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ORNL P
1218
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11450
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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
wATTATEARETTYYTTATIVT METRA
.
224Wo
- ORNG-D-1218
CFSTI PRICES
H.C. $ 1.00; MN 50
MONITORING OF LEACHED FUEL ELEMENTS WITH A NEUTRON GENERATOR*
W. J. Ross, J. W. Landry** and J. E. Strain
MAY 5 1966
*:
Analytical Chemistry Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
.
An apparatus has been designed and constructed to detect milligram amounts
of <?u. Such sin apparatus was needed to monitor the removal of nuclear reactor
fuel from spent fuel elements as part of a proposed process that involved seg-
.
.
.
-
-
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-
.
mentation of the metal cladding and dissolution of the fuel by acid leaches
and aqueous rinses. The segmented cladding produced by mechanically chopping
fuel rod bundles is in the form of an irregular section of tubing about 1-1/4"
long and 3/8" diameter. These segments, or hulls, are leached with an appro-
priate acid that dissolves fuel without attaching the cladding metal. When
all the fuel is removed from the hulls, the hulls are discarded. It the ends
of a segment happen to seal during the chopping operetion, there is a possi-
bility that all the fuel will not be removed in the first leach.
To prevent the loss of significant amounts of 235U, this process required
inspection of a large number of hulls and the means for recycling those that
contained 330 in quantities above a pre-set level. All operations must be per-
formed remotely in facilities that are designed to eliminate hazards from the
high levels of radioactivity associated with the hulls and spent fuel.
This prototype was designed to meet three major requirements: it must be
(1) capable of detecting milligram amounts of 530 in highly radioactive hulls
(2) adaptable to batch-type analysis on a continuous and routine basis; and
(3) amenable to remote-controlled operation with minimum maintenance.

*Research sponsored by the U. S. Atomic Energy Commission under contract
with Union Carbide Corporation.
**Chemical Technology Division
LEGAL NOTICE
RELEASED FOR ANNOUNCENENT
IN NUCLEAR SCIENCE ABSTRACTS
IN
W
.790
NOTITOR PARTE PRELEASES TARIFICIAL
S
*
YTU:
Dewasa
TEST
ShitBCN
W
The report mo prepared as an account of Governmt sponsored mork, Noluber the Vallad
Halo, por the Commioloo, Dor day porn acupy on bubut of the Commission:
A. Mukai vay maraty or representation, expressed or implied, with respect to the accu-
racy, completeness, or wtuloons of the laformation contaloed in the report, or what the wo
of way laformadoa, appenatu, molbod, or procesu disclosed to this report way dol lairinge
printsly owned righta; or
B. AINDAI nay lladillues muroopact to the use of, or for dengan rosulting from the
um of way wormation, apparatus, molbod, or procesu discloud in to report.
Al und in the aboro, "parxa acting oa bebuli of the Commissioo" lactades day om.
ploym or contractor of the Commission, or employee of such contractor, to the orient that
auch taplogn or contractor of the Commission, or employm of much coatructor prepares,
diseminauer, or promido, accosi lo, Lay Information purnant to do employabat or contract
with the Coomisjon, or blo employment with such contractor,
manovasan
wo more
Principle
When 2350 (also 2333 or 239 pu) is bombarded with thermal neutrons, fission
occurs with the production of short-lived nuclei among which are some that emit
delayed' neutrons during their decay. According to Dyer(1), 0.0158 + 0.0005
delayed neutrons are produced per Pission of 2350, and these neutron emitters
have half-lives of 0.23 to 56 seconds. The majority decay with half-lives of
2 to 23 seconds. Subsequent moderatica of these energetic neutrons and mea-
surement of the resulting thermal neutrons affords an analytical method that is
specific for 235u in fuels that do not contain 2333 or 239pu. Segments of the
leached fuel elements can, therefore, be analyzed rapidly and non-destructively
for-590 by irradiating a batch of hulls with thermal neutrons and measuring the
resulting delayed neutrons immediately after the end of the irradiation period.
Description of Apparatus
The initial working model of a leached-bull monitor, as shown in Figure 1,
(Figure 1)
consisted of two major components connected by a 24" tube of 2.5" diameter.
Irradiation Compartment. The upper section is a block of paraffin
(10" X 12" x 28") penetrated by (1) a vertical hole of 2.5" diameter, (2) a
4" horizontal of 3.5" diameter, and (3) a horizontal slit that encloses a movable
aluminum plate (1/8" x 2-1/2" 16"). The target end of a 150-kv, Cockcroft-
Walton neutron generator is placed in the horizontal hole so that the source of
14 MeV neutrons, a titanium tritide target, is separated from the vertical hole
by 2" of paraffin. The lower edge of the tritium target is on the same level
as the aluminum plate.
The aluminum plate functions as a sliding door that holds the hulls in
position during irradiation. At the end of irradiation the door is opened by
means of an electrically operated solenoid, allowing the bulls to fall through
a connecting tube into the detector compartment.
«?.
Detector Compartment. Another block of paraffin (3" x 12" x 28") forms
the lower section of the apparatus. Six 1" diameter holes were drilled to
within 1" of the bottom of this block and spaced so that they were equi-
distant from one another and separated from the edge of the central 2.5"-
diameter hole. by 1.5" of paraffin. Six -UBF, neutron counting tubes
(8" x 1" diameter) are positioned in the exterior holes and connected in
series to a pre-amplifier. The cases of the BF, tubes are connected, in
series, to a grounded, copper screen that is placed around the tubes to
minimize electronic noise.
The remainder of the detection apparatus (power supply, amplifier, and
scaler), as well as the electrical switch used to retract the sliding door,
is located in a seperate room along with the controls for the neutron
generator. The control room is separated from the generator room by concrete
walls 4 feet thick as shielding against neutron irradiation.
Method of Operation
This model was not designed to be operated completely by remote
control systems, therefore batches of hulls had to be loaded and removed
manually. The high voltage supply to the neutron generator was automatically
cut off before the generator room could be entered.
In most of the evaluation studies, the test batches consisted of 20
hulls that were jagged segments of steel tubing approximately 1-1/4" x 3/8"
diameter. Twenty bulls filled the irradiation chamber to a height of 2 inches.
When bulls containing-S9U were being analyzed, one standard and 19 empty
hulls were mixed and dropped into the irradiation compartment.
Each batch was irradiated for 20 seconds with neutrons that were generated
with a beam current of 700-800 lamps and subsequently thermalized in the paraffin
block. At the end of the irradiation, the sliding door was opened so that
the irradiated bulls fell into the detection chamber. A neutron count was
begun 2 seconds after the end of irradiation and was continued for 0.5 minute
to determine the presence of -in the batch of bulls.
Flux Mapping of the Irradiation Chamber
Variations in the thermal neutron flux throughout the volume of the
irradiation chamber filled by 20 bulls were measured with manganese flux
monitors. Weighed amounts of manganese powder were incapsulated in poly-
ethylene tubes and fastened to the sides and base of a 6-oz. polyethylene
bottle. Monitors #1, 2, 3, and 4 were spaced equidistantly 5.5 cm above
the base, monitors A, B, C, and D were 1.5 cm above the base, and #5 was
1.5 cm above the center of the base. The bottle was positioned in the
irradiation chamber so that the base rested on the aluminum door and
monitors #1 and A faced the tritium target. The monitors were irradiated
for 10 minutes and sun produced was determined by beta and gamma counting.
Similar measurements were made after 20 bulls had been added to the poly-
ethylene bottle. The relative activities of the monitors are presented
mil
in Table I.
TABLE I
Thermal Neutron Flux in Irradiation Container
Relative Flux
Without Hulls
With Hulls
Monitor #
#
1.00
1.03
1.03
1.00
0.75
0.76
0.81
oN mp cu A
0.97
0.95
0.90
0.88
0.90
1.21
1.06
0.94
0.96
0.98
1.01
.
-5-
During these irradiations the average neutron output was 6 x 109 n/sec
and the average thermal neutron flux was 10°n/cm?/ sec. Very little axial or
radial gradient was observed in the flux in the absence of hulls. When the
irradiation container held 20 hulls (~2" depth or 150 cc total volume) the
radial distribution of the fļux remained relatively unaffected. Although
the flux in the filled chamber 1.5 cm above the base was essentially the
same as that of the empty chamber, approximately 20-25%. attenuation of flux
was observed at the 5.5-cm level of the filled chamber. These data show
that the counts obtained from 2350 in a hull can be determined within a
variation of + 10-15%.
Evaluation of Monitor
The efficiency of this initial BF, detection system was calibrated with an
<*Am - Be neutron source and found to be only 0.2%. The sensitivity of the
monitor was then determined with weighed amounts of u encapsulated in steel
hulls. This procedure entailed mixing the standard with 19 other hulls and
irradiating the batch for 20 seconds as described above. The counts obtained
with a flux of ~ 10 n/cm²/sec are listed in Table II.
TABLE II
Sensitivity of Monitor
Counts/20 sec()
5,9,6,4,7,6
Sample
Average(a)
Background
(20 bulls)
1 zag 2350
2354
6 mg 2350
100 mg 235
17,15,15
32,47,52,54,48
38, 38,60,58
520,583,686,506,910,662,687
(a) Background not subtracted
Discussion
The data in Table II reveal that the background for such measurements
can be made to be very low and constant il sensitivity is sacrificed to
eliminate electronic noise. The number of counts above background produced
by 1 mg 235U is small but significant - being twice that of the background.
Larger amounts of 330 are very readily detected. These results were
encouraging because they were obtained under less than optimum conditions. .
The neutron yield can be increased to ~ 5 x 200 n/s with a fresh 5-10 curie
target. If the neutron flux increases proportionally to the neutron yield,
the sensitivity would be improved by a factor of ~ 7. The counting efficiency
can be enhanced as much as 25-fold by using matched BF, tubes and improving
the design and shielding of the detector configuration. Increased sen-
sitivity can also be achieved with complete automation by eliminating
the 2-sec delay between end of irradiation and start of counting. This
delay is necessary for manual operation but is detrimental because the major
group of delayed neutrons has a half-life of only 2.3 seconds. In fact, if
greater speed is desired, both the irradiation and counting periods could
be decreased to 5 or 20 seconds without decreasing the sensitivity more than
50%.
This prototype can be readily adapted for remote controlled operation
by elimination of the remaining manu.. operations. The same apparatus can
also be used to detect fişsionable material in solutions. Thus, both bulls
and rinse solutions could be monitored for 235U by a rapid, non-destructive
technique that would not interrupt the remainder of the leaching process.
These improvements are being incorporated in a second more sophisticated
model of this monitor. In addition, several structural modifications are
7-
being made, such as substitution of blocks of polyethylene for parallin as
a neutron moderator and enlarging the volume of the Irradiation chamber.
The final instrument, Figure 2, will employ a smaller neutron generator
(Figure 2)
with a higher neutron yield than the one used in this evaluation program.
-8
Reference
1. Dyer, F. F., Enery, J. F., and Ledaicotte, G. W., ORNL-3342,
October, 1962.
Generator port.
. Sample loading port
.
.

Solenoid
Irradiation
compartment
Rubber
-
Connector
shiling plate
Tujil
- BFg Tubes
Detector
... compartment
i tive.l.
er
Fig.1 - Leached-Hill Monitor
...
.......
......
.
time koncentrat en insatsemise
.
..
..
...
...
.....
po...
........ intern
et.......
UNCLASSIFIED
ORNL-DWG 64-10017
TARGET
CUP CONVEYOR
· NEUTRON MODERATOR,
REFLECTOR, AND SHIELD
(PARAFFIN):
HULLS IN NEUTRON-
IRRADIATION CHAMBER
TRAPDOOR UNIT
ORNL-AEC - OFFICIAL V

RAIL 00
NEUTRON GENERATOR
Grounded case is 18-in.-
diameter by 24-in.-long.
WINDOW
::::
SHAFT (TRANSPARENT)
NEUTRON MODERATOR, .
REFLECTOR, AND SHIELD
· (PARAFFIN)
HULLS IN DELAYED-NEUTRON
COUNTING CHAMBER
NEUTRON-DETECTOR ARRAY
OF SIX BF3 TUBES
TRAPDOOR UNIT.
GATE
:
::
.....
...
.:...
1. 6
CONVEYOR TO RELEACH :
CONVEYOR TO HULL DISPOSAL ,
- PERSONNEL ACCESS AREA 1.
1 WALL FUEL RÉPROCESSING CELL
: 7-36 cm OD-
54 cm : 18 cm
: d et.6 cm ID :
w outside radius*** :
..3 cm .
I k5 cm diam
inside radius
:::::5 cm of paraffin r
S ix holes for BF3 tubes.
Recess for target.
between tube
holes and inner
.. wall of chamber.
30 cm ..
.
.
I
:...
-
..
2 cm of paraffin ...
between end surface :
of recess and inner.
wall of chamber.
IRRADIATION CHAMBER.
..
.. COUNTING
-
ORNI-AEC - OFFICIAL
-
-
::Fig. 2Leached Hull Monitor.
**
ir..
**
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.
-..
END
.
DATE FILMED
3 / 20 / 66







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