Radioactive pollution of food chain - ΑAir - soil pathways of radioactive pollution

Source

Inhalation

( External radiation )

Direct pollution

FoodAir Soil

Plants

Animals

MAN

Pollution of – and from water sources

Radioactive pollution of food chain - ΒFresh and marine water pathways of radioactive pollution

Pollutionsource

Direct pollution

FoodAir

Plants

Fish

Lakes,

Sea

SoilSoil pathways to man

MAN

FishRivers

Washout, irrigation

Environmental monitoring

External dose rate

Air radioactivity

Radioactive deposition

Soil radioactivity

Radioactivity of waters

Radioactivity of food

Radioactivity of building materials

Specific indicators

Environmental monitoringRoutine network design principles

1. Optimal selection of sampling and measuring stationswith respect to the potential sources of pollution

2. Balance between a source-orientated and population-oriented sampling network

3. Monitoring of all the major pathways of potentialpropagation of the radioactive pollution

4. Balance between fast availability and quality of theinformation obtained

Environmental monitoringGreece: map of the routine monitoring network

Total beta in airOn daily basisLLD: 0.3 mBq m-3

Total beta in waterIntegrated monthly samplesLLD: 4 Bq m-3

Radioactive depositionOn monthly basisLLD: 0.2 Bq m-2

Food radioactivityOn monthly basis or elseLLD: 1 - 10 Bq kg-1

External dose rateEvery 6 hours + telemetricLLD: 2 nGy h-1

Marine and lake samplesOccasionally

Environmental monitoringBasic measuring techniques, advantages / disadvantages

1. External dose rate Fast information, telemetry

No data on the source nuclides

2. Total beta counting High sensitivity

No data on the source nuclides. No telemetry

3. Gamma spectrometry Data on specific radionuclides

No telemetry

4. Alpha spectrometry Data on specific radionuclides

High sensitivity. No telemetry

5. Radiochemical analysis Data on specific radionuclidesHigh sensitivity. No telemetry

Monitoring of external dose rate

- Provides fast information, especially in the case of a telemetric network

- Allows the detection of reliably low increases of the dose rate

- Problems: - No data on the composition of the pollutant- Stability problems with certain types of detectors- Possibility of false alarms

A temporary increase of the doserate, caused by deposition of

External dose rate radon daughters washed out by rain

The simultaneous measurement ofRainfall rainfall provides additional

information

Monitoring of external dose rate (2)

A telemetric network allows to comparequickly the data from different locations,which help to form a first picture of thepollution propagation and also to avoidfalse alarms.

The background dose rate in each station

has to be known with sufficient accuracy,

as long as it can vary considerably.

In the case of Greece, the backgrounddose rate varies within 30 – 120 nGy h-1.

Monitoring of external dose rate (3)

In the case of air pollution, the additional dose rate is composed by 2 parts:

1. The part determined by the radionuclides in the air2. The part determined by the radionuclides deposited in the ground

Depending on the deposition conditions, the second part may gradually dominateover the first and determine a significant residual dose rate, even after the endof air contamination.

Air radioactivity

External dose rateBackground

Dry deposition

Wet deposition

Monitoring of air radioactivity

A known volume of air is filtered and the filter activity is consequentlydetermined by use of different techniques.

It is possible - in principle - to perform real time telemetric measurements.However, the accumulation of decay products of Rn222 and Rn220 contributes unfavorably and reduces significantly the sensitivity.

Sample volumes of the order of 50 – 100 m3 are sufficient to achieve LLD valuesof the order of 0.2 mBq m-3, in the case of delayed total beta measurement.This ensures the monitoring of the bckg levels, which are of the order of 1 mBq m -3.

Much higher sample volumes are necessary in order to detect the backgroundlevels of specific artificial radionuclides in air.

Series of 24-h sampling are applied typically, to ensure continuous air monitoring.If necessary, the density of sampling can be enhanced.

Special filters are used in order to measure gaseous forms of radionuclidesin air (e.g. the non-particulate fraction of I131).

Monitoring of air radioactivity (2)

When using the total beta counting method, a delay of 4-5 days is necessarybetween the end of sampling and the start of measurement, in order to allowthe decay of the Rn222 and Rn220 decay products accumulated on the filter.

Note that the concentrations of these natural nuclides in open air are typicallywithin 2 – 3 Bq m-3 or 3 orders of magnitude higher than the total activityof the long-lived background radionuclides.

The delay mentioned is not necessary when high-resolution gamma spectrometryis applied instead. Nevertheless, the presence of radon daughters may reducesignificantly the accuracy of the measurement.

________

A derived quantity is the integrated air concentration - the time integral of the air concentration during a given period of time. This quantity characterizes the total‘exposure’ of a given atmosphere to the pollutant(s) and is used as an inputvalue for various environmental and dosimetric models.

ΕΡΓΑΣΤΗΡΙΟ ΡΑΔΙΕΝΕΡΓΕΙΑΣ ΠΕΡΙΒΑΛΛΟΝΤΟΣ

ΟΡΙΟ ΑΝΙΧΝΕΥΣΗΣ ΚΑΙ ΕΠΙΠΕΔΟ ΕΠΙΦΥΛΑΚΗΣ ΓΙΑ ΤΙΣ ΜΕΤΡΗΣΕΙΣ ΟΛΙΚΗΣ-β ΣΤΟΝ ΑΕΡΑ

Monitoring of air radioactivity (3)

The reasonably low alarm level of the total beta measurementsshould ensure low inhalation doses during the period necessary to perform

further analyses and consider the application of countermeasures

Annual dose limit

Alarm level

Typical levels

Weeks

LLD

Monitoring of radioactive deposition

The radioactive deposition is defined as the activity deposited in unit areaof the ground surface (during a given period of time).

The monitoring may refer to the total beta deposition, as well as to the depositionof specific radionuclides.

This quantity is of primary importance for the prediction of the additional externaldose rate due to the soil pollution, as well as the pollution of plants and surface waters and the propagation of the pollutants through various environmentalpathways and trophic chains.

Due to its integral nature, the radioactive deposition is a more easily detectablequantity than the air concentrations of the radionuclides from which it results.

Under given concentrations of radionuclides in air, the radioactive deposition isconsiderably enhanced during rainfalls. The difference between “dry” and “wet”deposition may depend on the chemical properties of the radioactive materials.

Radioactive deposition = Integrated air concentration x Deposition velocity

Radioactive depositionRelation between radioactive deposition and rainfall

0.1

1

10

100

1000

0 5 10 15 20 25 30

Βροχόπτωση, mm

Ρα

διε

νερ

γό

ς εν

απ

όθε

ση

, kb

q/m

2 Cs137

Zr95

Rainfall, mm

R

ad

ioa

cti

ve

de

po

sit

ion

, k

Bq

m-2

Monitoring of radioactive deposition (2)

The radioactive deposition is usually sampled by use of collective vessels ofknown area (typically 0.1 – 1 m2). To avoid re-suspension losses, an layer ofdistilled water has to be present in the vessel’s bottom.

The typical collection duration is 1 month, but shorter collection times may beapplied in the case of radiological emergency

After the end of collection, the water sample is transferred to the laboratory andmildly evaporated, to avoid loss of volatile components. The activity of theresidue is determined by total beta counting or gamma-spectrometry or some radiochemical method.

In the case of total beta counting, the LLD is of the order of 0.05 – 0.2 Bq m -2,depending on the area of the collecting vessel and for 1 month sampling duration.

In the case of enhanced accidental contamination (deposition values of the orderof 1 kBq m-2 or higher), gamma-spectrometry can be easily applied. To determinethe current background monthly deposition values of Cs137, vessels of largerarea ( 1 m2 ) have to be used.

Monitoring of soil radioactivity

Although not included in the group of standard monitoring procedures of the EU,the measurement of soil radioactivity is a useful additional method.

It is one of the simplest methods for a detailed post-accidental mapping of theradioactive pollution. In this case, samples of well defined area are collected,in order to enable the expression of results in deposition units as well ( Bq m -2).The sampling is done from a layer 0 – 5 or 0 – 10 cm.

The samples are homogenized, dried, shifted and analyzed by means ofgamma-spectrometry or certain radiochemical procedure.

The presence of natural radionuclides restricts the application of this methodto the cases of rather significant soil pollution. Depending on the radionuclide,the efficiency of the detector used and the duration of measurement, the LLDmay vary within 0.1 – 10 Bq kg-1 which, for a sample size of 10 x 10 x 10 cm, corresponds to deposition values of 10 – 100 Bq m-2.

When the measurements aim the determination of radioactive deposition,attention has to be paid to the selection of soil undisturbed since the pollutionevent. Areas of possible rainwater accumulation have to be excluded as well.

Monitoring of soil radioactivity

A map of Cs137 depositionafter the Chernobyl accidentderived on the basis of about 2000 soil measurementsperformed in Greece by NTUA( ~ 1500) and ERL ( ~ 500).

This type of maps are veryuseful not only for variousmodel evaluations, but also asbackground data in the caseof any consequent pollutionevent.

Depending on the region,the current concentrationsof Cs137 in the surface layerof undisturbed soil in Greecevary within 4 – 1000 Bq kg-1.

Monitoring of water radioactivity

The monitoring of surface waters is often source-oriented and concerns areasof routine or possible accidental release of specific radioactive liquid materials.In this cases the methodology applied depends on the specific radionuclides ofinterest.

In other cases the monitoring is population-oriented and concerns reservoirsof drinking water and major rivers, supplying these reservoirs.

The tap water is also monitored on regular basis, at least that of the largestpopulation centers.

The well drinking waters are often controlled for the possible presence of enhancedconcentrations of natural radionuclides.

The marine water is monitored on regular basis only near points of routine orpossible accidental releases. In Greece, where no major nuclear facilities exist,sea water monitoring is regularly performed during the visits of nuclear-poweredmilitary vessels.

Monitoring of water radioactivity (2)

The lake water is usually monitored on a weekly bases, except in regions ofroutine radioactive releases.

The river water should be sampled more often, preferably on daily basis,but composite samples may be measured instead.

The total beta measurements are usually applied in the cases of drinking watersamples. The samples are mildly evaporated in order to avoid losses of somevolatile components.

The results of total beta measurements have to be corrected for the presenceof K40, to provide the so-called “residual beta-activity” = beta activity – that of K40.

In the case of total beta activity exceeding certain level, additional analysis hasto be performed for the determination of the radionuclide composition.

Underground waters should be analyzed for determination of radon isotopesand other radiologically important natural radionuclides, e.g. Ra226.

Chernobyl accident - Greece Average 1st year and 50-year doses through various pathways, μSv

Pathway 1st year 50 years

Inhalation 22 22External exposure 52 190Food - water 420 500

TOTAL 490 710

The radiologically most important food chain pathways

Grass pollution with Ι131 > Sheep and goat milkGrass pollution with Cs137 > Sheep and goat milk and meatPollution of fruits and vegetables

Animal foodstuff pollution > Meat and milk products, 1986/87Cereals pollution > Bread and pastry, 1986/87

Monitoring of food radioactivity

Two basic types of food control are performed during the routineradioactivity monitoring:

1. Deternination of Cs137 and Sr90 in milk.

In some cases the milk sampling is location-oriented. In other cases a compositesample of diary milk from different producers, mixed in proportion to theircoverage of the market.

2. Deternination of Cs137 and Sr90 in the so-called “mixed diet” sample.

The “mixed diet” sample is composed by the most representative food productsof a given country, in proportions according to the consumption of the averagecitizen. Therefore, their constituents and the proportions they are mixed mayvary significantly from country to country.

In both cases specific radiochemical analysis is applied for determination ofSr90 and Cs137, after proper homogenization of the samples. The determinationof Cs137 can be based also on gamma-spectrometry measurements, if the sensitivity of the system is high enough.

Monitoring of food radioactivity (2)

After the Chernobyl accident (1986) a lot of countries require radioactivitycertificates from the exporting countries known to be affected by the resultedradioactive pollution. At the same time, many countries perform a sample or evena regular control of imported foodstuffs, depending on the country of origin.

Most of the above controls refer (today) to the concentration of Cs137.If a sertification of the type “Cs137 below 10 Bq kg-1” is acceptable (in mostcases it is), the analysis is relatively simple and includes sample homogenizationand gamma-spectrometry measurement for about 1 h.

Sample pre-concentration by low-temperature ashing may be necessary ifdetection of 1 Bq kg-1 levels is required.

Monitoring of building materials

The monitoring of building materials is not currently a part of the routine control in most EU countries. Nevertheless, when the existing suggestions regarding thenatural radioactivity of building materials will become regulations, this typeof monitoring may be included as a standard.

Even today, the requests for analyses of building materials (domestic and/orimported) are continuously growing.

The concentrations of natural radionuclides in building materials are usuallyhigh enough to be easily detected by gamma-spectrometry. The only difficultiesrefer to the sample preparation (in some cases) and to the application of properself-absorption corrections.

The compliance with future regulations regarding indoor radon may require thedetermination of radon exhalation rate from the major building materials and/ortheir components.

Interpretation of the resultsof an environmental monitoring program

1.