which began in 1994. These studies have suggested
substantially shorter circulation times for Sellafield
releases to northern Scottish coastal waters and
across the North Sea to the NCC than previously
accepted. For instance, the
99 Tc pulse reached the
NCC within 2.5 years, rather than three to four. It
has been suggested that this is a real difference between sampling periods, resulting from climatically
induced circulation changes in the North Atlantic.
Some of the difference may also be related to the fact
that much more detailed data, both in terms of seawater sampling and regarding the releases, are
available on this recent event. The continuing passage of the EARP
99
Tc signal promises to be very
useful in the Nordic Seas and Arctic Ocean as well.
Deep-water Formation in the North Atlantic and
Arctic Oceans
In addition to surface water flows, deep-water formation processes within the Arctic Ocean and Nordic Seas have been elucidated through the study of
reprocessing releases. In a classic presentation of reprocessing tracer data, Livingston showed that the
surface water distribution of
137
Cs in the Nordic Seas
in the early 1980s was marked by high concentrations at the margins of the seas, highlighting the
delivery of the isotope in the northward-flowing
NCC and WSC to the west and the return flow in the
southward-flowing EGC to the east. The opposite
distribution was found in the deep waters, with
higher concentrations in the center of the Greenland
Basin than at the margins, as a result of the ventilation of the Greenland Sea Deep Water by deep
convective processes in the center of the gyre.
In the Arctic Ocean, elevated
137 Cs and
90
Sr concentrations and
137 Cs/
90
Sr ratios at 1500 m at the
LOREX ice station near the North Pole in 1979 indicated that deep layers of the Arctic Ocean were
ventilated from the shelves. Similar observations in
deep water north of Fram Strait provided early evidence suggesting a contribution of dense brines from
the Barents Sea shelf to the bottom waters of the
Nansen Basin. Reprocessing tracers, particularly
those, like
129 I and
99 Tc, which have only a small
contribution from other sources such as weapons
fallout, hold great promise for illuminating eastern
Arctic Ocean deep water ventilation processes from
the Barents and Kara Sea shelves. The NCC delivers
reprocessing tracers to the Barents and Kara relatively rapidly (B 5 years) and more importantly in
high concentration. With high tracer concentrations
in the area of interest as a source water, the reprocessing tracers may be particularly sensitive
tracers of a contribution of dense shelf waters to the
deep Arctic Ocean.
In addition to the deep waters formed through
convection in the polar regions (most notably in the
Greenland Sea) which fill the deep basins of the
Nordic Seas, intermediate waters are formed which
subsequently overflow the sills between Greenland,
Iceland, and Scotland and ventilate the deep North
Atlantic. The presence of
137 Cs and
90 Sr from Sellafield was reported in the overflow waters immediately south of the Denmark Straits sampled during
the Transient Tracers in the Ocean (TTO) program in
1981. Later, it was demonstrated that reprocessing
cesium and strontium could be distinguished in the
deep waters as far as TTO Station 214, off the Grand
Banks of Newfoundland. No samples were taken for
reprocessing radionuclides further south as part of
that study, but it was clear in retrospect that the reprocessing signal had traveled even further in the
Deep Western Boundary Current (DWBC). Recent
work on
129 I has highlighted the utility of reprocessing radionuclides as tracers of northern
source water masses and the DWBC of the Atlantic.
Profiles in stations south of the overflows show much
clearer tracer signals for
129 I than for the CFCs, and
129 I has been detected in the DWBC as far south as
Cape Hatteras. As with the cesium studies of the
1980s, it is likely that sampling further south will
reveal the tracer there as well.
Conclusions
Releases of radionuclides from the nuclear fuel-reprocessing plants at Sellafield and Cap de la Hague
have provided tracers for detailed studies of the circulations of the local environment into which they
are released, namely the Irish Sea, English Channel,
and North Sea, and for the larger-scale circulation
processes of the North Atlantic and Arctic Oceans.
These tracers have very different source functions for
their introduction into the oceans compared to other
widely used anthropogenic tracers, and in some cases
compared to each other. The fact that they are released at point sources makes them highly specific
tracers of several interesting processes in ocean circulation, but the nature of the releases has complicated their quantitative interpretation to some extent.
Recent advances have been made in the measurement of
129 I and
99 Tc, long-lived tracers whose releases have increased in recent years and which
experience little complication from other sources.
These two tracers hold great promise for elucidating
deep water formation and ventilation processes in
296 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES
substantially shorter circulation times for Sellafield
releases to northern Scottish coastal waters and
across the North Sea to the NCC than previously
accepted. For instance, the
99 Tc pulse reached the
NCC within 2.5 years, rather than three to four. It
has been suggested that this is a real difference between sampling periods, resulting from climatically
induced circulation changes in the North Atlantic.
Some of the difference may also be related to the fact
that much more detailed data, both in terms of seawater sampling and regarding the releases, are
available on this recent event. The continuing passage of the EARP
99
Tc signal promises to be very
useful in the Nordic Seas and Arctic Ocean as well.
Deep-water Formation in the North Atlantic and
Arctic Oceans
In addition to surface water flows, deep-water formation processes within the Arctic Ocean and Nordic Seas have been elucidated through the study of
reprocessing releases. In a classic presentation of reprocessing tracer data, Livingston showed that the
surface water distribution of
137
Cs in the Nordic Seas
in the early 1980s was marked by high concentrations at the margins of the seas, highlighting the
delivery of the isotope in the northward-flowing
NCC and WSC to the west and the return flow in the
southward-flowing EGC to the east. The opposite
distribution was found in the deep waters, with
higher concentrations in the center of the Greenland
Basin than at the margins, as a result of the ventilation of the Greenland Sea Deep Water by deep
convective processes in the center of the gyre.
In the Arctic Ocean, elevated
137 Cs and
90
Sr concentrations and
137 Cs/
90
Sr ratios at 1500 m at the
LOREX ice station near the North Pole in 1979 indicated that deep layers of the Arctic Ocean were
ventilated from the shelves. Similar observations in
deep water north of Fram Strait provided early evidence suggesting a contribution of dense brines from
the Barents Sea shelf to the bottom waters of the
Nansen Basin. Reprocessing tracers, particularly
those, like
129 I and
99 Tc, which have only a small
contribution from other sources such as weapons
fallout, hold great promise for illuminating eastern
Arctic Ocean deep water ventilation processes from
the Barents and Kara Sea shelves. The NCC delivers
reprocessing tracers to the Barents and Kara relatively rapidly (B 5 years) and more importantly in
high concentration. With high tracer concentrations
in the area of interest as a source water, the reprocessing tracers may be particularly sensitive
tracers of a contribution of dense shelf waters to the
deep Arctic Ocean.
In addition to the deep waters formed through
convection in the polar regions (most notably in the
Greenland Sea) which fill the deep basins of the
Nordic Seas, intermediate waters are formed which
subsequently overflow the sills between Greenland,
Iceland, and Scotland and ventilate the deep North
Atlantic. The presence of
137 Cs and
90 Sr from Sellafield was reported in the overflow waters immediately south of the Denmark Straits sampled during
the Transient Tracers in the Ocean (TTO) program in
1981. Later, it was demonstrated that reprocessing
cesium and strontium could be distinguished in the
deep waters as far as TTO Station 214, off the Grand
Banks of Newfoundland. No samples were taken for
reprocessing radionuclides further south as part of
that study, but it was clear in retrospect that the reprocessing signal had traveled even further in the
Deep Western Boundary Current (DWBC). Recent
work on
129 I has highlighted the utility of reprocessing radionuclides as tracers of northern
source water masses and the DWBC of the Atlantic.
Profiles in stations south of the overflows show much
clearer tracer signals for
129 I than for the CFCs, and
129 I has been detected in the DWBC as far south as
Cape Hatteras. As with the cesium studies of the
1980s, it is likely that sampling further south will
reveal the tracer there as well.
Conclusions
Releases of radionuclides from the nuclear fuel-reprocessing plants at Sellafield and Cap de la Hague
have provided tracers for detailed studies of the circulations of the local environment into which they
are released, namely the Irish Sea, English Channel,
and North Sea, and for the larger-scale circulation
processes of the North Atlantic and Arctic Oceans.
These tracers have very different source functions for
their introduction into the oceans compared to other
widely used anthropogenic tracers, and in some cases
compared to each other. The fact that they are released at point sources makes them highly specific
tracers of several interesting processes in ocean circulation, but the nature of the releases has complicated their quantitative interpretation to some extent.
Recent advances have been made in the measurement of
129 I and
99 Tc, long-lived tracers whose releases have increased in recent years and which
experience little complication from other sources.
These two tracers hold great promise for elucidating
deep water formation and ventilation processes in
296 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES