WO1995010840A1

WO1995010840A1 - Thermal separation of tritium and apparatus therefor

Info

Publication number: WO1995010840A1
Application number: PCT/GB1994/002201
Authority: WO
Inventors: Geoffrey Vincent Atkins; Paulo Pacenti
Original assignee: European Economic Community
Current assignee: European Economic Community
Priority date: 1993-10-08
Filing date: 1994-10-10
Publication date: 1995-04-20
Anticipated expiration: 1996-04-08
Also published as: GB9320806D0

Abstract

A method and apparatus is provided for the thermal separation of tritium from a sample of nominally-radioactive material which comprises stripping tritium from said material into a stream of humidified gas. The sample of material is placed in a furnace and the humidified gas is introduced therein to pass over and/or through the sample. Any tritium in the sample is transferred to the passing gas. The humidified gas is then cooled and the water condensate subjected to scintillation counting. In one embodiment of the invention the humidified gas is initially an inert gas which after a pre-determined period is replaced by humidified air or oxygen which allows controlled combustion of the sample.

Description

THERMAL SEPARATION OF TRITIUM AND APPARATUS THEREFOR

This invention relates to thermal separation of tritium from nominally-radioactive material. In particular the present invention is concerned especially, though not exclusively, with methods and apparatus for thermal separation of tritium from samples of potentially-radioactive waste material. These methods and apparatus are required, for example, for radioactive-contamination assessment of waste material, and in particular for the declassification of the waste to a non-radioactive status in compliance with limits established by regulatory bodies.

Separation techniques previously used involve combustion of the material. In these, oxygen (as oxygen itself, or in air) is supplied in excess of the amount needed to fulfil the stoichiometric requirement for complete combustion of the heated test material so that combustion proceeds at the maximum rate. Tritium is released as tritium gas or as tritiated water vapour, but in some cases the sample commences aerobic (oxygen related) degradation prior to the ignition point, so that decomposition products, which may not be completely combusted upon exit from the heated section of the apparatus, are released as well.

The present inventors have developed a method for thermal separation of tritium that has a number of advantages over the known methods of separation for radioactive materials. This method is particularly useful for determining the amount of tritium in a bulk sample of material and therefore determining its status for the purposes of disposal.

According to the first aspect of the present invention a method for thermal separation of tritium from nominally-radioactive material is provided which comprises stripping tritium from said material into a stream of humidified gas.

The humidified gas is passed over and/or through said nominally-radioactive material which is positioned in a heated chamber or furnace. Tritium in the sample of material, generally in the form of tritiated water, is stripped into the humidified gas either by simple transfer of the tritiated water to the gas or by isotope exchange between the water vapour in the gas and the tritium in the sample.

The gas used, at least initially, in this first aspect of the invention is inert and may be, for example, nitrogen that is supplied, humidified and heated to pass over and/or through the material. The supply of metered, humidified inert gas may be maintained to complete the stripping process without combustion of the test material. Optionally, when the sample has acquired the desired pre-combustion temperature the supply of nitrogen may be replaced by a metered supply of humidified air (or oxygen) to allow controlled combustion of the sample.

In carrying out the method without combustion or prior to the step of controlled combustion in humidified oxygen or air, the sample material and the inert humidified gas may be heated to a temperature of about 200 to 400°C. The humidified gas is pre-heated to the appropriate temperature before coming into direct contact with the sample. This may be achieved by using the heat from the furnace to heat the gas, for example by passing the gas around the outside walls of the furnace before passing therein.

The humidified gas loaded with tritiated water, after being in contact with the sample of radioactive material, exits the furnace. For determination of the amount of tritium in the sample, it may be passed through condensing means and the condensate collected and subjected to scintillation counting.

The passage of an inert humidified gas over and/or through the sample for a pre-determined period of time without combustion may be sufficient to determine the tritium content thereof and is useful in situations where it is preferable if the sample material is not destroyed. However, as aforesaid optionally the method of the first aspect of the invention may include the additional step of replacing the flow of inert humidified gas with a metered supply of humidified air or oxygen once the desired pre-combustion temperature has been reached. The sample may then undergo combustion at a rate which is determined by the flow rate of the humidified air or oxygen. This controlled combustion is in contrast to prior art methods where air or oxygen are added to the sample in excess and there is thus no control over the rate of combustion. During this controlled combustion step more tritium is stripped into the humidified gas. For determination of the amount of tritium in the sample, the tritium loaded gas may be passed through the condensing means and the condensate collected for scintillation counting in conjunction with that from the inert gas stage.

In the embodiment of the invention which involves a controlled combustion step it is preferable if the temperature of the sample is measured independently of the temperature of the body of the furnace or chamber and the apparatus for carrying out the method of the invention, which is described hereinafter, has means for so doing. Knowing the temperature of the sample allows the real time of combustion at the required temperature to be determined.

In commencing the method of the invention the sample is introduced into the upper part of the furnace. Any air which may have entered therewith is removed by purging at least the furnace chamber, if not the rest of the apparatus with nitrogen before lowering the sample into the hottest part of the furnace.

It will be appreciated that the controlled combustion step alone, without prior exposure to a humidified inert gas may be sufficient for stripping tritium for a nominally-radioactive material in some applications. Thus, in its second aspect the invention provides a method for thermal separation of tritium from a sample of a nominally-radioactive material by controlled combustion which comprises stripping tritium from said material into a stream of humidified oxygen or air, the rate of combustion being controlled by the flow rate of said humidified air or oxygen stream.

Tritium is found as contaminant mainly in the form of tritiated water on waste materials of tritium extraction plants and other nuclear facilities. Experimental work using the methods of the first and second aspects of the present invention has demonstrated that a useful percentage of the total tritium content can be separated from the material using a lower temperature than required for the known, uncontrolled-combustion techniques. Condensation of the tritium-bearing water vapour from the gas stream and measurement of tritium in the condensate, enables the concentration of tritium per unit mass of the material to be determined.

More particularly in the latter respect, the method has been found to enable extraction from the material of a reproducible percentage of the total tritium content. This enables the total concentration to be calculated from the measurement of tritium in the condensate, using a scaling factor, and avoids the high-temperature requirements and other disadvantages of the known extraction methods. Data obtained in this way from a sample taken from a body of waste material can be utilised, in conjunction with determination of the concentrations of other forms of radioactivity in that material, for comparison with regulatory requirements for declassification of the waste as radioactive. In the United Kingdom, declassification is given by way of an exemption order under provisions of the Radioactive Substances Act.

In the known techniques of thermal extraction of tritium, the samples used are typically less than 1.5 grams in weight. The sample-size restriction applicable makes such techniques unsuitable for batches of mixed, radioactive waste where larger, more representative quantities of each contributing material type should be included to create a truly representative sample. Furthermore, the known methods attempt to give almost complete extraction and capture of the tritium present, and unlike the method of the present invention are not readily applicable to determining the classification of waste material using appropriately representative samples of, for example, 10 to 20 grams in weight.

In accordance with a third aspect the invention provides an apparatus for the thermal separation of tritium from a sample of a nominally-radioactive material which comprises a chamber for receipt of said sample, means for heating said chamber to a pre-determined temperature, means for supplying a humidified gas into said chamber, means for heating said gas to said pre-determined temperature before entry into said chamber, means for exiting said humidified gas from said chamber and means for condensing the water vapour in said humidified gas on exiting said chamber.

In practice the heated chamber will be a furnace and in a preferred embodiment the furnace also pre-heats the humidified gas. For example, a particularly useful embodiment is one in which the furnace chamber is double-walled, the space between the inner and outer wall forming a passage through which, in use, the humidified gas must pass before entry into the chamber thereby pre-heating the gas to the temperature of the chamber before presentation to the sample.

Preferably, the means for supplying humidified gas is capable of switching the supply from humidified inert gas to humidified oxygen or air, the gases being humidified by passage through a water bubbler or the like.

The sample is introduced into the furnace chamber in a holder, for example, a wire basket, which contains the sample throughout the stripping procedure. Such a holder is suitable for 10 to 20 grams of material. In order that the temperature of the sample may be determined independently of the furnace the sample holder may include a temperature sensing means such as a thermocouple.

The condensing means is preferably a temperature controlled condensing vessel which incorporates a condensing coil and a reservoir for collection of the condensate. Preferably, it also incorporates within the vessel the initial portion of the exhaust gas exit tube which may be mounted vertically from the reservoir. These components are all maintained at the same temperature during the processing cycle minimising the possibility of re-evaporation of trapped tritiated water.

A method and apparatus for thermal separation of tritium from samples of potentially-radioactive waste material, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of the apparatus using the thermal separation method of the invention; and

Figures 2 and 3 are longitudinal sections of a furnace forming part of the apparatus of Figure 1 and illustrating, respectively, successive stages of operation of the apparatus in the thermal separation method of the invention.

Referring to Figure 1, a vertical-tube furnace 6, into which the sample is inserted within a wire-mesh basket 9, is arranged to be supplied with nitrogen from a pressurised source 1, or with pressurised air from a source 20, according to the setting of a valve 5. The nitrogen or air supplied through the valve 5 passes via a flow meter 3, and hence through a heated distilled-water bubbler 4 to become laden with water vapour, before entering furnace 6. The humidified gas enters an outer stainless-steel tube 7 of two concentric tubes 7 and 8 of the furnace 6, to pass downwardly between them and become heated to several hundred degrees Celsius before entering the inner tube 8 through holes 81 in its wall.

The heated gas entering the tube 8 rises within the tube 8 to come into contact with the sample in the basket 9; the pre-heating of the gas within the tube 7 ensures that the sample is not cooled by it. The basket 9 is provided at its base with a thermocouple 9a for measuring the temperature in the immediate vicinity of the sample. Exchange of tritium from the sample to the water vapour occurs at the basket 9, and the gases then rise to the top of the furnace 6 to exit through glass tubing 10. The tubing 10 is heated by a heating jacket 11 to avoid early condensation from the gases, and includes a filter 12 for capturing any solids expelled from the furnace 6.

The gases pass from the filter unit 12 into the condensing coil 131 of a condensing vessel 13. The coil 131 is cooled by a coolant that is supplied and recirculated through the vessel 13 by a pump 18 of a refrigeration unit 17. The condensate collects within a reservoir 132 at the base of the condensing coil 131 and within the thermal environment of the vessel 13 and may be drained off into a vial 14 for submission to a liquid scintillation counter, via a tap 133. Gases exit the condensing coil 131 at low temperature (for example at 4 to 5 degrees Celsius) , and this minimises tritium losses; such losses can be checked by analysis of the contents of a water bubbler 41. A facility is provided, using an inlet 26, for flushing the condensing coil 131 internally with distilled water or the same cocktail of chemicals used for scintillation counting.

A small pressure depression is maintained inside the apparatus during operation, by use of a vacuum pump 16 acting on the point where gases exit the condensing vessel 13. This assists in avoiding the possibility of external contamination during operation and sample changing.

In use, the apparatus is first purged with nitrogen to ensure that there is no combustion when the sample (for example 15 grams) is initially introduced into the inner tube 8 in the wire basket 9 as illustrated in Figure 2. The supply of metered, humidified nitrogen is maintained as the sample is introduced fully into the furnace 6 along the tube 8, as illustrated in Figure 3, and retained there to complete the stripping process without combustion of the test material. The resultant tritiated water condensed inside the condensing coil 131 and as subsequently drained from the reservoir 132 into the vial 14, is submitted to liquid scintillation counting.

As an option with the present apparatus, the valve 5 may be operated to replace the nitrogen by air, after (but only after) the sample has acquired a desired pre-combustion temperature. The supply of metered, humidified air to the furnace 6 allows controlled combustion of the sample to take place; combustion is controlled especially in that the time of combustion at the required temperature is known, the temperature of the sample itself being known independently of the temperature of the heating chamber due to the thermocouple 9a on the basket 9. The tritium extracted from within the sample in this way is taken up into the water vapour so as to be added into the condensate collected in the reservoir 132 during the flow of humidified nitrogen. This condensate is drained into the vial 14 as before, and submitted to liquid scintillation counting.

The condensate collected in the vial 14 in both cases represents a percentage of the total tritium content of the test material. In each case the method gives substantially consistent results using a sensitive scintillation counter that gives a good counting accuracy. The level of consistency is such as to enable the tritium concentration of the bulk material to be calculated using a previously-derived scaling-factor; the appropriate scaling-factor is determined by calibrating the apparatus using a sample from material having a known total tritium content. The calculated value has an accuracy that is adequate for determing whether the material is significantly above or below the relevant regulatory classification threshold.

In one application of the method and apparatus described above, very good results have been achieved with a sample-temperature of 400 degrees Celsius. However, tests using known quantities of tritiated water have demonstrated recovery of a high percentage of the total, by use of humid nitrogen as the transporting medium at lower temperature, for example 200 degrees Celsius, over a short period of, for example, 30 minutes.

In further detail in the apparatus of Figure 1 the components not specifically described above are as follows: item 2 is a differential manometer; items 15, 21, 24, 25 and 28 are valves; item 27 is an electrical heater for the bubbler 4; item 23 is a gas-flow meter; and items 22 and 19 are, respectively, an electrical switch for the pump 16 and an electrical transformer for furnace heating-current.

Claims

CLAIMS :

1. A method for thermal separation of tritium from a sample of a nominally-radioactive material which comprises stripping tritium from said material into a stream of a humidified gas.

2. A method as claimed in claim 1 wherein the humidified gas is an inert gas unreactive with the said material.

3. A method as claimed in claim 2 wherein the inert gas is nitrogen.

4. A method as claimed in any preceding claim wherein the sample material and the humidified gas are heated to between about 200 to 400°C.

5. A method as claimed in claim 4 wherein said humidified gas is passed over or through said sample material at a temperature of about 200 C for about

30 minutes.

6. A method as claimed in any preceding claim wherein the sample is heated in a chamber or furnace.

7. A method as claimed in claim 6 wherein the temperature of the sample is measured independently of the temperature of the body of the chamber or furnace.

8. A method as claimed in any preceding claim wherein the tritium is taken up into the water vapour in said humidified gas.

9. A method as claimed in any preceding claim wherein any tritium in said sample material is in the form of tritiated water.

10. A method as claimed in any preceding claim which is carried out on about 10 to 20 grams of sample material.

11. A method as claimed in any preceding claim wherein after stripping any tritium into said humidified gas the gas is cooled to condense the water vapour.

12. A method as claimed in claim 10 wherein said condensed water vapour is subjected to scintillation counting.

13. A method as claimed in any one of claims 6 to 12 wherein prior to commencing the stripping stage the chamber or furnace is purged with nitrogen.

14. A method as claimed in any one of claims 2 to 13 which comprises the additional step of replacing the inert humidified gas with a metered supply of humidified air or oxygen once the desired pre-combustion temperature has been reached and allowing controlled combustion of the sample.

15. A method as claimed in claim 14 wherein the rate of supply of said humidified air or oxygen can be adjusted to control the rate of combustion.

16. A method as claimed in claim 14 or 15 wherein during said controlled combustion any tritium in said material is stripped into said humidified air or oxygen which is then cooled to condense any tritiated water vapour, the condensate then being subjected to scintillation counting.

17. A method as claimed in any one of claims 12 to 16 wherein following the scintillation counting of the condensed water vapour the amount of any tritium in the bulk material from which the sample was derived is calculated using a previously derived scaling factor.

18. A method for thermal separation of tritium from a sample of a nominally-radioactive material by controlled combustion which comprises stripping, tritium from said material into a gas stream of humidified oxygen or air, the rate of combustion being controlled by the flow rate of said humidified air or oxygen stream.

19. An apparatus for the thermal separation of tritium from a sample of a nominally-radioactive material which comprises a chamber for receipt of said sample, means for heating said chamber to a pre-determined temperature, means for supplying a humidified gas into said chamber, means for heating said gas to said pre-determined temperature before entry into said chamber, means for exiting said humidified gas from said chamber and means for condensing the water vapour in said humidified gas on exiting said chamber.

20. An apparatus as claimed in claim 19 wherein the means for heating the chamber also heats the humidified gas supplied to the chamber.

21. An apparatus as claimed in claim 20 wherein the chamber is double-walled, the space between the inner and outer wall forming a passage through which in use the humidified gas must pass before entry into the chamber thereby pre-heating the gas to the temperature of the chamber before presentation to the sample.

22. An apparatus as claimed in any one of claims 19 to 21 which is capable of receiving a sample of nominally-radioactive material of between 10 and 20 grams.

23. An apparatus as claimed in any one of claims 19 to 22 wherein the means for supplying the humidified gas are adapted to supply a first humidified gas which is an inert gas not reactive with the sample material and independently a second humidified gas which is humidified air or oxygen.

24. An apparatus as claimed in any one of claims 19 to 23 wherein the means for condensing the water vapour is a condensor which includes within the body thereof a reservoir for receipt of the condensate.