complicating the problem. Also, other sources of the
radionuclides under consideration must often be
taken into account. Depending on the tracer, these
sources may be important and/or numerous, and due
to the secrecy involved in many aspects of the operation of nuclear installations the necessary information may not always be available.
Other Sources of Anthropogenic Radionuclides
Other sources of radionuclides to the oceans complicate the interpretation of Sellafield and Cap de la
Hague tracers to varying extents, depending on the
isotope under consideration, the location, and the
time. First there is the need to understand the mixing
of signals from these two plants. Other sources may
include fallout from nuclear weapons tests, uncontrolled releases due to nuclear accidents, dumping
on the seabed, other reprocessing plants, atmospheric
releases from Sellafield and Cap de la Hague, and
unknown sources. Many of these other sources are
small compared to the Sellafield and Cap de la Hague
releases. The primary complicating source for many
isotopes is nuclear weapons test fallout, which
peaked in the 1950s and again, more strongly, in
1962–63. This source has been particularly important
for
137
Cs and
90
Sr. The Chernobyl accident in 1986
also released significant amounts of radioactive materials which have themselves found use as oceanographic tracers. In terms of comparison to releases
from Sellafield, the Chernobyl accident has been most
important with respect to
134
Cs and
137
Cs.
In addition, it has recently been noted that in the
case of some nuclides, particularly
137
Cs and Pu isotopes, the sediments of the Irish Sea have become a
significant ongoing source of tracers to the North
Atlantic. Although generally considered a conservative tracer,
137
Cs exhibits some affinity for particulate
material and a significant amount has accumulated in
the sediments around Sellafield. With the continuing
reduction of
137
Cs activities in liquid effluents from
Sellafield, release from the sediments, either by
resuspension of the sediments or by reequilibration
with the reduced seawater concentrations, has become a relatively large (though still small in an absolute sense compared to the liquid discharges of the
1970s) contributor to the current flux out of the Irish
Sea, and will continue to be such for years to come.
A further complication of the use of some tracers
derived from Sellafield and Cap de la Hague, but one
that cannot be lamented, is the continuing reduction
of the releases, as well as the radioactive decay of
those with the shorter half-lives, such as
137 Cs which
was released in large quantities over 20 years ago.
With respect to many of these complications, and for
several other reasons, the reprocessing radionuclides
attracting the most attention in the oceanographic
community today are
99 Tc and
129 I. Both are long
lived and have fairly small relative contributions
from weapons testing and other sources. Unlike most
radionuclides, their releases increased in the 1990s,
and the recent releases of each are largely dominated
by a single source:
99 Tc by Sellafield and
129 I by Cap
de la Hague. Significant advances have been and are
being made in the measurement of these isotopes,
allowing their measurement on smaller sample volumes. The activity of
129 I is so low that it is measured
by accelerator mass spectrometry. This technique
allows measurement of
129
I on 1 liter seawater
samples. Advances in
99
Tc measurement, using both
radiochemical and mass-spectrometric (ICP-MS)
techniques, are continuing. The primary limitation of
99 Tc studies continues to be the comparative difficulty of its measurement. This is also true to some
extent for
129 I, for although the sample sizes have
been greatly reduced the measurement requires
highly specialized technology. A further complication
for
129 I is its volatility, and the fact that as much as
10% of the reprocessing discharges have been released directly to the atmosphere. Nevertheless, the
promise of both tracers is such that these difficulties
are likely to be overcome.
Regional Setting and Circulation of Reprocessing
Discharges
A summary of the regional circulation into which the
liquid effluents from Sellafield and Cap de la Hague
are released is presented in Figure 2. It is particularly
important to note that studies of the reprocessing
discharges have contributed greatly to the development of this detailed picture of the regional circulation. Briefly, from the Sellafield site the waste
stream is carried north out of the Irish Sea, around
the coast of Scotland, through the North Sea, and
into the northward-flowing Norwegian Coastal Current (NCC). Transport across the North Sea occurs at
various latitudes: some fraction of the reprocessing
releases ‘short-circuits’ across the northern part of the
North Sea, while some flows farther south along the
eastern coast of the UK before turning east and north.
Recent studies of the EARP
99
Tc pulse from Sellafield
have suggested that the rate and preferred transport
path of Sellafield releases across the North Sea into
the NCC may vary in relation to climatic conditions
in the North Atlantic such as the North Atlantic
Oscillation (NAO).
Radionuclides discharged from the Cap de la
Hague reprocessing plant flow north-east through
the English Channel and into the North Sea,
294 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES
radionuclides under consideration must often be
taken into account. Depending on the tracer, these
sources may be important and/or numerous, and due
to the secrecy involved in many aspects of the operation of nuclear installations the necessary information may not always be available.
Other Sources of Anthropogenic Radionuclides
Other sources of radionuclides to the oceans complicate the interpretation of Sellafield and Cap de la
Hague tracers to varying extents, depending on the
isotope under consideration, the location, and the
time. First there is the need to understand the mixing
of signals from these two plants. Other sources may
include fallout from nuclear weapons tests, uncontrolled releases due to nuclear accidents, dumping
on the seabed, other reprocessing plants, atmospheric
releases from Sellafield and Cap de la Hague, and
unknown sources. Many of these other sources are
small compared to the Sellafield and Cap de la Hague
releases. The primary complicating source for many
isotopes is nuclear weapons test fallout, which
peaked in the 1950s and again, more strongly, in
1962–63. This source has been particularly important
for
137
Cs and
90
Sr. The Chernobyl accident in 1986
also released significant amounts of radioactive materials which have themselves found use as oceanographic tracers. In terms of comparison to releases
from Sellafield, the Chernobyl accident has been most
important with respect to
134
Cs and
137
Cs.
In addition, it has recently been noted that in the
case of some nuclides, particularly
137
Cs and Pu isotopes, the sediments of the Irish Sea have become a
significant ongoing source of tracers to the North
Atlantic. Although generally considered a conservative tracer,
137
Cs exhibits some affinity for particulate
material and a significant amount has accumulated in
the sediments around Sellafield. With the continuing
reduction of
137
Cs activities in liquid effluents from
Sellafield, release from the sediments, either by
resuspension of the sediments or by reequilibration
with the reduced seawater concentrations, has become a relatively large (though still small in an absolute sense compared to the liquid discharges of the
1970s) contributor to the current flux out of the Irish
Sea, and will continue to be such for years to come.
A further complication of the use of some tracers
derived from Sellafield and Cap de la Hague, but one
that cannot be lamented, is the continuing reduction
of the releases, as well as the radioactive decay of
those with the shorter half-lives, such as
137 Cs which
was released in large quantities over 20 years ago.
With respect to many of these complications, and for
several other reasons, the reprocessing radionuclides
attracting the most attention in the oceanographic
community today are
99 Tc and
129 I. Both are long
lived and have fairly small relative contributions
from weapons testing and other sources. Unlike most
radionuclides, their releases increased in the 1990s,
and the recent releases of each are largely dominated
by a single source:
99 Tc by Sellafield and
129 I by Cap
de la Hague. Significant advances have been and are
being made in the measurement of these isotopes,
allowing their measurement on smaller sample volumes. The activity of
129 I is so low that it is measured
by accelerator mass spectrometry. This technique
allows measurement of
129
I on 1 liter seawater
samples. Advances in
99
Tc measurement, using both
radiochemical and mass-spectrometric (ICP-MS)
techniques, are continuing. The primary limitation of
99 Tc studies continues to be the comparative difficulty of its measurement. This is also true to some
extent for
129 I, for although the sample sizes have
been greatly reduced the measurement requires
highly specialized technology. A further complication
for
129 I is its volatility, and the fact that as much as
10% of the reprocessing discharges have been released directly to the atmosphere. Nevertheless, the
promise of both tracers is such that these difficulties
are likely to be overcome.
Regional Setting and Circulation of Reprocessing
Discharges
A summary of the regional circulation into which the
liquid effluents from Sellafield and Cap de la Hague
are released is presented in Figure 2. It is particularly
important to note that studies of the reprocessing
discharges have contributed greatly to the development of this detailed picture of the regional circulation. Briefly, from the Sellafield site the waste
stream is carried north out of the Irish Sea, around
the coast of Scotland, through the North Sea, and
into the northward-flowing Norwegian Coastal Current (NCC). Transport across the North Sea occurs at
various latitudes: some fraction of the reprocessing
releases ‘short-circuits’ across the northern part of the
North Sea, while some flows farther south along the
eastern coast of the UK before turning east and north.
Recent studies of the EARP
99
Tc pulse from Sellafield
have suggested that the rate and preferred transport
path of Sellafield releases across the North Sea into
the NCC may vary in relation to climatic conditions
in the North Atlantic such as the North Atlantic
Oscillation (NAO).
Radionuclides discharged from the Cap de la
Hague reprocessing plant flow north-east through
the English Channel and into the North Sea,
294 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES