Abstract:
A method for in-situ measuring of a liquidus temperature of a supply of the molten salt, includes withdrawing a sample of the molten salt from the supply, placing it into a sample container, and cooling the sample of the molten salt from a first temperature above the liquidus temperature of the molten salt to a second temperature at which at least a portion of the sample of the molten salt solidifies. The method includes taking a plurality of temperature measurements of the sample of the molten salt during cooling of the sample and determining the liquidus temperature of the molten salt from the measurements. The sample of the molten salt is heated from the second temperature to the first temperature and returned to the supply of the molten salt.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The application claims the benefit of U.S. Application No. 62/251,410, filed Nov. 5, 2015, and U.S. Application No. 62/251,365, filed Nov. 5, 2015, the contents of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to molten salt nuclear reactors and more specifically to an in situ probe for measurement of liquidus temperature in a molten salt reactor. 
       BACKGROUND 
       [0003]    To improve on previous Light Water Reactor (LWR) technologies, Molten Salt Reactors (MSRs) have been researched since the 1950s. MSRs are a class of nuclear fission reactors in which the primary coolant, or even the fuel itself, is a molten salt mixture (e.g., fluoride or chloride salt). Compared to LWRs, MSRs offer projected lower per-kilowatt hour (kWh) levelized cost, comparatively benign fuel and waste inventory composition, highly efficient fuel utilization, and a combination of higher accident resistance with lower worst-case accident severity (due to more benign inventory composition). In various designs, the innate physical properties of MSRs passively and indefinitely remove decay heat and bind fission products. 
         [0004]    Early development of MSRs was primarily from the 1950s to 1970s, but a renewed interest in MSRs has recently developed. However, since less development effort has been devoted to MSRs than to other reactor types, various technical challenges remain to be solved in order to develop a commercially viable system. 
         [0005]    One of the challenges of operating an MSR arises from the fact that it is important to maintain the molten salt entirely in the liquid phase during the operation. Therefore, the temperature of the system must always be kept above the liquidus temperature of the molten salt. The liquidus temperature (or liquidus, T liq ) of a material specifies the temperature above which the material is completely liquid, and the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium (Askeland et al.,  Essentials of Materials Science and Engineering . Cengage Learning, 2014, p. 329). However, the MSR salt contains many constituents, and the concentrations of these constituents vary significantly during operation, resulting in phase behavior that is difficult to predict. In particular, variations in composition can alter the liquidus temperature (i.e., solidification temperature) of the molten salt. Changes of a molten salt composition during system operation will be difficult to predict or monitor (e.g., abnormal operational situations may cause the composition to go outside of the expected range, which may lead to salt freezing or precipitation). Therefore, molten salts with changing compositions have not been widely used in commercial reactor systems. 
         [0006]    To date, primary interest in molten salts for nuclear energy applications have been focused on pyroprocessing of metallic or oxide spent fuels, which also needs to anticipate how composition changes affect the salt phase behavior. This, however, requires a multi-year study to develop the theoretical foundation and obtain the empirical data needed to predictively model variations in phase properties for a given system (e.g., for a given MSR). Previous studies have used thermal analysis instruments to measure liquidus/solidus temperatures of small frozen samples of salts (Gutknecht, Fredrickson, and Utgikar, “Thermal Analysis of Surrogate Simulated Molten Salts with Metal Chloride Impurities for Electrorefining Used Nuclear Fuel,” No. INUEXT-11-23511, Idaho National Laboratory, 2012; Sridharan, et al., “Thermal Properties of LiCl-KC1 Molten Salt for Nuclear Waste Separation,” Final Project Report, NEUP Project No. 09-780, Nov. 30, 2012). Such analyses require manual sampling of the salt, crushing and dividing samples, and placing samples carefully into a small pan used for either differential scanning calorimetry or differential thermal analysis. 
         [0007]    In another approach, small samples of crystalline material are placed in sample tubes accommodated in an illuminated chamber within an aluminum sample block (i.e., the Omega, Inc. SMP30 Melting Point Apparatus), where the samples can be subjected to programmed heating and cooling cycles and their plateau (i.e., liquidus or solidus) temperatures are determined. 
         [0008]    However, none of the existing methods is adaptable to highly radioactive salts (e.g., a molten salt) as would be found in MSRs, and their application to model the operating characteristics of a given MSR would be prohibitively expensive and time consuming. Thus, there is a need for effective, efficient and economical means of determining the liquidus temperature of a molten salt during operation of a MSR, which can deliver nearly real-time results. 
       SUMMARY 
       [0009]    It is therefore an object of the invention to provide an effective, efficient, and economical solution to determine the liquidus temperature of a molten salt, particularly, in a molten salt reactor system where the composition of the molten salt changes continuously during operation. 
         [0010]    It is yet another object of the invention to mitigate the need for expensive computational or experimental studies to map out phase behavior of the molten salt, and to prevent costly and perhaps catastrophic salt freezes. Moreover, the invention may be applied to avoid potential zone freeze refining problems that would be encountered by immersing a probe in the larger pool of a molten salt. 
         [0011]    In one aspect of the present invention a method for in-situ measuring of a liquidus temperature of a supply of a molten salt is disclosed which includes withdrawing a sample of the molten salt from the supply and placing it into a sample container, cooling the sample of the molten salt in the sample container from a first temperature above the liquidus temperature of the molten salt to a second temperature at which at least a portion of the sample of the molten salt solidifies, taking a plurality of temperature measurements of the sample of the molten salt during cooling of the sample from the first temperature to the second temperature, and determining the liquidus temperature of the molten salt from the plurality of temperature measurements. The method further includes heating the sample of the molten salt in the sample container from the second temperature to the first temperature, and returning the heated sample of the molten salt from the container to the supply. 
         [0012]    In other aspects of the invention one or more of the following features may be included. The molten salt may be a molten salt nuclear fuel and the supply may be in a reactor system. The sample of the molten salt nuclear fuel may be a static sample removed from a flow of the molten salt nuclear fuel in the reactor system. The container may include a tube having proximal and distal ends and the step of withdrawing may include lowering the distal end of the tube into the molten salt nuclear fuel in the reactor system to a predetermined depth so that the molten salt nuclear fuel enters the distal end of the tube. The step of withdrawing may further include heating the tube in a sample region to the first temperature, the sample region being located between the distal and proximal ends of the tube. The step of withdrawing may include reducing a pressure within the tube proximate the proximal end of the tube relative to the distal end of the tube to cause the molten salt nuclear fuel in the tube to travel from the distal end of the tube to the sample region, the molten salt nuclear fuel in the sample region constituting the sample of the molten salt nuclear fuel. 
         [0013]    In yet other aspects of the invention one or more of the following features may be included. The step of cooling may include using a heater to linearly with time cool the sample region from the first temperature to the second temperature, wherein at least a portion of the sample of the molten salt nuclear fuel solidifies at the second temperature. The step of taking a plurality of temperature measurements of the sample of the molten salt nuclear fuel may include using a first temperature sensor to take the plurality of temperature measurements of the sample and a second temperature sensor to take a corresponding plurality of temperature measurements of the heater during cooling of the sample from the first temperature to the second temperature. The step of determining the liquidus temperature of the molten salt nuclear fuel may include determining temperature differences between the plurality of temperature measurements of the sample and the corresponding plurality of temperature measurements of the heater; determining a first temperature point of the sample where the temperature difference starts to substantially increase; and using the first temperature point to define the liquidus temperature of a molten salt nuclear fuel in a reactor system. 
         [0014]    In yet other aspects of the invention, the step of determining the liquidus temperature of the molten salt nuclear fuel may include comparing the plurality of temperature measurements of the sample to the corresponding plurality of temperature measurements of the heater and determining a first temperature point where the plurality of temperature measurements of the sample become substantially constant while the plurality of temperature measurements of the heater continue to decline; and wherein the step of determining may further include determining a second temperature point, lower than the first temperature, where the plurality of temperature measurements of the sample transition from being substantially constant to declining with the temperature measurements of the heater; and using the first temperature point to define the liquidus temperature of a molten salt nuclear fuel in a reactor system. 
         [0015]    In yet other aspects of the invention one or more of the following features may be included. The step of heating may include using the heater to heat the sample region to cause the temperature of the sample of the molten salt nuclear fuel in the tube to rise from the second temperature to the first temperature and cause the sample to transition from being at least partially solidified to a liquid state. The step of returning the heated sample of the molten salt nuclear fuel from the tube to the reactor system may include increasing the pressure within the tube proximate the proximal end of the tube relative to the distal end of the tube to cause the sample of the molten salt nuclear fuel in the tube to travel from the sample region out of the distal end of the tube and into the molten salt nuclear fuel in the reactor system. 
         [0016]    In yet other aspects of the invention one or more of the following features may be included. The method may further include interconnecting a first port of a vessel to the proximal end of the tube through a first valve and interconnecting a second end of the vessel to an external region of the nuclear reactor system through a second valve; and wherein before the step of lowering the distal end of the tube into the molten salt nuclear fuel in the reactor system, the method may include opening the first and second valves to allow gas to flow from the tube to the external region. The step of reducing the pressure within the tube proximate the proximal end of the tube relative to the distal end of the tube may include closing the first valve and opening the second valve to pump gas out of the vessel to reduce the pressure in the vessel to a level below that in the tube; the step of reducing may further include closing the second valve and opening the first valve to reduce pressure within the tube to the pressure level with the vessel to cause the molten salt nuclear fuel in the tube to travel from the distal end of the tube to the sample region and closing the first valve when the sample of the molten salt nuclear fuel is in the sample region. 
         [0017]    In yet other aspects of the invention one or more of the following features may be included. The step of withdrawing may further include increasing a pressure outside of the distal end of the tube to cause the molten salt nuclear fuel in the tube to travel from the distal end of the tube to the sample region, the molten salt nuclear fuel in the sample region constituting the sample of the molten salt nuclear fuel. The step of cooling may include passively cooling the sample region from the first temperature to the second temperature, wherein at least a portion of the sample of the molten salt nuclear fuel solidifies at the second temperature. The step of heating may include immersing the tube with the sample of the molten salt at the second temperature into the molten salt nuclear fuel in the reactor system to heat the sample region to cause the temperature of the sample of the molten salt nuclear fuel in the tube to rise from the second temperature to the first temperature and cause the sample to transition from being at least partially solidified to being molten. The step of returning the heated sample of the molten salt nuclear fuel from the tube to the reactor system may include increasing the pressure within the tube proximate the proximal end of the tube relative to the distal end of the tube to cause the sample of the molten salt nuclear fuel in the tube to travel from the sample region out of the distal end of the tube and into the molten salt nuclear fuel in the reactor system. 
         [0018]    In one aspect of the present invention a device for in-situ measuring of a liquidus temperature of a supply of a molten salt is disclosed which includes a sample container for holding a sample of the molten salt withdrawn from the supply, an extraction device in communication with the sample container and configured to withdraw the sample of the molten salt from the supply and place it in the sample container, and a first temperature sensor configured to measure the temperature of the sample of the molten salt in the sample container. The device further includes a control unit, the control unit configured to cause the extraction device to withdraw the sample of the molten salt from the supply and place it in the sample container, cool the sample of the molten salt in the sample container from a first temperature above the liquidus temperature of the molten salt to a second temperature at which at least a portion of the sample of the molten salt solidifies, cause the first temperature sensor to take a plurality of temperature measurements of the sample of the molten salt during cooling of the sample from the first temperature to the second temperature, determine the liquidus temperature of the molten salt from the plurality of temperature measurements, heat the sample of the molten salt in the sample container from the second temperature to the first temperature, and cause the extraction device to return the sample of the molten salt from the sample container to the supply. 
         [0019]    In yet other aspects of the invention one or more of the following features may be included. The molten salt may be a molten salt nuclear fuel and the supply may be in a reactor system. The sample of the molten salt nuclear fuel may be a static sample removed from a flow of the molten salt nuclear fuel in the reactor system. The sample container may include a tube having proximal and distal ends and the control unit may be further configured to cause the device to lower the distal end of the tube into the molten salt nuclear fuel in the reactor system to a predetermined depth so that the molten salt nuclear fuel enters the distal end of the tube prior to withdrawing of the sample. The device may include a heater in communication with the tube, and wherein the control unit may be configured to cause the heater to heat the tube in a sample region to the first temperature prior to withdrawing of the sample, the sample region being located between the distal and proximal ends of the tube. 
         [0020]    In yet other aspects of the invention one or more of the following features may be included. The extraction device may include a vessel having a first port interconnected to the proximal end of the tube through a first valve and a second port interconnected to an external region of the nuclear reactor system through a second valve; and wherein the control unit may be configured to open the first and second valves to allow gas to flow from the tube to the external region before lowering of the distal end of the tube into the molten salt nuclear fuel in the reactor system. The control unit may be further configured to close the first valve and open the second valve to pump gas out of the vessel to reduce the pressure in the vessel to a level below that in the tube. The control unit may be further configured to close the second valve and open the first valve to reduce pressure within the tube to the pressure level within the vessel to cause the molten salt nuclear fuel in the tube to travel from the distal end of the tube to the sample region and then to close the first valve when the sample of the molten salt nuclear fuel is in the sample region. 
         [0021]    In yet other aspects of the invention one or more of the following features may be included. The control unit may be configured to control the heater to linearly with time cool the sample region from the first temperature to the second temperature during cooling of the sample, wherein at least a portion of the sample of the molten salt nuclear fuel may solidify at the second temperature. The device may further include a second temperature sensor and the control unit may be configured to cause the second temperature sensor to take a corresponding plurality of temperature measurements of the heater during cooling of the sample from the first temperature to the second temperature. The control unit may be configured to determine temperature differences between the plurality of temperature measurements of the sample and the corresponding plurality of temperature measurements of the heater, determine a first temperature point of the sample where the temperature difference starts to substantially increase, and use the first temperature point to define the liquidus temperature of a molten salt nuclear fuel in a reactor system. The control unit may be configured to compare the plurality of temperature measurements of the sample to the corresponding plurality of temperature measurements of the heater and determine a first temperature point where the plurality of temperature measurements of the sample become substantially constant while the plurality of temperature measurements of the heater continue to decline; and the control unit may be further configured to determine a second temperature point, lower than the first temperature, where the plurality of temperature measurements of the sample transition from being substantially constant to declining with the temperature measurements of the heater, and use the first temperature point to define the liquidus temperature of a molten salt nuclear fuel in a reactor system. 
         [0022]    In yet other aspects of the invention one or more of the following features may be included. The control unit may be configured to control the heater to heat the sample region to cause the temperature of the sample of the molten salt nuclear fuel in the tube to rise from the second temperature to the first temperature and cause the sample to transition from being at least partially solidified to a liquid state. The control unit may be further configured to open the first and second valves to increase the pressure within the tube proximate the proximal end of the tube relative to the distal end of the tube after heating of the sample to cause the sample of the molten salt nuclear fuel in the tube to travel from the sample region out of the distal end of the tube and into the molten salt nuclear fuel in the reactor system. 
         [0023]    In yet other aspects of the invention one or more of the following features may be included. The extraction device may include an external pressure induction system; and wherein the control unit may be configured to cause the external pressure induction system to increase a pressure outside of the distal end of the tube to cause the molten salt nuclear fuel in the tube to travel from the distal end of the tube to the sample region during withdrawing of the sample, the molten salt nuclear fuel in the sample region constituting the sample of the molten salt nuclear fuel. The control unit may be configured cause the container to passively cool the sample region from the first temperature to the second temperature during cooling of the sample, wherein at least a portion of the sample of the molten salt nuclear fuel solidifies at the second temperature. 
         [0024]    The control unit may be configured to immerse the tube with the sample of the molten salt at the second temperature into the molten salt nuclear fuel in the reactor system to heat the sample region to cause the temperature of the sample of the molten salt nuclear fuel in the tube to rise from the second temperature to the first temperature and cause the sample to transition from being at least partially solidified to being molten during heating of the sample. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a schematic diagram depicting a molten salt reactor system. 
           [0026]      FIG. 2  is a schematic diagram depicting the chemical processing plant of the molten salt reactor system depicted in  FIG. 1 . 
           [0027]      FIGS. 3A-E  arc cross-sectional views of a probe according to an embodiment of this invention at different stages of measurement. 
           [0028]      FIGS. 4A-B  are plots for determining the liquidus temperature of a molten salt using data from temperature measurements by the probe in  FIGS. 3A-E . 
           [0029]      FIG. 5  is a flow diagram depicting the operation of the probe in  FIGS. 3A-E . 
           [0030]      FIG. 6  is a cross-sectional view of a probe according to another embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    In a preferred embodiment, a molten salt reactor system  100  for the generation of electrical energy from nuclear fission is depicted in  FIG. 1 . System  100  includes a molten salt reactor  102  containing molten salt  104 , which may include a mixture of chloride and fluoride salts. The mixture may comprise fissile materials, including thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm) (more specifically Th-225, Th-227, Th-229, Pa-228, Pa-230, Pa-232, U-231, U-233, U-235, Np-234, Np-236, Np-238, Pu-237, Pu-239, Pu-241, Am-240, Am-242, Am-244, Cm-243, Cm-245, Cm-247), and fertile materials, such as  232 ThCl 4 ,  238 UCl 3  and  238 UCl 4 . In this embodiment, the mixture comprises fissile materials including  233 UCl 3 ,  235 UCl 3 ,  233 UCl 4 ,  235 UCl 4 , and  239 PuCl 3 ; and carrier salts including sodium chloride (NaCl), potassium chloride (KCl), and/or calcium chloride (CaCl 2 ). 
         [0032]    Upon absorbing neutrons, nuclear fission may be initiated and sustained in the fissile molten salt  104 , generating heat that elevates the temperature of the molten salt  104  to, for example, approximately 650° C. 1,200° F. The heated molten salt  104  is transported via a pump (not shown) from the molten salt reactor  102  to a heat exchange unit  106 , which is configured to transfer the heat generated by the nuclear fission from the molten salt  104 . 
         [0033]    The transfer of heat from salt  104  may be realized in various ways. For example, the heat exchange unit  106  may include a pipe  108 , through which the heated molten salt  104  travels, and a secondary fluid  110  (e.g., a coolant salt) that surrounds the pipe  108  and absorbs heat from the molten salt  104 . Upon heat transfer, the temperature of the molten salt  104  is reduced in the heat exchange unit  106  and the molten salt  104  is transported from the heat exchange unit  106  back to the molten salt reactor  102 . A secondary heat exchange unit  112  may be included to transfer heat from the secondary fluid  110  to a tertiary fluid  114  (e.g., water), as fluid  110  is circulated through secondary heat exchange unit  112  via a pipe  116 . 
         [0034]    The heat received from the molten salt  104  may be used to generate power (e.g., electric power) using any suitable technology. For example, the water in the secondary heat exchange unit  112  is heated to a steam and transported to a turbine  118 . The turbine  118  is turned by the steam and drives an electrical generator  120  to produce electricity. Steam from the turbine  118  is conditioned by an ancillary gear  122  (e.g., a compressor, a heat sink, a pre-cooler, and a recuperator) and transported back to the secondary heat exchange unit  112 . Alternatively, the heat received from the molten salt  104  may be used in other applications such as nuclear propulsion (e.g., marine propulsion), desalination, domestic or industrial heating, hydrogen production, or a combination thereof. 
         [0035]    During the operation of the molten salt reactor  102 , fission products will be generated in the molten salt  104 . The fission products will include a range of elements. In this preferred embodiment, the fission products may include, but are not limited to, rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), an element selected from lanthanides, palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), technetium (Tc), Xenon (Xe) or Krypton (Kr). 
         [0036]    The buildup of fission products (e.g., radioactive noble metals and radioactive noble gases) in molten salt  104  may impede or interfere with the nuclear fission in the molten salt reactor  102  by poisoning the nuclear fission. For example, xenon-135 and samarium-149 have a high neutron absorption capacity, and may lower the reactivity of the molten salt. Fission products may also reduce the useful lifetime of the molten salt reactor  102  by clogging components, such as heat exchangers or piping. 
         [0037]    Therefore, it is generally necessary to keep concentrations of fission products in the molten salt  104  below certain thresholds to maintain proper functioning of the reactor  102 . This may be accomplished by a chemical processing plant  124  configured to remove at least a portion of fission products generated in the molten fuel salt  104  during nuclear fission. During operation, molten salt  104  is transported from the molten salt reactor  102  to the chemical processing plant  124 , which may processes the molten salt  104  so that the molten salt reactor  102  functions without loss of efficiency or degradation of components. An actively cooled freeze plug  126  is included and configured to allow the molten salt  104  to flow into a set of emergency dump tanks  128  in case of power failure or on active command. 
         [0038]      FIG. 2  shows additional detail of the chemical processing plant  124 . During a typical state of reactor operation, the molten salt  104  can be circulated continuously (or near-continuously) by way of pump  202  from the molten salt reactor  102  through one or more of the functional sub-units of the chemical processing plant  124 . In addition to removing fission products, the chemical processing plant  124  is also configured to limit or reduce the corrosion of the molten salt reactor  102  by the molten salt  104  by way of a corrosion reduction unit  204 ,  FIG. 2 . 
         [0039]    The chemical processing plant  124  also includes a froth flotation unit  206  configured to remove at least part of the insoluble fission products (e.g., krypton (Kr), Xenon (Xe), palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), technetium (Tc)) from molten salt  104 . Froth flotation unit  206  is also configured to remove at least part of the dissolved gas fission products (e.g., Xenon (Xe) or Krypton (Kr)). The froth flotation unit  206  generates froth from the molten salt  104  that includes insoluble fission products and dissolved gas fission products. The dissolved gas fission products are removed from the froth, and at least a portion of the insoluble fission products are removed by filtration. 
         [0040]    Also included in chemical processing plant  124  is salt exchange unit  208  which is configured to remove at least a portion of the fission products (e.g., rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), and an element selected from lanthanides) soluble in the molten salt  104 . The removal of soluble fission products may be realized through various mechanisms. 
         [0041]    As indicated above, in order to limit corrosion of the molten salt reactor  102 , the chemical processing plant  124  includes a corrosion reduction unit  204  configured to protect the corrosion of the molten salt reactor  102  by the molten salt  104 . The molten salt reactor  102  is typically constructed of metallic alloy including one or more of the following elements: iron (Fe), nickel (Ni), chromium (Cr), manganese (Mn), carbon (C), silicon (Si), niobium (Nb), titanium (Ti), vanadium (V), phosphorus (P), sulfur (S), molybdenum (Mo), or nitrogen (N). The molten salt  104  may include uranium tetrachloride (UCl 4 ), which can corrode the molten salt reactor  102  by oxidizing chromium (Cr→Cr 2+ +2e−; Cr+2UCl 4 →CrCl 2 +2UCl 3 ). 
         [0042]    During reactor operation, the molten salt  104  is transported from the reactor  102  to the corrosion reduction unit  204  and from the corrosion reduction unit  204  back to the reactor  102 . The transportation of the molten salt  104  may be driven by pump  202 , which may be configured to adjust the rate of transportation. The corrosion reduction unit  204  is configured to process the molten salt  104  to maintain an oxidation reduction (redox) ratio, E(o)/E(r), in the molten salt  104  in the molten salt reactor  102  (and elsewhere throughout the system) at a substantially constant level, wherein E(o) is an element (E) at a higher oxidation state (o) and E(r) is the element (E) at a lower oxidation state (r). 
         [0043]    During operation of the molten salt reactor system  100 , the temperature of the molten salt  104  needs to be maintained above its liquidus temperature to prevent solidification of the molten salt  104 . Therefore, it may be important to obtain real-time liquidus temperature of the molten salt  104 , which often varies over time due to the changing composition of the molten salt  104 . A probe  300  is disposed in the molten salt reactor system  100 , preferably in a position (e.g., the headspace of a molten salt reactor  102 ) to access the molten salt  104  during operation of the molten salt reactor system  100 . 
         [0044]      FIGS. 3A-E  further illustrate a preferred embodiment of the probe  300 . The probe  300  is configured to withdraw a sample  400  ( FIG. 3C ) from a molten salt pool  302 , measure the liquidus temperature of the sample  400 , and return the sample  400  to the molten salt pool  302 . In various embodiments, the probe  300  may be permanently installed through a surface of a vessel or a pipe in the molten salt reactor system  100 , or be inserted as a detachable device (e.g., via a feedthrough into a gas headspace above a portion of the molten salt  104 ). 
         [0045]    During a typical measurement, the molten salt pool  302  (whose liquidus temperature is to be measured) is surmounted by a gas phase  304  (e.g., gas contained within the headspace of a molten salt reactor  102 ). The molten salt pool  302  is flowing at sufficiently low velocity with respect to the probe  300  so that splashing, bow-wave formation and other hydrodynamic effects are negligible to the probe  300 . In other embodiments, the molten salt pool  302  may be static. 
         [0046]    The probe  300  includes a tube  306  within in which the sample  400  is held, an internal thermocouple  310  configured to measure the temperature of the sample  400 , and a furnace  308  configured to heat a portion of the tube  306  and the sample  400  therein. The tube  306  (preferably cylindrical in shape) includes a proximal end  307 , and a distal end  309 , through which the sample  400  enters the tube  306 . 
         [0047]    The probe  300  is further configured to induce a pressure difference between the proximal end  307  and the distal end  309  of the tube  306 . Probe  300  includes a tank  318 , a first pressure line  314  through which gas travels between the tank  318  and the proximal end  307 , and a second pressure line  315  through which gas travels between the tank  318  and the atmosphere or other components (e.g., one or more valves or a pump). The first pressure line  314  is in communication with the proximal end  307  through a gas port  313 . The probe  300  further includes a first valve  316  configured to control gas flow through the pressure line  314  and a second valve  320  configured to control gas flow through the second pressure line  315 . A control unit  319  is included in the probe  300  and configured to control the first valve  316  and second valve  320  independently, as well as to control the overall operation of probe  300 . 
         [0048]    Alternative or additional embodiments to create the pressure difference between the proximal end  307  and distal end  309  of the tube  306  are considered within the scope of the invention. For example, the probe  300  may include an external pressure induction system  317  configured to increase the pressure of the gas phase  304  to a level higher than in the tube  306 . The external pressure induction system  317  may be controlled by the control unit  319 . 
         [0049]    At the beginning of the measurement,  FIG. 3A , the probe  300  is positioned so that the distal end  309  of the tube  306  resides in the gas phase  304  and is not in contact with the molten salt pool  302 . The first valve  316  and second valve  320  are open, allowing the communication between the proximal end  307  of the tube  306  and a body of gas (e.g., the atmosphere) that at around the same pressure as the gas phase  304 . 
         [0050]    As shown in  FIG. 3B , the probe  300  is then lowered to a position so that at least a portion of distal end  309  of the tube  306  is submerged in the molten salt pool  302 , but the internal thermocouple  310  is not in contact with the molten salt pool  302 . The lowering of probe  300  may be monitored (e.g., by a sensor not shown) and controlled so that tube  309  is submerged to a target depth. 
         [0051]    During the lowering of the probe  300 , a portion of the molten salt pool  302  enters the tube  306  through the distal end  309 . Since the first valve  316  and second valve  320  are open, gas in the proximal end  307  of the tube  306  exits through the first pressure line  314  as the probe  300  is lowered. The surface of the molten salt within the tube  306  is maintained at the same level as the surface of the molten salt pool  302 . Upon lowering of the probe  300 , the gas trapped in the proximal end  307  of the tube  306  stabilizes at the pressure in the pressure line  314  (e.g., the pressure of the gas phase  304 ). 
         [0052]    The first valve  316  is then closed, and the gas within the tank  318  is then withdrawn (e.g., by a pump through the second pressure line  315 ). As a result, the pressure within the tank  318  is lower than the pressure within the distal end  309  of the tube  306 . The second valve  320  is then closed, and the furnace  308  heats a portion of the tube  306  to a temperature at or above the temperature of the molten salt pool  302 . 
         [0053]    Referring to  FIG. 3C , the first valve  316  is then opened, allowing gas to exit from the tube  306  into the tank  318  through the first pressure line  314 . Since the gas pressure within the tube  306  is lower than the pressure of the gas phase  304 , a sample  400  of the molten salt is drawn into the tube  306  from the distal end  309  towards the proximal end  307  and occupies sample region  401 . The volume of the sample region  401  may be adjusted by controlling the reduced gas pressure within the tank  318  (e.g., by controlling the close of the second valve  320 ). Preferably, the sample  400  immerses at least a portion of the internal thermocouple  310  to allow the temperature measurement of the sample  400  by the internal thermocouple  310  (monitored by the control unit  319 ), and the sample  400  does not immerse the gas port  313  so that the sample  400  does not enter the first pressure line  314 . The first valve  316  is then closed, isolating the tube  306  from the tank  318 . 
         [0054]    The temperature of the furnace  308  is gradually lowered (e.g., linearly with time) from the initial temperature, i.e. the temperature of the molten salt pool  302 , thus the temperature in sample region  401  and the sample  400  therein are likewise lowered. As a result, the sample  400  is cooled from a first temperature above the liquidus temperature of the sample  400  to a second temperature at which at least a portion of the sample  400  solidifies. A furnace thermocouple  312  is included to monitor the temperature of the furnace  308 , and the control unit  319  is further configured to receive the monitored temperature from the furnace thermocouple  312  and control the temperature of the furnace  308  (e.g., by controlling the power input of the furnace  308 ). 
         [0055]    During the cooling process, a plurality of temperature measurements of the sample  400  are taken by the interior thermocouple  310 , while a plurality of temperature measurements of the furnace  308  arc performed by the furnace thermocouple  312 . The two sets of measured temperatures (by the interior thermocouple  310  and by the furnace thermocouple  312 ) may be compared over time. 
         [0056]      FIG. 4A  shows a typical temperature-time plot  500  that may be used for the comparison of the two sets of measured temperatures. In the plot  500 , the two sets of measured temperatures are independently plotted. At the beginning of the cooling (region I in plot  500 ), the sample  400  is a liquid, and the two sets of measured temperatures reduce at a similar rate (e.g., the two sets of measured temperatures closely track). As the temperature of the sample  400  reaches its liquidus temperature, at least a portion of the sample  400  begins to solidify. The measured temperatures by the internal thermocouple  310  level off (“plateau”, region II in plot  500 ) compared to the measured temperatures by the furnace thermocouple  312 , which continues to linearly decline. Upon further cooling the sample  400 , two sets of measured temperatures then begin to approximate each other and decline as determined by the furnace  308  (region III in plot  500 ). Upon cooling to the second temperature, the sample  400  at least partially solidifies, and preferably, the sample  400  completely solidifies. The liquidus temperature of the molten salt pool  302  may be determined by control unit  319  by identifying a temperature at which the two sets of measured temperatures start to diverge from each other (i.e. plateau region II). 
         [0057]      FIG. 4B  shows a typical plot  500 ′ that may be alternatively used for the comparison of the two sets of measured temperatures. In plot  500 ′, the difference between the two sets of temperatures is plotted over time, and independently, the set of temperatures measured by the internal thermo couple  306  is plotted over time. At the beginning of the cooling (region I′ in plot  500 ′), the sample  400  is a liquid, and the temperature difference maintains at a low level. As the temperature of the sample  400  reaches its liquidus temperature, at least a portion of the sample  400  begins to solidify, resulting in an increased temperature difference (“peak”, region II&#39; in plot  500 ′). Upon further cooling the sample  400 , the sample  400  is a liquid, and the temperature difference reduces to a low level (region III′ in plot  500 ′). The liquidus temperature of the molten salt pool  302  may be determined by control unit  319  by identifying a temperature at which the temperature difference starts to increase (the peak starts to form, e.g., 340.56° C. at 73.93 min); the temperature can be calculated by performing a linear extrapolation of the peak back to the baseline. 
         [0058]    Referring to  FIG. 3D , upon completion of the temperature measurements, the furnace  308  then heats the sample region  401  of the tube  306  and the sample  400  therein to the first temperature so that the sample  400  liquefies. The first valve  316  and the second valve  320  are then opened to allow the gas pressure within the tube  306  to reach a similar pressure of the gas phase  304 , thereby releasing the sample  400  from the sample region  401  into the molten salt. 
         [0059]    The probe  300 ,  FIG. 3E , is then lifted to a position so that the distal end  309  resides in the gas phase  304  and is not in contact with the molten salt pool  302 . Preferably, the probe  300  is lifted to the same position as at the beginning of the measurement. Gas pressure within the tube  306  continues to equilibrate throughout the lifting of the probe  300  so that at least a majority of the molten salt exits from the tube  306  into the molten salt pool  302  through the distal end  309 . 
         [0060]    A flow diagram  600 ,  FIG. 5 , further illustrates the operations of the probe  300  controlled by the control unit  319 . As a first step,  602 , both of the first valve  316  and the second valve  320  are opened and the probe  300  is lowered to a position so that at least a portion of distal end  309  of the tube  306  is submerged in the molten salt pool  302 . However, the internal thermocouple  310  is not in contact with the molten salt pool  302 . 
         [0061]    At step  604 , the first valve  316  is closed, isolating the tank  318  from the tube  306  while the second valve  320  remains open and the pressure within the tank  318  is then reduced by withdrawing gas from the tank  318 . Next, in step  606 , the second valve  320  is closed and the furnace  308  heats the sample region  400  of the tube  306  to a temperature at or above the temperature of the molten salt pool  302 . At step  608 , the first valve  316  is opened, the sample  400  is withdrawn into the sample region  401  and then the first valve  316  is closed. 
         [0062]    In step  610 , the sample  400  is cooled from a first temperature to a second temperature, and a plurality of temperature measurements of both the sample  400  and the furnace  308  are taken during the cooling. The two sets of measured temperatures are compared to determine the liquidus temperature of the molten salt pool  302  in step  612 . In step  614 , the furnace  308  heats the tube  306  and the sample  400  therein, and both of the first valve  316  and the second valve  320  are opened to return the sample  400  from the sample region  401  into the molten salt. Finally, the probe  300  is lifted to a position so that the distal end  309  resides in the gas phase  304  and is not in contact with the molten salt pool  302 , step  616 . 
         [0063]    The in-situ measurement of the liquidus temperature of the molten salt pool  302  as described above may be repeated. The repeated measurements may allow the monitoring of the liquidus temperature of the molten salt pool  302  in real-time, and preferably, to further determine the affect of the changes of the molten salt composition to its liquidus temperature. The composition of the molten salt  104  may then be changed (e.g., by adding substance to the composition of the molten salt  104 ) in order to adjust the liquidus temperature of the molten salt  104  to a desired level. 
         [0064]      FIG. 6  illustrates another embodiment of the probe  300 ′ according to this invention. The probe  300 ′ does not include the furnace  308  and furnace thermocouple  312  of the probe  300  as depicted in  FIG. 4A-E . Additionally, the tube  306 ′ included in the probe  300 ′ carries sufficient volume so that the sample  400 ′ cools slowly enough to allow the determination of its liquidus temperature from a plateau observed in the set of measured temperatures of the sample  400 ′. The cooled sample  400 ′ may be returned to the molten salt pool  302 ′, or alternatively, the cooled sample  400 ′ may be discarded or recovered. 
         [0065]    In other embodiments, returning the sample  400  ( 400 ′) to the molten salt pool  302  ( 302 ′) may be accomplished by removing the tube  306  ( 306 ′) from the molten salt reactor  102  ( 102 ′), heating the tube  306  ( 306 ′) or processing the tube  306  ( 306 ′) to remove the sample  400 , and returning the remove sample  400  ( 400 ′) to the molten salt pool  302  ( 302 ′). 
         [0066]    Alternatively, the tube  306  ( 306 ′) may be further lowered to submerge the sample region  401  ( 401 ′) into the molten salt pool  302  ( 302 ) upon completion of temperature measurements, so that the sample  400  ( 400 ′) therein liquefies. The tube  306  ( 306 ′) may then be lifted from the molten salt pool  302  ( 302 ′) to allow the molten salt exit from the tube  306  ( 306 ′) to the molten salt pool  302  ( 302 ′). 
         [0067]    A particular implementation has been described above. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.