(Ci), a unit which was based on the activity of one
gram of radium. These two units are related as follows:
1Bq ¼ 1disintegration per second
1Ci ¼ 3:7 Â 10
10 Bq
The conversion from activity to mass or molar units
is dependent on the half-life of the isotope in question. Activity (A) is the product of the number of
atoms of the isotope present (N) and its decay constant (l): A ¼ lN. The decay constant, l, is related to
the half-life by t 1=2 ¼ ðln2Þ=l. Using this equation, it
is simple to convert from Bq to mol, given the decay
constants in units of s
À1
, and Avogadro’s number of
6.023 Â 10
23 atoms mol
À1
.
It is also worth noting that the total activity of
naturally occurring radionuclides in sea water is
approximately 12 kBq m
À3 . Most of this activity
is from the long-lived naturally occuring isotope,
40 K(t 1/2 ¼ 1.25 Â 10
9 years). Activities contributed
from anthropogenic radionuclides greatly exceed this
(millions of Bq m
À3 ) in the immediate vicinity of the
Sellafield and La Hague outfall pipes. However, most
oceanographic studies of these releases involve
measurements of much lower activities: up to 10s of
Bq m
À3 in the case of
137 Cs, and generally even lower
for other radionuclides, for instance mBq m
À3 or less
for plutonium isotopes and
129 I.
Nuclear Fuel Reprocessing and
Resulting Tracer Releases
Origin and Description of Reprocessing Tracers
Nuclear fuel reprocessing involves the recovery of
fissile material (plutonium and enriched uranium)
and the separation of waste products from ‘spent’
(used) fuel rods from nuclear reactors. In the process,
fuel rods, which have been stored for a time to allow
short-lived radionuclides to decay, are dissolved and
the resulting solution is chemically purified and
separated into wastes of different composition and
activity. Routine releases from the plants to the environment occur under controlled conditions and are
limited to discharge totals dictated by the overseeing
authorities of each country. The discharges have
varied over the years as a function of the amount and
type of fuel processed and changes in the reprocessing technology. The discharge limits themselves have
changed, in response to monitoring efforts and also
spurred by technological advances in waste-treatment capabilities. As a general rule, these changes
have resulted in decreasing releases for most nuclides
(most notably cesium and the actinide elements), but
there are exceptions. For instance, the Enhanced
Actinide Removal Plant (EARP) was constructed at
the Sellafield site in the 1990s in order to enable the
additional treatment and subsequent discharge of a
backlog of previously stored wastes. Although the
new technology enabled the removal of actinide
elements from these wastes, it is not effective at removing
99
Tc. Therefore an allowance was made for
increased discharge of
99 Tc, up to 200 TBq per year.
The resulting pulse of increased
99
Tc discharges from
Sellafield beginning in 1994 is currently being followed with great interest, as is discussed in more
detail below.
The end result of these processes is the availability
of a suite of oceanographic tracers with different
discharge histories (e.g., in terms of the timing and
magnitude of spikes) and a range of half-lives, and
thus with a range of utility and applicability to
studies of oceanographic processes at a variety of
spatial and temporal scales. A brief summary of the
reprocessing radionuclides most widely applied in
oceanography is shown in Table 1, and examples of
discharge histories are given in Figure 1. It has also
been particularly useful in some cases to measure the
ratio of a pair of tracers, for instance
134 Cs/
137 Cs,
137 Cs/
90 Sr, or potentially,
99 Tc/
129 I. The use of isotope ratios can (1) provide temporal information and
aid the estimation of rates of circulation, (2) mitigate
the effects of mixing which will often alter individual
concentrations more than ratios, and (3) aid in distinguishing the relative contributions of different
sources of the nuclides in question. Finally, differences in the chemical behavior of the different
elements released can be exploited to study different
processes. Although most of the widely applied tracers are largely conservative in sea water and therefore serve as tracers of water movement, the
actinides, particularly Pu, have a high affinity for
particulate material and accumulate in the sediments. These tracers are then useful for studying
sedimentary processes.
The Reprocessing Tracer Source Function
The primary difference between the north-western
European reprocessing releases and other anthropogenic tracers used in oceanography is the nature of
their introduction to the oceans. Reprocessing releases enter the oceans essentially at a point source,
rather than in a more globally distributed fashion as
is the case for weapons test fallout or the chlorofluorocarbons. Thus, reprocessing tracers are excellent, specific tracers for waters originating from
north-western Europe. Because these waters are
transported to the north into the Nordic Seas and
292 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES
gram of radium. These two units are related as follows:
1Bq ¼ 1disintegration per second
1Ci ¼ 3:7 Â 10
10 Bq
The conversion from activity to mass or molar units
is dependent on the half-life of the isotope in question. Activity (A) is the product of the number of
atoms of the isotope present (N) and its decay constant (l): A ¼ lN. The decay constant, l, is related to
the half-life by t 1=2 ¼ ðln2Þ=l. Using this equation, it
is simple to convert from Bq to mol, given the decay
constants in units of s
À1
, and Avogadro’s number of
6.023 Â 10
23 atoms mol
À1
.
It is also worth noting that the total activity of
naturally occurring radionuclides in sea water is
approximately 12 kBq m
À3 . Most of this activity
is from the long-lived naturally occuring isotope,
40 K(t 1/2 ¼ 1.25 Â 10
9 years). Activities contributed
from anthropogenic radionuclides greatly exceed this
(millions of Bq m
À3 ) in the immediate vicinity of the
Sellafield and La Hague outfall pipes. However, most
oceanographic studies of these releases involve
measurements of much lower activities: up to 10s of
Bq m
À3 in the case of
137 Cs, and generally even lower
for other radionuclides, for instance mBq m
À3 or less
for plutonium isotopes and
129 I.
Nuclear Fuel Reprocessing and
Resulting Tracer Releases
Origin and Description of Reprocessing Tracers
Nuclear fuel reprocessing involves the recovery of
fissile material (plutonium and enriched uranium)
and the separation of waste products from ‘spent’
(used) fuel rods from nuclear reactors. In the process,
fuel rods, which have been stored for a time to allow
short-lived radionuclides to decay, are dissolved and
the resulting solution is chemically purified and
separated into wastes of different composition and
activity. Routine releases from the plants to the environment occur under controlled conditions and are
limited to discharge totals dictated by the overseeing
authorities of each country. The discharges have
varied over the years as a function of the amount and
type of fuel processed and changes in the reprocessing technology. The discharge limits themselves have
changed, in response to monitoring efforts and also
spurred by technological advances in waste-treatment capabilities. As a general rule, these changes
have resulted in decreasing releases for most nuclides
(most notably cesium and the actinide elements), but
there are exceptions. For instance, the Enhanced
Actinide Removal Plant (EARP) was constructed at
the Sellafield site in the 1990s in order to enable the
additional treatment and subsequent discharge of a
backlog of previously stored wastes. Although the
new technology enabled the removal of actinide
elements from these wastes, it is not effective at removing
99
Tc. Therefore an allowance was made for
increased discharge of
99 Tc, up to 200 TBq per year.
The resulting pulse of increased
99
Tc discharges from
Sellafield beginning in 1994 is currently being followed with great interest, as is discussed in more
detail below.
The end result of these processes is the availability
of a suite of oceanographic tracers with different
discharge histories (e.g., in terms of the timing and
magnitude of spikes) and a range of half-lives, and
thus with a range of utility and applicability to
studies of oceanographic processes at a variety of
spatial and temporal scales. A brief summary of the
reprocessing radionuclides most widely applied in
oceanography is shown in Table 1, and examples of
discharge histories are given in Figure 1. It has also
been particularly useful in some cases to measure the
ratio of a pair of tracers, for instance
134 Cs/
137 Cs,
137 Cs/
90 Sr, or potentially,
99 Tc/
129 I. The use of isotope ratios can (1) provide temporal information and
aid the estimation of rates of circulation, (2) mitigate
the effects of mixing which will often alter individual
concentrations more than ratios, and (3) aid in distinguishing the relative contributions of different
sources of the nuclides in question. Finally, differences in the chemical behavior of the different
elements released can be exploited to study different
processes. Although most of the widely applied tracers are largely conservative in sea water and therefore serve as tracers of water movement, the
actinides, particularly Pu, have a high affinity for
particulate material and accumulate in the sediments. These tracers are then useful for studying
sedimentary processes.
The Reprocessing Tracer Source Function
The primary difference between the north-western
European reprocessing releases and other anthropogenic tracers used in oceanography is the nature of
their introduction to the oceans. Reprocessing releases enter the oceans essentially at a point source,
rather than in a more globally distributed fashion as
is the case for weapons test fallout or the chlorofluorocarbons. Thus, reprocessing tracers are excellent, specific tracers for waters originating from
north-western Europe. Because these waters are
transported to the north into the Nordic Seas and
292 NUCLEAR FUEL REPROCESSING AND RELATED DISCHARGES