EP1070266B1 - Method for measuring tritium activity in a radioactive waste drum - Google Patents

Description

Technical area

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

State of the art

Tritium is a radionuclide present in radioactive waste. It is an unstable isotope of hydrogen. Its disintegration into ³ helium is accompanied by the emission of a particle β with a negative charge which corresponds to an electron. The pathways of these β particles are extremely reduced. 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 drum walls or in solid or liquid waste.

To measure the rate of tritium in radioactive waste, we must therefore move towards other measurement methods that take advantage of the main features of tritium detect his presence. Its particular atomic mass allows to separate it by mass spectrometry and in chromatography if it is in the gas phase. grace β radiation, it is possible to implement scintillation counting technique, the most often in liquid medium. The absorption of radiation β in the surrounding environment induces a release of heat. This heat flow can be quantified by the calorimetry.

These are, in most applications, these physico-chemical properties that are exploited to measure tritium. The field of tritiated waste does not escape this rule. Before you can put the aforementioned techniques, it turns out often necessary to go through stages of preparation of the tritiated samples. These phases prerequisites, sometimes complex, depend on the state physico-chemical composition of the tritium and the matrix found trapped. The level of activity sought, the form chemical tritium and matrix therefore define the criteria for choosing measurement techniques.

Tritium is also present in organic solid waste, for example poly (ethylene / vinyl acetate) or EVA or poly (vinyl chloride) or PCV, from overboots, gloves, prégants, etc ... The diversity of nature of this waste reinforces the difficulty of characterization. Experience shows a strong heterogeneity of activity distribution. When all these types of products are mixed, the representativeness of a few grams of samples is very random. However, a sample burner (weighing from 0.1 to 1 g) under an oxygen stream allows to estimate the activity of some samples to levels of some Bq / g.

The determination of tritium in the gas phase or aqueous uses very classical techniques well controlled such as scintillation and the Mass spectrometry for the most common. When the medium is homogeneous a simple sample allows to obtain a reliable quantitative result in a range exhaustive concentration.

However the determination of the activity relatively contaminated tritiated waste remains particularly difficult. In this case, the use of destructive analytical techniques is unsatisfactory because of the uncertainty of the representativeness of the sample. To date no simple and reliable method exists to characterize the tritiated technological waste.

A rigorous and efficient management of tritiated waste imposes the measurement of the quantity of tritium contained in the packages. For the management of this waste by a specialized body, it must be ensured that the tritium activity is below a certain 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 this time. Calorimetry on 200 liter drums can be practiced but does not allow to measure tritium activities lower than 1.8.10 ¹⁴ Bq (5000 Ci).

Except in liquid or gaseous form, Tritiated waste with little contamination is very difficult to characterize, especially organic waste each sample is marked by a very high degree of uncertainty important as to its representativeness. The problem can be solved by homogenizing waste by grinding. The lack of sensitivity of the calorimetry does not allow it to be used. Point analytical checks an important gap remains. The solution is to achieve control overall, non-destructive and without generating waste.

Presentation of the invention

To remedy this problem, it is proposed according to the present invention a non destructive, reliable and economical, which allows quantify the activity of tritiated waste enclosed in bags, for example PVC, placed themselves in a barrel of unknown volume and leakage.

The principle adopted is to measure the amount of helium ³ 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. By way of example, waste with an activity of 10 ⁹ Bq (0.027 Ci) causes in one year a release of ³ helium leading to a concentration of 0.0055 ppm of this gas in a volume of 200 liters.

Tritium is radioactive β ^- . it disintegrates by giving ³ helium, an electron and an antineutrino according to the reaction: T → ³ He + e ^- + ν

N_3He =

quantité d'³hélium formée pendant la durée t,

N_τ =

quantité de tritium initialement présent,

λ =

constante radioac tive du tritium.

The half-life of tritium is 12.34 ± 0.02 years. The number of ³ helium atoms generated is directly related to the number of tritium atoms present by the relation: NOT 3He = N τ · (1-e -λt ) with:

N _3He =

quantity of ³ helium formed during the duration t,

N _τ =

amount 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 drum containing a quantity of radioactive waste and a free volume, consisting in measuring the amount of ³ He produced by the disintegration of the tritium contained in it. in said radioactive waste for a determined period of time and to deduce therefrom the corresponding activity of the tritium contained in said radioactive waste. The amount of ³ He produced can advantageously be evaluated by a leak detector.

The tritiated waste contained in the bags behave globally as a single source of ³ helium which discharges into the free volume of the drum. The partial pressure of the gas present is a function of the ³ helium flow rate of the tritium source (thus the activity), the free volume and the tightness of the barrel (a part of the gas created is escaping from it), the confinement time.

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

a) prélèvement d'un échantillon du gaz contenu dans le fût et évaluation de la quantité d'³He contenue dans l'échantillon par le détecteur de fuite,

b) mesure du volume libre dudit fût,

c) mesure de l'étanchéité du fût pour en déterminer le taux de fuite,

d) calcul du débit d'³He grâce aux données procurées par les opérations a), b) et c),

e) détermination de l'activité tritium du fût en fonction du débit d'³He calculé lors de l'opération d).

The process according to the invention may therefore comprise the following operations:

a) taking a sample of the gas contained in the drum and evaluating the amount of ³ He contained in the sample by the leak detector,

b) measuring the free volume of said drum,

c) measurement of the tightness of the drum to determine the leakage rate,

(d) calculation of the ³ He flow using data from operations (a), (b) and (c),

e) determination of the tritium activity of the drum as a function of the flow rate of ³ He calculated during the operation d).

Preferably, during step a), parasitic gases are removed from the sample taken before evaluating the amount of ³ He. During this same operation, said evaluation may comprise the comparison of the sample taken with a gas at the same pressure and known ³ He concentration.

The operation b) can be carried out by injecting a known amount of ⁴ He into the drum, then measuring the partial pressure of ⁴ He in the drum, and finally by determining the free volume of said drum from the known amount of ⁴ He and the measurement of the partial pressure of ⁴ He. This ⁴ He partial pressure measurement can be obtained by taking a sample of the gas contained in the drum and evaluating the amount of ⁴ He contained in this sample by a leak detector.

The operation c) can be carried out by injecting a known quantity of ⁴ He into the drum, then by measuring a first time the partial pressure of ⁴ He in the drum, then, after a determined duration with respect to said first time , by measuring a second time the partial pressure of ⁴ He in the drum, the leakage rate of the drum being calculated from these values of partial pressure of ⁴ He and of the determined duration between these partial pressure measurements.

In the case where the apparent leakage of ³ He from the drum is estimated to be equal to the amount of ³ He produced by the disintegration of the tritium contained in the said radioactive waste, the measurement of the said quantity of ³ He produced can simply be obtained by placing the barrel in a chamber designed to collect the ³ He fleeing the barrel and evaluating this amount of ³ He by the leak detector.

Brief description of the drawing

The invention will be better understood and others advantages and particularities will appear on reading description which follows, given as a non-limiting example, accompanied by the figure annexed illustrating a process for measuring activity tritium from a radioactive waste drum, according to the present invention.

Detailed description of an implementation mode of the invention

The attached figure shows a closed barrel 1 by a lid 6 and containing radioactive waste 2 arranged in PVC bags and leaving a free volume 3 in the drum. Reference 4 designates a leak detector provided with a spectrometer of mass. A test-tube 5, with a capacity of about 2 liters, allows to take a sample of gas in the free volume 3 of the barrel, through the lid 6.

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

The ³ He concentration of the test specimens is deduced after calibration under the same conditions of specimens with a known ³ He concentration.

The ³ He concentration of the sample specimens is low, of the order of one part per trillion. However, after the elimination of the gases other than ³ He, ⁴ He and Ne of the test piece by means of an activated carbon trap, a pressure of the order of 10 millibars can be reached in the test specimen. Under these conditions, the concentration of ³ He in the test tube is multiplied by a factor of 100.

In the attached figure, there is shown the sampling specimen 5, disconnected from the barrel 1 and connected to an activated carbon trap 7 soaked in liquid nitrogen. After the trap 7 has exerted its action by attracting gases other than ³ He, ⁴ He and Ne, the test piece 5 is connected to the leak detector via a micrometer valve 8. The sample to be tested is is then placed upstream of the calibrated leak of the detector 4.

The calibration is performed by replacing the sample gas by a gas at the same pressure and of known ³ He concentration.

The measurement of free volume (which is the volume offered to the ³ He, ie the volume surrounding the bags of waste and the volume in the bags in which the ³ He can diffuse) of the drum 1 is made by injection into the barrel of a known quantity of ⁴ He. After diffusion of this gas into the volume of the drum, the partial pressure of ⁴ He is measured by means of the technique used for ³ He. We deduce the free volume by the relation: V free = Amount 4 Hey Partial pressure (t = 0)

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

A calculation program makes it possible to obtain the value of the flow rate of the source ³ He as a function of the concentration in ³ He, the free volume, the tightness of the drum 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 drums (10 ^-3 Pa.m ³ / s), after a few years of confinement, the partial pressure of ³ He inside the drum is such that the system stabilizes. The apparent leak of the barrel (the ³ He coming out of the barrel) is equal to the flow of the tritium source (the ³ He that is created). Under these conditions, sampling in the drum is no longer necessary. It is sufficient to measure the apparent leakage of the drum to know its tritium activity. This measurement can be done by placing for about 20 hours the barrel of 200 liters in a slightly larger enclosure, to limit the dead volume to about twenty liters, and measuring as previously the concentration of ³ He.

As a demonstration, this method was used to measure the tritium activity of two steel containers containing a tritium source (consisting of 0.1 liter of tritiated water) and having 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 process of the present invention. The results shown in Table I (for the first container) and in Table II (for the second container) include the theoretical values of the expected concentrations of ³ He and the concentration values deduced from the measurements as a function of the number of days elapsed since the establishment of sources 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 above mentioned threshold of 10 ⁹ Bq (27 mCi) can be characterized by the method of the present invention. 100 ml of water - total activity of 12.3 mCi (4.55.10 ⁸ Bq). Number of days Theoretical concentration
in ³ He (ppm volume) to
± 5% Measured concentration
in ³ He (ppm volume) Difference (%) 14 0.0021 0.0029 38 21 0.0033 0.0043 30 28 0.0041 0.0051 24 35 0.0050 0.0060 20 42 0.0058 0.0070 21 49 0.0067 0.0083 24 72 0.0085 0.0079 -7 77 0.0105 0.0104 -1 100 ml of tritiated water - total activity of 123 mCi (4.55.10 ⁹ Bq). Number of days Theoretical concentration
in ³ He (ppm volume) to
± 5% Measured concentration
in ³ He (ppm volume) Difference (%) 0 0 - 8 0,008 0,009 13 17 0,019 0,018 -5 28 0.031 0,028 -10 50 0,056 0,052 4

Claims (9)

1. Method of measuring the tritium activity of a drum (1) of radioactive waste containing a quantity of radioactive waste (2) and a free volume, consisting of measuring the quantity of ³He produced by the decay of the tritium contained in said radioactive waste (2) during a defined period of time and deducing from it the corresponding activity of the tritium contained in said radioactive waste (2).
2. Method according to Claim 1, characterized in that the quantity of ³He produced is evaluated by a leak detector (4).
3. Method according to Claim 2, characterized in that it comprises the following operations:

   a) the taking of a sample of the gas contained in the drum (1) and evaluation of the quantity of ³He contained in the sample using a leak detector (4),

   b) measurement of the free volume of said drum (1),

   c) measurement of the tightness of the drum (1) to determine the leakage rate from it,

   d) calculation of the flow rate of ³He using data obtained from operations a), b) and c),

   e) determination of the tritium activity of the drum (1) in relation to the flow rate of ³He calculated in operation d).
4. Method according to Claim 3, characterized in that during operation a), parasitic gases are removed from the sample before making the evaluation of the quantity of ³He.
5. Method according to one of Claims 3 or 4, characterized in that during operation a), said evaluation comprises the comparison of the sample taken with a gas at the same pressure and of known ³He 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 ⁴He into the drum (1) and then measuring the partial pressure of ⁴He in the drum (1), and finally by determining the free volume of said drum (1) from the known quantity of ⁴He and the measurement of the partial pressure of ⁴He.
7. Method according to Claim 6, characterized in that the measurement of the partial pressure of ⁴He is obtained by taking a sample of gas contained in the drum (1) and evaluating the quantity of ⁴He 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 ⁴He into the drum (1), then measuring for the first time the partial pressure of ⁴He in the drum (1) and then, after a period of time defined in relation to said first time, measuring the partial pressure of ⁴He in the drum (1) for a second time, the leakage rate from the drum (1) then being calculated from these values of the partial pressures of ⁴He and said defined period of time between these partial pressure measurements.
9. Method according to Claim 2, characterized in that the apparent leakage of ³He from the drum (1) being estimated as being equal to the quantity of ³He produced through the decay of the tritium contained in said radioactive waste (2), the measurement of said produced quantity of ³He is obtained by placing the drum (1) within an enclosure intended to collect the ³He leaking from the drum (1) and by evaluating this quantity of ³He using the leak detector (4).

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