GB2031863A

GB2031863A - Storing hydrogen isotopes

Info

Publication number: GB2031863A
Application number: GB7932171A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-09-27
Filing date: 1979-09-17
Publication date: 1980-04-30

Abstract

A method and apparatus for the storage of a hydrogen isotope which includes bombarding a retentive material with ions or atoms of the hydrogen isotope so as to implant the hydrogen isotope in the retentive material, the retentive material being selected such that and the bombarding being effected such that a concentration of the hydrogen isotope is formed in and entrapped in the retentive material and means for such a process.

Description

SPECIFICATION Improvements in or relating to the storage of material The present invention relates to the storage of material and more particularly to the storage of a hydrogen isotope or a mixture of two or more hydrogen isotopes.

According to one aspect of the present invention there is provided a method for the storage of a hydrogen isotope which includes bombarding a retentive material with ions or atoms of the hydrogen isotope so as to implant the hydrogen isotope in the retentive material, the retentive material being selected such that and the bombarding being effected such that a concentration of the hydrogen isotope is formed in and entrapped in the retentive material.

The retentive material should be receptive in the sense of being capable of being implanted with ions or atoms to an extent compatible with forming a concentration of the hydrogen isotope suitable for given storage purposes. Also the retentive material should be retentive in the sense that the entrapped hydrogen isotope has a diffusivity therein which is sufficiently low that no unacceptable quantity of the hydrogen isotope is released at the temperature at which the retentive material is stored after implantation and entrapment of the hydrogen isotope.

It is preferred that the retentive material also is such that no unacceptable quantity of hydrogen isotope is released under attack by agents present in the environment (e.g. corrosion) or at temperatures which might be reached during a fire.

What is an unacceptable quantity of hydrogen isotope to be released can vary with the particular chosen application of the invention. However, by way of example, for long term storage of hydrogen containing tritium at ambient temperature, there lease rate will be dependant upon environmental considerations.

Hydrogen, deuterium, tritium or mixtures of two or more of these isotopes may be stored in accordance with the present invention.

In accordance with one embodiment of the present invention the retentive material is provided as a layer on a substrate.

In accordance with another embodiment of the present invention the retentive material is provided as a powder.

In accordance with a further embodiment of the present invention the retentive material is provided as a layer of powder sintered onto a substrate.

In the nuclear energy field there is a requirement for a method of immobilising and storing tritium (half-life 12.5 years) or hydrogen containing tritium which arises, for example, from the reprocessing of nuclear fuels.

Thus, in accordance with a preferred embodiment of the present invention there is provided a method for the storage of tritium, or hydrogen containing tritium, which includes bombarding a retentive material with ions or atoms of tritium, or with ions or atoms of hydrogen and tritium, so as to implant tritium or hydrogen and tritium in the retentive material, the retentive material being selected such that and the bombarding being effected such that a concentration of tritium, or hydrogen and tritium, is formed in and entrapped in the retentive material.

Examples of suitable retentive materials for use in accordance with the present invention are alumina, carbon, silicon carbide and boron nitride.

The retentive material (together with its substrate where used) is preferably encapsulated or sealed in a container after the bombardment and entrapment and prior to storage. This provides further containment of the hydrogen isotope(s) should any release from the retentive material occur during storage (e.g. in the event of exposure to high temperatures in a fire or attack by agents present in the environment (e.g. corrosion)).

Where the hydrogen isotope is a radioactive isotope, encapsulation or sealing of the retentive material in a container after bombardment and entrapment also provides a convenient means for extracting radioactive decay heat to inhibit heating of the retentive material. Thus, for example, the container may be a finned container which can be cooled by air convection.

In one embodiment a layer of retentive material is carried on a sheet or foil substrate which after the bombardment and entrapping can be rolled into a cylindrical configuration convenient for sealing in a container prior to storage.

A layer of retentive material may be provided for bombardment on both sides of a sheet or foil.

By way of example, a layer of alumina may be provided on a foil for bombardment by anodising an aluminium foil, or by oxidising (e.g. by heating in air) a foil of an aluminium bearing ferritic alloy such as Fecralloy (Registered Trade Mark) alloy or Kanthal (Registered Trade Mark) alloy.

The foil can be, for example, 0.001 to 0.002" in thickness and the layer of alumina formed by the anodising or the oxidising can be about lum in thickness. By use of a roll or coil of foil the anodising or oxidation may be carried out in a continuous process. For example, aluminium foil can be continuously anodised by feeding through an electrolytic bath and an aluminium bearing alloy foil (e.g. of Fecralloy (Registered Trade Mark) alloy) may be oxidised by multiple passes through a furnace at 1000-1200"C.

For use in accordance with the present invention a layer of retentive material on a substrate may be provided by any convenient means which, for example, modifies the surface to give the layer or produces a coating to give the layer.

For example, the layer of retentive material may be formed on a substrate by a wash-coating process (for example as disclosed in British Patent Specification No. 1,490,977 (corresponding to U.S. Patent No.

3,957,692), German Offenlegunsschrift No. 2647702, and co-pending British Application No. 45471/77 followed by subsequent heating to give a layer of a ceramic retentive material on the substrate. Thus, for example, a foil of Fecralloy (Registered Trade Mark) alloy, or a foil of Kanthal (Registered Trade Mark) alloy (either of which foils may optionally be preoxi dised to give an oxide layer) can be washed coated with an alumina orceria sol in accordance with British Patent Specification No. 1,490,977 (corresponding to U.S. Patent No. 3,957,692), German Offenlegunschrift No. 2,647,702, and co-pending British Patent Application No. 45471/77) and heated to produce a foil carrying a layer of ceramic retentive material comprising alumina or ceria or a mixture thereof.

It will be appreciated that where the substrate is a foil intended for rolling up after implantation, the layer should be sufficiently flexible and tenaciousiy bonded to the substrate that it is not removed when the substrate is rolled.

It is believed that, in a further embodiment, it is possible to deposit a layer of retentive material on a substrate and implant an hydrogen isotope, by ion bombardment, in the layer so deposited in substantially simultaneous operations. The deposition in accordance with the immediately foregoing embodiment may be, for example, by means of sputtering, ion plating, or plasma activated vapour deposition (PAVD).

In yet a further embodiment, subsequently to implanting a hydrogen isotope into a retentive material a layer of retentive material can be applied over the implanted retentive material and the implantation repeated to store a hydrogen isotope in the said layer. The application of still further layers and subsequent implantation may be carried out.

Optionally after implantation of the hydrogen isotope in a retentive material, a "barrier layer" of material may be applied to the retentive material.

The barrier layer would not be subsequently bombarded with ions or atoms and may serve further to inhibit any tendancy for the implanted hydrogen isotope to be released. A barrier layer may be applied by any suitable process (e.g. sputtering).

According to another aspect of the present invention there is provided apparatus for use in the storage of a hydrogen isotope in accordance with the method of the invention comprising, a chamber, a feedspool and take-up spool within the chamber for carrying a sheet or foil carrying a layer of retentive material, means for implanting ions or atoms of a hydrogen isotope in the layer of retentive material and means for transferring the sheet or foil from the feed spool to the take-up spool past the means for implanting the ions or atoms.

The means for implanting the ions or atoms in the retentive material may be any suitable type of device or arrangement which is capable of producing ions or atoms and bombarding the retentive material with the ions or atoms at sufficient energy to implant them in the retentive material. Examples of such devices and arrangements are acceleterators, ion guns (e.g. of the glow discharge or filament type), glow discharge systems, ion plating systems and PAVD systems.

In order to reduce heating of the sheet or foil by the implantation process the sheet or foil can be supported by water cooled rollers or spools. Where an ion gun is used the heating effect may also be reduced by spreading the ion beam over as large an area of the sheet or foil as possible thereby reducing the ion beam density.

It is believed that the hydrogen isotope to be implanted can be fed to the means for implanting ions or atoms in a number of forms. Thus, for example, gaseous hydrogen, deuterium ortritium or gaseous mixtures thereof can be fed to the means for implanting ions or atoms. By way of further example, the hydrogen isotope can be fed to the means for implanting in the form of a compound of the isotope and another element. Thus, for example, tritiated or deuterated water vapour may be fed to the means for implanting.

Where the hydrogen isotope fed to an ion gun is in the form of a compound of the hydrogen isotope and oxygen, some of the oxygen produced will be negatively charged and remain in the gun, whilst the remainder will be implanted in the retentive material at a shallower depth than the hydrogen isotope.

Providing the retentive material is sufficiently thick the sputtering of the retentive material caused by the oxygen should not cause any significant effect on the retention of the implanted hydrogen isotope by the retentive material.

Alternatively, or additionally, a magnetic field may be provided around the ion gun to give some degree of mass analysis to reduce the oxygen bombardment of the retentive material.

When implanting ions and atoms of a hydrogen isotope it is believed that it may be advantageous also to bombard the retentive material with ions or atoms of a heavier gas such as argon or oxygen either before or during implantation in order to improve the hydrogen isotope entrapment efficiency of the retentive material by increasing the number of trapping sites for the isotope.

Where a PAVD system is used as the means for implanting the hydrogen isotope to be implanted, the hydrogen isotope may be included with, or in, the vapour which is decomposed to give the retentive material.

The chosen source of hydrogen isotope may be fed to the means for implanting the hydrogen isotope by any convenient arrangement appropriate to the particular type of means for implanting used, such arrangements will be apparent to those skilled in the art. For example the chosen source of - hydrogen isotope may be fed directly to the means for implanting (e.g. an ion gun) or to a chamber enclosing the means for implanting (e.g. an ion gun or a glow discharge system).

For the storage of tritium it is preferable from the point of view of process efficiency that the source of tritium contains between 1 % to 10%tritium atoms/hydrogen atoms. This may be achieved by known isotope separation process such as electrolysis or distillation. The higher the tritium content the smaller the overall volume to be implanted and entrapped in order to store a given amount of tritium.

To maintain suitable low pressure conditions in a chamber used for implantation, it is believed that some external pumping may be required. The pumping may remove gas adsorbed on the retentive material and also maintain a low pressure in the chamber for implantation purposes. Gas removed by pumping can be recycled to the chamber for storage.

The size of the apparatus required to store 500 Ci of tritium per day depends upon the dilution of the tritium and its form. Ion beam current required is given by 19/cz mA where c is the atomic fraction of tritium in the source of hydrogen isotope and Z the implant efficiency which is 80% for doses of ~ 1.5 1017 ions/cm2 (25 milli coulomb/cm2).

The area of retentive material required (at a dose of 1.5 1017 ions/cm2) is 0,773/cm cm2/sec or 7/cZ m2/day.

Therefore, using a moving foil 10 cm wide having a layer of retentive material on each side, if the tritium concentration in the source is 10%, a storage rate of 500 Ci/day may be obtained by using two ion guns each of 100 mA beam current and each irradiating a different side of the foil moving ats 30 cm/minute.

The invention will now be further described, by way of example only, with reference to the single figure of the accompanying drawing which shows a diagrammatic representation of an apparatus for implanting a hydrogen isotope into a retentive material.

Referring now to the drawing, a sealable chamber 1 is provided with a pipe 2 connected to a vacuum system 3 via a valve 4.

Within the chamber 1 there is supported a feed spool 5, take-up spool 6, and carrier spools 7 and 8 for carrying a sheet or foil 9, said foil 9 carrying a layer of retentive material on each of its sides.

Multiple slot saddle-field ion guns 10 and 11 are provided for bombarding the retentive layer on the foil 9 in operation.

Pipes 12 and valves 13 are provided for the introduction of a hydrogen isotope for implantation (or a source thereof) to the ion guns 10 and 11.

Also electrical connections represent diagrammatically by lines 14 are provided for operating the ion guns 10 and 11. The detailed form of the electrical connections 14 is in accordance with known ion gun technology.

In operation the chamber 1 is evacuated to a pressure of about 10-4torr by means of pipe 2, vacuum system 3 and valve 4.

Subsequently a hydrogen isotope to be implanted (or a source thereof) is fed to the ion guns 10 and 11 by means of pipes 12 and valves 13 and the ion guns 10 and 11 are operated by means of electrical connections 14, in accordance with known ion gun technology, to produce ion beams represented by arrows 15 and bombard the layers of retentive material of the foil 9 with hydrogen ions thereby to implant the hydrogen isotope in the layers. It will be appreciated that in the apparatus shown each ion gun bombards one side of the foil. By use of the spools 5 to 8 the foil 9 is moved relative to the ion guns 10 and 11 so that fresh portions of the layers of retentive material are presented for bombardment and implantation of hydrogen isotope. Thus the foil 9 on the feed spool 5 becomes implanted and wound onto the take-up spool 6.

Any one or more of the spools 5 to 8 may be cooled (e.g. by circulation of water therethrough) to remove heat generated during bombardment with ions.

When the desired amount of hydrogen isotope has been entrapped in the layers of retentive material the spool 6 may be removed from the chamber 1 for storage following optional encapsulation.

Means (not shown in the drawing) for cooling the ion guns may be provided if required as will be apparent to those skilled in the art.

Where the source of hydrogen is such that condensation could occur in the pipes 12 and guns 10 and 11 (e.g. in the case of tritiated water), means (not shown) may be provided for warming the pipes 12 and guns 10 and 11.

It will be appreciated that alternativeiy the arrangement could have been such that the hydrogen isotope, or source thereof, can be fed to the chamber 1 generally rather than specifically to the ion guns 10 and 11.

The invention will now be further described, by way of example only, as follows: Example A 0.003" thick foil of Fecralloy (Registered Trade Mark) alloy was oxidised by heating in air at 1000 C for 12 hours to give an oxide (retentive material) layer of 5500 A thickness on the foil. The oxide layer was bombarded with deuterium ions of energy 1 5KeV by means of a linear accelerator to a dose of 5.6 x 1017 deuterium atoms/sq. cm thereby to implant deuterium ions and to form a concentration of deuterium in the oxide layer (conc~10%).

The foil and oxide layer after implantation was heated at a rate of 80 deg C/min and the deuterium release measured as a function of temperature.

The results were as follows: 50% released by 550"C Complete release by 1300 C

Claims

1. A method forthe storage of a hydrogen isotope which includes bombarding a retentive material with ions or atoms of the hydrogen isotope so as to implant the hydrogen isotope in the retentive material, the retentive material being selected such that and the bombarding being effected such that a concentration of the hydrogen isotope is formed in and entrapped in the retentive material.

2. A method as claimed in claim 1 for the storage of tritium, or hydrogen containing tritium, which includes bombarding a retentive material with ions or atoms of tritium, or with ions or atoms of hydrogen and tritium, so as to implant tritium or hydrogen and tritium in the retentive material, the retentive material being selected such that and the bombarding being effected such that a concentration of tritium, or hydrogen and tritium, is formed in an entrapped in the retentive material.

3. A method as claimed in claim 1 or claim 2 wherein the retentive material is provided as a layer on a substrate.

4. A method as claimed in claim 1 or claim 2 wherein the retentive material is provided as a powder.

5. A method as claimed in claim 1 or claim 2 wherein the retentive material is provided as a layer of powdersintered onto a substrate.

6. A method as claimed in any one of claims 1 to 3 wherein a layer of retentive material is carried on a sheet or foil substrate.

7. A method as claimed in claim 6 wherein a layer of retentive material is provided on both sides of a sheet or foil.

8. A method as claimed in any one of claims 1 to 7 wherein the retentive material is alumina, carbon, silicon carbide or boron nitride.

9. A method as claimed in any one of claims 6 to 9 wherein a layer of alumina is provided on an.

aluminium foil by anodising.

10. A method as claimed in any one of claims 6 to 8 wherein a layer of alumina is provided on a foil of an aluminium bearing ferritic alloy by oxidising.

11. A method as claimed in any one of claims 1 to 3 wherein a layer of retentive material is formed on a substrate by a wash-coating process followed by heating to give a layer of a ceramic retentive material on the substrate

12. A method as claimed in any one of claims 1 to 3 wherein a layer of retentive material is deposited on a substrate and an hydrogen isotope is implanted, by ion bombardment, in the layer so deposited in substantially simultaneous operations.

13. A method as claimed in claim 12 wherein the layer of retentive material is deposited by means of sputtering, ion plating, or plasma activated vapour deposition.

14. A method as claimed in any one of the preceding claims wherein subsequently to implanting a hydrogen isotope into a retentive material a layer of retentive material is applied over the implanted retentive material and the implantation repeated to store a hydrogen isotope in the said layer.

15. A method as claimed in any one of the preceding claims wherein after implantation of a hydrogen isotope in a retentive material, a "barrier layer" of material is applied to the retentive material.

16. A method as claimed in any one of the preceding claims wherein the retentive material is bombarded with ions or atoms of a heavier gas before or during implantation of the hydrogen isotope to improve the hydrogen isotope entrapment efficiency of the retentive material by increasing the number of trapping sites for the isotope.

17. A method as claimed in claim 16 wherein the heavier gas is argon or oxygen.

18. Apparatus for use in the storage of a hydrogen isotope, in accordance with the method of Claim 1 comprising, a chamber, a feedspool and take-up spool within the chamber for carrying a sheet or foil carrying a layer of retentive material, means for implanting ions or atoms of a hydrogen isotope in the layer of retentive material and means for transferring the sheet or foil from the feed spool to the take-up spool past the means for implanting the ions or atoms.

19. Apparatus as claimed in claim 18 wherein the means for implanting ions or atoms of a hydrogen isotope in the layer of retentive material comprises an accelerator, an ion gun, a glow discharge system, an ion plating system or a plasma activated vapour deposition system.

20. Apparatus as claimed in claim 18 or claim 19 wherein means are provided for feeding gaseous hydrogen, deuterium ortritium, or gaseous mixtures thereof, or titritated or deuterated water vapour, to the means for implanting.

21. A method for the storage of a hydrogen isotope substantially as hereinbefore described with reference to the Example.

22. Apparatus for use in the storage of a hydrogen isotope substantially as hereinbefore described with reference to the accompanying drawing.

23. A method for the storage of a hydrogen isotope substantially as herein before described with reference to the accompanying drawing.