Other artificial radionuclides in seawater

loviisa

In addition to ⁴⁰K (naturally-occurring), ¹³⁷Cs, ¹³⁴Cs (from Chernobyl fallout),

90Sr (mainly from global fallout) and tritium, observations of other artificial radionuclides have been relatively few in the seawater samples of the regular environmental monitoring programme. The detected gamma-emitting radio-nuclides were mainly fission or activation products of local origin, such as ⁵⁴Mn,

58Co, ⁶⁰Co, ^110mAg, ¹²⁴Sb and ¹²⁵Sb, and were mainly detected in the seawater samples taken just in front the cooling water outlet.

Predominantly, local discharge nuclides appeared in seawater samples during the first operational years of the power plant. Small amounts (< 60 Bq m^-3) of e.g. ⁹⁵Nb, ⁹⁵Zr, ¹²⁴Sb and ¹²⁵Sb were then detected even more widely, i.e., at the sampling stations Loviisa 1, 2, 4 and 10 (Koivulehto et al. 1979, 1980). In the special discharge-monitoring-survey in December 1979 (see above), relatively high concentrations of ¹²⁴Sb (maximum 5 500 Bq m^-3), ⁶⁰Co (maximum 1 000 Bq m^-3)

Fig. 82. Annual mean concentrations of ¹³⁷Cs in surface seawater (Bq m^-3) at the sampling stations at Loviisa in 1975 – 2007.

Fig. 83. Annual mean concentrations of ¹³⁷Cs in surface seawater (Bq m^-3) at the sampling stations at Olkiluoto in 1977 – 2007.

and ¹²⁵Sb (maximum 670 Bq m^-3) were also detected in samples taken from the immediate vicinity (< 300 m) of the cooling water outlet, but almost without exception, not at the more distant stations. On the 28^th of December 1979, four days after the last discharge batch, small amounts of ⁶⁰Co (5.2 Bq m^-3) and ¹²⁴Sb (35 Bq m^-3) were detected in a seawater sample taken from the strait leading to the south from Hästholmsfjärden Bay (distance 1 km from the outlet).

Since 1980, local discharge nuclides were detected only at the cooling water outlet (Station 02), except for three separate observations of ¹²⁵Sb at Stations 1 and 4 in Klobbfjärden and Vådholmsfjärden in 1988 (Klemola et al. 1991). Small amounts of local discharge nuclides were detected for the last time in seawater samples in March 1993. The activity concentrations of

54Mn, ⁵⁸Co, ⁶⁰Co and ^110mAg were then 19.5, 1.6, 5.6 and 3.9 Bq m^-3, respectively, at Station Loviisa 02 (Klemola et al. 1998). Since then, local discharge nuclides were not detected in the seawater samples of the regular monitoring programme.

The average activity concentrations of ⁹⁰Sr in the surface water at the cooling water outlet (Station 02) and at Reference Station R1 during the operational period of the power plant are given in Table 13. The year 1986 was passed over in the average calculations, because of the momentarily higher values, which were certainly caused by the Chernobyl fallout. On the 9^th of May 1986, the activity concentration of ⁹⁰Sr was 91 Bq m^-2 at Reference Station R1 and 36 Bq m^-3 at Station 02 (Ilus et al. 1987). In August, the values had already decreased to 25 and 27 Bq m^-3, respectively.

The concentrations decreased in accordance with the half-life of ⁹⁰Sr, and the results of the two stations did not significantly differ from each other. The slow decrease of strontium is explained by its slow transfer into sediments, while it remains in the water phase. The effective ecological half-life of ⁹⁰Sr in seawater was 16.4 ± 0.1 years between 1989 and 2007, i.e., about half of the physical half-life. The difference is due to several ecological factors, such as sedimentation of strontium (slower than that of caesium), mixing of water masses, removal from coastal areas by sea currents, etc.

olkiluoto

At Olkiluoto, local discharge nuclides were detected in regular seawater samples more often and more widely, but in smaller quantities than at Loviisa. All in all, ⁶⁰Co was detected 35 times, ⁵⁴Mn 15 times, ¹²⁵Sb 9 times,

58Co 4 times and ^110mAg once in the water samples taken from the Olkiluoto 2, 3, 10, 13 and 15 stations in 1981 – 1998. The highest activity concentration was 32 Bq m^-3 of ⁶⁰Co at the Station 13 in Iso Kaalonpuhti Bay in May 1991.

Once (in August 1993), ⁶⁰Co was detected in the seawater at Station 15,

situated 10 km north of the power plant (5 Bq m^-3). Since 1998, local discharge nuclides were not detected in the seawater samples of the regular monitoring programme.

The activity concentrations of ⁹⁰Sr in the seawater were slightly lower than at Loviisa during the whole monitoring period. In 1977, the background level was 28 Bq m^-3 (Koivulehto et al. 1979). The Chernobyl fallout in 1986 raised the values only slightly (maximum 29 Bq m^-3), but on the other hand, the values returned to their former level only after five years. In 2001 – 2007, the average activity concentration of ⁹⁰Sr was 11.9 ± 2.5 Bq m^-3 at Station 13 in Iso Kaalonpuhti Bay and 11.4 ± 2.6 at Station 15 situated 10 km north of the power plant. At Olkiluoto, the effective ecological half- life of ⁹⁰Sr in seawater was about 20.6 years between 1989 and 2007.

In conclusion

Tritium (³H) is ubiquitous in the aquatic environment and has a variety of sources (Jacobs 1968). It is a cosmogenic radionuclide with a half-life of 12.43 years, and is continuously produced in the upper atmosphere as a result of cosmic-ray-induced spallation and particle interaction with atmospheric nitrogen and oxygen (McCubbin et al. 2001). These processes produce most of the world’s natural tritium (IAEA 2004a). The levels of tritium rose as a legacy of the atmospheric nuclear weapons tests between 1952 and 1962 that resulted in the injection of about 240 EBq into the earth’s atmosphere (UNSCEAR 1993).

During recent decades, the levels of fallout ³H have continuously decreased, but plenty of tritium remains in the environment, mostly diluted in the oceans (IAEA 2004a).

Nevertheless, ³H is also produced by a variety of processes in nuclear power plants, and consequently, it is the predominant radionuclide both in airborne and liquid discharges from the nuclear power plants, not only in Finland but also Table 13. Average activity concentrations of ⁹⁰Sr (Bq m^-3) in the surface water of the Loviisa 02, 2 and R1 stations during 1973 – 2007.

Period Loviisa 02 Loviisa 2 Loviisa R1

1973 – 1976 30.5 ± 2.4*

elsewhere. Discharges from nuclear power plants into the aquatic environment result in locally enhanced water concentrations of tritium (McCubbin et al. 2001).

Tritium is produced in reactors by neutron activation of ²H, ³He, ⁶Li and ¹⁰B (IAEA 2004a). The formation of tritium by the activation reactions in pressurised water reactors (Loviisa) is considered to be mainly from the boron in the coolant water, which is used for reactivity control. In boiling water reactors (Olkiluoto) the contribution of tritium by activation reactions is mainly from the boron in the control rods. Tritium activities discharged from boiling water reactors into the environment are lower than those of pressurised water reactors, because less tritium is produced in or diffuses into the primary coolant (IAEA 2004a). Tritium is produced in light-water reactors in quantities that are relatively copious compared to other radionuclides. Fortunately, tritium, which emits beta particles of very low energy, ordinarily enters the environment in the form of water. It does not concentrate significantly in biological systems, and has a relatively rapid turnover rate (Eisenbud and Gesell 1997). Tritium emits low-energy β-particles and therefore does not pose an external radiological hazard (Phillips and Easterly 1981).

Annual discharges of tritium from the Loviisa NPP into the aquatic environment have varied between 1.2 ∙ 10¹² Bq (1977) and 1.7 ∙10¹³ Bq (2004 and 2006) during the operational period of the power plant, composing 1.2 – 11.3%

of the annual release limit (1.5 ∙ 10¹⁴ Bq) set for the liquid effluents of tritium from this power plant. The total discharge of tritium from the Loviisa NPP into the aquatic environment since 1977 was 3.30 ∙ 10¹⁴ Bq, and the decay-corrected cumulative discharge until 2007 was 1.82 ∙ 10¹⁴ Bq.

Annual discharges of tritium from the Olkiluoto NPP into the aquatic environment have varied between 8.6 ∙ 10⁹ Bq (1978) and 3.6 ∙10¹² Bq (1993) during the operational period of the power plant, composing 0.05 – 20% of the annual release limit (1.8 ∙ 10¹³) set for the liquid effluents of tritium from this power plant. The total discharge of tritium from the Olkiluoto NPP into the aquatic environment since 1978 was 4.14 ∙ 10¹³ Bq, and the decay corrected cumulative discharge until 2007 was 2.28 ∙ 10¹³ Bq.

In Finnish coastal waters, the concentrations of ³H from natural sources and nuclear weapons fallout decreased from about 10 – 15 kBq m^-3 to less than 4 kBq m^-3 between the late 1970s and the early 2000s (Ilus et al. 2008). The detection limit of our analyses of tritium in seawater was 7 kBq m^-3 until 1992, and 4 kBq m^-3 since 1993. Elevated ³H concentrations were more frequent at Loviisa (Figs. 73 – 80), which was due to the larger discharges and to the slower exchange of water in the Loviisa discharge area.

In 2007, the mean activity concentration of ¹³⁷Cs in seawater was 28.3 ± 2.8 (range 23 – 32 Bq m^-3) at Loviisa and 43.7 ± 5.5 (33 – 50 Bq m^-3) at Olkiluoto. Since

the Chernobyl accident (1986), the caesium concentrations have decreased more rapidly at Loviisa, and in the whole Gulf of Finland, than at Olkiluoto and in the Bothnian Sea. In Figure 84, Station LL3a represents the open sea off Loviisa and Station EB1 that off Olkiluoto. Between 1986 and 2007, the decrease in ¹³⁷Cs values was about 94% at Loviisa and 87% at Olkiluoto (including the physical decay of ¹³⁷Cs). The main reason for the differently decreasing rates of caesium was the more effective water exchange between the Gulf of Finland and the Baltic Proper than that between the Bothnian Sea and the Baltic Proper. On the other hand, in the Loviisa area, the archipelago also retards the exchange of water more effectively than at Olkiluoto, and consequently, the exit rate of caesium from seawater in the discharge area. The activity concentrations of ¹³⁷Cs in the vicinities of the Loviisa and Olkiluoto power plants did not differ from those in the open Gulf of Finland and Bothnian Sea (Fig. 84).

Fig. 84. Late-summer mean concentrations of ¹³⁷Cs in surface seawater (Bq m^-3) at Loviisa and Olkiluoto, and the nearest offshore stations LL3a (Gulf of Finland) and EB 1 (Bothnian Sea) in 1972 – 2006.