ORNL-CF-65-10-18.txt

UCN-2383
(3

 

11.60)

OAK RIDGE NATIONAL LABORATORY
Operated by

UNION CARBIDE NUCLEAR COMPANY

Division of Union Carbide Corporation

=

 

Post Office Box X

Oak Ridge, Tennessee

Internal Use Only

 

 

ORNL
CENTRAL FILES NUMBER

 

65-10-18
DATE: October 29, 196Y | COPY NO. /
SUBJECT: Results of lleutron Fluctuation Measurements Made During
the MSRE Zero-Power Experiment
TO: Distribution |
FROM: D.N. Fry and D.P., Roux

ABSTRACT

Neutron fluctuation measurements were made during the MSRE zero-power
experiment as part of the dynamic tests requested by the project. A first
attempt to measure subcritical reactivity was unsuccessful, because the
detection efficiency of a l-atm BFy ionization chamber installed in the
nuclear instrument penetration was too low. A 2-atm BF3 chamber was in-
stalled in the thermal shield, and with the reactor operating at 10 w, 1t
was determined that the effective delayed-neutron fraction Be increased
1.58 + 0.21 when fuel circulation was stopped. From measurements made
with a low pump-bowl level, it is believed that neutron iluctuations are

strongly affected by entrained gas in the i1uel salt.

NOTICE

This document contains information of a preliminary nature
and was prepared primarily for internal use at the Oak Ridge
National Laboratory. It is subject to revision or correction
and therefore does not represent a final report. The information
is not to be abstracted, reprinted or otherwise given public
dissemination without the approval of the ORNL patent branch,
Legal and Information Control Department.

 

 
CONTENTS
Page
Abstract ..............,................;.,......,.............. 1
Description Of TNSETUMENEALION +eureneeseesensossnssasensaasenss L
Measurements with Detector in Nuclear Instrument Penetration ... L
.Nbasurements with Detector in Thermal Shield .cccccsveecveeescscs b
CONCLUSLONS o v v eceoosesacoscossesssoscssossssssossssoscsoansseose Ll

Recomendations 000.0.0.00.0.CO‘.O..OO..0‘.00'0000000...‘000.0..0. ll

 
DESCRIPTION OF INSTRUMENTAT ION | —

The current fluctuations from a 2-in. BF; ionization chamber output
were amplified with a wide-band ac amplifier [ORNL model Q-2591) and then
recorded on magnetic tape by a Precision Instrument, PS 200 A, tape recorder.
The recorded signal was later analyzed using a multichannel spectral-
density analyzer. | 4 | |

MEASUREMENTS WITH DETECTOR IN NUCLEAR INSTRUMENT PENETRATION

A l1-atm BF; ionization chamber was installed in the MSRE nuclear
instrument penefration (Fig. 1). A l-hr tape record was made for each
major fuel addition during the approach to criticality and with the reactor
critical at an estimated power of 10 w. :

The chamber location was not favorable for subcritical reactivity
measurements since a detection efficiency (that is, the ratio of the neutrons
detected in the chamber to the total neutron population in the core) of at
least 10-5 is required for this gype of measurement. We estimated the de-
tection efficiency to be 1 x 10™°. When the detection efficiency is too
low, the observed spectrum will be constant with frequency and will contain
no information about the reactor. Owing to insufficient detection effi-
ciency, we were unable to measure subcritical reactivity by use of the
neutron fluctuation technique. N

A neutron spectrum of the reactor while critical at a power of 10 w
and with the fuel circulating (Fig. 2) 1llustrates the detection inef-
ficiency. Although there appears to be some reactor information at fre-
quencies below 3 cps, the spectrum, for the most part, shows a constant
output over the frequency range of 0.1 to 50 cps.

To improve the measurement, a second test was made with a chamber
more sensitive to neutrons which was installed in the thermal shield of
the reactor.

MEASUREMENTS WITH DETECTOR IN THERMAIL SHIELD

A 2-atm BF, ionization chamber was installed in a vertical penetration
Of the thermal éhield at the midplane of the core (Fig. 1l). The measured
overall detection efficiency was increased by a factor of 25, of which 1.7
was due to increased neutron sensitivity of the chamber.

 

lC.W. Ricker, S.H. Hanauer, and E.R. Mann; Measurement of Reactor
Fluctuation Spectra and Suberitical Reactivity, ORNL-TM-1066 (April 1965).

 

 
ORNL DWG. 65-12627

Chamber

  

 

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Thermal Shield —
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ELEVATION

Fig. 1. BFs Ion Chamber Locations in Initial Critical Experiment.
10°13 | ORNL IMG. 65-12628

lo‘lk

» o e we

(amp-sec)

&(r
<>

100

10‘16 \

0.1 | T | 10 | | 100
FREQUENCY (cpe) |

 

Fig. 2. Measured Neutron Power Spectrum Normairized to dc
Current in Chanber. The ion chamber was in position No. 1 (see
Fig. 1), and the reactor power was 10 w.
With the reactor operating at 10 w, data were collected for three
different conditions: (1) with the fuel not being circulated, (2) with
the fuel being circulated, and (3) with the fuel being circulated but with
the fuel in the pump bowl at a low level.

The measurements with the fuel not being circulated and then being
circulated were made to determine the change in the effective delayed-
neutron fraction B.

A mathematical model based on point reactor kinetic equations for a
zero-power critical reactor, assuming one average delayed group of neutrons,
yields an expression for power spectral density of the form

3(e) = SN&Z | <A/B >2 - (1)
V() ("L - bor)

where

C = a constant,
A = neutron generation time,

Be = effective delayed neutron fraction,
X = effective one-delay group neutron precursor decay constant,

w = frequency in radians per second, |
Q = uniform spectral component attributed to the neutron detection

process, |

e = detection efficiency,

N = mean neutron density in the reactor.

This model 1s valid only for the reactor at zero power (no thermal feedback).

From a least-squares fit of the parameters of the model to the experi-
mental data we determined that B, increased 1.58 + 0.21 when circulation of
the fuel was stopped (Fig. 3). ]hdependent measurements using control-rod
calibration showed an increase of 1.47 (ref 2).

Examination of Eq. 1 shows that the shape of the spectrum will not be

~affected by a change in neutron power level. In Figs. 4 and 5 the measured
reactor spectrum minus the constant texrm eNQ is plotted for noncirculating

and circulating fuel, respectively. The large limits of error for the date
taken at higher frequencies are attributed to the subtraction of one number
from another of equal magnitude.

Measurements made with a low pump-bowl level are plotted in Fig. 3.
From these data we infer that neutron fluctuations are strongly affected by
~entrained gas in the fuel salt, although when we made these measurements,
we were not able to obtain sufficient experimental data to quantitatively
estimate the amount of entrained gas in the fuel salt.

2.

 

B.E. Prince, private communication.
1012

ORNL IWG. 65-12629

& - Circulating fuel, lov pump-bowl level

b - Circulating fuel, normal pump-bowl level

¢ - Static fuel, normal pump-bowl level

(aup-aec)

é(r
<>

10

 

1 10 100
FREQUENCY (cpe)
Fig. 3. Measured Neutron Power Spectra Normalized to de
Current in Chamber.

The ion chamber was in position No. 2, and
the reactor power was 10 w.
1013 | ORNL ING. 65-12630

. Theoretical Curve Using
A= 2.9 x 10°%
\ . Be = 0.00676
1071
A= 1,261

   

1016

10 | 100

0.1 ‘ o 1l | | o
FREQUENCY (cps)
"Fig. 4. Measured Neutron Power Spectrum Minus eNQ. The

ijon chamber was in position No. 2, and the reactor power was
10 w with fuel not being circulated.

 
10

10713

 

ORNL DWG. 65-12631

Theoretical Curve Using
A= 2,90 x 107
Be = 0.00428
A = 0.8702
10 |

"fg - e (m-B&(!)i

10715

10716

0.1

| 10 | 100
FREQUENCY (ecps)

Fig. 5. Measured Neutron Power Spectrum Minus ¢NQ. The

ion chamber was in position No. 2, and the reactor power was
10 w with fuel circulating.
-~

11

CONCLUSIONS

- By use of neutron fluctuation measurement techniques during the MSRE
zero-power experiment we have measured the effective delayed-neutron loss
due to circulation of the fuel salt. Also, this method might be used to
measure the amount of gas entrained in the MSRE fuel salt.

RECOMMENDATIONS

We recommend that neutron fluctuation measurements be continued when
operation -of the MSRE is resumed. The following items should be investigated:

l.

Increase the detection éfficiency in the nuclear instrumentation
penetration by using an ion chamber that is more sensitive to

neutrons.

Investigate further the effect of entrained gas in the fuel salt
on the neutron power spectral density.

Determine the neutron power spectral density at various reactor
power levels up to 10 Mw. Our experience at the ORR leads us to
believe that the detection efficiency requirement for measurements
at power are not as stringent as they are at zero-power because
of the additional thermal feedback effect. Therefore, measure-
ments could be taken with the detector located in the nuclear

instrument penetration,

 
O OO~ O\ FHFW P

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ORNL-CF-65-10-18

DISTRIBUTION

Ball
. Borkowski
Briggs
Engel
Epler
Fry
. Gabbard
. Haubenreich
Jordan
Kasten
Kerlin
Kryter
Roux
.S, Stone
J.R. Tallackson
Central Research Library
Document Reference Section
ILaboratory Records Department
ILaboratory Records, ORNL R.C.
ORNL Patent Office |

2

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