ORIG. ENGLISH: EXPERIMENTAL STUDY OF SOME METHODS OF COMPENSATING EXCESS REACTIVITY IN INTERMEDIATE REACTORS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP88-00904R000100110022-5
• Release Decision: RIPPUB
• Original Classification: U
• Document Page Count: 21
• Document Creation Date: December 22, 2016
• Document Release Date: August 26, 2009
• Sequence Number: 22
• Publication Date: May 1, 1964
• Content Type: STUDY

 Approved For Release 2009/08/26: CIA-RDP88-00904R000100110022-5
Third United Nations
International Conference
on the Peaceful Uses
of Atomic Energy
Confidential until official release during Conference
EXPERIMENTAL STUDY OF SOME METHODS OF
COMPENSATING EXCESS REACTIVITY IN INTERMEDIATE
REA CTORS.
A/CuIJ1 t;/i'/s",
uasil
(Jriginul; ENGL1Sli
V.A.Kuznetsov, A.I.Mogilner, L.A.Chernov, V.V.Chekunov,
V. I.Sharadin.
The work which is in part discussed in this paper aimed at
analysing, in a comparative way, three possible methods of compen-
sating excess reactivity in an intermediate reactor so as to de-
termine their potentialities, advantages and drawbacks:
a) by means of a system of absorbing rods;
b) by means of control cylinders placed in the side reflec-
tor with their surface partially covered by a layer of absorber
(See Refs. I and 2);
c) by separating two halves of the core in height.
No generality is chaimed for the absolute values of reactivi-
ty changes and distortion of the fields cited in the paper due to
the obvious fact that such values are dependent on the size and
composition of the core and reflectors as well as on the neutron
spectrum in the reactor.
The experiments described below were carried out on the
PP-4 assembly designed for studying the physical characteristics
of intermediate reactors; the detailed description of this assem-
bly is given in Ref. (1).
Reactivity changes were measured during the experiments by
the reverse counting method and by method of increasing the core
height.
The first method was used in subcritical experiments. It is
based on a well-known equation which relates the changes in the
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counting rate of an external detector (N1-N2) to the changes of
reactivity (e K) in a subcritical system with a nouree :
4K= C ^-?' _ )V.I.
JV1.Nt x'11
Coefficient C was found with the help of a reactivity standard -
one of the regulating rods of the reactor. In the course of the
experiments the value of C varied quite substantially due to
the distortions of the neutron fields in the reactor. To elimi-
nate the resultant error C was measured for e.aoh condition of
the system.
Under the second method the negative reactivity given to a
critical reactor was mainly compensated by addition to the core
of fuel and moderator layers (in the same ratio as in the core)
previously calibrated in the units of reactivity, and, to a les-
ser degree, by small displacement of the regulator.
The neutron flux distribution in the core was measured with
the help of a small-size fission chamber having an outer diame-
ter of 7 mm. The measurements were made at a power level of
about 25 mW maintained by automatic regulator APPM (Ref.1), the
ionization chamber of the regulator being used as a monitor.
The small diameter of the fission chamber enabled it to be placed
between the tubes so as to minimize the field distortion in mea-
surement s.
Experimental Investigation of Mutual Effect (Interference)
of Absorbers x).
A considerable amount of excess initial reactivity has of-
ten to be compensated by a large number of control rods-absor-
bers. Physical investigation of such reactors is hampered by the
difficulties in determining the reactivity margin, d:ae to mutual
interference of the control rods as a result of which the full
reactivity margin actually differs from the sum of control rod
worths mesured separately in an initially critical reactor.
A special experimental study was undertaken on the PF-4
assembly t o =: = e st igat e the mutual effect (interference) of two
absorbing control rods in uranium-beryllium assembly (PF-4-16)
x)The work was participated in by Y.G.Pashkin.
375 -2-
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without reflector ( / ( n 75).
gc v s
The cylindrical control rods 20 mm in diameter contained bo-
ron carbide with boron density of 1.7 gr/em3. One control rod
(No.1) was placed near the centre of the core, while the other
was capable of radial displacement. Measurements were carried out
in subcritical experiments.
The interference term was measured using the following tech-
nique. First the worth P10 of one rod (rod No.1 , for example)
was measured in the absence of the other rod; then the worth f 12
of the same (lst) rod was measured in the presence of the other
rod. The interference term JO 12 describing the effect of the se-
cond rod on the worth of the first was found from the relation:
(2)
The worth of the second control rod in the presence of the
12 ' P10 + Q 12
first is equal to:
f021520 + 21
(3)
The total worth of two control rods 1+2 is equal to:
_ jj?4j(J4f+, (4)
whence '0 = jA +t
21 I2 p i t p P o o + ?a, Uz
When studying the interference a relative unit of reactivi-
ty was used; this unit represented the worth of the assembly re-
gulator per centimeter of the linear portion of its characteris--
ties. The results of the measurements are summarized in Table 1.
The results of the experiments were analysed on a simple
assumpt.-on that if 12 is relatively small (4 ' f4 ; J ), its 112 value is proportional both to _P 10 and to J 20' the proport io-
nality factor depending on the distance X 12 between the control
rod axes:
'612 J 10 20 (x12) (5)
Fig.1 shows the experimental points which were calculated
on the basis of the data given in Table 1 with the help of rela-
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tion (5). The analysis of these data prompts the following conclu-
sions :
a) the perturbation introduced by the central control rod
extends to the entire reactor;
b) when distance between the control rods is small their to-
tal effectiveness is below the sum of unperturbed worths of these
control rods;
c) beginning with a certa,..-~l distance (in the given case it
is 164 mm) the interference term changes its sign;
d) the dependence of the factor f(x) -A 1211010 ?20 on
the distance is almost linear. Deviation of the proportionality
factor f (x) from the linear dependence towards higher values of
d (x) at short distances may be explained both by the fact that
in this case the inequality b .P is satisfied in the least deg-
ree and by the fact that the worth of the second control rod is
additionally decreased by the shadow effect of the first rod.
T a b 1 e 1.
M
Distance between 'ontrol 20 36 73 109 1/i6 182
rods (_mm).
Efficiency of a single 216 2,13 191 148 110 77
Tod _(linear cm).
Inter-ferentional term -95 -67 -34 -21 - 8 3
-642-(linear cm).
The conclusion cited above refer to two control rods one of
which is capable of radial displacement. However, the applicabi-
lity of these conclusions is much wider. Fig.2 gives the data ob-
tained on PP-4 assembly (with the same ratio fe.1 assemb-
ly PF-4-17) with the excess reactivity compensated by a group of
control rods. In analysing the results of the experiments it was
assumed that the combined effect of group of control rods on one
of the rods can be treated as superposition of the effects of
each rod in the group. In other words, it is assumed that if &4