GB2040555A

GB2040555A - Gettering vacuum systems

Info

Publication number: GB2040555A
Application number: GB7937273A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1978-11-21
Filing date: 1979-10-26
Publication date: 1980-08-28
Also published as: IT7941649A0; IT1124483B; FR2442493A1; DE2945548A1; JPS5574487A

Abstract

A gettering system for removing radioactive hydrogen isotope containing hydrocarbon bases from a hermetically sealed fusion reactor system to which the gettering system is coupled, includes an electron source 32 and means for accelerating the electrons to produce collisions with the hydrocarbon gas present in the system to dissociate the gas and a hydrogen and radioactive hydrogen isotope gas regenerable getter pumping means 36, e.g. a zirconium- aluminium alloy getter, is included within the system for adsorbing these gases from the system. The hydrocarbon may be tritiated methane which is generated in a fusion reactor. The getter 36 is heated to an activated operating temperature of 400 DEG C to getter the hydrogen and its isotope, and the carbon component is also removed e.g. by being deposited within the system. In other arrangements described a ring filament, or a helical grid or a magnetic field are utilised. The absorbed hydrogen and its isotope are recovered from the getter 36 by heating to 700 DEG C. <IMAGE>

Description

SPECIFICATION Gettering vacuum systems This invention relates to getter vacuum systems, and more particularly to such systems used with a nuclear fusion reactor for removing residual gases generated in the reactor. These gases may be radioactive hydrocarbon gases which must be isoiated from the environment.

A getter is a material which is very effective to absorb or adsorb gases in a vacuum system. The getter binds the gas to the getter surfaces and permits achieving high vacuum conditions.

Specific getter materials have been developed for specific gases, with one of the more difficult to pump gases being hydrogen. A very efficient hydrogen gas getter is a zirconium-aluminium alloy described in U.S. Patents 3,203,901; U.S.

Patents 3,858,709; and U.S. 3,975,304. These getter materials are themselves non-evaporable types of getters, and are typically coated on a supporting substrate which may or may not be a resistance heating element for heat activating the getter material. The getter materials are typically very temperature responsive and the absorbed or adsorbed gases can be regenerated from the getter typically by heating the getter above a particular temperature for a particular time. The gettering structure typically has a high surface area as described in U.S. Patents 3,609,062; 3,662,522 and 3,780,501.

In proposed fusion reaction systems now under development, it is expected that hydrogen isotopes such as deuterium and tritium will be used as fuel gases. The required fusion temperature is lower for a deuterium-tritium fusion reaction for a deutrium reaction. Tritium is a radioactive hydrogen isotope gas which must be isolated from the environment. It has been observed in experimental reactors that considerable methane gas as well as other hydrocarbons are produced, presumably as a result of carbon impurities present in the system.

Thus when tritium is used as a fuel gas some tritiated methane will be produced, and because it is radioactive must not be discharged to the environment.

The conventional hydrogen gettering materials described above can effictively and safeiy pump hydrogen and hydrogen isotope gases permitting effective isotope separation and isolation.

However, methane is non-polar and relatively inactive chemically, and is not conveniently gettered or adsorbed. A means of removing the radioactive methane is needed. It had originally been thought that thermal cracking of the tritiated methane to separate the tritium from the methane molecule would be an effective way to permit gettering of the tritium and pumping of methane.

The use of a hydrogen gettering means with a thermal cracking element for pumping methane is suggested in U.S. Patent 3,961,897. Investigation and experiments have shown that thermal cracking is extremely inefficient even at temperatures of approximately 25000 K. To achieve such temperatures would be difficult and inefficient in terms of energy input as well as producing marginal pumping capability.

Accordingly, the present invention resides in a regenerable gettering vacuum system for separating and isolating radioactive hydrogen isotope from hydrocarbon gas produced in a fusion reactor, which system comprises: (a) means for hermatically coupling the regenerable gettering system to the fusion reactor so that-the radioactive hydrogen isotope containing hydrocarbon gas from the reactor is introduced to the regenerable gettering system; (b) means for generating and accelerating an electron beam of sufficient energy to produce electron collision with the radioactive hydrogen isotope containing hydrocarbon gas to dissociate the gas and produce free radioactive hydrogen isotope;; (c) means for gettering the free hydrogen isotope, which gettering means is thermally activated at a first temperature to efficiently getter the free radioactive hydrogen isotope without gettering the hydrocarbon gas, and which gettering means is regenerable by being thermally activated at a second higher temperature at which the gettered hydrogen isotope is released; and (d) means for pumping and collecting the radioactive hydrogen isotope released from the regenerated gettering means to effect collection and isolation of the radioactive hydrogen isotope.

In discussing the present invention and applications of the gettering vacuum systems, reference to hydrogen is meant to include the hydrogen isotopes deuterium and tritium as well.

Also, in referring to methane or tritiated methane it should be understood that a variety of hydrocarbon gases may also be present and likewise dissociated, and the hydrogen component gettered per the present invention.

In order that the invention can be more clearly understood, convenient embodiments thereof will now be, described by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an application system for a gettering vacuum system for use in combination with a fusion reactor system; Figure 2 is a schematic representation of a gettering vacuum system of a first embodiment; Figure 3 is a schematic representation of a gettering vacuum system of a second embodiment; Figure 4 is a gettering vacuum system of a third embodiment; and Figure 5 is a gettering vacuum system of a fourth embodiment.

A gettering vacuum system 10 is seen in Figure 1 in an application with a fusion reactor 12, which can be a "Tokamak" type reactor, or a reactor such as the toroidal fusion test reactor. The gettering vacuum system of the present invention can be used as part of any vacuum system in which it is desired to getter pump hydrocarbon gasses. The fusion reactor 1 2 is connected by a high-vacuum valving system valve 14 to a highvacuum pump 16, which is capable of maintaining a high vacuum within fusion reactor 12. The outlet side of the high-vacuum pump 16 is connected by a valve 18 to a roughing vacuum pump 20. The outlet side of the high-vacuum pump 1 6 is also connected by a valve 22 to the gettering vacuum system 10.Periodically, tritiated methane which is generated within the fusion reactor 1 2 is removed via the high-vacuum pump 16 and collected in the gettering vacuum system 1 0. The tritiated methane is dissociated within the gettering vacuum system 10, and tritium absorbed by the getter material with the dissociated carbon being deposited within the system.

The gettering vacuum system 10 is seen in schematic form in Figure 2, wherein the hermetically sealed enclosure or envelope 30 is connectable by a valve 22 to the high-vacuum system and fusion reactor. An electrical filament 32 is disposed within the envelope 30 with suitable electrical lead-ins 34a and 34b being provided through the envelope to permit a filament drive current to be passed through the filament 32 to generate an electron current within the chamber defined by the envelope 30. The envelope 30 can be formed of an insulating material such as high temperature glass, or can be formed of a non-reactive metal, such as stainless steel, with appropriate insulating means permitting the electrical lead-ins 34a and 34b to be brought through the envelope. A gettering means 36 is disposed within envelope 30 and spaced from the filament 32.The gettering means 36 can be a high surface area non-evaporable hydrogen getter, such as described in U.S. Patents 3,609,062; 3,662,522 or similar high hydrogen gas and tritium gas gettering devices. An electrical lead-in 38 penetrates the enclosure 30 and is connected to the gettering means 36 within the enclosure, and is externally connected to a source of potential. It should also be understood that electrical lead-ins, not shown, are brought to the getter means for heating the getter means to an activated operating temperature of about 4000C for a zirconium-aluminium alloy getter medium.

The getter means 36 is shown connected to a source of positive potential relative to the filament 32 so that the filament serves as the cathode of the system, with electrons generated therefrom being directed to the getter means36 which in effect acts as an anode. The electrons in passing from the filament 32 to the getter 36 interact with the tritiated methane within the enclosure 30 causing electronic dissociation and producing free tritium. The tritium is adsorbed or absorbed on the getter means 36. The carbon component of the methane will be deposited within the enclosure both on the gettering means 36, which being at a positive potential will tend to ion pump carbon therto, and the carbon can also be deposited on the interior walls of the enclosure or ion bombarded into the filament.

The potential maintained between the filament and the getter which accelerates the electrons to a sufficient energy to dissociate the methane need only be approximately 50 to 1 50 electron volts. In this way a gettering vacuum system has been tested which is capable of pumping tritiated methane at up to about 10 litres per second.

In another embodiment of the present invention in Figure 3, the gettering vacuum system 40 includes enclosure 42 with a ring type filament 44 disposed about a gettering means 46 within the space defined by the enclosure 42. In this embodiment the getter 46 is connected to a more negative source of potential than the filament 44, with the enclosure 42 being a conductive member and connected to a more positive potential source. Thus, electrons generated at the filament are accelerated by the potential gradient to the enclosure wall 42, with the electrons impacting the methane and electronically dissociating it to permit the dissociated tritium to be gettered or sorbed on getter means 46, while the carbon will be deposited on the interior of enclosure 42, or ion pumped at the negatively charged getter.

In another embodiment illustrated schematically in Figure 4, the gettering vacuum system 50 comprises non-reactive metal enclosure 52, gettering means 54 disposed within the enclosure 52, and electron generating means 56. The electron generating means 56 comprises a central filamentary electrode which is connected to a potential source for passing a heating current through the filamentary electrode sufficient to generate electrons from the filament. Helical grid 58 with widely spaced turns is coaxially disposed about the filamentary electrode 56, which grid 58 is electrically connected to a positive potential source that may be electrically common with the enclosure 52 relative to the filamentary electrode.

Thermionic electrons are generated at the filamentary electrode and accelerated toward the grid and enclosure, which are typically at +150 volts relative to the filament. The combination of the filament and grid serve to provide high electron mobility, high electron-molecule collision rate, and dissociation rate of the methane molecules present within the enclosure. The grid can also be used to increase the attainable, space charge limited current, thereby increasing the rate of dissociation and overall pumping speed. The dissociated hydrogen and/or tritium gas is then gettered by gettering means 54.

In order to further increase the electron collision rate and gas dissociation rate in the gettering systems of the present invention, a magnetic field B illustrated schematically may be applied in a direction transverse to the accelerated electron path, which is generally from the filament outward the grid and beyond to the enclosure. The transverse magnetic field may be generated by a permanent or electromagnet which may be disposed within the enclosure or outside the enclosure.

The gettering vacuum system of the present invention has a finite gettering capacity, and when this capacity has been utilized the getter material may be regenerated for reuse. This regeneration of the getter material is accomplished by heating the getter material to the requisite temperature of about 7000C for the zirconium-aiuminium alloy getter, which drives off the adsorbed or gettered gases which can be collected in an isotope separation system, with the regenerated getter means then availabie for reuse.

In the embodiment of Figure 5, the getter system 60 includes enclosure 62, gettering means 64, and a simple electrically heated filament 66 which is externally connected to a 110 volt, 60 cycle AC source. The potential difference across the filament provides an accelerating potentials field which reverses with time, for accelerating electrons generated thermionically from the filament body. This is the simplest system which has been tested that will effectively electronically dissociate methane present within the enclosure and getter the dissociated hydrogen. This system does not have the pumping speed of soMe of the other systems described above. The filament need only be heated to a temperature sufficient to generate electrons and is not heated to a temperature sufficient to thermally crack the methane molecule.

Claims

1. A regenerable gettering vacuum system for separating and isolating radioactive isotope from hydrocarbon gas produced in a fusion reactor, which system comprises: (a) means for hermetically coupling the regenerabie gettering system to the fusion reactor so that the radioactive hydrogen isotope containing hydrocarbon gas from the reactor is introduced to the regenerable gettering system; (b) means for generating and accelerating an electron beam of sufficient energy to produce electron collision with the radioactive hydrogen isotope containing hydrocarbon gas to dissociate the gas and produce free radiactive hydrogen isotope;; (c) means for gettering the free hydrogen isotope, which gettering means is thermally activated at a first temperature to efficiently getter the the free radio-active hydrogen isotope without gettering the hydrobcarbon gas, and which gettering means is regenerable by being thermally activated at a second higher temperature at which the gettered hydrogen isotope is released; and (d) means for pumping and collecting the radioactive hydrogen isotope released from the regenerated gettering means to effect collection and isolation of the radioactive hydrogen isotope.

2. A system according to claim 1, wherein the electron generating and accelerating means and the greater pumping means are spaced apart disposed within a hermetically sealed enclosure which is connectable to a nuclear fusion reactor.

3. A system according to claim 1 or 2 wherein the hydrocarbon gas is tritiated methane.

4. A system according to claim 1, 2, or 3 wherein the generated electrons are accelerated to an energy of at least 50 eV.

5. A system according to any of claims 1 to 4, wherein the coupling means includes a hermetically sealable enclosure which is conductive and is maintained at a positive potential with respect to the electron generating means to permit acceleration of the electrons.

6. A system according to any of claims 1 to 5, wherein the electron generating means is an electrically heated filament, and a potential gradient is maintained between the filament and the getter pumping means.

7. A system according to any of claims 1 to 6, wherein 9 conductive grid is disposed coaxially about a filamentary electron generating means, with the grid maintained at the same positive potential with respect to the filamentary electron generating means as the enclosure.

8. A system according to any of the preceding claims, wherein a magnetic field is applied in a direction transverse to the direction of electron acceleration.

9. Regenerable gettering vacuum systems for separating and isolating radioactive hydrogen isotope from hydrocarbon gas produced in a fusion reactorand as claimed in claim 1, said systems being substantially as described herein with particular reference to the accompanying drawings.