Patent Number: 040100685
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, a reactor coolant such as liquid sodium, is heated to reactor operating temperature on passage through a nuclear core 10 contained within a reactor vessel 11. The hot liquid sodium exits from the reactor vessel 11 and enters a main reactor coolant flow line 12. The hot reactor coolant transfers its heat to another flow system coupled in sealing arrangement with the reactor primary system within item 15, which may be either a steam generator or a heat exchanger, depending upon the particular type of nuclear reactor to which this invention is applied. Upon exiting from item 15, the cooled reactor coolant is circulated back into the reactor vessel 11 by means of a main coolant circulating pump 16, repeating the described flow cycle. An oscillating cold trap system 17 is provided across the main coolant circulating pump 16. Valves 13 and 14 isolate the oscillating cold trap system 17 from the reactor primary system during normal reactor operation. After the reactor has been operated for an extended period of time, or when the reactor coolant has become contaminated by radioactive fission product nuclides such as tridium, barium-130, cesium-141, zirconium-95, iodine-131, and iodine-125, it then becomes desirable to remove this contamination. The oscillating cold trap system 17 is put into operation by opening isolation valves 13 and 14. This causes a portion of the hot reactor coolant to be diverted from the reactor primary system and flow through the oscillating cold trap system 17. The hot contaminated reactor coolant is purified within the oscillating cold trap system 17 and then flows back into the main reactor coolant flow line 12 of the primary system where it joins with the bulk of the reactor coolant prior to entering the main coolant circulating pump 16. The reactor coolant flowing through the oscillating cold trap system 17 is only a portion of the total reactor coolant flow. Therefore, the reactor coolant is not completely purified of all the radioactive fission products nuclides. But, contained operation of the system lowers the concentration of radioactive contamination in the total reactor coolant to an effectively safe level. The details of the oscillating cold trap system are more completely shown in FIG. 2 to which reference is now made. With valves 13, 18, 19 and 14 open and valves 20 and 21 closed, the portion of reactor coolant flowing through the oscillating cold trap system 17 flows in the direction indicated by arrows A, B, C, D, E, F and Z. Hot contaminated reactor coolant, such as liquid sodium at approximately 1000.degree. F, enters the first cold trap 22 which has been preloaded with the desired reactant or mixture of reactants. These may include high concentrations of isotopic diluents such as sodium iodide and sodium hydride, and reacting chemical species such as sodium oxide. The high temperature of the reactor coolant causes dissolution of the reactants into the reactor coolant. The diverted stream of reactor coolant, now saturated with the reactants exits from cold trap 22 which is not being cooled and enters a mixing and reacting tank 23. The mixing and reacting tank 23 provides mixing to assure adequate reaction between the fission product contamination contained within the reactor coolant and the dissolved reactants which are also contained within the reactor coolant. The still contaminated reactor coolant flows into a second cold trap 24 where the temperature of the reactor coolant is lowered to approximately 250.degree. F. At this temperature, the radioactive fission product contamination and the excess reactants are precipitated out of solution. The precipitation or nucleation occurs within cold trap 24 on a suitable surface provided therein, such as wire mesh. Purified reactor coolant exits from cold trap 24 and is reheated to approximately 1000.degree. F prior to being reintroduced into the main coolant flow line 12. An impurity monitor 25, such as an electrochemical oxygen meter or a hydrogen diffusion meter, is installed between cold traps 22 and 24. When the reactants in cold trap 22 are exhausted, the impurity monitor 25 senses the decrease in impurity level in the contaminated reactor coolant. At a predetermined decreased impurity level, the impurity monitor 25 actuates an automatic control system to reverse the direction of flow in the oscillating cold trap system 17 (FIG. 1) and to reverse the heating and cooling of the cold traps. Flow reversal is effectuated by closing valves 18 and 19 and opening valves 21 and 20. The reactor coolant now flows in a direction indicated by arrows A, G, H, I, J, K, L, M, and Z. Since cold trap 24 now contains the excess reactants, that is, reactants which have not isotopically exchanged or chemically compounded with the fission product nuclides, it operates as the reactant supply for the contaminated reactor coolant. On flowing through cold trap 24, the temperature of the reactor coolant is maintained at approximately 1000.degree. F so that the contaminated reactor coolant becomes saturated with the excess reactants. The mixing and reacting tank 23 functions as previously by mixing the reactor coolant to assure adequate reaction between the reactant and the radioactive fission product nuclides. The temperature of the reactor coolant is decreased to approximately 250.degree. F within cold trap 22 which now acts as the depository for the precipitated fission product nuclides and the excess reactants. The precipitated products in cold trap 22 again comprise excess reactants and isotopically exchanged or chemically compounded radioactive fission products. However, the amount of excess reactants is less than that previously contained in cold trap 24, while the amount of precipitated fission products is greater than the amount previously contained in cold trap 24. When the impurity monitor 25 senses the previously set decreased impurity level indicating the supply of reactants in cold trap 24 is exhausted then the automatic control system again reverses the direction of flow of reactor coolant through the oscillating cold trap system 17 (FIG. 1) and the heating and cooling of the col traps. The system continuously operates in this oscillating mode, transferring the excess reactants and the precipitated radioactive fission products from one cold trap to the other until there is no longer any excess reactants. At this point, it finally becomes necessary to service the oscillating cold trap system. This may be accomplished by flushing cold traps 22 and 24 by hot reactor coolant which may be obtained from an auxiliary source. Another method is to remove the wire mesh material contained within cold traps 22 and 24 on which the radioactive fission products have deposited. An impurity monitor 26 is provided in the return line of the oscillating cold trap system 17 (FIG. 1). The impurity monitor 26 senses a failure to remove the impurities and actuates an automatic control system which closes valves 13 and 14 thereby isolating the oscillating cold trap system 17 (FIG. 1) from the primary system of the nuclear reactor. Although not shown in FIG. 2 heating means are provided in the oscillating cold trap system 17 (FIG. 1) for heating the reactor coolant from the cold trap exit temperature of 250.degree. to 1000.degree. F prior to being reintroduced into the main coolant flow line 12. For optimal efficiency the heat rejected by the operating cold trap, 22 or 24, may be used as a heat source for heating the purified reactor coolant prior to being reintroduced into the nuclear reactor primary system. Typical impurities removed from a primary system of a nuclear reactor such as a liquid metal-cooled fast breeder reactor are: 1. NaH (sodium hydride will isotopically exchange with H-3 (tritium) to form sodium tritide. Precipitation of sodium tritide in the operating cold trap will retain most of the H-3 in the cold trap system, thus preventing its diffusion from the primary system to the environment. 2. Na.sub.2 O (sodium oxide) will react with rare earth and alkaline earth fission products to form insoluble oxides. For example: EQU Ba + Na.sub.2 O .fwdarw. BaO + 2Na EQU Zr + 2Na.sub.2 O .fwdarw. ZrO.sub.2 + 4Na Since the oxygen concentration in the primary system will be one to five ppm, the kinetics of the above reactions will be rate limited by the availability of oxygen; hence, reaction and precipitation will be favored in the cold trap system provided by this invention because the oxygen level will be maintained at several hundred ppm in the reactant supply tank. 3. NaH (sodium hydride) will remove cesium-134 and cesium-137 by either adsorption or occlusion. It has been shown that sodium hydride is more effective in adsorbing cesium than sodium oxide. 4. NaI (sodium iodide) will isotopically exchange with fission product isotopes, iodine-131 and iodine-135 to form Na.sup.131 I + Na.sup.135 I. Since solubility of sodium iodide is highly temperature dependent and its solubility at cold trap temperatures is extremely low, more than 99.9% of these isotopes have been shown to be removed by this method. From the foregoing description, taken in connection with the drawing it is seen that this invention provides an oscillating cold trap system which effectively removes radioactive fission product contamination from a liquid metal coolant of a nuclear reactor. Further, the oscillating cold trap system makes full use of reactants which enhance precipitation of the radioactive contamination such that reactor downtime is minimized and the time interval between required servicing of the cold trap is maximized. Since numerous changes may be made in the above-described apparatus, different embodiments of the invention may be made without departing from the spirit and scope thereof. It is intended that all the matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.