GB2073938A

GB2073938A - Single-fluid type accelerator molten-salt breeder

Info

Publication number: GB2073938A
Application number: GB8111129A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-04-15
Filing date: 1981-04-09
Publication date: 1981-10-21
Also published as: FR2480482B1; DE3113238A1; DE3113238C2; CA1183287A; FR2480482A1; GB2073938B

Abstract

A single-fluid type accelerator molten-salt breeder characterized in that, in a system comprising a molten- salt nuclear reactor vessel 1, a heat exchanger 7 and a pump 6 for circulating a molten salt from said nuclear reactor vessel via said heat exchanger to the same, said nuclear reactor vessel is a cylindrical vessel having closed top and bottom made of Hastelloy N for a single-phase molten fluoride containing ThF4 and/or UF4 with some alkali fluoride and/or alkaline earth fluoride acting as the function of target and blanket, which is provided with a graphite shield along the inner wall and has an opening 2 on the top, and a linear accelerator tube for generating a fast charge particle such as proton is set up on said vessel so that said particle is injected directly onto the liquid surface of molten salt in the vessel through said opening. <IMAGE>

Description

SPECIFICATION Single-fluid type accelerator molten-salt breeder Background of the Invention The present invention relates to an apparatus for molten-salt nuclear chemical reaction.

More particularly, the present invention relates to a single-fluid type accelerator molten-salt breeder utilizing neutron produced by a spallation reaction of heavy nucleus caused by a fast particle such as proton.

A molten-salt reactor is a reactor in which a liquid fuel (molten salt) is passed through moderator graphite to cause a nuclear fission, and such idea has been developed by ORNL during a period of 1947 to 1976. In the molten-salt reactor a high temperature is obtained under low operation pressure for the nature of molten salt, and, from a fact that the fuel is liquid, the reactor has the following characteristics:: (1) It is not necessary to fabricate a fuel pin and assembly; (2) It is not suffered radiation damage and can combine heat transport; (3) The turnover rate of nuclear fuel is good for the time lag of fuel reprocessing is very small since it can be done on site; (4) It can be operated with a low excess reactivity since the fuel can be continuously exchanged during operation; (5) Therefore, it can be a thermal fission breeder reactor since protoactinium and fission products can be continuously removed.

These characteristics are very significant in nuclear fuel cycle, particularly in the total amount of natural uranium required, and a molten-salt reactor of 1.07 in breeding ratio has a performance almost equal to an effect of fast breeder reactor of about 1.4 in that ratio.

In this field researches are now being conducted on an accelerator breeder used for producing fissible materials for absorbing tens of neutron into fertile materials, which are released by a spallation reaction of heavy nucleus caused by proton, etc. in order of 0.5 to 1.5 GeV, and for incinerating transuranic elements and other radioactive wastes. The present inventors have been concerned with this problem and have put forth some proposals.

In the production of neutron released by a spaliation reaction of heavy nucleus caused by a fast particle such as proton in energy of about 0.5 to 1.5 GeV, the present inventors have previously researched on utilizing a molten salt free from a danger of radiation damage due to the particle and having a function of heat removal and fuel-shuffling, and on absorbing neutrons into a fertile material in the molten salt to produce fissible materials and separating spallation products such as, for example tritium produced and simultaneously generating electricity by heat generated, and have already filed a Japanese Patent Application (Japanese Patent Application No.

27178/78) on an invention reiating to an acelerator molten-salt breeder in which a proton beam from an accelerator tube is injected into a target molten salt, for example KF-NaF-BeF2-AnF3 in a target vessel through a window to incinerate transuranic element An and to produce neutrons and a blanket molten salt containing 232Th, 238U, etc. in a blanket vessel surrounding the target vessel is transmutated to fissile materials such as 239Pu, etc. absorbing the above neutrons.

Summary of the Invention An object of the present invention is to provide an accelerator molten-salt breeder having improved structure which is higher in technical reliability, safety and economical efficiency.

Another object of the present invention is to provide an accelerator molten-salt breeder capable of producing fissible materials and in some case tritium effectively.

Further, another object is to provide an accelerator molten-salt breeder capable of taking out tritium produced into a coolant salt through a heat exchanger and separating it and simultaneously recovering heat.

As the result of studying diligently for accomplishing these objects, the present inventors have designed an accelerator molten-salt breeder improved by integrating the target vessel and the blanket vessel to use a singlephase molten fluoride containing ThF4 and/or UF4 acting as the function of target and blanket, thereby solving the problem on radiation damage of target vessel and simplifying the structure. However, the accelerator molten-salt breeder has still such a defect that the window for proton beam injected has a double structure which is so complicated as to force helium gas into an intermediate space thereof and, in case the window is broken, the reactor has to be suspended operation for replacing the window with another remotely.The present inventors have further gone on with the research to find a fact that the amount of fume from the molten salt is so small that it can be managed without the window by utilizing a differential pumping system and to design a single-fluid type accelerator moltensalt breeder characterized in that, in a system comprising a molten-salt nuclear reactor vessel, a heat exchanger and a pump for circulating a molten salt from said nuclear reactor vessel via said heat exchanger to the same vessel, said nuclear reactor vessel is a cylindrical vessel made of Hastelloy N having closed top and bottom, which is provided with a graphite shield along the inner wall and has an opening on the top, and a linear accelerator is set up on said cylindrical vessel without lieing a window therebetween so that a fast charge particle such as proton is injected directly onto the surface of molten salt in the vessel through a differential pumping system, and have completed the present invention.

Brief Description of the Drawing Figure 1 is a schematic view of the accelerator molten-salt breeder of the present invention in the first step; Figure 2 is a schematic view of the accelerator molten-salt breeder previously proposed by the present inventors and being now pending under Japanese Patent Application No.

27178/78; and Figure 3 is a schematic view of the singlefluid type accelerator molten-salt breeder of the present invention in the second step.

Detailed Description of the Preferred Embodiment In order to facilitate the understanding on the structural characteristics and advantages of the accelerator molten-salt breeder of the present invention, the breeder as shown in Fig. 2 will be first explained. In Fig. 2, a beam from an accelerator tube 1 2 located at one end of target vessel 11 is injected to a target molten salt 1 4 in the interior of target vessel 11 through a window 1 3. The target molten salt 1 4 is cooled by a blanket molten salt 1 6 in a blanket vessel 1 5 surrounding the target vessel 11 while circulated by a pump or natural convection.The blanket molten salt 1 6 used contains 232Th, 28U, etc. so that fissible materials are prepared by the absorption of neutrons produced in the target molten salt 1 4. Energies for electricity generation, and others can be recovered by the blanket or target molten salts at elevated temperatures.

In Fig. 1 showing an accelerator molten-salt breeder of the present invention in the first step, which is characterized in that the target vessel and blanket vessel are integrated to one vessel thereby a single molten fluoride containing ThF4 and/or UF4 can act as the function of target and blanket, a beam from an accelerator tube located at one end of reactor vessel 1 is injected to a molten salt 5 in the reactor vessel 1 through the windows 3 and 4. The molten sale 5 is circulated from the reactor vessel 1 via a heat exchanger 7 to the same vessel by a pump 6.

By adopting such structure an accelerator molten-salt breeder has the following advantages: (1) The problem of radiation damage on target vessel goes away; (2) The window can be designed widely and the problem of radiation damage on window material can be greatly lightened; and the core structure becomes most simple. The accelerator molten-salt breeder having a structure as shown in Fig. 1 will be explained more in detail with an embodiment.

1 GeV 300 mA of proton was employed as a beam. The beam was dispersed by about 5 by a magnetic field and was injected almost uniformly into a window of 1 500 mm in effective diameter, which is composed of windows 3 and 4. The window 3 was made of Zircalloy thin plate and the window 4 was made of Nb or Mo thin plate. A thermal shielding plate of Al foil was place on the upper surface of window 4. He gas was flowed into the space between the windows 3 and 4 under 0.1 to 0.5 atm. Even if the window 3 is broken, He leaks only. In case the window 4 is broken it is not detected until about 1.2 atm of molten salt (nearby the window) begins to leak into the He space.

When a leakage occurs, the reactor is stopped operation and the window is replaced with new one. All the reactor vessel 1, pump 6, heat exchanger 7, small tubes 8 and piping 10 were made of Hastelloy N (Ni-Mo-Cr alloy). The composition of molten salt 5 was LiF-BeF2-ThF4 (72-16-12 Mol %, melting point 500"C).

Incidentally, Li was used as it is in natural composition. The inlet temperature was 550"C and the outlet temperature was 700"C. And the reactor vessel was designed in about 6 m each of diameter and depth.

NaBF4- 8 mol % NaF was used as a coolant salt. A very small amount (about 300 ppm) of water remaining in the coolant and tritium produced run a transmutation through the small tubes 8 made of Hastelloy N to come out as THO, T2O, etc. into the covering gas in the coolant salt piping system. These are separated and recovered to produce tritium. Tritium was about 30 9 in daily output.

233U produced was about 300 9 in daily output, which was not particularly taken out for using a portion thereof as a neutron source for nuclear fission.

The above described accelerator molten-salt breeder has additionally the following characteristics and advantages: (1) The general constitution of reactor is very simple; (2) Although the amount of molten salt used comes up to about 800 ton, it is very inexpensive since natural Li and Th are used; (3) Be can be utilized in neutron multiplying reaction (n, 2n); (4) Graphite can be directly immersed into the molten salts and there is no problem on Hastelloy N; (5) The behaviour and separation technique of tritium are simple and well known so that any additional developement of new technique and apparatus is not particularly required; (6) The effusion of molten salt (contamination of accelerator tube, etc.) does not come about so long as the windows 3 and 4 are not broken simultaneously. The molten salt is maintained under sufficient low pressure; (7) The fissile materials are very low in concenlretion and comprise chemically stable substanese only, and the control of tritium is simple. IF there is a fear of leakage of tritium from the reactor vessel and pipings, it will be a good chilly If a double container is used; and (8) If mixing of H20 into T20 is not preferable, H20 in the molten salt may be previously substituted with D20.

Then, in Fig. 3 showing an accelerator molten-salt breeder of the present invention in the second step, which is characterized by havine a dferential pumping system with many ori-fices for a beam from an accelerator tube in replace of the window in the above described accelerator molten-salt breeder of the present invention in the first step, thereby the beam being injected directly onto the surface cf molten salt without passing through the wincllow, 4 indicates a cylindrical nuclear reactor vessel of 7 m in diameter and 8 m in height mace of Hastelloy N (Ni-Mo-Cr alloy) of 50 mn in thickness having closed top and bottom.The interior of vessel 4 is covered with two layers of graphite shield 5 and 6 of each about 50 cm in thickness all over the surface except a portion of opening. In the center 9f tRe top of container 4, an opening 1 of about 5 cm in diameter is opened through the vessel 4 and two graphite layers 5 and 6, and 300 mA of proton accelerated by a 1 GeV linear accelerator 1 7 provided upper the vessel 4 is injected directly onto the liquid surface cf molten salt 7LiF-BeF2-ThF4 (64-18-18 mol %) in the vessel 4 through the opening 1.In the upper end of the side of vessel 4, four molten salt inlets 2 of 60 cm in diameter are opened through the side wall of vessel 4 and graphite layers 5 and 6, and in the side near the bottom four molten salt outlets 3 of 60 cm in diameter are provided, which are crooked so as to prevent the streaming of neutron. A molten salt at 580'C flowed into the vessel 4 in total flow rate of about 5 m3/second through the inlets 2 is heated to 680"C by heat generated by proteon injected from the 1 GeV linear accelerator and flows out through the outlets 3. The molten salt flowed out from the vessel 4 is introduced to a heat exchanger 8 through a piping 7 and transfers the heat to a coolant salt NaBF4-NaF (92-8 mol %) thereby electricity generation by steam is carried out.The generation efficiency is 42% and about 800,000 SWe of electricity generation is possible. a free liquid surface type of centrifugal pump is employed for circulating the molten salt, above which a liquid sink is provided.

The sink is small in diameter so as to be sensitive for the fluctuation of liquid level to serve to control the liquid level in the vessel 4. A portion of molten salt circulated is spouted into the system at about 10 m/se cond through a conduit 1 8 provided near the opening 1 to form a vortex so that proton is injected in the center of vortex. This is for the sake of not making the liquid surface of molten salt stagnant and of producing neutron in a deep zone of the system which is not near the surface for making the adsorption and shield of neutrons easy.The control of liquid surface is done first by keeping the difference of liquid level in about 3 m to balance to the vacuum pump and covering gas (He) pressure 0.99 atm and also by revolution number of pump, a throttling valve 10, control of gas pressure in the upper zone and others. Molten salts increased excessively in the pump is drained out into a storage tank 1 2 through an overflow piping 11. Above the top of vessel 4, an orifice 13, a shutter 14 and a vacuum system 1 5 are provided and off-gas is removed therethrough.The vapour of molten salt attaches not only on a vapour trap 1 6 but also on the plate of orifice 1 3 and others, but the vapour on a locaton up to the lower portion of shutter 14 and the vapour trap 1 6 can be washed by ascending the liquid level of molten salt, if necessary. However, the vapour trap 1 6 has to be frequently replaced since spallation products are trapped therein. The shutter 14 is closed in case of no injection of proton beam. Above the shutter 1 4 a differential pumping system having tens of orifice plate of about 50 mm in diameter is provided for maintaining a high degree of vacuum. The reactor is continuously operated during a period of a half to one year and about 800 kg of 233U are produced by full operation for one year.The graphite layer 6 is cut a groove to utilize for adsorbing and removing a part of spallation product. 233UF4 produced in the molten salt during the operation is below 0.1 mol % so that there is no fear of criticality and others (the neutron absorption of 233U for nuclear fission is contrariwise useful for neutron breeding and therefore there is no substantial loss). However, in order to hasten the removal, on and after the 3rd month about 30 tons of salt may be chemically treated batchwise for 400 tons in total to remove 233U and a part of nuclear spallation reaction products.Even if not separated for one year the total amount of nuclear spallation reaction products is 30 to 40 Kg a year, and He and other noble gas elements as well as 'H, 2H, 3H and the like existing therein can be easily separated and it is not difficult to keep various kinds of remaining impurity elements below 1 ppm in concentration. Noble gas element products can be removed by a vacuum system and pump gas system.

Tritium produced comes out as water largely in the off-gas system of secondary coolant salt and is collected. The management of fission products has been known.

Although 7LiF-BeF2-ThF4 (64-18-18 mol was used as a molten salt in this embodiment, also the following mixtures of fluorides of alkali metal and/or alkaline earth metal, in which at least parent nuclear materials ThF4 and UF4 are as higher as possible in concentration and which is low in melting point and low in viscosity and is good in the compatibility with Hastelloy N, are suitable as a molten salt: 7LiF-BeF2-ThF4 76-16-12 mol 67-18-15 mol % 64-18-18 mol % 71-9-20 mol % 7LiF-ThF4 71-29 mol % 7LiF-NaF-ThF4 43.5-32.5-24 mol % 54.5-13.5-32 mol % 7LiF-UF4 71-19 mol % 7LiF-NaF-UF4 43.5-24.3-32.3 mol % 35.4-24.6-40 mol % 7LiF-RbF-UF4 60-10-30 mol % 57-10-33 mol % NaF-KF-UF4 47-29-38 mol % NaF-RbF-UF4 47-31-22 mol % 45-27-28 mol % Incidentally, in the above molten salts 7LiF may be partially replaced by LiF for the production of tritium By using a molten salt as exemplified above, for example Li F- BeF2- ThF4 ternary molten salt, tritium is produced and at the same time 233U is produced thereby a fuel for molten salt breeder is gradually formed.In the accelerator molten-salt breeder of the present invention actionoids are formed though they are in a very small amount, however, this can serve the extinction of actionoids by replacing a porton (below about 0.1 mol %) of ThF4 in a molten salt, for example Li F-BeF2-Th F4 with AnF3 (herein An is an actinoid element). That is, in the accelerator molten-salt breeder of the present invention, the breeding of 233U and production of tritium, which are the primary object of the present invention, can be attained and simultaneously the extinction treatment of actinoids and fission products can be performed by changing the composition of molten slat suitably.

As understood from the above descriptions, the single-fluid type accelerator molten-salt breeder of the present invention has the following advantages: 1) Fissile materials can be produced without utilizing any fissile material. And about 800 Kg of 233U (or 239Pu) can be produced for one year with this reactor of about 800,000 KWe in power capacity; 2) The molten salt used can be utilized as a fuel salt for molten-salt fission breeder (or converter) as it is by regulating its composition. In that case it can be done without using U or enriched U or Pu at all. That is, this is a system independent from other nuclear engineering technologies; 3) Electric power generated is possible to exceed the power for home consumption by about 200,000 KWe. That is, electric power may not be consumed; 4) In a viewpoint of safety, (a) the structure is very simple so that any trouble is not feared; (b) any combustible and explosive substance is not contained; and (c) it is sufficiently in a subcritical system so that the adjustment of chemical composition or use of control rod mechanism is not necessary at all for controlling the nuclear reactivity.

Therefore, the reactor can be reasonably designed without any restriction as reducing the inventory of nuclear matter; and 5) The molten salt technique including chemical treatment can be extensively utilized.

Moreover the nuclear spallation product is low in concentration so that it can be treated without problem.

Claims

1. A single-fluid type accelerator moltensalt breeder characterized in that, in a system comprising a molten-salt nuclear reactor vessel, a heat exchanger and a pump for circulating a molten salt from said nuclear reactor vessel via said heat exchanger to the same, said nuclear reactor vessel is a cylindrical vessel having closed top and bottom made of Hastelloy N for a single-phase molten fluoride containing ThF4 and/or UF4 with some alkali fluoride and/or alkaline earth fluoride acting as the function of target and blanket, which is provided with a graphite shield along the inner wall and has an opening on the top, and a linear accelerator tube for generating a fast charge particle such as proton is set up on said vessel so that said particle is injected directly onto the liquid surface of molten salt in the vessel through said opening.

2. The breeder as set forth in Claim 1 wherein said graphite shield is composed of two layers.

3. The breeder as set forth in either of Claims 1 and 2 wherein said vessel has one or more inlets for molten salt in the upper end of cylinder side and one or more outlets formed crooked in the graphite shield near the bottom.

4. The breeder as set forth in any one of Claims 1 to 3 wherein said opening is provided with a window composed of two plates, the space between them being flowed with He gas.

5. The breeder as set forth in Claim 4 wherein one of said plates is made of Zircalloy thin plate and another is made of Nb or Mo thin plate covered with Al foil on its upper surface.

6. The breeder as set forth in Claim 1 wherein said opening is a differential pumping system with many orifices.

7. The breeder as set forth in Claim 6 wherein near said opening a jet is provided for spouting a portion of circulating molten salt into said vessel to form a vortex in the center of liquid surface of molten salt.

8. The breeder as set forth in any one of Claims 1 to 7 wherein said heat exchanger heat and tritium are recovered by coolant salt from the circulating molten salt.

9. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-BeF2-ThF4 in such a composition as 76-16-12 mol %, 67-18-15 mol %, 64-18-18 mol % and 71- 9-20 mol %.

10. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-ThF4 in such a composition as 71-29 mol %.

11. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-NaF-ThF4 in such a composition as 43.5-32.5-24 mol % and 54.5-13.5-32 mol %.

1 2. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-UF4 in such a composition as 71-19 mol %.

1 3. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-NaF-UF4 in such a composition as 43.5-24.3-32.2 mol % and 35.4-24.6-40 mol %.

14. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is 7LiF-RbF-UF4 in such a composition as 60-10-30 mol % and 57-10-33 mol %.

1 5. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used in NaF-KF-UF4 in such a composition as 47-29-38 mol %.

16. The breeder as set forth in any of Claims 1 to 8 wherein said molten salt used is NaRbF-UF4 in such a composition as 47-31-22 mol % and 45-27-28 mol %.

1 7. The breeder as set forth in any of Claims 9 to 1 4 wherein said 7LiF is partially or totally replaced by LiF for the production of tritium.

1 8. A single4luid type acceleration molten-salt breeder substantially as hereinbefore described with particular reference to the drawings.