Abstract:
Use in a nuclear fission reactor of a sacrificial metal in a molten salt fuel containing actinide halides in order to maintain a predefined ratio of actinide trihalide to actinide tetrahalide without reducing actinide trihalide to actinide metal. A method of maintaining oxidation state of a molten salt containing actinide halides. The method comprises contacting the molten salt continuously with a sacrificial metal, the sacrificial metal being selected in order to maintain a predefined ratio of actinide trihalide to actinide tetrahalide without reducing actinide trihalide to actinide metal. A fuel tube containing a sacrificial metal is also described.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to chemical optimisation of a molten salt fuel for a fission reactor. 
       BACKGROUND 
       [0002]    Nuclear fission reactors using fissile fuels in the form of molten halide salts have many advantages over solid fuelled reactors but generally suffer from problems due to continuous changes in the chemical composition of the molten fuel salt during operation as fission products accumulate and a net release of halogen from the actinide tri or tetrahalide fuel occurs. Most designs of molten salt reactors incorporate a continuous chemical treatment process in the fuel circulation to manage this problem, however doing so involves adding complex chemical engineering systems into a highly radioactive environment. 
         [0003]    A much simpler design of molten salt reactor was described in GB 2508537 in which the fuel salt was held in static tubes in which convection or other mixing processes allowed heat to pass from the fuel salt to the tube wall at a sufficient rate for the reactor to have a practical energy production. Such static fuel tubes do not permit continuous active adjustment of the chemistry of the fuel salt. In GB 2508537 it was suggested that inclusion of metals such as niobium, titanium or nickel in the fuel salt or on the fuel tube would be useful in scavenging excess halogen released during fission but no specific suggestions were made for controlling deleterious effects of fission products. 
       SUMMARY 
       [0004]    According to an aspect of the present invention, there is provided use in a nuclear fission reactor of a sacrificial metal in a molten salt fuel containing actinide halides in order to maintain a predefined ratio of actinide trihalide to actinide tetrahalide without reducing actinide trihalide to actinide metal. 
         [0005]    According to a further aspect of the present invention, there is provided a method of maintaining oxidation state of a molten salt containing actinide halides. The method comprises contacting the molten salt continuously with a sacrificial metal, the sacrificial metal being selected in order to maintain a predefined ratio of actinide trihalide to actinide tetrahalide without reducing actinide trihalide to actinide metal. 
         [0006]    According to a further aspect of the present invention, there is provided a fuel tube for use in a nuclear fission reactor. The fuel tube is configured to contain a molten salt comprising actinide halides. The fuel tube comprises a sacrificial metal such that in use the sacrificial metal is in contact with the molten salt, or with liquid condensed from vapour evolved from the molten salt. The sacrificial metal is selected in order to maintain a predefined ratio of actinide trihalide to actinide tetrahalide without reducing actinide trihalide to actinide metal. 
         [0007]    According to a further aspect of the present invention, there is provided a method of managing gas production in a fission reactor comprising fuel tubes containing a molten salt fissile fuel. The method comprises contacting the molten salt fissile fuel with a sacrificial metal. The sacrificial metal is selected in order to control a level of volatile iodine compounds released from the molten salt. The method further comprises permitting gasses produced during fission of the molten salt fissile fuel to pass out from the fuel tubes into a coolant surrounding the fuel tube or into a gas space in contact with the coolant. 
         [0008]    Further aspects are set out in claim  2  et seq. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Some preferred embodiments will now be described by way of example only and with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1  shows examples of fuel tubes containing molten fuel salt; 
           [0011]      FIG. 2  shows examples of three methods to allow fission gas emission from fuel tubes. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    A systematic analysis of the effects of incorporating sacrificial metals into the fuel salt or fuel tube has been carried out resulting in the identification of particularly effective metals for this purpose. Three factors dictate the suitability of any particular sacrificial metal. These are
       maintaining a low redox state and hence low metal corrosive power and low concentration of actinide tetrahalides as indicated by a high ratio of trivalent to tetravalent actinides in the molten salt while not reducing actinide (usually uranium) halides to the metal form at temperatures approaching the boiling point of the salt mixture   chemically binding potentially volatile fission products in the molten salt and preventing their entering the gaseous phase above the salt. Particularly important is to minimise volatile iodine compounds especially Tel 2 .   Converting reactive tellurium to stable tellurides to prevent tellurium induced embrittlement of metals, especially nickel alloys, in contact with the molten salt       
 
         [0016]    Thermodynamic calculations of these three factors have been carried out using a software program HSC Chemistry 7. The results are shown in Table 1. 
         [0017]    The parameters of the thermodynamic calculation were as follows. The sacrificial metal was provided as a separate pure metallic phase in excess over other reactants. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Salt composition in moles: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 NaCl 
                 428 
               
               
                   
                 UCl 3   
                 225 
               
               
                   
                 UCl 4   
                 10 
               
               
                   
                 Cd 
                 0.38 
               
               
                   
                 I 
                 0.84 
               
               
                   
                 In 
                 0.04 
               
               
                   
                 Sb 
                 0.14 
               
               
                   
                 Se 
                 0.24 
               
               
                   
                 Te 
                 1.47 
               
               
                   
                   
               
             
          
         
       
     
         [0018]    This represents a typical fuel salt towards the end of its useful life in a fast spectrum nuclear reactor. The group 1 and 2 metals, lanthanides, noble metals and noble gasses have been excluded as they were shown to have no effect on the chemistry involved. Gas composition was determined at 600° C. and reduction of uranium to the metal at 1500° C. 
         [0019]    Examination of table 1 indicates that zirconium, titanium, niobium, vanadium, zinc, chromium, silver and manganese are suitable as sacrificial metals to control redox state without producing uranium metal in situations where control of volatile species is not important. 
         [0020]    Where, in addition, control of dangerous volatile species such as iodine is important then only zirconium, titanium, vanadium, chromium and silver are useful. These same metals with the exception of vanadium are also effective in controlling tellurium levels. 
         [0021]    Silver as a sacrificial metal appears to have unique properties. Despite its high Pauling electronegativity, it is very effective at reducing UCl 4  concentrations, reducing volatile iodine species and scavenging tellurium. The high affinity for iodine is a known property of silver but the efficacy in reducing UCl 4  to UCl 3  is unexpected. 
         [0022]    Combinations of multiple sacrificial metals produce still more favourable results where particular sacrificial metals are more effective against the three factors set out above. 
         [0023]    While data has been presented for chloride salts, the same principles and useful sacrificial metals can be applied to fluoride salt systems. 
         [0024]    While passive control of molten salt chemistry with sacrificial metals is of general value for molten salt reactors, it is particularly important for reactors such as that described in GB 2508537 where access to the molten salt for active management of the chemistry, for example by adding small amounts of reactive metals, is challenging. In such a reactor it is useful for the sacrificial metal to be applied to the vessel containing the molten fuel salt both above and below the level of the salt. This prevents occlusion of the sacrificial metal by deposited noble metal fission product. It can also be advantageous, particularly where the sacrificial metal has a significant neutron absorption, for the sacrificial metal not to be located near the centre of the reactor core so that any neutron absorption is minimised. 
         [0025]    The sacrificial metal can be provided in a variety of ways.  FIGS. 1 a  to 1 e    show examples of fuel tubes incorporating sacrificial metal.  FIG. 1 a    shows a fuel tube  101   a  containing molten salt  103   a  and an internal coating  102   a  of the sacrificial metal applied to the inner wall of the fuel tube. The sacrificial metal can be applied to the inner wall of the fuel tube by a variety of methods including, but not restricted to, electroplating, plasma spraying, dipping into molten metal, brazing, welding, chemical vapour deposition, sputtering, vacuum deposition, conversion coating, spraying, physical coating and spin coating. Alternatively, as shown in  FIG. 1   b,  the internal coating  105   b  may be applied to only part of the fuel tube  101   b,  provided that part is in contact with the fuel salt  103   b.    FIG. 1 c    shows a further embodiment, in which a metal insert  104   c  made from or coated with the sacrificial metal is placed within the molten salt  103   c  inside the fuel tube  101   c.  This insert may be shaped so as to aid the convective mixing of the fuel salt, e.g. spiral shaped.  FIG. 1 d    shows a yet further embodiment, where the sacrificial metal is provided as particles  107   d  suspended in the molten salt  103   d  within the fuel tube  101   d,  or as coatings on such particles.  FIG. 1 e    shows an embodiment where the sacrificial metal is provided as particles  106   e  which are allowed to sink in the fuel salt  103   e  to the bottom of the fuel tube  101   e.    
         [0026]    Use of a sacrificial metal such as titanium, vanadium, chromium or silver reduces the vapour pressure of many radioactive species produced by the fuel salt to very low levels. This makes possible much simpler methods to manage the gasses released from the fuel which, with suitable sacrificial metals present, are predominantly the noble gasses, xenon and krypton, cadmium and zirconium halides although the concentration of the latter is substantially reduced if zirconium is used as the sacrificial metal. 
         [0027]    Accumulation of these gasses in fuel elements is a major limitation in the longevity of such fuel elements as if the gas is permitted to accumulate it generates high pressures which can rupture the cladding of the fuel elements. 
         [0028]    It is known that, particularly in sodium cooled fast reactors, fission gasses can be allowed to vent from the fuel elements into the sodium coolant. This practice was used in the early days of development of such reactors but was abandoned because of the presence of highly radioactive, relatively long half life, cesium in the vented gas. The cesium contaminated the sodium coolant and made disposal of the sodium extremely challenging as well as creating a major hazard in the event of a sodium fire. The practice was therefore discontinued. Similar venting procedures have never been suggested for reactors other than sodium cooled reactors. 
         [0029]    Molten salt reactors are unique in not accumulating cesium in the form of the volatile metal, which is released as a gas from metallic nuclear fuel elements and accumulated in partially leaking high pressure gas microbubbles in ceramic nuclear fuel elements. In molten salt reactors the cesium forms non-volatile cesium halide which has negligible vapour pressure at the temperatures involved. It is therefore possible to vent fission gas from molten salt reactors into the coolant without causing serious levels of contamination. This is particularly relevant for the molten salt reactor design described in GB 2508537 where the alternative is a relatively complex pipework arrangement to collect the gasses. 
         [0030]    The gasses released in this way still contain appreciable quantities of radioactive iodine but of short half life. The radioactive iodine will contaminate the coolant but will decay to harmless levels in a relatively short time period. However, inclusion of a sacrificial metal such as magnesium, zirconium, scandium, titanium, manganese, aluminium, vanadium, chromium and/or silver reduces the amount of volatile iodine to a lower level. There is thus a major advantage to combining the use of sacrificial metals as described above with a gas venting system for the fuel tubes. Suitable gas venting systems are described in the literature (ORNL-NSIC-37, Fission Product release and transport in liquid metal fast breeder reactors) and include “diving bell” apparatus, narrow or capillary tubing and gas permeable sinters located above level of the fuel salt. The gas can be vented into the gas space above the coolant salt or directly into the coolant salt where it will bubble to the surface. 
         [0031]      FIG. 2 a  to  c    shows examples of three methods to allow fission gas emission from fuel tubes. The method shown in  2   a  uses closure of the upper opening of the fuel tube  203   a  with a sintered metal plug  201   a  where the sinter pore size is adjusted to allow gasses to pass but not to allow liquids, either the fuel salt  202   a  or the coolant outside the fuel tube to pass.  FIG. 2 b    shows a fuel tube  203   b  containing fuel salt  202   b  where the fuel tube is capped by a diving bell assembly  205   b.  The diving bell assembly  205   b  allows gas to pass from the fuel tube  203   b  to the coolant  207   b  via vents  206   b  in the wall of the fuel tube, but coolant  207   b  sucked into the diving bell assembly  205   b  cannot mix with the fuel salt  202   b.    FIG. 2 c    shows a fuel tube  203   c  vented directly to the gas space above the coolant  207   c  via a narrow tube or capillary tube  208   c.