What Type Of Reaction Occurs When Carbon-14 Changes To Carbon-12

Final update on nine August 2012

Contents

Summary

Emitting b radiation with a half-life of 5730 years, Carbon fourteen follows the wheel of the stable chemical element C, ane of the components of the living materials, in which information technology is diluted. Carbon-14 is indeed around ten^-12 times less abundant than stable carbon. The master source of exposure is due to naturally occurring ^fourteenC (cosmogenic origin).

With regard to the impact of chronic releases, the general consensus is that ¹⁴C behaves in the same mode as the stable ¹²C isotope (representing 99% of carbon). Carbon-xiv transfers between two compartments of the surround are generally evaluated based on the assumption that the isotopic ratio between the radioactive carbon and the stable carbon (considered to be ¹²C) is maintained, between the organism and the surrounding environs. This assumes that the transfer of the trace radionuclide ¹⁴C is identical to that of ¹²C and that equilibrium between the two compartments is achieved. Nether this assumption, the affect on the surroundings and populations can merely exist evaluated for environmental releases and concentrations that are abiding over fourth dimension, generally past using average annual values.

The ecology toxicity of ^fourteenC is just related to radioactive emissions of the pure, low-energy b type. This toxicity is mainly the upshot of internalisation, essentially by ingestion.

Characteristics

Chemical characteristics

Carbon-14 (¹⁴C) is a radioactive carbon isotope present in minute quantities in the atmosphere. Carbon-12 and carbon-13 are the stable carbon isotopes and respectively represent 98.9% and 1.1% of the full carbon. Carbon-14 but exists in trace quantities. The chemical forms of ¹⁴C vary co-ordinate to the method of production. In the environment, ¹⁴C exists in two primary forms:

- as^xivCO₂, it acts as stable carbon dioxide, which means it can remain in gas form in the air, condign bicarbonate and carbonate in water

- during photosynthesis, ¹⁴CO₂ is incorporated in the organic material, forming its carbon skeleton. Equilibrium between the specific activity of atmospheric carbon and that of organic material is then finally reached and maintained by carbon recycling.

In the gaseous effluents of humid water reactors, ^xivC is 95% ^xivCO₂, ii.5% carbon monoxide (¹⁴CO) and 2.5% hydrocarbon. In the gaseous effluents of pressurised water reactors, it is assumed that eighty% of ¹⁴C is in organic grade (¹⁴CH_four) whereas 20% is in the course of ¹⁴CO₂. In the liquid releases, the carbon chemical species are carbonates and diverse organic compounds, their relative quantities being currently unknown.

Nuclear characteristics

Carbon has 15 isotopes, with masses of viii to 22. Simply isotopes 12 and thirteen are stable. The radioactive half-life is higher than a year only for carbon-14, its maximum value for the other isotopes being around xx minutes.

Carbon-xiv, a beta ^- emitter, gives ascension to stable ¹⁴Due north with 100% yield.

(Nucleonica; CE, 2011)

O rigins

Natural origins

Natural ¹⁴C results from cosmic neutrons acting on nitrogen atoms in the stratosphere and in the upper troposphere (¹⁴N +due north →¹⁴C+¹p). The annual production level is around 1.40 x 10¹⁵ Bq and the atmospheric stock of carbon-14 at equilibrium is around 140 ten 10¹⁵ Bq (UNSCEAR, 2008). Production fluctuates due to variation in cosmic ray intensity. This fluctuation results from various factors that are not nevertheless well understood, just mainly include the 11-twelvemonth solar cycle and, on a larger temporal scale, variations in the terrestrial magnetic field that serves every bit a shield against cosmic rays (Garnier-Laplace et al., 1998).

Artificial origins

Fallout from atmospheric nuclear explosions

During nuclear explosions, the emitted neutrons interact with atmospheric nitrogen, as catholic neutrons do, to course carbon-fourteen, according to the aforementioned reaction every bit to a higher place: ¹⁴N +n →^xivC+¹p.

Nuclear explosions carried out before 1972 released around 3.5 x 10¹⁷ Bq of carbon-14. Later on explosions increased this amount by around 1% (UNSCEAR, 2008).

Nuclear reactor releases

In nuclear reactors, carbon-14 is produced from reactions in the fuel, the core structural materials and the moderator. The production rate depends on the spectrum and the neutron flux, on cross-sections and on the concentration of the post-obit target elements: uranium, plutonium, nitrogen and oxygen. H2o in the master coolant circuit of pressurised water reactors contains backlog hydrogen that combines with oxygen from radiolysis. In this reducing environment, compounds such as marsh gas (CH₄) and ethane (C₂H₆) form. Most of the carbon-14 released in a pressurised water reactor is in the form of alkanes. Various estimations indicate that the annual production charge per unit for a light water reactor (pressurised or humid water reactor) is between 0.v and 1.nine x 10¹² Bq/GWe/year, with carbon-14 mainly taking organic forms (CH₄). The rest is released during reprocessing, or remains in the fuel cladding and is later disposed of equally solid waste (Garnier-Laplace et al., 1998).

Releases past irradiated fuel reprocessing plants

Spent nuclear fuel ¹⁴C is released during the dissolution step in reprocessing plants. Depending on the operating fashion, these releases are continuous or discontinuous. In reprocessing plants that apply the PUREX process (e.g. the AREVA NC La Hague plant), the ¹⁴C is mainly released as CO_ii. Commissioning of the UP3 and UP2-800 plants at La Hague resulted in increased annual gaseous ¹⁴C releases starting in the early 1990s. In 2009, the gaseous releases of carbon-14 at the site corresponded to 1.45 x 10¹³ Bq and the liquid releases corresponded to 6.12 10 ten¹² Bq. Carbon-fourteen in fuel cladding is not released during dissolution and remains trapped. It is disposed of later as solid waste.

At the Sellafield plant in the UK in 2009, the ^xivC gaseous releases reached iii.8 x x¹¹Bq and the liquid releases, 8.2 ten 10¹²Bq (Sellafield Ltd, 2009).

Diverse sources (medical, industrial, research)

In enquiry, carbon-14 is widely used in carbonate form for isotopic labelling of molecules. The activities used are greater than ane GBq. For example, carbon-14 is used to study metabolic dysfunction related to diabetes and anaemia. Information technology tin can likewise be used as a mark to track the metabolism of new pharmaceutical molecules. More generally, carbon-xiv tin exist used to uncover new metabolic pathways, and to identify their normal functioning and any departures from it, e.g. for photosynthesis (Calvin and Benson, 1948) or, more recently, for the methylaspartate cycle in halobacteria (Khomyakova et al., 2011).

It is assumed that all ¹⁴C used for labelling molecules will exist released into the atmosphere as CO₂. According to UNSCEAR, the annual product of ¹⁴C is equivalent to 3 x 10¹⁰Bq per 1000000 inhabitants in developed countries and to 5 x x^xiiiBq worldwide. This estimation is based on the results of a 1978 US study. A 1987 British interpretation led to values at least twice every bit high (UNSCEAR, 1993).

Environmental concentrations

Carbon-14 groundwork in the environs and changes over the terminal 60 years

In the terrestrial environment, the consensus (relatively well supported past observations) is that the specific activity, expressed in becquerels of ¹⁴C per kilogram of total carbon, is constant in the ecology components and at equilibrium with the specific action of atmospheric CO₂(Roussel-Debet et al., 2006, Roussel-Debet, 2007, 2009). Uninfluenced by nuclear facilities, the ^fourteenC specific activities for the biological compartments of the terrestrial environment reached their maximum values (more than than 400 Bq/kg of C) in the mid-1960s, due to fallout from atmospheric nuclear artillery testing, and then at its pinnacle ( Figure 1 ). These activities have slowly decreased since then (by less than 0.5% per year) with the end of testing and the continuous increase in CO₂ from fossil fuels (gasoline, coal, gas). The specific activities of terrestrial biological compartments are currently effectually 238 Bq¹⁴C/kg C (2009 measurements), which is very close to 1950 values (226 Bq/kg C), before atmospheric testing.

Figure i:Changes in average specific activity of carbon-fourteen (Bq/kg C) for biological compartments sampled in terrestrial environment, during the last threescore years

In aquatic environments, the specific activity of ¹⁴C varies with its dilution in carbon substances, particularly carbonates from onetime sedimentary rocks lacking carbon-fourteen. Unlike the terrestrial surround, ¹⁴C in freshwater ecosystems is not in equilibrium with atmospheric CO₂: freshwater specific activity is and then lower, around 200 Bq/kg C.

Based on the specific activity and the total proportion of carbon in the diverse environmental matrices (air, plants, animals and thus nutrient products), the activity concentration for the ^fourteenC in these matrices tin be estimated ( Effigy ii ). The more than carbon the product contains (sugars, oils, grains, etc.), the higher the activeness.

Figure 2: Carbon-14 activeness concentration range for food products

Depending on the proportion of carbon per wet mass unit of food product, the action concentration of these products varies between less than 15 (lettuce, mussels) and more than 80 (grains) Bq/kg wet. Atmospheric activities vary from 3 ten ten^-2 to seven x 10^‑ii Bq/yardⁱⁱⁱ. Carbon-14 thus has the highest environmental activities amidst the radionuclides released from nuclear facilities.

Influence of nuclear facilities

With atmospheric releases of around 2 x 10^thirteen Bq/year of ¹⁴C, mainly as CO₂, the AREVA-NC La Hague plant causes an added carbon-14 activity (higher up the natural background) regularly detectable in the site'south terrestrial environment, leading to specific activities of 500 to 1000 Bq/kg C, and occasionally 2000 Bq/kg C. The corresponding action concentrations range from 20 to 140 Bq/kg of moisture grass or vegetables, compared with a background of around 5 to 20 Bq/kg of moisture fabric in this type of matrix. In milk and meat, this contamination is also meaning although much less so, probably due to a feeding component outside the surface area influenced past the atmospheric releases. Note that the maximum radioactivity in the air at basis level after dispersion, gear up at 1 Bq/mⁱⁱⁱ past the French social club authorising AREVA-NC La Hague releases, would correspond to specific activeness in plants of 5000 Bq/kg C, if attained at all times throughout the yr.

The carbon-14 addition around nuclear ability plants (atmospheric releases of 0.2 to 1x10¹² Bq/twelvemonth) is extremely express: the associated specific activity is around 3 Bq/kg C in improver to the 243 Bq/kg of C representing the average background for 1994-2003 (Roussel-Debet et al., 2006), i.e. an added activity of around i%. This low level is the outcome not simply of low releases, but likewise of a articulate predominance of releases in the course of methane (CH₄), which plants cannot digest.

In rivers, the carbon-14 released by nuclear ability plants is diluted in the dissolved stable carbon from carbonates, which are found in sediment. This significantly decreases the specific activity of carbon-14 in physical components. For semi-underwater aquatic plants, dilution too occurs in the atmospheric CO_two used during photosynthesis; the associated specific activities rarely exceed 400 Bq/kg C. For reasons that remain to exist elucidated, fish practise non seem to benefit from these dilution phenomena. Their specific action under the influence of nuclear power plants regularly exceeds 600 Bq/kg C and may reach 1000 Bq/kg C.

Metrology, belittling techniques and detection limits

Carbon-14 in an environmental sample may be quantified by activity measurement or by atom counting. These two destructive techniques crave converting the sample to CO₂(Maro et al., 2008).

Activity measurement

Principle

The carbon contained in the test portion is transformed to carbon dioxide from which a sample is prepared for measurement by liquid scintillation (AFNOR, 2006).

Two sample preparation methods are mainly used: combustion past oxydiser and benzene synthesis (Fournier et al., 1999).

Preparation of samples by oxydiser

The sample is placed in a cellulose cone, which is inserted in a platinum filament. The unabridged unit is placed in a combustion sleeping accommodation. Voltage applied to the ends of the filament in the presence of O₂ causes combustion of the sample. The combustion gases are pushed by nitrogenin a column containing Carbosorb®, which traps CO_two in the form of carbamate. This mixture is eluted from the column by the scintillation liquid so nerveless for measurement.

The oxydiser allows to prepare several samples per mean solar day for counting. The test portions are by and large less than 0.5 g of the dry sample. They must exist rich enough in carbon to undergo a complete oxidation.

Combustion yield must exist determined on a reference sample labelled for ^fourteenC. This reference sample must exist as shut equally possible in nature and composition to the samples to be analysed.

The ¹⁴C naturally independent in the combustion cone cellulose contributes to the increase in groundwork and thus in higher measurement uncertainty. Background must thus be determined equally precisely as possible.

The expression of the sample's activity in Bq of ^fourteenC per kg of carbon also requires measuring its uncomplicated carbon content, generally by gas chromatography.

The measurement uncertainty, around xxx to 40% (k=2) for activities of around 260 Bq.kg^-ane of carbon (natural level in the environment), makes it difficult to discover low concentrations with this method. This incertitude can, all the same, be reduced past increasing the test portions or by combining the measurements of several test portions from the aforementioned sample.

¹⁴ C analysis by benzene synthesis

The sample is burned in the presence of under pressure oxygen in a combustion bomb. The CO_twoproduced is and so reduced by a heated reaction with lithium to obtain lithium carbide (Li_twoC_ii), the hydrolysis of which produces acetylene (C₂H_two), which is trimerised past catalysis in benzene (C₆H₆).

The counting vial is prepared by weighing out synthesised benzene and scintillants. Spectroscopy-quality benzene is added if needed.

The activity of the ¹⁴C present in the vial is and so measured using liquid scintillation. The consequence can be directly converted into Bq/kgof carbon.

The test portions consist of 7 to x g of finely ground, dry out sample. The chemic processing fourth dimension for ane sample is 3 days, 2 more than days being necessary for counting. Uncertainty at the level of the environmental background corresponds to vi to 7% (k=2).

This method is suitable for solid dry out samples containing high carbon and for water matrices in the grade of carbonate (e.g. barium carbonate). For h2o matrices, CO_two is extracted from the sample by acid attack (east.k. addition of orthophosphoric acrid) rather than by combustion bomb. The rest of the protocol does not change.

The analysis methods involving oxydiser or benzene synthesis are not well suited for carbon-poor matrices, such as soil and sediment.

Atom counting

Principle

The carbon present in the sample is extracted in the form of ions. The carbon ions are accelerated, sorted by mass in a magnetic field which alters their trajectory. They are so counted.

¹⁴ C measurement by accelerator (AMS)

Subsequently decarbonation and combustion of the sample, the CO₂ obtained is reduced by H₂in the presence of powdered iron. The carbon is deposited on the powdered iron and the mixture is pressed into a target to allow for measurement by mass spectrometry. The sample's ^fourteenC activity is calculated by comparing ¹⁴C, ¹³C and ¹²C beam intensities, measured sequentially, with the CO₂ reference intensities.

The test portions consist in around 0.10 thousand of material. Incertitude at the level of the environmental background corresponds to 2 to 3% (one thousand=2).

Accelerator Mass Spectrometry (AMS) is characterised by high sensitivity, which is obtained past good separation of ¹⁴C from other ions having the aforementioned mass (specially nitrogen). It is favoured for low-quantity samples or those containing low levels of organic materials (soil, sediment, body of water water, air samples, etc.).

Expression of results

The activity concentration results are expressed in Bq/kg of dry textile, Bq/kgof wet fabric or Bq/kgof carbon.

Mobility and bioavailability in terrestrial environments

Carbon-14 data and the models on the fate of this radionuclide in terrestrial environments (Scott et al., 1991; Sheppard et al., 1994; Garnier-Laplace et al., 1998; Fontugne et al., 2004; Tamponnet, 2005a and b) are based on knowledge of the carbon cycle at equilibrium (Ouyang and Boersma,1992). Carbon-14 is integrated in the carbon bike, which is very complex due to the presence of inorganic and organic carbon, in solid, liquid or gaseous forms ( Effigy three ).

Effigy 3: Carbon cycle in soil-plant-animal systems

Soil

The average quantity of carbon in organic material of cultivated soils is in French republic effectually 20 g of carbon per kg of dry out soil. The soil solution carbon can be in the grade of CO_two, carbonate (CO_three ^ii-) or bicarbonate (HCO₃ ^-), depending on the pH and the quantity of calcium ions.

Plants

The average CO₂quantity of gaseous soil phase varies from 0.v to ane%. It increases in the presence of plants (due to root respiration, the pH decreases and the dissolved CO_ii increases past around 38% per pH unit).

Root assimilation of carbon by plants is negligible. Root incorporation from carbonate ions, poorly understood, appears to represent 5% maximum of the full carbon incorporated in a found. Virtually of the carbon is assimilated by leaves every bit CO₂ during photosynthesis. Isotopic discrimination, which depends on the plant's photosynthetic cycle, is negligible (¹⁴C /¹²C ratio less than v% maximum betwixt the plant and the atmospheric CO₂).

CO₂emanation from the mineralisation of organic soil residues and root respiration tends to increase the concentration of CO_two in the air, at the plant cover level. The daily flux of CO_ii released past the soil appears to be 2 to 13 g per m². This flux appears to contribute effectually 10% to the full carbon assimilated by leaves during photosynthesis (Le Dizès-Maurel et al., 2009).

Animals

More than 99% of the carbon incorporated by livestock comes from their feed. Carbon from inhalation is negligible, as is carbon from ingestion of water or soil.

Mobility and bioavailability in continental aquatic environments

Carbon-14 information and the models on the fate of this radionuclide in continental aquatic environments (Sheppard et al., 1994; Garnier-Laplace et al., 1998) are based on knowledge of the carbon wheel at equilibrium (Stumm and Morgan, 1981; Amoros and Petts, 1993).

The ¹⁴C organic compounds released by nuclear facilities are incorporated into the organic carbon of the hydrosystem that receives them (Figure 4).

Figure 4: Carbon cycle in freshwater hydrosystems

The inorganic carbon released by nuclear facilities or nowadays in the hydrosystem takes the class of species in the carbonate system (CO_{two aqueous}/HCO₃ ^-/CO₃ ^2-), which is one of the main chemical systems involved in controlling freshwater pH. In most running waters, pH varies from half-dozen to 9, with bicarbonate forms dominating. Carbon-14 in liquid effluents, released equally carbonates, is incorporated in the inorganic carbon. Isotopic dilution varies according to atmospheric exchanges, run-off contribution and exchanges with hydrogeological systems. In all cases, the specific action of inorganic ¹⁴C must exist considered in terms of the value measured in situ for total CO₂ , co-ordinate to the post-obit equation: [COtwo]total = [CO2]aq + [HCO3 -] + [COthree ii-].

Water and sediment

Carbon-14 is integrated in the carbon bicycle of continental hydrosystems where the principal forms are organic carbon (dissolved organic carbon/Doc, 1 to three mg of carbon per litre; and particulate carbon, which is highly variable from one hydrosystem to another) and inorganic carbon (essentially in the class of dissolved bicarbonate, 1 to 120 mg of carbon per litre). Humic and fulvic acids represent from 50 to 75% of the Doc, whilst the colloidal forms represent 20%. The particulate forms are also varied: allogenic detrital forms, living organisms and compounds from their decay.

Plants

Transfers to plants are governed by photosynthesis. Photosynthesis is mainly carried out by higher plants, periphytic and planktonic algae, and cyanobacteria. In schematic terms, it tin be considered the dominant biological procedure that influences the concentration of inorganic carbon in the hydrosystem; respiration and bacterial fermentation tin can be considered negligible. On boilerplate, the concentration of total carbon in freshwater plants is 5 x 10⁴ mg of carbon per kilogram of moisture material.

Animals

Transfers to animals are governed past ingestion. For aquatic organisms, the processes of respiration and osmoregulation that utilize inorganic carbon are similarly negligible in the animal's carbon balance compared to transfers via food ingestion. Carbon concentration in animals varies from i species to another.

Mobility and bioavailability in marine environments

The mechanisms of ¹⁴C transfer in marine and freshwater environments are identical, and the models are based on the assumption that equilibrium is reached due to environmental carbon recycling. About of the ¹⁴C released into the sea is in dissolved inorganic grade and is incorporated by organic cloth. Close to release points, when the variations in the quantities released are rapid and big, equilibrium between the specific activities of the organic material and the sea water is not e'er reached (Fiévet et al., 2006).

Ocean water

In the Channel, the research of Douville et al. (2004) indicates that the ¹⁴C in sea water at Cap de La Hague mainly takes the course of dissolved inorganic carbon (dissolved CO_ii,HCO^3-, CO₃ ^2-), which is the predominant form of carbon in ocean water, with activities between 300 and 800 Bq.kg^-i of carbon.

Seeweed

As in the case of freshwater plants, the transfer of ^xivC to seaweed occurs by photosynthesis. The total carbon concentration in seaweed is roughly equivalent to the freshwater constitute concentration. This concentration was found to be eight ´ 10^iv mg of carbon per wet kilogram of the chocolate-brown seaweed Fucus serratus, an case of the algal flora of north-western European coasts. Used as a model compartment for ^fourteenC exchanges between sea water and a photosynthetic organism, this alga was used to estimate a biological half-life for ¹⁴C of around 5 months. The value of this parameter explains the absence of equilibrium close to the release betoken (Cap de La Hague), where the variations in seawater ¹⁴C concentration are large and rapid, due to the history of releases past the AREVA NC reprocessing found (Fiévet et al., 2006).

Animals

Equally in the instance of the terrestrial and freshwater animals, transfers to marine animals are mainly governed by ingestion. Although prison cell membranes are permeable to bicarbonates dissolved in water, the quantity of absorbed carbon that they represent is depression compared to the carbon incorporated in organic fabric. The carbon concentration by unit of wet weight in marine animals varies a peachy deal from ane organism to another, particularly due to the unlike h2o contents (e.g. jellyfish, bivalves, gastropods, echinoderms, crustaceans, fish, etc.). The limpet has been used as a model compartment for ^xivC exchanges between sea h2o and a grazing beast, making it possible to estimate a biological half-life for ¹⁴C of effectually 8 months. This half-life integrates all the transfer pathways between the sea water and the gastropod'southward flesh, including ^xivC incorporation from the animal'southward food source. Biological half-life is estimated to exist effectually 1 month in mussels, which are used as a model of filtering organisms (Fiévet et al., 2006). Although there is great variability in the speed of carbon recycling between sea water and the dissimilar biological compartments, these one-half-life values conspicuously explain why a land of equilibrium is non reached where the sea h2o ¹⁴C concentration may vary rapidly, close to release points for example.

Mobility and bioavailability in semi-natural ecosystems

This section is based on the international literature review conducted for the revision of the IAEA handbook on parameter values for predicting radionuclide transfer in terrestrial and temperate continental aquatic environments (IAEA, 2010).

Forests

There is no specific information on the mobility and bioavailability of carbon-14 in forest ecosystems.

Artic ecosystems

There is no specific information on the mobility and bioavailability of carbon-fourteen in arctic ecosystems.

Alpine ecosystems

There is no specific information on the mobility and bioavailability of carbon-14 in tall ecosystems.

Ecology dosimetry

The effects of exposure to ionising radiation depend on the quantity of free energy absorbed by the target organism, expressed by a dose rate (µGy/h). This dose charge per unit is evaluated past applying dose conversion coefficients (DCCs, µGy/h per Bq/unit of mass or volume) to radionuclide concentrations in exposure environments or in organisms (Bq/unit of mass or volume).

The characteristic ¹⁴C DCCs were determined without considering decay products and without RBE weighting. Version 2.3 of EDEN software (Beaugelin-Seiller et al., 2006) was used, taking into account shape, dimensions and chemic limerick of the organisms and of their environments, as well every bit their geometrical relations. The modelled species were chosen every bit examples.

Except in the particular case of the fescue (10^-3µGy/h per Bq/kg moisture), internal exposure is by and large characterised by DCCs of effectually x^-5 µGy/h per Bq/kg moisture.

External exposure is characterised by lower DCCs that vary according to the organism, within a range of 10^-x and 10^-5 µGy/h per Bq/kg.

For more details on how to calculate DCC, see the Environmental Dosimetry Canvas.

Environmental toxicity

Element chemotoxicity

Not applicative

Radiotoxicity of the radioactive isotope 14C

Carbon-14 is a low b emitter, with a low penetrating power which causes radiation stress mainly due to internal irradiation, if the ¹⁴C is incorporated. Carbon-xiv is interesting from a radiobiological standpoint because it is integrated in cellular components (proteins, nucleic acids), particularly cellular DNA (Le Dizès-Maurel et al., 2009). The resulting Deoxyribonucleic acid damage, involving molecular breaks, may lead to cell death or induce potentially inheritable mutations.

Yet, in that location is currently not plenty information to determine whether the ecosystem protection threshold criterion of 10 µGy/h is relevant for ¹⁴C (Le Dizès-Maurel et al., 2009). This criterion is consensual in Europe relative to chronic exposure to external gamma radiation.