GB2052835A - Spallation source target, its cooling and use - Google Patents

Abstract

Target for a liquid cooled spallation source comprising a liquid reserve (target liquid) (1) in the path of incident radiation (4) capable of vapourizing the liquid, a heat sink (3) for condensing vapour positioned above the liquid reserve and a gravity-flow return path from the heat sink to the reserve (1). The phase transition of the liquid to vapour is utilized to remove heat from the target, and the vapourized liquid is cooled, condensed and recirculated. By mixing suitably processed heavy alpha -emitters or long-lived radio-isotopes with the liquid these can be destroyed by spallation and/ or neutron capture, or alternatively nuclear fuel can be improved by being mixed into the liquid and being subjected to breeder reactions. <IMAGE>

Description

SPECIFICATION Spallation source target, its cooling and use This invention relates to a method of remov ing heat from a spallation source target by means of a liquid, as well as to a target so cooled and to its use.

The object of the invention is particularly a target for a spallation source, in which high energy densities are permitted and intensive removal of heat takes place.

Spallation sources are part of the known state of the art. In the designs known to date there are considerable difficulties in coping with the extremely high heat production in the target material. As is known the proton beam is shot into a rapidly flowing liquid metal and the liquid metal which has been heated through a certain temperature range is circulated through a heat exchanger by a pump and cooled off again. In doing this large quantities of liquid metal have to be moved (for example with 2MW power input and an increase due to heating of 100"K, 140kg/s of liquid lead) which considerably stress the bends of the pipe system and for whose movement considerable pumping power is necessary.The necessity of using a pump is already in itself a disadvantage, as for example centrifugal pumps work under difficult conditions in liquid metal and make the system prone to malfunctions due to narrow finishing tolerances and as the case may be due to the necessary passages and seals.

Further safe containment of the radioactive material cannot be absolutely guaranteed. Induction pumps though they do not have this constructional drawback, are very unattractive from the point of view of energy and limited in their power due to their poor efficiency, particularly in the case of lead. Further it is disadvantageous that the whole of the large quantity of liquid metal becomes radioactive, which leads to difficulties when the plant is closed down. In particular when using the lead-bismuth eutectic mixture for the liquid metal circulation a-active polonium is created by neutron capture, which has a higher vapour pressure than the target liquid and thus represents a danger.

Another design proposed by the applicants consists of a so-called rotary target: in this type of spallation source a ring of spallation metal in solid form is fastened to a top-like construction with a vertical axis over whose surfaces cooling water flows. Steady rotation of the top results in only a small sector of the target ring being exposed for a short space to the proton beam at any one time, so that local heating is not very great. A disadvantage of this construction is that the target must be driven, to do which vacuum and cooling water tight shaft lead-throughs, bearings and motor become necessary. Furthermore considerably mechanical energy is stored in the rotating target wheel, which if a shaft breaks could lead to considerable destruction.Although the amount of material that becomes highly radioactive is considerably less in the case of the rotary target than in the case of fluid metal circulation, it is still in the region of tons.

It is the object of the invention to create a constructionally simple spallation target, which can be run with little effort, which meets the safety requirements and which creates the smallest possible amount of highly radioactive material.

According to a first aspect of the invention there is provided a target for a liquid cooled spallation source comprising a liquid reserve in the path of incident radiation capable of vapourizing the liquid, a heat sink for condensing vapour positioned above the liquid reserve and a gravity-flow return path from the heat sink to the reserve.

According to a second aspect of the invention there is provided a method of cooling a spallation source target by a liquid wherein the phase transition of the liquid to vapour is utilized to remove heat from the target, and the vapourized liquid is cooled, condensed and recirculated. Recirculation is preferably achieved under gravity. Due to the relatively high specific enthalpy of vapourization only relatively small amounts of metal take part in the circulation in this process (for example 2.35 kg/s of lead at a lower input of 2MW).

According to a third aspect of the invention there is provided a method of cooling a liquid spallation source target wherein radiation impinges on a target liquid and vapourizes it and thereafter the vapour is cooled, condensed and recirculated.

According to a fourth aspect of the invention there is provided a process of removing a-emitters or long-lived radio-isotopes producing nuclear fuel by utilizing a target according to the invention.

The continuous recondensation preferably takes place on suitably dimensioned water cooled ribs or condensation surfaces above the fluid metal reserve, so that the condensate flows back to the reserve under gravity. Due to this closed self circulating system no driving means and no corresponding energy supply (for example in the form of pumps or motors) are necessary.

It has turned out that without more ado it is possible for the removal of heat hitherto thought necessary by means of an intensive forced circulation of large amounts of fluid, to be replaced by cooling by means of a relatively small amount of liquid in a self circulating system, as obviously the phase transition of the liquid takes place rapidly enough and no "heat flow barrier" is formed at the place of heat transfer to the liquid (for instance due to vapour layers).

However, cooling of the liquid to temperatures which are close to its melting point is particularly suitable, and preferably the flow of vapour formed is let through the nozzles of an arrangement functioning as a steam jet ejector, which is particular-as will be further shown with reference to examples-is arranged concentrically about the proton beam.

A particular advantage of the arrangement, called a vapourization target, of the invention is its relatively small dimensions and the possibility to adapt its geometric form to the requirements of a favourable distribution of radiation pipes, for example, by a cylindrical form with targentially directed beam pipe axes in several levels.

Further it is advantageous that according to the invention-particularly in a geometrically favourable configuration of the target-only a comparatively very small amount of highly radioactive material is produced: for estimating purposes one can assume, that the necessary amount of material will substantially be determined by the necessity for completely stopping the proton beam and may be about 300-400 kg.

A further advantage consists in the fact that the temperature level of the arrangement does not change during operating even if the incoming proton power is varied within certain limits, whereby for example the mechanical and thermal loading of the container stays constant and transients which might reduce the operational life are excluded.

Due to its simple construction and as a result of the fact that the target material is permanently exposed both to the incident proton beam as well as to a high neutron flux, it is possible for example to destroy particularly efficiently long-lived heavy and highly radioactive nuclei in particular the a-active isotopes 237-Np, 238-Pu and 239-Pu. These substances could be added to the liquid metal in a suitable processed form, whereby due to boiling turbulance and a suitable directed return flow of the target liquid the particles are for example kept distributed in suspension and cooled by the target liquid. After sufficiently advanced reduction of the long-live aemitters the whole target container is exchanged and sent for processing or final storage respectively.

In a similar manner it would also be possible to mix breedable nuclear fuel, for example thorium, from which thermally fissionable Uranium-233 is created by neutron capture, into the liquid and convert it into fissionable nuclear fuel.

In a particular embodiment the condensor part of the target container is-as has been already indicated-constructed as a steam jet or diffusion pump. By these means volatile substances created in the target can be concentrated in the annular chamber of the condensation container, and pumped out of this and separated and isolated in a suitable manner.

Due to its small dimensions, its low weight and simple construction the target of the invention is further distinguished over known constructions by its low cost of production,easire handling during mounting and repair and increased operational safety.

Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 shows a target with a proton beam incident from above.

Figure 2 shows a target with a proton beam incident from the side Figure 3 shows a target with a condensation chamber former as a diffusion pump.

The cylindrical arrangement shown in Fig. 1 comprises a liquid metal reserve 1 with an auxiliary heater 2 (which is required in particular when starting up) and a condensor 3 above the liquid metal reserve 1. Arrows 4 indicate the proton beam, incident from above parallel to the cylindrical axis of the arrangement, which impinges without hindrance on the liquid metal reserve 1 and there (as indicated) leads to the creation of metal vapours, which rise and are cooled in the condensor 3 and condensed, after which the liquid is returned to the liquid metal reserve 1 under gravity. Some externally positioned cooling pipes 5 are capable of providing additional cooling from outside.

The arrangement shown in Fig. 2 comprises analogous elements indicated by the same reference numbers as in the arrangement of Fig. 1, however here the proton beam is laterally incident on the fluid reserve 1 and the metal circulation comprises a condensation path 3, from which the condensed liquid is returned to the liquid metal reserve 1 via a return conduit.

In the embodiment shown in Fig. 3 the proton beam entering at 4 impinges vertically from above onto the liquid reserve 1 (with auxiliary heater 2) and the vapours formed are cooled and condensed by a main cooler 3 (with main coolant supply 3'). An auxiliary cooler 6 with coolant supply 7 serves to further cool the returning portions of the liquid. The vapours rising frm the fluid reserve 1 of necessity enter the nozzle system 8 of the diffusion pump inside the condensation chamber. A thermally isolating intermediate wall 9 is provided to protect the exterior wall 10, which is cooled by cooled returning target liquid which re-enters the target container at 12.

Volatile parts created in the target liquid can be separated out by means of the annular chamber 13, after they have left the nozzles of the diffusion pump together with the condensable vapours.

With this construction of the condensation region together with subsequent additional cooling of the condensed liquid down to close to its melting point it is possible to cool particularly effectively the exterior wall exposed to the radiation, so that radiation damage in this region is lessened.

Claims (14)

1. Method of cooling a spallation source target by a liquid wherein the phase transition of the liquid to vapour is utilized to remove heat from the target, and the vapourized liquid is cooled, condensed and recirculated.

2. Method of cooling a liquid spallation source target wherein radiation impinges on a target liquid and vapourizes it and thereafter the vapour is cooled, condensed and recirculated.

3. Method according to claim 1 or claim 2 wherein recirculation is achieved by gravity.

4. Method according to any preceding claim wherein the liquid is a liquid metal.

5. Method according to claim 4 wherein the metal is lead.

6. Method according to any preceding claim wherein the liquid vapour is passed through a diffusion pump or steam jet ejector.

7. Method according to claim 6 wherein volatile impurities are separated out from the vapour.

8. Target for a liquid cooled spallation source comprising a liquid reserve in the path of incident radiation capable of vapourizing the liquid, a heat sink for condensing vapour positioned above the liquid reserve and a gravity-flow return path from the heat sink to the reserve.

9. Target according to claim 8 wherein the heat sink is annular surrounding a central hole.

1 0. Target according to claim 9 wherein incident radiation passes through the central hole to impinge on the reserve.

11. Target according to claim 8 or 9 further comprising arrangement of the type of a diffusion pump or steam jet ejector in the vapour path.

1 2. Target according to claim 8 wherein the liquid reserve is contained in the inner chamber of a double-walled container, and is returned from the heat sink via the outer chamber of the double-walled container.

1 3. Target according to claim 2 wherein the heat sink has oversize cooling surfaces.

14. Target according to claim 12 or 1 3 wherein the liquid is cooled to close to its freezing point before being returned to the reserve.

1 5. Target for a liquid cooled spallation source substantially as hereinbefore described with reference to any one of the embodiments of Fig. 1 or 2 or 3 of the accompanying drawings respectively.

1 6. Process of removing heavy a-emitters or long-lived radio-isotopes by using the target claimed in any one of claims 8-1 5.

1 7. Process of producing nuclear fuel by using the target claimed in any one of claims 8-15.

1 8. Method of cooling a spallation source target substantially as hereinbefore described with reference to any one of the embodiments of Fig. 1 or 2 or 3 of the accompanying drawings respectively.