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NATIONAL BUREAU OF STANDARDS - 1963
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Con7-661001-18
OPTIMIZATION OF REACTOR SHUTDOWN MARGIN MEASUREMENTS IN HIGH
Tr
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URSIL - ACC-urricio
GAMMA FLUXES *
CFSTI PRICES
ORNI - AEC - OFFICIAL
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F
D. P. Roux
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AUG 10 1966
Oak Ridge National Laboratory
Oak Ridge, Tennessee
H.C. $ 1.00;
Since the quality of reactor shutdown margin measurements made by
neutron noise analysis is degraded by high gamma-ray intensities produced
by residual fission products, a study was made to derive an equation
that can be used to quantitatively predict this degrading effect. In
this equation are indicated the significant parameters controlling the
ratio of the signal to background. With this information ionization
chamber characteristics were optimized.
Using a point reactor model approximation, one can derive the
measured Power Spectral Density (PSD) associated with the fluctuating
current of a neutron-sensitive ionization chamber.,2 Because of the
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statistical nature of the detection process, an uncorrelated detector
neutron noise Q. is superimposed upon the correlated neutron population
fluctuations. The ratio of the correlated to the uncorreiated PSD can
be expressed as follows, using Stegemann's formilation,' and taking in
account a one-lumped delayed neutron group of decay constant ili

... RELEASED FOR ANNOUN CITU TANT
IN NUCLEAR SCIENCE ABSTRACTS
where
B = effective fraction of delayed neutrons,
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= reactivity in B units or dollars,
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TC
ORNL - AEC - OFFICIA!
*Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation,
ORNL - AEC - OFFICIAL
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LEGAL NOTICE
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& = prompt neutron lifetime.
TWroport mu prorood u an account of Government sponsored work. Neither the Vallad
Sala, por the Commission, nor any parroa acung on behall of the Commission:
A. Makes any warrwty or representation, exprossed or implied, with respect to them accu-
racy, complotonets, or wotulacus of the information contained in the report, or that ibo use
of way lalormation, apparatus, metbod, or process declosed le Whio report may not infringe
privately owned rights; or
B. Asmumos vay liabluues with respoct to the wo of, or for damages resulting from the
use of any laformation, apparatus, matbod, or process disclosed in this roport.
As wand in the above, "person ucting on behalf of the Commission" includes way em-
ployee or contractor of the Commission, or employce of such coatrictor, to the oxtent that
such employee or contractor of the Commission, or employee of such contractor preparos,
dlukeminates, or providos uCCOsı 63, way Information pursuant to his employment or contract
with the Commission, or his employment with such contractor,
ORNL - AEC - OFFICIAL :
The symbol E is defined as
E = WX,6-2R,
rr-!!
!!!
where
W = efficiency of the detector,
R = factor accounting for the statistical fluctuations of the
ionization phenomena,
X = number of neutrons per fission.
The ratio of c/u (critical) to c/u (subcritical) 1.8 approximated for :
12 «w? « (8/2)2 as follows:

.
Thus, by measuring c/u for p = 0 at w, and calling it E meas, c/u can
be predicted for any subcritical state using
(3)
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:
The approximation determined by use of Eq. 2 is excellent if w, is well
chosen. For example, using the parameters 1, l, and ß determined by
Ricker2 at the PCA, the approximation at 1 cps 1s within 3% for any p
to -10 dollars. The value of Eneas at reactor critical is an indication
of the chamber performance for a given location in the reactor.
In presence of a gamma flux background, an additional frequency-
independent background Q also due to the statistical nature of the
garoma detection process occurs. Since at shutdown, this gawna flux
background is uncorrelated to the neutron population fluctuations, Q,
=
.
.
ORNI - AEC - OFFICIAL
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can be taken into account in Eq. 3 as follows:
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a
©??!!
meas
Cin
illr-!!!!!
ORNI - AEC - OFFICIAL
where Q, and Qn are proportional to the gamma and neutron fluxes
respectively ( = 5.5.Q.R, where s 18. the detector sensitivity and ī is
the average charge released per detection event). Tierefore, Eq. 4 can
be rewritten as
*
***
IU
(5)
' Holtz
or lo
,
----
mm
where Z in nv-hr/r units is a value characteristic of the chamber used.
The term (1 + 02/0) represents the degradation resulting from the gamma
background. The value of 2 can be as low as 2 for optimized chambers,
and If it is so, c/u is not appreciably degraded as long as , < (this
emphasizes the neec for a strong reactor neutron source). When
, > , the degradation factor approaches 0,2/0, and the ratio clu
becomes nearly proportional to (1 - p) -2.0-2 or 0-3, being proportional to
0-2. Consequently, if the gamma background is too high for an accurate
measurement of a given subcritical reactivity, the accuracy will be
improved considerably with a slight increase in reactivity. E can still
be measured at critical, even in presence of high gamma backgrounds, 11
the fission rate is high enough such that go » 0,2.
These considerations apply to single detector experiments. When
the outputs of two detectors are cross correlatea', partial cancelation
of both Q, and Q., results, and the ratio Qvdan will not change; the
improvement appears as an increase in Emeas only.
ORNL - AEC - OFFICIAL
:-
As a result of this analysis, an effort WELB made to obtain an ionization
• ORNL - AEC - OFFICIA!
· ORNL - AEC - OFFICIAL
chamber with maximum E/Z ratio. The best chamber was found to be an
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RSN-76 boron-lined chamber filled with "He at a pressure of 2 atm. For
-.-.
a given reactor configuration, the E/Z ratio obtained for this chamber
was slightly better than that obtained for an RSN-252 chamber that con-
sisted 01 34 individual 1/4-in. OD elements and was designed especially
for measurement of reactor shutdown margin in a high gamma flux.Both
chambers have similar external dimensions.
LE
L.
- NE
ORNL -- AEC - OFFICIAL

References
1. W. Seifritz, D. Stegemann, W. Väth, Two-Detector Cross Correlation
Exteriments in the Fast Thermal Argonaut Reactor Stark; paper to be
published in the Proceedings of the International Symposium on
..
2
Neutron Noise, Gainesville, Florida, February 14-16, 1966.
4
RE
2. C. W. Ricker, S. K. Hanauer, and E. R. Mann, Measurement of Reactor
S
Fluctuation Spectra and Subcritical Reactivity, ORNL-IM-1066
(April 1965).
3. M. A. Shultz et al., Shutdown Reactivity Meter Development,
Phase II, Operation in High Gamma Fields, NYO-3122-35 (Dec. 1965).
.
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END
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10/10 /66
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