Journal of Radioanalytical and Nuclear Chemistry, Vol. 263, No. 2 (2005) 427–436
0236–5731/2005/USD 20.00 Akadémiai Kiadó, Budapest
© 2005 Akadémiai Kiadó, Budapest Springer, Dordrecht
Recent developments in the analysis of transuranics (Np, Pu, Am) in seawater
J. La Rosa,* J. Gastaud, L. Lagan, S.-H. Lee, I. Levy-Palomo, P. P. Povinec, E. Wyse
International Atomic Energy Agency, Marine Environment Laboratory, 4, quai Antoine 1er, B.P. 800, Monte-Carlo, MC 98012 Monaco
(Received April 6, 2004)
A procedure is described to extend the current radiochemical method of seawater analysis for Pu and Am including Np. Short-lived
239
Np tracer
was prepared by separation from its
243
Am parent. Irish Sea Water reference material (IAEA-381) containing known concentrations of
237
Np, Pu
isotopes and
241
Am was used to test the procedure for small water volumes. Inductively-coupled plasma mass spectrometry (ICP-MS) was used in
addition to alpha spectrometry for measurement of
237
Np in the purified final Np fractions.
Introduction
Anthropogenic Np, Pu and Am are present in
seawater worldwide as a result of atmospheric nuclear
weapons testing in the 1950s and early 1960s, controlled
and uncontrolled releases of nuclear fuel reprocessing
facilities (e.g., the 1986 Chernobyl accident). The
measurement of transuranics may be needed for
assessment of the radiological situation or may be useful
for understanding their chemical behavior in the marine
environment. They can be used as tracers for following
water movements, sedimentation and interaction with
biological systems. Because of their relatively low
concentration in seawater, large volume samples and
extensive chemical separations are necessary to provide
highly purified fractions for measurement by sensitive
radiometric and mass spectrometry methods.
At the IAEA’s Marine Environment Laboratory
(MEL) in Monaco, a procedure for the separation of Pu
and Am from large volumes (up to 500 liters) of
seawater has been in routine use for many years.
1
It is
based on the co-precipitation of Pu and Am
radionuclides at environmental levels with freshly
precipitated manganese dioxide (MnO
2
).
2
After further
chemical separation and purification, the Pu isotopes are
measured by means of alpha spectrometry and/or by
means of mass spectrometry [inductively-coupled
plasma mass spectrometry (ICP-MS) or accelerator mass
spectrometry (AMS)].
241
Am is measured exclusively
by alpha-spectrometry.
In recent years, interest in transuranic radionuclide
measurement has extended to include Np (specifically
237
Np) as well as Pu and Am. The long half-life of
237
Np (T
1/2
=
2.14
.
10
6
y) and consequently low specific
activity suggest that mass spectrometry will have a
lower detection limit for sub-nanogram amounts of
237
Np than alpha-spectrometry. Methods have been
published for mass spectrometric determination of
237
Np
in environmental samples.
3–7
One very significant
* E-mail: J.LaRosa@iaea.org
impediment has been the lack of a widely available,
long-lived isotopic spike such as high-purity
236
Np.
Our goal was to develop a method for the
determination of
237
Np in large volume seawater
samples which could be carried out in conjunction with
the current method for Pu and Am. Preliminary
experiments with short-lived
239
Np (T
1/2
=
56.52 h)
indicated that the MnO
2
pre-concentration step used for
Pu and Am would also carry down Np as well. This
became the starting point for development of a
combined Np–Pu–Am procedure from small volumes of
Irish Sea Water (IAEA-381).
Experimental
Preparation of
239
Np tracer
The chemical recovery of the purified Np fraction is
determined using
239
Np tracer (T
1/2
=
56.52 h).
239
Np
undergoes beta-minus decay with the emission of
gamma-rays. It is conveniently obtained by separation
from its long-lived
243
Am parent (T
1/2
=
7380 y). Just
before the beginning of a seawater sample analysis, an
243
Am “cow” is “milked” by separating the ingrown
239
Np
8
by the procedure described below and illustrated
schematically in Fig. 1.
A source of 500–3000 Bq of
243
Am containing
239
Np (usually in radioactive equilibrium with the
243
Am) is mixed with 10 mg of Nd as nitrate,
evaporated and prepared in 9 ml of 10M HCl. A fresh
solution of 0.15 g NH
4
I, dissolved in 1 ml of de-ionized
water is added. Iodide rapidly adjusts Np to the
tetravalent oxidation state. The resulting solution is
passed slowly (0.5–1 ml/minute) through the 1
st anion-
exchange resin column (6 ml of AG 1-X4, 100–200
mesh anion-exchange resin, chloride form, in a Bio-Rad
20 ml polypropylene column), which was previously
conditioned with 50 ml of 9M HCl. After complete
passage of the feed solution and four-1.5 ml 9M HCl