GB2123805A

GB2123805A - Separating hydrogen isotopes

Info

Publication number: GB2123805A
Application number: GB08316825A
Authority: GB
Inventors: Bruno Ferrario
Original assignee: SAES Getters SpA
Current assignee: SAES Getters SpA
Priority date: 1982-06-28
Filing date: 1983-06-21
Publication date: 1984-02-08
Also published as: FR2529097B1; JPS5932947A; IT1157286B; IT8222087A0; NL193238C; DE3322637C2; GB2123805B; NL8302294A; GB8316825D0; FR2529097A1; DE3322637A1; NL193238B

Abstract

A non-evaporable getter metal is encapsulated in a shell of porous sintered metal powder to provide a hydrogen isotope sorption pellet. The pellets may be used for the purification of rare gases or for the recovery, storage and regeneration of hydrogen isotopes in fusion reactors.

Description

SPECIFICATION Hydrogen isotope sorption pellets and process for their use There are many cases in which it is desirable or necessary to remove large quantities of hydrogen and/or its isotopes from a particular ambient. For instance it may be required to remove hydrogen from a stream of natural gas to separate it from other component gases such as methane or other hydrocarbons. Large quantities of gas are involved at high gas flow rates. To separate the hydrogen and gas mixture can be passed over a bed of hydrogen sorbing material or nonevaporable getter material. These materials are usually inert to hydrocarbons which are therefore not sorbed. Most non-evaporable getter materials are metallic or have a metallic component which, upon sorption of large quantities of hydrogen, embrittle and form a very fine powder.This very fine powder can be carried away in the gas stream and can be difficult to recover efficiently.

Loss of the particles implies loss of the recovered hydrogen. Also uncontrolled, very fine metal particles in the ambient are well known to be an explosion hazard. Fine hydrided metal particles in the gas stream are also abrasive and can cause breakdown of such components as valves or pumps due to this abrasive action.

Fusion reactors transform mass into energy by joining light atoms. Many different nuclear fusion reactions are possible but only a few are of practical value for energy production. These involve the isotopes of hydrogen. Three isotopes of hydrogen are known; they are hydrogen, deutrium and tritium.

To produce net power fusion reactions must take place at high temperatures. The power production process which can occur at the lowest temperature and hence the most readily attainable fusion process in practice is the combination of a deuterium nucleus with one of tritium.

The products are energetic helium-4 (4He) the common isotope of helium (which is also called an alpha particle) and a more highly energetic free neutron. The helium nucleus carried about onefifth of the total energy released and the neutron carries the remaining four-fifths.

Deuterium may be readily extracted from ordinary water. The surface waters of the earth are estimated to contain more than 1018 tons of deuterium, an essentially inexhaustable supply.

The tritium may be prepared on a large scale by the bombardment of enriched 6Li by 14 MeV neutrons 6Li (n, a) 3H from a fission reactor.

In order to extract energy from the reactor it is surrounded by a neutron absorbing "blanket". The neutrons yield their kinetic energy as heat in the blanket which heat can be utilized, for instance, to drive conventional turbines for the generation of electricity.

The heat from the blanket may be extracted in many different ways. The blanket itself may consist of a liquid metal which is continuously circulated through a heat exchanger and then returned to the blanket environment.

Unfortunately this involves pumping the liquid metal through high magnetic fields and complex geometries. Alternatively, the blanket may be a solid neutron absorber over which flows a liquid or gaseous coolant such as high pressure steam or a rare gas such as helium.

As the tritium required as a fuel for the reactor is expensive, the blanket itself can be used as a source of tritium. If the blanket is lithium or an alloy of lithium with other elements such as hydrogen, deuterium, lead or lead and aluminium or other lithium based compounds such as Li2SiO3 the lithium of the blanket material produces tritium when it is irradiated with neutrons from the fusion reaction.

Although the following disclosure will refer frequently to tritium it will be realized that some hydrogen and deuterium will be formed in the blanket by (n, p) and (n, d) reactions and these will behave in a similar manner to tritium.

Tritium has only a low solubility in the blanket material and therefore it quickly starts to diffuse out of the solid or liquid breeding material creating a high tritium gas partial pressure and creating considerable difficulties in tritium confinement especially if the coolant is a liquid breeder. Some of these difficulties can be alleviated by using a lithium based breeder in the solid form as only a breeder and using a rare gas coolant or purge gas to take away the tritium as it is produced. The tritium must then be separated in a pure form from the rare gas coolant or purge gas.

The tritium-rare gas mixture may be passed through a purification chamber containing powdered gettering material to sorb the tritium only, as the rare gas is inert and is not sorbed.

However as the quantities of tritium involved are large the gettering powder can easily embrittle and crumble to such a fine powder that it is difficult to manipulate safely. If the purification chamber is damaged there may escape particles of the very fine powder which contain radioactive tritium. If the powder accidentally ignites this may also cause release of radioactive powder into the ambient.

It is not possible to mix the getter powder with the lithium breeding material as the same crumbling effect and dangers may take place.

Furthermore it is difficult to separate completely the radio-active tritium-containing getter powder from the breeding material without the use of complex and expensive procedures.

It has been proposed to place the non evaporable getter powder in trays between flat porous backing plates but this leads to a non continuous process as the gas flow has to be stopped periodically for removal of the trays.

It is therefore an object of the present invention to provide a hydrogen isotope sorption pellet which prevents the escape of loose particles of getter material and which can be used in a continuous process for the sorption of hydrogen isotopes in a gas stream.

It is another object of the present invention to provide a tritium sorption pellet and method for tritium recovery in fusion reactor technology free from one or more of the disadvantages of previous getter devices or tritium recovery methods.

It is a further object of the present invention to provide a tritium sorption pellet and method for tritium recovery from a rare gas coolant or purge gas of a fusion reactor breeder blanket.

These and other objects and advantages of the present invention will become clear to those skilled in the art by reference to the following detailed description and drawings wherein: Fig. 1 is a diagrammatic cross-sectional view of a hydrogen isotope sorption pellet of the present invention, and Fig. 2 is a diagrammatic cross-sectional view of a rare gas purifier using hydrogen isotope sorption pellets of the present invention for the removal of hydrogen isotopes from a rare gas or a purge gas of a fusion reactor breeder blanket.

Fig. 3 is a diagrammatic representation of a process for the removal of hydrogen from a hydrogen rich zone.

The present invention provides a hydrogen isotope sorption pellet having a shell of porous sintered metal powder which encloses a nonevaporable getter material at least part of the getter material being a powdered getter metal.

The porosity of the metal shell is sufficient to allow sorption of hydrogen isotopes after activation of the non-evaporable getter material, while preventing the escape of loose particles of getter material.

Each hydrogen isotope sorption pellet preferably comprises a substantially spherical shell of porous metal enclosing a non-evaporable getter material. The porous metal shell may be conveniently formed by partially sintering a metal powder placed round the getter material. Any metal may be used which can withstand the working environment and is available in powder form and which forms a cohesive porous mass on heating at a temperature sufficiently low to cause no damage to the non-evaporable getter material.

Steel, iron, nickel and cobalt, among others, are suitable. One preferred metal is stainless steel.

Another preferred metal is nickel as it is magnetic and its magnetic properties can be utilized in handling the pellets. The metal powder can have any diameter suitable for forming the porous shell and conveniently can be from 5 ,um to 200 ,um and preferably from 40 ym to 120 ym. With smaller diameters it is more difficult to control the partial sintering process and there is the risk that the shell is not sufficiently porous to allow adequate passage of hydrogen isotopes to the getter material. With larger diameters the porosity is such as to allow particles of getter material to escape through the shell.

The external diameter of the shell can be between 0.2 and 5 cm and is preferable between 0.3 and 1.5 cm whereas the shell thickness can be about 0.5 to 2 mm thick.

The getter material enclosed by the shell is any non-evaporable getter material capable of the reversible sorption of hydrogen isotopes such as titanium, zirconium, tantalum or niobium as well as alloys and or mixtures of two or more of the above or with other metals that do not materially reduce their sorptive capacity. The preferred nonevaporable getter materials are those which comprise a sintered mixture of powdered zirconium or titanium and an antisintering agent.

The zirconium or titanium is present as a fine powder which passes through a U.S. standard screen of 200 mesh/inch (79 mesh/cm) and preferably through a U.S. standard screen of 400 mesh per inch (158/cm). The antisintering agent can be chosen from the group comprising C, Zr-Al alloys and Ti-V-Fe or Zr-V-Fe alloys.

The Zr-V-Fe and Ti-V-Fe alloys are particularly useful when the getter material must be rendered capable of hydrogen isotope sorption at relatively low temperatures.

The antisintering agent is present as a powder that passes through a U.S. standard screen of 60 mesh per inch (24 mesh/cm) and preferable through a U.S. standard screen of 120 mesh per inch (47 mesh/cm), they are also generally larger than the zirconium or titanium particles.

In operation the sorption pellets are placed in the stream of gas which contains the hydrogen isotope to be recovered. This may for instance be a flow of natural gas containing hydrogen or it may be the rare gas coolant or purge gas of a fusion reactor breeder blanket. They may be activated by, for instance, induction heating prior to introduction in the gas stream or, if the getter material is activatable at low temperatures, the temperature of the gas may be sufficient to cause their activation and the sorption of hydrogen isotope.

The pellets may also be situated in the breeding blanket in close spacial relationship with the breeding material.

The sorption pellets can therefore be used in any application where large quantities of hydrogen and/or its isotopes must be sorbed and the formation of fine getter metal particles and their release into a gas stream could be dangerous.

The hydrogen which is useful in the present invention is all of the isotopes of hydrogen and can be H2, D2,T2,HD, HT or DT.

The invention is especially useful with heavy hydrogen by which is meant deuterium and or tritium.

Referring now to the drawings and in particular to Fig. 1, there is shown a diagrammatic crosssectional view of a hydrogen isotope sorption pellet 10 having a substantially spherical shell 12 of porous sintered metal powder, preferably stainless steel powder having a particle size of between 5 ,um and 200,tom and preferably between 40 ,um and 120 ,um. The diameter of the shell is between 0.2 cm and 5 cm and its thickness is between 0.5 and 2 mm.A non-evaporable getter material 1 4 is enclosed by spherical shell 12 and comprises a sintered mixture of zirconium and an antisintering agent which may be chosen from the group comprising C, a Zr-Al alloy and preferably an 84% Zr-16% Al (by weight) alloy or a Ti-V-Fe alloy or Zr-V-Fe alloy and preferably an alloy whose composition in weight percent when plotted on a ternary composition diagram in weight percent Zr, weight percent V and weight percent Fe lies within a polygon having as its corners the points defined by:: i) 75% Zr20% V5% Fe ii) 45% Zr20% V35% Fe iii) 45% Zr50% V5% Fe The sorption pellet is prepared by mixing together zirconium powder and the anti-sintering agent, placing the mixture in a spherical mould and heating in vacuum at about 8000C-1 2000C for several minutes. After cooling to room temperature the sintered sphere of getter material is placed in a second larger spherical mould lined with the shell metal powder. the second mould is then heated under vacuum to approximately the same temperature for a sufficient time to give the spherical shell the required porosity.The porosity of the shell must be sufficient to allow sorption of hydrogen isotopes from a gas mixture after activation of the non-evaporable getter material while preventing the escape of loose particles of getter material resulting from the sorption of large quantities of hydrogen isotopes.

Alternatively the getter powder mixture may be simply mechanically compressed to form a cohesive spherical shape and then coated by immersion in a bath of metal powder mixed with a binder to form a shell. This pellet is then heated in a vacuum to cause the simultaneous sintering of the getter material and the shell.

Referring now to Fig. 2 there is shown a rare gas purifier 16 for the removal of tritium from helium in a fusion reactor. Rare gas purifier 16 comprises a gas inlet 18 attached to a tritium sorption chamber 20 and a gas outlet 22. A feeding hopper 24, containing tritium sorption pellets 26, 26' etc., identical to pellet 10, is also attached to sorption chamber 20 by means of a non-metallic pipe 28 and two valves 30, 32. An induction heating coil 34 surrounds pipe 28. A tritium sorption pellet outlet 36 is also provided with two valves 38, 40.

By suitable operation of valves 30, 32, 38, 40 tritium sorption pellets are caused to pass through sorption chamber 20. The pellets are prevented from entering gas inlet 18 or gas outlet 22 by means of wire gauze limiters 42, 44 respectively. Hot helium, from the reactor blanket, mixed with tritium is passed through sorption chamber 20 and the tritium contacts the tritium sorption pellets whereupon it is sorbed. If the temperature of the helium is insufficient to activate the getter material of the tritium sorption pellets then induction heating coil 34 can be used to activate the material during passage of the pellets through non-metallic pipe 28 before they enter sorption chamber 20.After removal of the pellets from the sorption chamber they can be handled safely without loss of getter material particles and they can be heater in vacuum to recover the sorbed tritium.

Referring now to Fig. 3 there is shown a diagrammatic representation 300 of a process using pellets of the present invention for removing hydrogen from a hydrogen rich zone 302. A source 304 of pellets of the present invention is provided and is connected to hydrogen rich zone 302 by suitable connecting means 306 suitably adapted to allow a continuous stream of pellets to pass into the hydrogen rich zone 302. The pellets contact the hydrogen present in the hydrogen rich zone where they sorb the hydrogen. The pellets are removed from the hydrogen rich zone 302 by means of a second connecting means 308 leading to a pellet collector 310. The pellets can then be heated to remove the hydrogen.

The hydrogen rich zone could be a rare gas contaminated with heavy hydrogen, hydrogen rich meaning any percentage of heavy hydrogen which is desired to be removed from the rare gas.

Although the invention has been described in detail with reference to certain preferred embodiments variations and modifications can be performed within the scope and spirit of the invention as described and defined in the following claims.

Claims

Claims 1. A hydrogen isotope sorption pellet comprising 1. a spherical shell of porous sintered metal powder; and 2. a non-evaporable getter material enclosed by said shell comprising a powdered getter metal, wherein the porosity of the metal shell is sufficient to allow sorption of hydrogen isotopes from a gas mixture after activation of the nonevaporable getter material, while preventing the escape of loose particles of getter material. 2. A tritium sorption pellet comprising:

1. A substantially spherical shell of porous sintered stainless steel powder, the shell having a diameter of between 0.2 cm and 5 cm and a thickness of 0.5 to 2 mm, and the stainless steel powder having a particle size of between 40 ,am and 120,us; and

2. a non-evaporable getter material enclosed by said shell of stainless steel powder and comprising a sintered mixture of powdered zirconium and an antisintering agent chosen from the group comprising: C, Zr-Al alloys, Ti-V-Fe alloys and Zr-V-Fe alloys.

wherein the porosity of the stainless steel shell is sufficient to allow sorption of tritium from a rare gas tritium gas mixture after activation of the non evaporable getter material while preventing the escape of loose particles of getter material resulting from the sorption of large quantities of tritium.

3. A process for sorbing hydrogen isotopes said process comprising contacting the hydrogen isotope with a hydrogen isotope sorption pellet comprising:

1. a shell of porous sintered metal powder; and

2. a non-evaporable getter material enclosed by said shell comprising a powdered getter metal, wherein the porosity of the metal shell is sufficient to allow sorption of hydrogen isotopes after activation of the non-evaporable getter material while preventing the escape of loose particles of getter material.

4. heating the pellets to remove the hydrogen.

4. A process for sorbing tritium from a rare gas tritium mixture in a fusion reactor said process comprising contacting the tritium with a tritium sorption pellet comprising:

1. a substantially spherical shell of porous sintered stainless steel powder, the shell having a diameter of between 0.2 cm and 5 cm and a thickness of 0.5 to 2 mm, and the stainless steel powder having a particle size of between 40 ym and 120cm; and

2. a non-evaporable getter material enclosed by said shell of stainless steel powder and comprising a sintered mixture of powdered zirconium and an antisintering agent chosen from the group comprising:C, Zr-Al alloys and Ti-V-Fe or Zr-V-Fe alloys, wherein the porosity of the stainless steel shell is sufficient to allow sorption of tritium from the rare gas tritium gas mixture after activation of the nonevaporable getter material while preventing the escape of loose particles of getter material resulting from the sorption of large quantities of tritium.

5. A process for removing heavy hydrogen from a rare gas contaminated with heavy hydrogen comprising the steps of;

1. contacting the gas contaminated with heavy hydrogen with a pellet comprising i) a spherical shell of porous sintered metal powder; and ii) a non-evaporable getter material enclosed by said shell comprising a powdered getter metal.

2. Removing the pellets from the gas.

6. A process for removing hydrogen from a hydrogen rich zone comprising the steps of;

1. passing pellets into a hydrogen rich zone wherein the pellets comprise i) a spherical shell of porous sintered metal powder; and ii) a non-evaporable getter material enclosed by said shell comprising a powdered getter metal.

2. contacting the pellets with the hydrogen present in the hydrogen rich zone to sorb the hydrogen;

3. removing the pellets from the hydrogen rich zone;