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Carbon-14 in Terrestrial and Aquatic Environment of Ignalina Nuclear Power Plant:Sources of Production, Releases and Dose Estimates Jonas Mazeika Nature Research Centre Vilnius University Lithuania

1. Introduction The development history of nuclear power in world already has over passed the limit of 50 years. This time span was sufficiently long for many nuclear reactors to complete their operation stage and to enter the decommissioning stage. The Ignalina NPP (INPP), Lithuania, is one of them. Its operation history only lasted for 26 years for different reasons but mainly the political ones. The INPP consists of two RBMK-1500 reactor units, Unit 1 and Unit 2 (Almenas et al., 1998). The ‘‘1500’’ refers to the designed electrical power in units of MW. Its designed thermal rating is 4800 MW. The nominal thermal power is 4250 MW, and the nominal electrical power is 1300 MW. The RBMK is a graphite-moderated boiling water channel-type reactor with the principle of electricity generation the same as for boiling water reactors (BWRs). The Ignalina NPP is located in the north-eastern part of Lithuania……..

The routine monitoring of radiation in environment of NPPs often does not include some important nuclides, namely carbon-14 (14C), which have or may have significant contribution to effective dose of human exposure in the whole nuclear fuel cycle. ……….

At the beginning of the 19th century, the concentration of 14C, compared to the stable isotope 12C, in the atmosphere began to decrease due to the increased burning of fossil fuel and consequent anthropogenic emissions via the Suess effect (Fairhall & Young, 1970).

In the 1950s, when many atmospheric nuclear weapon tests took place, the 14C concentration in the air rose sharply to a maximum (double the natural concentration), and gradually decreased in 1960s and later after cessation of atmospheric tests. The 14C activity in the atmosphere has decreased steadily due to CO2 uptake by the oceans and by the biosphere (Hertelendi & Csongor, 1982; Levin & Kromer, 2004).

14C is also artificially produced in all types of nuclear reactors by the similar neutron-induced reactions on isotopes of carbon, nitrogen and oxygen present in the fuel, cladding, coolant, moderator and structural materials of the reactor (NCRP, 1985; IAEA, 2004). A fraction of the generated 14C is released continuously during normal operation of NPPs, mainly in two chemical forms; oxidized, i.e. carbon dioxide (CO2), and reduced, which mostly is in the form of CH4 (Levin et al., 1988).

For all types of reactors, except pressure water reactors (PWRs), most of the gaseous releases of 14C are in the form of 14CO2 (IAEA, 2004). 14C is referred to as one of the difficult-to-measure nuclides (pure beta emitters) due to the presence of other radionuclides in a sample. 14C is rarely measured in process media of nuclear reactors and even in gaseous releases from nuclear plants notwithstanding that equipment for monitoring 14C in gaseous releases is today commercially available.

However, releases of gaseous 14C from nuclear power reactors result in prevailing dose fraction compared to all radionuclides. This is a case for the INPP (Nedveckaite et al., 2000). In 2002, the Government of Lithuania approved the adoption of the ‘‘immediate dismantling’’ strategy for decommissioning of both INPP units.

A key component of this decommissioning strategy is to dispose of operating and decommissioning waste in a nearsurface repository (NSR). 14C as a long-lived and mobile in the environment radionuclide and due to large inventory has been recognized as one of the most important nuclides in the assessments of doses for future generations arising from INPP NSR for low- and intermediate-level, short-lived radioactive waste (LILW) disposal. Based on the scaling factor method (Lukauskas et al., 2006; Plukis et al., 2008), a preliminary inventory of 14C for NSR with capacity for disposal of approximately 100 000 m3 of waste was evaluated as 1.43×1013 Bq.

Similar figures of 14C inventory were evaluated for NSRs in other countries: Japan, Rokkasho-Mura disposal facility, first 40 000 m3, maximal inventory – 3.37×1012, Spain, El Cabril disposal facility, maximum permissible activity – 2.00×1013 Bq (IAEA, 2004). From radioactive waste long-term management point of view, the importance of 14C is also due to the large total mass of graphite per RBMK-1500 reactor (more than 1800 tons) and consequently there is a large 14C inventory in graphite (IAEA, 2006).

Based on calculations of graphite impurities activation, 14C and tritium contribute mostly (respectively 3.91×1014 Bq and 2.19×1014 Bq) to the total activity of all the irradiated graphite constructions of the reactor (Ancius et al., 2004). It is the rough upper limit estimate, derived assuming that activation products are retained in the graphite. Activated graphite of INPP due to the high specific activity (up to 5×105 Bq/g) of the long-lived 14C is classified as the long-lived radioactive waste.

The worldwide nuclear power operational experience gives evidence that 14C is continuously released to environment from NPPs, is key radionuclide of NSRs for LILW disposal, forms significant fraction of irradiated graphite radionuclide inventory, retains in the spent nuclear fuel and consequently will be disposed of in geological repositories for long-lived high activity waste in the form of spent fuel or radioactive waste arising from spent fuel reprocessing. In this sense, we can consider 14C as one of the most powerful environmental tracers of nuclear fuel cycle.

Because of the biological importance of carbon ( 12C, 13C and 14C) and, consequently, the biological incorporation of radioactive 14C through photosynthesis, it is of great interest to run 14C measurements in environment, especially if 14C is not monitored in releases as was a case in the INPP operational history.

Considering that 14C specific activities of terrestrial and aquatic plants may vary according to the atmospheric 14C/12C ratio, many studies have investigated atmospheric and sometimes aquatic 14C activities in the vicinities of nuclear power plants using vegetation as bioindicators (Obelic et al., 1986; Levin et al., 1988; Loosli & Oeschger, 1989; Buzinny et al., 1995; Milton et al., 1995; Milton & Kramer, 1998; Stenström et al., 1996; Mikhajlov et al., 1999; Magnusson et al., 2004; Mazeika et al., 2008).

The main aim of this study is to investigate how terrestrial and aquatic vegetation (organic carbon, Corg) and dissolved inorganic carbon (DIC) of aquatic environment recorded the 14C activity excess compared to the contemporary 14C background level during the whole period of the INPP operation (the end of 1983–the end of 2009). The 14C activity excess data allow evaluating the order of magnitude of gaseous and liquid release rates due to operation of the NPP what can partially substitute direct 14C release monitoring data and give basis for approximate dose estimates.

The other goals are: to give closer look on 14C activity aerial www.intechopen.com……….. For all types of reactors, except pressure water reactors (PWRs), most of the gaseous releases of 14C are in the form of 14CO2 (IAEA, 2004). 14C is referred to as one of the difficult-to-measure nuclides (pure beta emitters) due to the presence of other radionuclides in a sample. 14C is rarely measured in process media of nuclear reactors and even in gaseous releases from nuclear plants notwithstanding that equipment for monitoring 14C in gaseous releases is today commercially available. However, releases of gaseous 14C from nuclear power reactors result in prevailing dose fraction compared to all radionuclides. This is a case for the INPP (Nedveckaite et al., 2000). ………

….. to present first experimental data on 14C activities in different process water systems of the INPP reactor and in various water clean-up systems, namely, in evaporator concentrates and spent ion-exchange resins (taken in 2002). This chapter mostly presents the summarization of 14CO2 specific activities in terrestrial and aquatic plants and H14CO3 – specific activities in aquatic environment measured during various research projects performed in the INPP vicinity.

2. Methods 2.1 14C production in nuclear reactors The normal operation of NPPs produces various radionuclides by fission within the fuel or by neutron activation in the structural materials and component systems of the reactor. The escape of these radionuclides from the reactor and its auxiliary process systems during operation time generates a variety of solid, liquid and gaseous radioactive waste. Despite that the design of the reactor ensures minimized escaping of radionuclides from technological systems, some radionuclides, namely 14C, are continuously discharged in various effluents to environment. The major 14C producing neutron activation reactions in NPP reactors are (IAEA, 2004; Buchuev et al., 2002): ……….

……………….. Conclusion The 14C concentration near the INPP has been studied by analysing various samples from terrestrial and aquatic environments and groundwater using conventional liquid scintillation counting. A comparison between the specific activity of 14C at short distances from the INPP and 14C specific activity of the background level has revealed the 14C excess over the whole operation period of the INPP (the end of 1983–the end of 2009).

By studying tree rings and annual terrestrial plants in the terrestrial environment mostly continuous 14C excess was delineated recently approaching 10 pMC that can be formed by 14CO2 annual release of magnitude 1013 Bq with the maximal value of normalized release rate 11 TBq×GW(e)–1×year–1. In comparison with the other radionuclide releases from the INPP and respective doses (Motiejunas et al., 1999), the effective dose resulting from the 14C is the highest reaching 2.0×10–3 mSv/year. Nevertheless, this dose estimate is much lower than the 14C dose occurring from the 14C of natural production in the atmosphere (~12×10-3 mSv/year). www.intechopen.com308 Nuclear Power In the aquatic environment, the 14C activity in DIC from the cooling basin increased only in 2002–2006 with the 14C excess of 30–35 pMC above the background level what could be caused by the 14C release rate from the INPP to water 108 Bq/year. The highest 14C excess in aquatic plants (up to 65 pMC) with significant variability was detected in the period 1995–2008. The groundwater system was rarely and insignificantly influenced by the INPP. Thus the effective dose resulting from the 14C in the aquatic environment was very low.

It was confirmed that the majority of 14C contained in the gaseous waste (e.g. reactor offgases) was released from the INPP to the atmosphere during the operation time with the total 14C activity roughly of 2.0×1014 Bq. Only a small quantity of 14C was released via liquid effluents from the INPP. A large quantity of 14C is retained in the structural materials of the reactor (graphite moderator mainly with 14C of 7.82×1014 Bq per two units) and in the radioactive waste (LILW with 14C of 1.43×1013 Bq), which will be handled during the INPP decommissioning phase.

For waste containing high levels of 14C, such as graphite, that may exceed the waste acceptance requirements for the near surface disposal, no disposal facility is available at present. This waste only can be kept in an interim storage until a suitable disposal facility (e.g. deep geological disposal) or other disposal and treatment alternatives become available. 5. References …….http://cdn.intechopen.com/pdfs-wm/11575.pdf