Benchmark Data on the Transmutation of 129I, 139La and 237Np
- The reduction of long-lived nuclear waste -
W. Westmeier1,7 ,
R. Brandt1, E.-J. Langrock2, H. Robotham7,
K. Siemon7, R. Odoj3,
V.M. Golovatyuk4, M.I. Krivopustov4, S.R.
Hashemi-Nezhad5, M. Zamani6
1
Institut f¸r Physikalische Chemie, Kernchemie und Makromolekulare Chemie,
Philipps-Universit”t,
D 35032 Marburg (Germany)
2
Forschungsb¸ro Langrock, D 02977 Hoyerswerda (Germany)
3
Institut f¸r Sicherheitsforschung und Reaktortechnik, Forschungszentrum J¸lich
GmbH, D 52425 J¸lich (Germany)
4
Joint Institute for Nuclear Research, 141980 Dubna (Russian Federation)
5
Dept. of High Energy Physics, University of Sydney, Sydney, NSW 2006
(Australia)
6
Physics Department, Aristotle University, GR 52124 Thessaloniki (Greece)
7
Dr. Westmeier GmbH, D 35085 Ebsdorfergrund, (Germany)
Transmutation
was proposed [1] as a hypothetical means to reduce the amount of very
long-lived radioactive waste from technological applications of nuclear
fission. With the advent of new technologies this idea came closer to reality
and high-precision experimental data are now required to check the feasibility
of the concept.
Experiments
were carried out with the GAMMA-2
target setup [2] at the NUCLOTRON accelerator using protons in the energy range
from 0.53 GeV to 4.15 GeV. Fig.1 gives a schematic view of the GAMMA-2
experimental setup together with its beam monitoring system.
Figure 1: Schematic view of the GAMMA-2 setup.
The target is composed of 20 lead disks with 8 cm diameter and 1 cm thickness,
the paraffin moderator shell has 20 cm outer diameter, 6 cm thickness and 31 cm
length. The Al- monitor contains a stack of three thin aluminium foils where
the center foil is used. Polaroid films were used for beam alignment before
each
irradiation.
Five scintillation detectors C1 to C5 and a 1 g/cm2
PE target were used to monitor the beam. Aluminium activation foils were used
to determine the integral proton fluence on the target. The Al monitor foil
stack was placed approx. 60 cm upstream the Pb target in order to avoid
activation from backwards emitted particles. In each experiment a stack of
three Al foils with a thickness of 31 mm (1.883*1020
atoms*cm-2 ) was mounted in an aligned position with the target and
perpendicular to the beam axis as shown in Fig. 1, and irradiated during the
whole run. The beam intensity was determined via the 27Al(p,3pn)24Na
reaction in the center foil.
Samples containing 1 gram of lanthanum each were placed on top of the
target assembly at distances of 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm from the
front side of the paraffin block, i.e. the first sample sits just above the
location where the proton beam hits the Pb. B-values for each of the five
samples (corrected for neutron anisotropy) were measured in every experiment.
The B-value is an absolute cross section which is specific for each
experimental setup and defined for the example nuclide 140La as :
B(140La)
= Atoms of 140La produced in 1 gram of 139La sample by 1
primary proton
In order to compare neutron densities from various experiments we have
calculated the integrated B(140La ) for 140La on the
GAMMA-2 setup by fitting the five data points with a modified (skewed) Gaussian
function. The function is used because it has a suitable shape and not because
of any physical significance.
Figure
2:
B-values for 140La along the top of the paraffin moderator in the
irradiation with 0.53 GeV protons on the GAMMA-2 target. The distance d=0 cm
corresponds to the upsteam end of the 20 cm long Pb target, i.e. the point of
proton impact.
The fitted distributions quantify findings from
earlier experiments [3,4] that the shapes of B-value distributions (i.e. the
neutron densities over the target) are almost identical over the entire proton
energy range studied. The maximum of the B-values is always found at about 10
cm downstream the beginning of the lead target and the widths of the
distributions are essentially the same for each energy in the 0.53 GeV £ Ep £ 4.15 GeV range.
The integrated B(140La )ñvalues divided by the proton beam
energy EP are plotted in Fig. 3 as a function of proton energy EP.
This picture shows the effectiveness of the GAMMA-2 setup for transmutation of 139La
via neutron capture reactions. Thus, it also displays the effectiveness of the
GAMMA-2 setup for the production of low-energy neutrons. It is interesting to
note that the effectiveness of GAMMA-2, which has only 20 cm Pb target length,
for low-energy neutron production is best at low proton energies.
Figure 3: Normalized B-values for 140La
on the GAMMA-2 setup. The dotted line serves to guide the eye.
Uncorrected data
points at 0.65 GeV, 1 GeV and 1.5 GeV proton energy show the necessity of the
anisotropy correction of measured B-values.
In Figures 4 and 5 the corresponding functions of
B-values/Ep are shown for the transmutation of 129I and 237Np.
In these experiments samples of approx. 1g of radioactive target material,
which was weld sealed into Al-containers, were exposed to the secondary neutron
fluence on top of the paraffin moderator on the GAMMA-2 target setup.
Figure 4: Normalized
B(130I)/Ep measured Figure
5: Normalized
B(238Np)/Ep measured
on
the GAMMA-2 setup on
the GAMMA-2 setup
The lines in Figs. 4 and 5 serve to guide
the eye. Considering results from Figures 3 to 5 it is clear that the
transmutation effectiveness B/Ep (also called Ñneutron economyì [5])
on the GAMMA-2 target is always highest at low proton energy and gradually
falls off with rising bombarding energy. This may favour the use of proton beam
energies that are lower than it has been assumed in other design studies.
Operating at lower energy would of course be commercially attractive. However,
the gradual fall may be a consequence of the size of the target where the small
diameter and short length do not allow the intra- and inter-nuclear cascades
originating from incident protons to be completed. Further experiments shall
answer that question very soon.
References
:
[1] K.D. Tolstov, JINR preprint 18-89-778,
Dubna, Russia (1989) and
C.D. Bowman et al., Nuclear Instruments
and Methods in Physics Research A320 (1992) 336
[2] Adam J. et al., ÑFirst
nuclear activation experiments using the new accelerator
NUCLOTRON in Dubnaì , Kerntechnik 68 (2003) 214
[3] Wan J.-S. et al., ÑTransmutation of 129I and 237Np
using spallation neutrons produced by 1.5,
3.7 and 7.4 GeV
protons",
Nuclear
Instruments and Methods in Physics Research A463 (2001) 634
[4] Adam J. et al.,
ÑTransmutation of 239Pu and other nuclides using spallation neutrons
produced
by
relativistic protons reacting with massive U- and Pb-targets",
Radiochimica Acta 90 (2002) 431
[5] A. Letourneau et al., ÑNeutron
production in bombardments of thin and thick W, Hg, Pb
targets
by 0.4, 0.8, 1.2, 1.8 and 2.5 GeV protonsì,
Nuclear Instruments
and Methods in Physics Research B170 (2000) 299