Development of a Neutron Detector Facility at Kansas State University

haddockhellskitchenΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

85 εμφανίσεις

Development of a Neutron Detector Facility at Kansas State University


P. M. Whaley


INTRODUCTION


Construction of the Kansas State University nuclear research reactor began in 1960, with
initial criticality in 1962
, with

a

license maximum

power level of 1
00 kW
. In 1968 the

license was revised
for
250 kW

operations
, pulsing
to $2.00.

A license renewal
request
with
power
upgrade
(
1.25 MW
,

pulsing to $3.00
) was submitted in 2002
.


E
xperiment facilities include thermal columns,
a well in the graphite reflect
or, in
-
core
irradiation sites, and 4 beam ports.
One beam
port
penetrates
the reflector
,

one is

tangential to the reflector
; t
he remaining
p
orts
extend from the reflector
radial
to
core
center. Beams exit shie
lding 30 inches f
r
o
m
the
floor.


In 2002, the

Semiconductor Materials and Radiological T
echnologies (SMART)
Laboratory
was installed at K
-
State
,
dedicated to design and
pr
oduction of
s
emiconductor
-
based
and gas
-
filled
radiation detectors
.

I
ndividual laboratories support
specific
fabrication
processes
, including
material preparation and crystal growth, low
pressure condensation/deposition, specialized crystal growth processes (cadmium zinc
telluride and mercuric iodide), a large class 1000 clean room and adjacent chemistry lab,
and a vacuum processing
lab.


One
SMART lab
focus
is neutron detectors,
testing
using
reactor
beams
. The
reactor
committed one beam port (northwest beam port, NWBP) for development as a dedicated
test facility
,
using neutron diffraction

to provide
a nearly monoenergetic beam
wit
h low
gamma

content.

Design considerations included floor loading constraints

and

shielding
construction, motion controls, beam intensity, and collimation.


DESIGN CONSIDERATIONS


The reactor bay floor
,

built to Uniform Building Code standards
, is

rated to
1
1
0 pounds
psf
. Since concrete
is typically 150
psf
,
concrete extend
ing

above
a
beam port exceed
s

the
limit.
The University Architect review
ed

building design,
providing
a new rating of
350
psf
, still limiting but manageable

with
shielding

elevated near t
he beam
.
Stacked
blocks are commonly used for shielding,
interlocked and offset to minimize streaming.
E
ffort is required to build and tear down block walls.
S
tacked block

are sometimes
stabilized
.
A principal design
concept
was therefore a monolithic
shield on a stand
capable of being manipulated with
facility equipment. The reactor bay polar crane is
rated to 8000 lbs. The facility maintains two pallet jacks rated to 5500 lbs, one manual
and one powered.


Motion controls
are

expensive, and
initially

SMART lab resources
established
the
facilities.
T
wo
-
axis control (rotational and
azimuthal
)
is required
to manipulate the
monochromator.

Another
axis
may be

required to
orient

the crystal plane, but adjustment
can be manual.
Testing showed the reactor f
loor will not vibrate under impulse loads,
but
vibration mount
ing

is a good practice.


The NWBP flux is 6e7
n cm
-
2

s
-
1
.
E
nergy distribution
is
nearly
M
axwellian
,

peak
at
50
meV
. Calculations indicate
1%

of flux is
available near the peak.
With
neutron
attenuation in air, neutron flux at the monochromator
is

1e5
n cm
-
2

s
-
1
.


NWBP g
amma radiation is significant
; collimation

size
and filtering can
inf
luence

shielding

requirements
.

Beam degradation
in air
can be minimized by
helium fill
or
vacuum
.

B
eam port

casings

extend into the reactor pool,
with
potential
leakage
path.
Beam port doors
are sealed against
leakage from the pool to reactor bay
. C
ollimators are
built with thin alum
inum windows
and sea
ls around the casing at the biological shielding.



INITIAL

DIFFRACTION SYSTEM


F
irst efforts at neutron diffraction
used
a
Newport 2
-
axis controller

for r
otation

and
g
oiniometer
,
mounted on vibration damper
. A
Lab
V
iew
application
controlled the
monochromator, rotating the crystal at elevation angles.

The monoch
romator (from a
single silicon crystal) could be
adjusted
in the holder

in two dimensions
.
S
hielding was
fabricated in cylind
rical forms, with penetrations for
neutron extraction at two angles
,

and
a main bean. Additional shielding minimize
d

leakage at c
ollimator exit.


S
hielding was on a large
, elevated, rotating ring
. Rotation of the shielding provided 2
-
wall thicknesses
with
beam “off,” and a thickness
with
beam “on.”
A wire fence
enclosure segregated the shielding and beam stop.
The collimator
had

three 1.5 inch
penetrations
for future developments

and filtering.


P
enetrations
allow instrument
and
tubing lines.
F
light tubes
terminated at
thin aluminum plates
, with unused penetrations
shielded. A
flange seals
the beam tube f
r
o
m the reactor bay
.



The shielding configuration and beam control proved adequate, with small foot print and
acceptable weight.
A
clear, but small set of peaks at locations consistent with diffraction
from silicon

were observed. P
eak intensity was too low
,
not unexpected

bec
ause
a
perfect crystal
was used as the monochromator.
Resources were
in
adequate to
continue
optimization.


MODIFIED DIFFRACTION SYSTEM


A research

assistant was
assigned, and initially
focus
ed

on
generating
a mo
saic spread in
crystal silicon. A
cquisitio
n of equipment provided an alternative.
As directed,
a
large
aperture
collimator
and new shielding was developed
.
M
onochromator and filter testing
was accomplished on the tangential port. Pyrolytic graphite provide
d

an adequate
signal
.

Sapphire and bism
uth filters were tested, with intensity reduced 78% and 57%
respectively, with significant

gamma
reduction
.


Prior experience
with NAA
used
large
concrete blocks
(3600 lbs) and ½ height blocks
to
isolate
detectors.
A concrete vendor
indicated the
modifica
tion was possible.

An
inverted cylinder was
inserted in the form as a beam catcher. A

slot for
take
-
off ang
l
e

was inserted in a form.
A
modification matched the biological shielding.
T
ables were
manufactured to hold the blocks.
T
ables
bracing
was provi
de
s

clearance for the pallet
jacks. A test fixture
was mounted on
phenolic resin containing boron.
L
inear stage
s
position

an aluminum

beam and
shutter
.


CONCLUSIONS


The facility is functioning.
Minor changes to eliminate streaming
are planned
.
A better

layout for the a
rea radiation monitor

is planned
. A
status
sensor
will be mounted to the
shutter, indicating in the control room.


S
ome
system
aspects are not optimal. The beam
is larger
than the exposed
monochromator
surface
, and s
hielding requirements

and background
are more sever
e

than
necessary.
The
shielding foot
print
is
large, and requires
more
effort for
managing
the shield.
The blocks
do not
have stepped interfaces,
consequently

there is
radiation
streaming

at the interface
.
The large rotating
stage is not necessary, and requires
a larger
monochromator cavity.

A lack of knowledgeable resources
created some difficulties in

development.