EP1070266A1

EP1070266A1 - Method for measuring tritium activity in a radioactive waste drum

Info

Publication number: EP1070266A1
Application number: EP99911877A
Authority: EP
Inventors: Philippe Bugeon; Bernard Bachet; Didier Devillard
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1998-04-07
Filing date: 1999-04-06
Publication date: 2001-01-24
Anticipated expiration: 2019-04-06
Also published as: FR2777090B1; EP1070266B1; FR2777090A1; DE69909074T2; RU2225016C2; CA2324890C; CA2324890A1; UA57136C2; JP2002510800A; DE69909074D1; US6731714B1; WO1999051997A1

Abstract

The invention concerns a method for measuring the tritium activity in a radioactive waste drum (1) containing an amount of radioactive waste (2) and a free volume, which consists in measuring the amount of <3>He produced by the disintegrating tritium contained in said radioactive waste during a predetermined time interval and in deducing therefrom the corresponding activity of tritium contained in said radioactive waste (2).

Description

METHOD FOR MEASURING THE TRITIUM ACTIVITY OF A FUT

RADIOACTIVE WASTE

Technical area

The present invention relates to a method for measuring the tritium activity of a radioactive waste drum.

State of the art

Tritium is a radionuclide found in radioactive waste. It is an unstable isotope of hydrogen. Its disintegration into ³ helium is accompanied by the emission of a β particle with a negative charge which corresponds to an electron. The paths of these β particles are extremely short. They do not exceed 6 μm in water and 5.7 mm in air. This property excludes any possibility of detection of tritium radiation through the walls of drums or in solid or liquid waste.

To measure the level of tritium in radioactive waste, it is therefore necessary to turn to other measurement methods which take advantage of the main characteristics of tritium to detect its presence. Its particular atomic mass makes it possible to separate it by mass spectrometry and in chromatography if it is in the gas phase. Thanks to β radiation, it is possible to implement the scintillation counting technique, most often in a liquid medium. The absorption of β radiation in the surrounding medium induces the release of heat. This heat flow can be quantified by calorimetry. In most applications, these physicochemical properties are used to measure tritium. The field of tritiated waste is no exception to this rule. Before being able to implement the aforementioned techniques, it very often proves necessary to go through stages of preparation of the tritiated samples. These preliminary phases, sometimes complex, depend on the physico-chemical state of the tritium and the matrix where it is trapped. The level of activity sought, the chemical form of the tritium and of the matrix therefore define the criteria for choosing the measurement techniques.

Tritium is also present in organic solid waste, for example poly (ethylene / vinyl acetate) or EVA or poly

(vinyl chloride) or PCV, coming from overboots, gloves, scrubs, etc. The diversity of nature of this waste reinforces the difficulty of their characterization. Experience shows a strong heterogeneity in the distribution of activity. When all these types of products are mixed, the representativeness of a few grams of samples is very random. However, a sample burner (weighing 0.1 to 1 g) under oxygen flow allows the activity of a few samples to be estimated at levels of a few Bq / g.

The dosage of tritium in the gaseous or aqueous phase calls on very well mastered conventional techniques such as scintillation and mass spectrometry for the most common. When the medium is homogeneous, a simple sampling makes it possible to obtain a reliable quantitative result in an exhaustive concentration range.

However, the determination of the activity of relatively little contaminated tritiated waste remains particularly difficult. In this specific case, the use of destructive analytical techniques is not satisfactory due to the uncertainty relating to the representativeness of the sample. To date, no simple and reliable method exists for characterizing tritiated technological waste.

Rigorous and efficient management of tritiated waste requires measurement of the quantity of tritium contained in the packages. For treatment of this waste by a specialized body, it must be guaranteed that the tritium activity is below a determined threshold, for example 10 ⁹ Bq (0.027 Ci) in a 200 liter drum. No reliable and economically acceptable technique can meet this requirement at present. Calorimetry on 200 liter drums can be practiced but does not allow the measurement of tritium activities below 1.8.10 ¹⁴ Bq (5000 Ci).

With the exception of liquid or gaseous form, poorly contaminated tritiated waste is very difficult to characterize, especially organic waste, each sample of which is marked by very significant uncertainty as to its representativeness. The problem can possibly be resolved by homogenizing the waste by grinding. The lack of sensitivity of the calorimetry does not allow to use it. From the point of view of analytical controls, an important gap remains. The solution is to achieve a global control of the barrel, non-destructive and without generating waste.

Statement of the invention

To remedy this problem, it is proposed according to the present invention a technique not destructive, reliable and economical, which makes it possible to quantify the activity of tritiated waste enclosed in bags, for example PVC, placed themselves in a barrel of free volume and of unknown leakage. The principle adopted consists in measuring the quantity ^{| 3} helium resulting from the decay of tritium from a sample of the gaseous atmosphere surrounding the waste. This amount of helium formed is proportional to the tritium activity present. For example, waste with an activity of 10 ⁹ Bq (0.027

Ci) result in one year a clearance ^d, helium ³ leading to a concentration of 0.0055 ppm of this gas in a volume of 200 liters.

Tritium is radioactive β ^" . It decays giving ³ helium, an electron and an antineutrino depending on the reaction:

T → 3ie + e ^"

The radioactive period of tritium is

12.34 ± 0.02 years. The number of ³ helium atoms generated is directly linked to the number of tritium atoms present by the relation:

N _3H e = N _τ • (the ^"λt )

with: N _3He = quantity of ³ helium formed during the period t,

N _τ = quantity of tritium initially present, λ = radioactive constant of tritium.

The subject of the invention is therefore a method for measuring the tritium activity of a radioactive waste barrel containing a quantity of radioactive waste and a free volume, consisting in measuring the quantity of ³ He produced by the disintegration of the tritium contained in said radioactive waste for a determined period and to deduce therefrom the corresponding activity of tritium contained in said radioactive waste. The amount of ^3.3 He produced can advantageously be evaluated by a leak detector.

The tritiated waste enclosed in the bags generally behaves like a single source of ³ helium which flows into the free volume of the barrel. The partial pressure of the gas present is a function of the flow rate of ³ helium from the tritium source (therefore of the activity), the free volume and the tightness of the barrel (part of the created gas escapes), the confinement time.

According to the invention, the measurement can be carried out in three stages: calibration and measurement of the concentration of ³ helium, measurement of the free volume in the barrel, measurement of the tightness of the barrel.

The method according to the invention can therefore comprise the following operations: a) taking a sample of the gas contained in the barrel and evaluating the quantity d ^{| 3} He contained in the sample by the leak detector, b) measuring the free volume of said barrel, c) measurement of the tightness of the barrel to determine the leak rate, d) calculation of the flow rate of ³ He using the data obtained by operations a), b) and c), e) determination of the tritium activity of the barrel as a function of the flow rate d ^{, 3} He calculated during the operation d). Preferably, during operation a), parasitic gases are removed from the sample taken before evaluating the amount of ^IJ He. During this same operation, said evaluation may include the comparison of the sample taken with a gas at the same pressure and with a known ³ He concentration.

Operation b) can be carried out by injecting a known quantity of ⁴ He into the barrel, then by measuring the partial pressure d ^{, 4} He into the barrel, finally by determining the free volume of said barrel from the known quantity d ^, He and the measurement of the partial pressure of ⁴ He. This partial pressure measurement d ^{, 4} He can be obtained by taking a sample of the gas contained in the barrel and evaluation of the quantity of ⁴ He contained in this sample by a leak detector.

Operation c) can be carried out by injecting a known quantity of ⁴ He into the barrel, then by measuring the partial pressure of ⁴ He in the barrel for the first time, then after a period determined relative to said first time , by measuring a second time the partial pressure of ⁴ He in the barrel, the leak rate of the barrel being calculated from these partial pressure values of ⁴ He and the said time determined between these partial pressure measurements.

In the case where the apparent leakage of ³ He from the barrel is estimated to be equal to the quantity of ^3.3 He produced by the disintegration of the tritium contained in said radioactive waste, the measurement of said quantity of ³ He produced can simply be obtained by placing the barrel in an enclosure intended to collect the ³ He leaking from the barrel and evaluating this quantity of ³ He by the leak detector.

Brief description of the drawing

The invention will be better understood and other advantages and features will appear on reading the description which follows, given by way of 7

non-limiting example, accompanied by the appended figure which illustrates a method for measuring the tritium activity of a radioactive waste drum, according to the present invention.

Detailed description of an embodiment of the invention

The appended figure shows a barrel 1 closed by a cover 6 and containing radioactive waste 2 placed in PVC bags and leaving a free volume 3 in the barrel. Reference 4 designates a leak detector provided with a mass spectrometer. A test tube 5, with a capacity of the order of 2 liters, allows a sample of gas to be taken from the free volume 3 of the barrel, through the cover 6.

The signal delivered by a leak detector depends, among other things, on the value of the flow of tracer gas and the pressure of the tracer gas located upstream of this leak.

The ³ He concentration of the sample tubes is deduced after calibration under the same conditions as test tubes with known ³ He concentration. The ³ He concentration in the sample tubes is low, of the order of one part per billion. However, after the removal of gases other than ³ He, ⁴ He and Ne from the test piece by means of an active carbon trap, a pressure of the order of 10 millibars can be reached in the test piece. Under these conditions, the ³ He concentration in the test tube is multiplied by a factor of 100.

In the appended figure, there is shown the sampling test tube 5, disconnected from the barrel 1 and connected to an active carbon trap 7 soaked in liquid nitrogen. After trap 7 has exercised its 8

action by attracting gases other than ³ He, He and Ne, the test tube 5 is connected to the leak detector via a micrometric valve 8. The sample to be checked is then placed upstream of the calibrated leak of the detector 4.

The calibration is carried out by replacing the sampled gas with a gas at the same pressure and with a known ³ He concentration.

The measure of free volume (which is the volume offered at ³ He, i.e. the volume surrounding the bags of waste and the volume in the bags in which 1 ^J He can diffuse) of the barrel 1 made by injecting a known quantity of ⁴ He into the barrel. After diffusion of this gas in the barrel volume, the partial pressure of He is measured using the technique used for ³ He. We deduce the free volume by the relation:

Quantity ⁴ He V _llbre

Partial pressure _{(t = 0)}

In order to measure the tightness of the barrel, the partial pressure of ⁴ He is measured again three months after the first measurement. The leak rate Q _barrel of the barrel is a function of the pressure variation in the free volume. This function follows the law of the diffusion of helium through polymers:

_Initial pressure _(t , ₀₎ - Pressure _{iim3 →} \ x V

Q _w „= function of - t (3 months) A calculation program makes it possible to obtain the value of the flow rate of the ³ He source as a function of the ³ He concentration, the free volume, the tightness of the barrel and the duration of confinement of the waste. The tritium activity is deduced from the value of the flow rate of the ³ He source.

Given the order of magnitude of the tightness of the waste barrels (10 ^{~ 3} Pa.m ³ / s), after a few years of confinement, the partial pressure of ³ He inside the barrel is such that the system stabilizes. The apparent leakage from the barrel (the ³ He which leaves the barrel) is equal to the flow from the tritium source (the ³ He which is created). Under these conditions, sampling from the barrel is no longer necessary. It is enough to measure the apparent leakage of the barrel to know its tritium activity. This measurement can be made by placing the 200-liter barrel for about 20 hours in a slightly larger enclosure, in order to limit the dead volume to around twenty liters, and by measuring the concentration in ° He as before.

By way of demonstration, this process was applied to the measurement of the tritium activity of two steel containers containing a source of tritium (consisting of 0.1 liters of tritiated water) and offering a free volume of 12.7 liters . The activities of the sources enclosed in these containers were 0.455.10 ⁹ Bq (12.3 mCi) for the first container and 4.55.10 ⁹ Bq (123 mCi) for the second container. The ambient gas contained in these containers was analyzed at regular time intervals, in accordance with the method of the present invention. The results indicated in table I (for the first container) and in table II (for the second container) include the theoretical values of the expected concentrations in ³ He and the concentration values 10

deducted from the measurements based on the number of days elapsed since the sources were placed in the containers. The difference between the theoretical and measured concentrations is also indicated.

The results obtained with the source of 0.455.10 ⁹ Bq (12.3 mCi) show that a source of activity below the threshold mentioned above of 10 ⁹ Bq (27 mCi) can be characterized by the process of the present invention.

Table I

100 ml of water - total activity of 12.3 mCi f4.55.10 ⁸ Bq).

Number of days Theoretical concentration Measured concentration Deviation in ³ He (ppm volume) at. in He (ppm volume) (%) ± 5%

14 0.0021 0.0029 38

21 0.0033 0.0043 30

2S 0.0041 0.0051 24

35 0.0050 0.0060 20

42 0.005S 0.0070 21

49 0.0067 0 _Λ 00S3 24

72 0.00S5 0.0079 -7 77 0.0105 0.0104 -1

Table I I

100 ml of water t itiéc - total activity of 123 mCi f4.55.10 Bq).

Number of days Theoretical concentration Measured concentration Deviation in "Ηc (ppm volume) to in ³ He (ppm volume) (%) ± 5%

0 0 -

S O.OOS 0.009 13

17 0.019 0.01 S -5

2S 0.0 1 0.02S -10

50 0.056 0 052 4

Claims

1 1 CLAIMS

1. Method for measuring the tritium activity of a barrel (1) of radioactive waste containing a quantity of radioactive waste (2) and a free volume, consisting in measuring the quantity of ³ He produced by the disintegration of the contained tritium in said radioactive waste (2) for a determined period and to deduce therefrom the corresponding activity of the tritium contained in said radioactive waste (2).

2. Method according to claim 1, characterized in that the quantity of ³ He produced is evaluated by a leak detector (4).

3. Method according to claim 2, characterized in that it comprises the following operations: a) taking a sample of the gas contained in the barrel (1) and evaluating the amount of ³ He contained in the sample by the leak detector (4), b) measuring the free volume of said barrel (1), c) measuring the tightness of the barrel (1) to determine the leak rate, d) calculating the flow rate of ³ He using to the data provided by operations a), b) and c), e) determination of the tritium activity of the barrel (1) as a function of the flow rate of ³ He calculated during operation d).

4. Method according to claim 3, characterized in that, during operation a), parasitic gases are removed from the sample taken before making the evaluation of the quantity d ^{, 3} He.

5. Method according to one of claims 3 or 4, characterized in that, during operation a), said evaluation comprises the comparison of 12

the sample taken with a gas at the same pressure and known ³ He concentration.

6. Method according to any one of claims 3 to 5, characterized in that operation b) is carried out by injecting a known quantity of ⁴ He into the barrel (1), then by measuring the partial pressure of ⁴ He in the barrel (1), finally by determining the free volume of said barrel (1) from the known quantity of ⁴ He and from the measurement of the partial pressure of ⁴ He.

7. Method according to claim 6, characterized in that the measurement of the partial pressure of ⁴ He is obtained by taking a sample of the gas contained in the barrel (1) and evaluation of the quantity of He contained in this sample by a leak detector (4).

8. Method according to any one of claims 3 to 7, characterized in that operation c) is carried out by injecting a known quantity of ⁴ He into the barrel (1), then by measuring the partial pressure for the first time d ^, H in the barrel (1), then, after a determined period with respect to said first time, by measuring a second time the partial pressure of ⁴ He in the barrel (1), the leak rate of the barrel (1 ) being calculated on the basis of these partial pressure values of He and of said time determined between these partial pressure measurements.

9. Method according to claim 2, characterized in that, the apparent leakage of ³ He from the barrel (1) being estimated equal to the quantity of ³ He produced by the disintegration of the tritium contained in said radioactive waste (2), the measurement of said quantity of ³ He produced is obtained by placing the barrel (1) in an enclosure intended to collect the ³ He leaking from the barrel (1) and by evaluating this quantity of ³ He by the leak detector ( 4).