Additional Flux Measurements on the Raleigh Reactor
5 pp.
November 1, 1954


A. W. Waltner
I. E. Daytona
November 1, 1954


During the past several weeks a series of flux measurements have been made on
the Raleigh Reactor. These measurements followed no comprehensive plan but were
rather intended to supply information useful to anyone working with the reactor.
Unless noted, conventional foil activation techniques ware used and will not be
discussed further.

1. Effect of Poison Near PCP Chamber. A BF3 counter was set up opposite beam hole
No. 15 to monitor the actual power level of the reactor. In beam hole No. 13, the
hole nearest the controlling PCP, a volume 10.5 cm long was left empty and than
filled successively with graphite and with textile samples, To within 1% counting
statistics the reactor power level remained the same in all three cases. A weekly
absorbing sample or a void thus exerts a negligible effect on the power level
calibration. The reactivity is, of course, altered by the presence of either a void
or a sample.

2. Neutron Flux at Sample Positions. In order to make possible more accurate
calculation of exposures, relative flux measurements were made at the positions of
all samples then in the reactor. Those data are not of general interest since they
apply only to the particular loading. They are omitted from this report but are
available to anyone who is interested. It is of interest to note that with these
weakly absorbing samples the flux depression is very slight, certainly loss than 25%.

For subsequent measurements all samples were removed from the reactor. The
graphite stringer in Upper No. 8 was made identical to that in Upper No. 7 by
drilling holes for foil holders. The eight sample holes in the inner graphite

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stringers of No. 13 and No. 15 were provided with graphite plugs, thus restoring the
reactivity to its original value.

3. Flux Asymmetry Produced by Control and Shim Rods. The profile of the neutron
flux in the graphite was measured by means of a set of foils in Upper No. 7 and
Upper No. 8, which are on opposite sides of the core. A control and shim rod con-
which would produce a maximum asymmetry was chosen. A second run with
the same foils in each position and a mirror image of the control and shim rod con-
was made to check the possibility of asymmetries due to poison in a
beam hole. To within the accuracy of the measurement the neutron profiles obtained
in the two runs were mirror images. A third run designed to separate the effects of
shim and control rods was made by balancing the shims so that the reactor was
critical at the same control rod position as used in the two preceding runs. The
results are exhibited in Figure 1. The maximum asymmetry varied from 15% next to
the core to 10% in the last position. With the shims balanced, the safety rod all
out, and the control rod out 4.1", the asymmetry varied from 4% to 2%. This is not
the maximum asymmetry that could be produced by the rods alone, since the shims
could be raised and the control rod run in. It might be valuable to extend those
measurements over a series of control rod positions.

4. Neutron Flux in Beam Hole. In order to design experiments using an external
neutron beam it is necessary to know neutron fluxes and cadmium ratios under various
conditions. Measurements were made in No. 13 with two, one, and no graphite
stringers in position. The results were normalized to unity at the center of the
core and are summarized in Table I. It should be noted that all those fluxes were
measured with bare In foils and hence include both thermal and epithormal neutrons.

5. Glory Flux by Wire Activation. The measurement of neutron flux in a region
which has sharp discontinuities requires a more continuous record than that
ordinarily available from foil data. In the reactor core there arc two such
discontinuities near each other, the surface of the fuel and the top of the reactor

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Table I. Beam Hole Measurements
2 graphite
1 graphite
no graphite
Center of core 1.00 1.00 1.00
Edge of core .42 .43 .35
End of 1st graphite .118 .093
End of 2nd graphite 4.7 x 10-3
Beam hole opening 7.6 x 10-6 3.9 x 10-5 1.17 x 10-4
In/ Cd/In/Cd -1]
at beam opening
11.5 5.5 3.2

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vessel. To measure the flux through this region Ca wire was placed along the axis
of the glory hole and activated. The counting was delayed until the 5.2 min Cu66
activity became negligible leaving only the 12.9 hour Cu64 activity. The wire was
counted in a load shielded GM counter assembly which exposed about 1/4" of the wire.
Counts were taken at 1/4" intervals.

The resulting flux profile showed the expected general behavior but exhibited
unexplained "fine structure". Confidence in the results is also diminished by the
fact that it was difficult to reproduce a given point--rotation of the wire seemed
to have some effect. The copper thermocouple wire used was presumably pure and had
desirable nuclear properties, but its physical properties were unsatisfactory in
that it was very soft, kinked easily, and was probably non-uniform.

In order to obtain a wire with more desirable physical characteristics, a
number of likely samples were irradiated to check their nuclear properties. Of
those tried, pure nickel and piano wire both exhibited a single half life of suit-
magnitude. Constantan, nichrome, tinned copper, and pure copper all seemed
less desirable.

If this technique were developed farther and automatic data recording intro-
, it might prove valuable in other flux measurements.

We would like to thank Mr. C. W. Terrell for assistance in the foil counting
and Professor E. J. Brown for help in plotting the data.


aVisiting Fellow from the Atomic Energy Division of the Babcock and Wilcox Company,
Akron, Ohio.