National laboratory
Oak Ridge, Tennessee
H.C.S 1.00
.50
ABSTRACT
A series of critical experiments has been performed on a close mock-up
of the SORA reactor. SORA 18 a Nak-cooled, repetitively pulsed fast reactor
which will be used as a high intensity neutron source for tima-02-flight ex-
periments. The reactivity of this reactor is periodically varics by means
of a movable reflector. Those parameters which are related to the kinetics
of the reactor have been investigated thoroughly in the critical experiments.
The total reactivity value of the movable reflector determines the
ratio of the peak power to that between pulser. The reactivity of the
movable reflector as a function of its position and the prompt neutron
decay constant determine the half width of the neutron pulses. These pere-
meters have been measured for beryllium and for iron reflectors of several
sizes and for various core and reflector configurations. It has been showni,
for example, that a 24-cm-high, 7-cm-thick, and 21-cm-wide beryllium re-
flector operating under reactor design conditions will produce pulses about
50 usec wide with a peak-to-minimum power ratio of approximately 4 x 10".
*Research sponsored by the U. 5. Atomic Energy Commission under contract
with the Union Carbide Corporation.
**On assignment to Oak Ridge National laboratory from EURATOM, Lepra, Italy.
LEGAL NOTICE

RELEASED FOR ANYCOMINI
IN NUCLE&F SCIENOS GESIRACTS
This report was prepared as an account if Government sponsored work. Neither the United
States, nor the Commission, nor any person acting on behalf of the Commission:
A. Makes any warranty or representation, expressed or implied, with respect to the accu-
racy, completeness, or usefulness of the information contained in thio report, or that the use
uf any information, apparatus, method, or proceso disclosed in this report may not fafrige
privately owned righto; or
.
B. Assumes any liabilities with respest to the uso of, or for damages rosulting 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" Inoludes any em-
ployeo 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,
disonninates, or provides access to, any Information pursuant to his employment or contract
with the Commisslon, or his employment with such contractor.
Lindsay untament
mirrorpinu
INTRODUCTION
A program of static critical experiments has been completed at the
Oak Ridge National Iuboratory Critical Experiments Facility in cooperation
with EURATOM supporting the design of SORA (Sorgente Rapida ).
SORA' is a fast, liquid metal cooled, repetitively pulsed reactor,
which will be used as a high intensity neutron source for time-of-flight
experiments. The core volume is about 5 liters, the fuel is highly en-
riched uranium and the mean power is planned to be 1 MW ultimately. The
reactivity is periodically varied by a movable reflector which passes by
one side of the core and increases the reactivity above prompt critical
for ~100 usec with a repetition rate of 50 pulses per second.
This report emphasizes the measurements related to the kinetics of
SORA.
DESCRIPTIO"? OF THE ASSEMBLY
The mock-up of SORA, shown in Fige. 1 and 2, closely resembled the
reactor: design. It differed principally in the use of iron to represent
NaK and of uranium instead of uranium-molybdenum alloy as fuel. The core
of the mock-up was a hexagonal shaped iron natrix containing unclad uranium-
metal rods with a 2350 enrichment of 93.18 wt%. The rods were 1.379 cm in
diameter and 24 cm long and were arranged in a triangular pattern with a
1.630 cm pitch. With this geometry the core volume fractions were 0.649
U, 0.344 Fe, and 0.007 void. The core was mounted between a 17.6-cm-thick
bottom reflector and a 16.3-cn-thick top reflector, both of iron.
A 28-cm-thick stationary iron reflector, containing six control rods
and two 14.1-cm-diam scatterers (used for moderating neutrons leaking out

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BLANK PAGE
ORNL-DWG 66-3244
-88-
CONTROL RODS7
MOVABLE
REFLECTOR
BERYLLIUM
DIPON
CORE
DE CORE MATERIAL
FIXED STEEL
REFLECTOR
PRIRED STEEL _
e
Ø POLYETHYLENE
SCATTERERS
DIMENSIONS IN cm

the horizontal beam holes ) enclosed five sides of the core. Adjacent to
the sixth side of the core a movable beryllium or iron reflector with a
height of 24 cm, a thickness of 7 cm and various widths ranging between
11 and 26 cm could be positioned. The reflector block was mounted on an
arm which could be moved remotely through + 450 from its central position
for purposes of reactivity calibration. The core was mounted on the
movable platform of the ORNL Critical Testing Unit (CTU) and was raised
into the stationary reflector by a pneumatic-hydravlic mechanism in the
approach to criticality. Figure 3 shows the core mounted on the CTU in
a position withdrawn from the reflector.
Eighty-six rods, containing 56 kg of uranium, were required for
criticality.
DISCUSSION AND RESULTS
The first parameter, related to reactor kinetics, is the total re-
activity value of the movable reflector. This valve determines the ratio
of the power in the pulse to that between pulses from the following for-
mula:
beleid
where
Po = pulse peak power,
P = average power between pulses,
o = total reactivity value of the movable reflector, dollars,
0 = half width of neutron pulse, sec,
f = pulse repetition rate, sec .

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With a reflector reactivity of 5 dollars, which has been set as a lower
acceptable design limit, about 80% of the power is developed within the
pulses. The values of this reactivity measured in these experiments
ranged between 6 and 12 dollars for beryllium reflectors and between 4
and 7 dollars for iron reflectors om various sizes.
The second parameter related to the kinetics describes the reactivity
of the movable reflector as a function of its position. This dependence
on position is parabolic and is characterized by the constant in the equa-
tion for the parabola. It is designated by a The third parameter is
the prompt neutron decay constant. These two parameters determine the
neutron pulse shape and, in particular, the half width of the pulse. The
pulse width of SORA has been estimated using an expression obtained from
one group, space independent kinetics with the approximation used by
Bondarenko and Stavisskii,
1.67
1
covje een
where
a = constant of the parabola describing the reactivity of the
movable reflector, dollars/cm,
On = prompt neutron decay constant, sec,
V = linear speed of the movable reflector, cm/sec,
€
= superprompt critical reactivity, dollars.
a. The reactivity measurements were performed with an analog computer
which has as input the current from a BF, ionization chamber responding
power changes in the assembly produced by reactivity changes.
The value of E is obtained as a finction of
V, and f
from a conditiør* which states that the mean power during repetitively
pulsed operation remains constant in time:
was exp ( 1.3 V8 D-1-tt.
In order to obtain
the reactivity of the movable reflector has
been measured as a function of the linear displacement (arc length) from
the position of maximum reactivity. The data were fitted to a parabola
in the range important to pulsed operation. If (0) is the maximum re-
activity, the reactivity at position x is:
P(x) = p(0) - Qux? .
The measured values of Q, were between 8 and 10 cents/cm for beryllium
reflectors and between 3 and 6 cents/cm² for iron reflectors.
The prompt neutron decay constants have ieen obtained by the Rossi-a
technique using instrumentation described previously.” For assemblies
with beryllium reflectors a decay constant of 0.3 x 10° sect, which corres-
ponds to a lifetime of 2.2 x 10-8 sec was obtained. The decay constant of
assemblies with iron reflectors was 0.5 x 100 sec , corresponding to a
lifetime of 1.3 x 10-8 sec.
The results, listed in Table 1, show the feasibility of the SORA
design, and indicate that pulse widths will be between 40 and 60 usec.
They also show that the reactivity values of movable reflectors will be
larger than 5 dollars in a variety of configurations.
T Mass of
Uranium
in Core
(kg)
Table 1. Parameters Determining the Pulse Characteristics of SORA
Prompt
Predicted
Movable Reflector
Fast Neutron Neutron Pulse
Material width Reactivity
Decay Constant Half Width
(cm) (dollars) (cents/cm) 1a, (usec)
o (usec)
5.9
8.6
0.32
8.8
9.5
0.34
11.2
9.7
0.30
3.7
0.55
5.2
0.48
4.0
0.49

57.2
54.0
5.3
51.2
57.7
56.4
4.2
6.5
55.2
&. The height and thickness of the movable reflector were l'ixed at 24 and 7 cm,
respectively.
b. These values, assume a pulse repetition rate of 100 sec® and a reflector speed
of 2.83 x 104 cm/sec.
For these experiments the polyethylene scatterers and the berylliun con crol
rods in the reflector were replaced by iron.
10
In addition to the measur ments related to the kinetics, the re-
activity of various parts, the reactivity coefficients of several materials
·in the core, and the reactivity temperature coefficient of the core have
been measured and will be reported elsewhere. .
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·
11
REFERENCES
.1.
2.
3.
4.
V. Pievski, "The Pulsed Fast Reactor as a Source for Pulsed Neutron
Experiments," in Proceedings of a Symposium on Pulsed Neutron Research,
Karlsruhe, May 10, 1965, Vol. II, p. 553, IAEA, Vienna (1965).
M. Stievenart, Beligo-Nucleaire Report BN 6203-25 (1964).
C. A. Sastre, Nucl. Sci. Eng. 8, 443 (1960).
I. I. Bondarenko and Yu. Ya. Stavisskii, Reactor Science and Technology
14, 55 (1961).
J. T. Mihalczo, Nucl. Sci. Eng. 20, 60 (1964).
5.
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