Method and apparatus for removing hydrogen from secondary cooling systems of fast breeder reactors
BACKGROUND OF THE INVENTION 
     This invention relates to the removal of hydrogen, a main impurity in a secondary cooling system of a liquid sodium cooled fast breeder reactor, from the system through a cover gas phase, to thereby prolong the life of a secondary cold trap. 
     The secondary cooling system of a fast breeder reactor is provided therein with a steam generator, in which heat exchange is carried out between a liquid sodium coolant and water via a wall of a heat-transfer tube to generate steam. It is known from past experience in operating a fast breeder that an extremely small part of the steam thus generated in the steam generator is decomposed, and that hydrogen, a product of the decomposition of the steam is, diffused, to passes through the wall of the heat-transfer tube in the steam generator, and enters the sodium in the secondary system. The hydrogen, thus diffused to and entered into the sodium in the secondary system, accounts for the greater part of the impurities therein. Consequently, the greater part of the impurities collected in a secondary cold trap, a refiner for sodium in the secondary system, consists of hydrogen. According to the results of rough calculations of a diffusion rate of hydrogen on the basis of the data collected on the experiments conducted heretofore, it can be predicted that, when several years have passed after starting an operation of a fast breeder reactor, the secondary cold trap is plugged with hydride, and its sodium-refining capability is lost. 
     According to conventional techniques, the cold trap thus plugged with the collected impurities, is replaced by a new cold trap. Although the replacement of such an impurity-plugged cold trap permits recovering the sodium-refining capability of the cold trap system, it is accomplished by the difficult work of cutting the secondary sodium pipes. In addition, with the existing state of knowledge of cold traps, it is almost impossible to accurately predict the life of the cold trap. Hence, in view of the possibility that the cold trap, in use, is plugged sooner or later, it is necessary to manufacture a spare cold trap in advance. 
     Thermally regenerating an impurity-plugged cold trap, instead of replacing the same, has been proposed in recent years. According to this method, an impurity-plugged cold trap is made vacuous, the cold trap is heated to increase the temperature thereof to thereby thermally decompose the collected hydrogen impurities therein, and the decomposed hydrogen impurities are vacuum-discharged out of the system. This method does not accompany an operation cutting of the sodium pipes, which is required in the above-described replacement of an impurity-plugged cold trap. Accordingly this thermally regenerating method can be practiced easily, and permits achieving a predetermined object is a comparatively short period of time. However, it takes at least several weeks to completely regenerate a cold trap by this thermal method. In view of the corrosion of a filler material in the trap with sodium, this thermal method cannot be practiced frequently. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a method, which permits removing hydrogen easily and effectively from a secondary cooling system of liquid sodium cooled fast breeder reactors. 
     Another object of the present invention is to provide a method, which permits preventing a secondary cold trap in a fast breeder from being plugged with hydrogen impurities, and thereby prolonging the life thereof. 
     Still another object of the present invention is to provide a method of prolonging the life of a secondary cold trap in a fast breeder reactor without replacing or regenerating the same when it is plugged with hydrogen impurities. 
     A space or gas phase zone in a component or apparatus, such as a tank in a secondary cooling system of a fast breeder reactor functions as a hydrogen-removing container and is filled with a cover gas, such as argon, and sodium mist is generated from vapor evaporated from the circulating liquid sodium coolant and is deposited; after on on an inner surface of the gas phase zone, which may be a wall of the component, which is in contact with the cover gas phase to constantly form a thin layer of the deposit thereon. The inventor of the present invention has found that this sodium mist deposit has the property of easily absorbing the hydrogen in the cover gas. Namely, the liquid drops of sodium absorb the ambient hydrogen gas to easily produce sodium hydride. Such a property of the deposit of sodium mist indicates that the deposit can be utilized as an absorbent, i.e., a &#34;getter&#34;, for the hydrogen gas in the cover gas. 
     The present invention has been achieved by utilizing the hydrogen-gettering action of the sodium mist deposit formed on the wall surface of a component in the secondary cooling system which is in contact with a cover gas contained therein. 
     According to the present invention, the removal of hydrogen impurities from a secondary cooling system of liquid sodium cooled fast breeder reactor is accomplished by providing a hydrogen-removing tank in the secondary cooling system. The hydrogen-removing tank has an inlet and an outlet of the liquid sodium and a cover gas phase therein. On the inner surface of such tank which faces the cover gas phase, a sodium mist deposit is formed. Hydrogen gas contained in the cover gas is captured by the sodium mist deposit due to its hydrogen-getter property. 
     In order to release the hydrogen thus captured by the sodium mist deposit into the cover gas phase, the inner surface of the tank may intermittently be heated. The hydrogen which has been released into the cover gas phase can be sent away to the outside of the secondary cooling system by replacing the hydrogen-containing cover gas with a fresh cover gas or refining the hydrogen-containing cover gas using a suitable refining means and reusing it. 
     A secondary cooling system for a fast breeder reactor is generally constituted of an intermediate heat exchanger, a steam generator, a secondary cold trap, a secondary mechanical sodium pump and the like, and secondary sodium pipes interconnecting these components to thereby put them in fluid communication with one another. If a cover gas phase zone is provided in at least one of these components, the method according to the present invention may be carried out by heating the inner surface of the gas phase zone, i.e., the wall, of any one of these components, without additionally providing the above-mentioned hydrogen-removing tank in the secondary cooling system. 
    
    
     The above and other objects as well as advantageous features of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the concentration of hydrogen gas in a cover gas; 
     FIG. 2 illustrates an embodiment of the present invention; and 
     FIG. 3 is a sectional view of a reticulated structure attached to an inner surface of a wall member according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows the results of an experiment conducted for measuring the concentration of hydrogen gas in a cover gas (argon gas) statically contained in a tank provided in a loop system in a sodium loop having a cold trap (the temperature of a stainless steel mesh inserted in this cold trap is maintained at 150° C.). As may be understood from the graph in FIG. 1, the actual concentration (curve B) of hydrogen in a cover gas is at least not less than ten times as high as a theoretical value (curve A) of concentration of hydrogen in a cover gas, which is in equilibrium with a partial pressure of hydrogen in the sodium in the loop, when the temperature of the sodium is in the range of about 300°-500° C. Although the cause of this phenomenon is unknown, an unexpectedly high concentration of hydrogen exists in the cover gas phase in a component in the secondary cooling system. As is well-known, a sodium mist deposit is accumulated constantly to a small thickness on that surface of a wall of a component which is in contact with the cover gas phase. 
     As hereinbefore described, the sodium mist deposit has the property of easily absorbing the hydrogen in a cover gas. Therefore, during a normal operation of the fast breeder reactor, hydrogen is concentrated gradually. Namely, the hydrogen is concentrated by accumulation more in the cover gas than in the sodium placed in the system, and more in the deposit of sodium mist than in the cover gas, due to the above-mentioned property of the same deposit. 
     It was ascertained by experiments that, when the sodium hydride thus concentrated in the deposit of sodium mist is heated, it is decomposed easily to release hydrogen therefrom. A curve C in FIG. 1 shows this fact. In other words, curve C shows that, when such sodium hydride is heated, the concentration of hydrogen in the cover gas becomes extremely high. During the above experiment, the temperature of the wall of the tank, which was normally kept at 150° C., was increased rapidly to about 350° C. to measure the concentration of hydrogen in the cover gas. The surface area of that portion of the tank which was in contact with the cover gas was about 1.7 m 2 , and the thickness of sodium mist deposited on this portion of the surface of the tank was about 1 mm. Therefore, the amount of sodium in the sodium mist, which was calculated on the assumption that the apparent specific density of the deposit is 0.3 g/cm 3 , was about 500 g. 
     The temperature, to which the sodium mist deposit is heated, i.e., the temperature to which the inner wall of a component on which sodium mist is deposited is heated, will now be described hereinbelow. A hydride normally starts being decomposed at about 250° C., but, in practice, it is heated at not less than about 300° C. There is not an upper limit temperature in particular, at which a hydride is heated to be decomposed, but about 500° C. is a practical upper limit temperature. Even when the temperature of the wall surface during a normal operation of the fast breeder reactor is as high as not less than 300° C., the method according to the present invention can be applied by increasing such a temperature by not less than about 100° C. 
     Thus, according to the present invention, the hydrogen condensed in the sodium mist deposit on that surface of a wall member which is in contact with a cover gas phase in a component in a secondary cooling system is released into the cover gas by heating the deposit of sodium mist to a temperature not lower than the decomposition temperature of the hydride therein for a short period of time at regular time intervals of, for example, one month, and the resulting cover gas is replaced with a cover gas not containing any hydrogen impurities, i.e., a fresh cover gas to immediately send away from the system the hydrogen contained in the old cover gas. Such operations are repeated intermittently during the operation of a fast breeder reactor to intermittently send away the hydrogen from the secondary cooling system, and thereby reduce the burden of collecting hydrogen impurities from the secondary cold trap. This allows the life of the cold trap to be prolonged. 
     A secondary cooling system for a fast breeder reactor is generally constituted of an intermediate heat exchanger, a steam generator, a secondary cold trap, a secondary mechanical sodium pump and the like, and secondary sodium pipes interconnecting these components. If a cover gas phase zone is provided in these components, the method according to the present invention may be practiced by heating the inner surface of the zone, i.e., the wall, of any one of these components. 
     The method according to the present invention may also be practiced preferably by heating the surface of a wall of a tank which is used only for removing hydrogen from the sodium coolant and is provided additionally in the secondary system. A hydrogen-removing tank 11 shown in FIG. 2 has a cover gas phase 12 in an upper portion thereof. Sodium 13 used as a coolant is introduced into the interior of the tank 11 through an inlet 22 to stay therein for a predetermined period of time, and then circulated out therefrom through an outlet 23. A heater (not shown) is buried in the wall of the tank 11, so that the temperature of the wall can be increased to not less than about 300° C. The tank 11 is further provided with an inlet port 14 and an outlet port 15 for cover gas so as to permit the replacement of cover gas to be carried out while the surface of the wall thereof is heated. This hydrogen-removing tank 11 may be provided in any part of a secondary cooling loop to obtain the desired effect, but it is most preferably provided in the position shown in FIG. 2, which is on the upstream side of a cold trap 16, and on the downstream side of a steam generator 17. The reason why this position is most preferable is that, since the steam generator 17 is the source of generated hydrogen, which is mixed in the sodium in the secondary system as mentioned previously, the concentration of hydrogen impurities becomes high on the downstream side thereof. Reference numeral 18 denotes an intermediate heat exchanger, and 19 a secondary pump. While the fast breeder reactor is in normal operation, the hydrogen gas released from the sodium into the cover gas phase is absorbed in the sodium mist deposit naturally formed on that portion of the inner surface of the tank 11 which faces the cover gas phase 12. The heater buried in the wall of the tank 11 is operated periodically, for example, at an approximate rate of once a month to heat the wall of the tank to about not less than 300° C., and release the hydrogen gas, which has been absorbed in the deposit of sodium mist, into the cover gas. At the same time, i.e., contemporaneously, a cover gas not containing impurities is introduced into the inlet port 14 to immediately send away the released hydrogen gas with the cover gas from the outlet port 15. Such operations are repeated intermittently to discharge the hydrogen contained in the sodium in the secondary cooling system to outside the system and thereby prolong the life of the cold trap. 
     In place of the above-described cover gas-replacing method, there may be employed a method for refining the cover gas, in which, after the hydrogen has been released into the cover gas phase by heating, the hydrogen-containing cover gas is led into a suitable refining means such as, for example, a platinum-palladium catalytic column, a hydrogen-absorbing agent of hydridable metal and the like, and the thus refined cover gas is reused in the cover gas phase in the tank. 
     In order to send away from the system the hydrogen released into the cover gas, a method of vacuum sucking a hydrogen-containing cover gas may also be thought of. However, when the interior of a component in the system is evacuated, the outer air would be sucked therein. Therefore, the vacuum suction method cannot be employed in the present invention. 
     As stated previously, the present invention utilizes as a hydrogen-getter the sodium mist deposit on a wall member contacting a cover gas, so that the amount of hydrogen collected increases in proportion to the amount of the deposit of sodium mist. Accordingly, in order to increase the amount of deposit of sodium mist, it is preferable that the surface area of the wall member contacting the cover gas be increased. Methods for increasing the surface area of such a wall member include, for example, a method of providing the inner wall of the component in the secondary cooling system with a wavy surface or a method of pleating the inner surface of such a wall. Also, as shown in FIG. 3, there is a method of attaching to the inner surface of such a wall 20 a reticulate structure 21 consisting of a material having a high heat transfer rate, such as stainless steel. Such a reticulate structure can be installed reasonably in a hydrogen-removing tank 11 as shown in FIG. 2. 
     Since the present invention is directed to a method of removing hydrogen from a secondary cooling system for a fast breeder reactor constructed as described above, it permits discharging a considerable amount of hydrogen from the system extremely easily in a short period of time. Moreover, the method can be practiced without hindering the operation of the breeder at all, and permits prolonging the life of the secondary cold trap. Therefore, the present invention can be carried out most effectively as a method capable of settling at once various problems concerning a cold trap, which is, according to a conventional technique, plugged with hydrogen impurities frequently, and which is, according to a conventional technique, not treated excellently when it is plugged with such impurities. 
     The present invention is not, of course, limited to the above embodiments; it may be modified in various ways within the scope of the appended claims.