A Photo-neutron Source for a Sub-Critical Nuclear Reactor
Program
M.A. REDA1, J.F. HARMON1 and S.B. SADINENI2
1
2
Idaho State University, Idaho Accelerator Center, P.O. Box. 8263, Pocatello, ID 83209.
University of Nevada, Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4027.
Abstract. Experiments to benchmark photo-neutron production calculations for an Accelerator Driven Sub-Critical
System (ADS) are described. A photo-nuclear based neutron source with output > 1013 n/sec has been proposed as a
driver for a program using the sub-critical assembly at Idaho State University. The program is intended to study ADS
control issues arising from coupling an accelerator neutron source with a sub-critical assembly. The experiments were
performed using the 20 MeV electron linear accelerator at the Idaho Accelerator Center (IAC). Results of calculations,
that were made using ACCEPT, PINP, MCNP, and MCNPX codes to optimize photo-nuclear based neutron conversion
targets, are compared to experimental data for a single energy measurement.
INTRODUCTION
of electron energy, for the range 15 to 60 MeV, for
lead and uranium targets. Our work extends the
calculations to a variety of target materials, shapes and
dimensions. Photo-neutron production was experimentally studied using a linear electron accelerator
located at the Idaho Accelerator Center (IAC). The
Linac served as the source of electrons with energies
variable from 15 to 22 MeV. Measurements were
performed on lead targets with different shapes and
showed good agreement with the calculations.
Accelerator Driven Sub-Critical System (ADS) has
been proposed[1,2,3] as a scheme to transmute nuclear
waste. The ADS concept consists of an accelerator, a
target module and a sub-critical blanket (which may
contain spent fuel). The idea is to couple an accelerator
with a subcritical system and produce neutrons,
through (p, n) spallation reactions at proton energies
∼1GeV, to transmute transuranic (TRU) isotopes
(plutonium and minor actinides) and long-lived fission
products into short-lived radioisotopes or stable nuclei
for the possibility of closing the fuel cycle [4].
The effort at Idaho State University is to couple an
electron linac of energy >20 MeV with an available
subcritical assembly to study the ADS control issues
arising from this coupling. In this case the neutrons
are generated via photo-nuclear reactions. The electron
beams, from the Linac, with energy greater than the
neutron emission threshold energy are incident upon a
thick target and induce bremsstrahlung radiation inside
the target which in turn produces (γ, n) reactions.
This paper presents calculations carried out with the
intention of optimizing the composition and geometry
of conversion targets for maximum neutron
production. Results of preliminary benchmark
measurements are also given. Our calculations were
made for beams of monoenergetic electrons incident
perpendicularly on the target.
ACCEPT, PINP,
MCNP, and MCNPX codes were used to simulate the
resulting electron – photon – neutron cascade in the
target. The total neutron yields from targets
bombarded by electrons were calculated as a function
METHOD AND CALCULATIONS
Electron, photon and neutron transport calculations
are usually conducted using the Monte Carlo code,
MCNP. This code takes into account most of the
interactions of electrons, photons and neutrons with
matter. However, photonuclear reactions are not taken
into account. Therefore, for simulating the (γ, n), (γ,
2n) and (γ,f) processes, PINP (Photon Induced Neutron
Production) code, was used. From the bremsstrahlung
photon spectrum determined with the ACCEPT code,
PINP calculates the photo-neutron production. An
alternative way to obtain the photo-neutron yield is to
run MCNPX, which is a new version of MCNP that
takes the photo-neutron process into account. We have
used these two ways to obtain the number of neutrons
produced per incident electron. The calculated neutron
production rate R is then given by the equation
R ( s −1 ) =
I
f τ r
Q
in which, I is the electron peak current (A), Q is the
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800
electron charge (C), f is the repetition rate (s-1), τ is the
pulse width (s), r is the photo-neutron production
efficiency (neutron/electron).
We have simulated the photo-neutron production for
different electron energies and different target
materials, shapes and dimensions. The total neutron
yields from targets bombarded by electrons were
calculated as a function of electron energy, for the
range from 15 to 60 MeV. Figure 1 shows the
calculated photoneutron yield (Neutrons/Sec.KwMeV)
as a function of the incident electron energy, for lead
and uranium targets having the same dimensions. It
can be seen that for the electron energy > 40 MeV
there is no significant increase in the neutron yield.
One way to improve the neutron yield from a target
is to reduce the amount of electron and photon losses
due to the scattering at the target surface. This can be
done by considering a target with a hole drilled up to a
certain depth and having the electron beam hit inside
the hole as illustrated in figure 3.
(a)
3.50E+012
Neutrons/ sec.kw.MeV
FIGURE 3. The irradiating position (a) On the target
surface. (b) Inside the hole in the target.
Lead
Uranium
3.00E+012
(b)
2.50E+012
For constant target thickness and variable irradiation
point inside the target to different depths, we obtained
the neutron production curve of Fig. 4. Note that the
irradiation point, inside the hole, in the middle of the
target is the optimum position for 10 cm thick target.
2.00E+012
1.50E+012
1.00E+012
5.00E+011
0
10
20
30
40
50
60
70
80
90
100
Electron Energy (MeV)
0.0026
FIGURE 1. Neutron yield as a function of incident electron
energy for Pb and U targets
Elect E=20 MeV
Lead Target
Neutron / Electron
0.0024
The dependence of the neutron yield on the target
thickness, for incident electron energy of 20 MeV, is
shown in figure 2. There is no significant increase in
the neutron yield for the target thickness greater than
10 cm, for Pb and U targets, at this particular energy.
0.0022
0.0020
0.0018
0.0016
Thickness 10cm
Diameter 6cm
Hole D=2cm
0.0014
0.0012
0.01
0
2
4
6
8
10
Neutron / Electron
Irradiation position
Depth of the hole(cm)
FIGURE 4. The dependence of the value of the neutron
yield on the irradiation position inside the target at 20 MeV.
1E-3
Elect. E=20MeV
Uranium Target
Lead Target
The calculations were performed for different target
thicknesses, for 40 MeV incident electron energy, to
find the optimum irradiation position for each. The
results show that the optimum irradiation position is
always in the middle of the target regardless of its
thickness as can be seen from figure 5.
1E-4
0
5
10
15
20
Target Thickness (cm)
FIGURE 2. The neutron yield as a function of the target
thickness for lead and uranium
801
MeV). The corresponding electron peak currents were
in the range (30 – 80 mA).
Our target was a lead cylinder with a 5 cm diameter
and 10 cm length. The target was divided into five
small cylinders, between which, lead foils of the same
diameter were placed. Thus, lead foils were distributed
along the target length and having the same diameter
of the target, allowed the axial and lateral distribution
of the neutron yield to be measured. Sufficient activity
in the foils was obtained in about 20 min. Another lead
target of the above dimensions was prepared with a
hole drilled up to the middle of its length, to examine
this geometry as well.
To obtain the neutron production, the foils were
taken immediately after irradiation to a gamma
spectrometer to measure the γ- rays yield emitted from
the 203Pb isotope. The photo-nuclear reaction,
204
Pb (γ, n) 203Pb, on the isotope 204Pb, produces 203Pb
which can be detected by its characteristic γ- ray of
279 keV. The gamma spectrometer consists of a high
purity germanium (HPGe) detector, an amplifier and a
multichannel analyzer interfaced to a PC for data
processing. The total and photo peak efficiencies of
the detector were determined by using Ba-133 source
which had an activity of 42.96 KBq.
Figure 7 shows the measured spectrum from a lead
foil counted after irradiating the target with the 20
MeV electron beam for 20 min.
Elec E=40MeV
Pb Target
0.009
Neutron / Electron
0.008
0.007
0.006
Thickness 10cm
15cm
20cm
0.005
0.004
-2
0
2
4
6
8
10
12
14
16
18
20
Irradiation position
Depth of the hole(cm)
FIGURE 5. The neutron yield as a function of the irradiation
position for different target thickness irradiated at 40 MeV.
Since the photonuclear-based neutron source with
output > 1013 n/sec is required as a driver for the
proposed program using the sub-critical assembly, we
have calculated the required average power in kW to
generate this number at various electron energies.
Figure 6 shows the dependence of the required power
on the incident electron energy.
160
For production of 5E13 n/s
Required Power (kW)
140
2500
120
Lead
Uranium
100
2000
80
1500
60
40
1000
20
10
20
30
40
50
500
60
Counts
Electron Energy (MeV)
FIGURE 6. The required power (kW), to produce 5x1013
n/s, as a function of the incident electron energy.
0
50
100 150 200 250 300 350 400 450 500
Energy (keV)
FIGURE 7. The obtained spectrum for the γ- ray (279 keV)
emitted from the foil
EXPERIMENTAL SETUP AND
MEASUREMENTS
Using the measured γ- ray yield, the neutron yield
can be calculated by taking into account the detector
efficiency, the abundance ratio of the lead isotopes and
the volume ratio between the foils and the target. The
measured neutron production rate from 20 MeV
electron Linac was 1011 n/s with an error of about 20%
which is in good agreement with the calculated value
from the simulation.
Photo-neutron measurements were carried out using
a pulsed electron linac located at the Idaho Accelerator
Center (IAC). The electron pulse width is 2 µs with 60
Hz repetition rate. The particular accelerator used
delivered electrons with energy in the range (15 – 22
802
CONCLUSION
REFERENCES
High neutron yields can be achieved through the
use of an electron accelerator. They reach the values
on the order of ∼1012 n/s.kW in the case of lead target
and can be doubled with the hole geometry. In order to
increase the neutron number it is necessary to increase
the incident electron beam energy and power.
Calculations were compared with preliminary
experimental data for lead targets at 20 MeV and
agreement was satisfactory giving confidence in the
calculations. These results have useful implications in
the development of the photonuclear-based neutron
source as a driver for accelerator/subcritical assembly.
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ACKNOWLEDGEMENTS
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The authors wish to thank the operating staff of the
IAC Linac. This work is supported by DOE contract
number DE - FG04 - 02AL68026.
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