Patent Number: 048308161
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The getter trap 1 (FIG. 1) of the present invention includes an elongated, closed housing 3 having an inlet 5 at one end thereof and an outlet 7 at the other end. A getter material 9 (FIG. 2) is randomly disposed within the housing 3. The getter material 9 is in the form of hollow, tubular sections 11, which are held in place within the housing 3 by a pair of baffle plates 13. The tubular sections 11 have a zirconium-containing substrate 15, formed of a material such as Zircaloy-2, which is an alloy 1.2 to 1.7 percent tin, 0.07-0.20 percent iron, 0.05-0.15 percent chromium, 0.03-0.08 percent nickel, and the balance zirconium; or Zircaloy-4, which is an alloy 1.2 to 1.7 percent tin, 0.12-0.18 percent iron, 0.05-0.15 percent chromium, and the balance zirconium. Zircaloy-4 tubes having a length of 1 inch, and outer diameter of 0.375 inch, and a wall thickness of 0.023 inch are particularly useful in the getter material 9 for the getter trap 1 of the invention. The hollow, tubular sections 11 are coated with a coating 17 of the gettering alloy disclosed in U.S. Pat. No. 4,312,669 of zirconium, vanadium and iron, whose composition, in weight percent, when plotted on a ternary diagram (FIG. 3), lies within a triangle having as its corners the points defined by: (a) 75 percent zirconium, 20 percent vanadium and 5 percent iron; PA1 (b) 45 percent zirconium, 20 percent vanadium and 35 percent iron; and PA1 (c) 45 percent zirconium, 50 percent vanadium and 5 percent iron. Preferably, the composition of the gettering alloy, when plotted on the ternary diagram of FIG. 3 lies within a polygon having as its corners the points defined by: PA1 (d) 70 percent zirconium, 25 percent vanadium and 5 percent iron; PA1 (e) 70 percent zirconium, 24 percent vanadium and 6 percent iron; and PA1 (f) 66 percent zirconium, 24 percent vanadium and 10 percent iron. PA1 (g) 47 percent zirconium, 43 percent vanadium and 10 percent iron; PA1 (h) 47 percent zirconium, 45 percent vanadium and 8 percent iron; and PA1 (i) 50 percent zirconium, 45 percent vanadium and 5 percent iron. One particularly suitably gettering alloy has a composition of 45 percent zirconium, 50 percent vanadium and 5 percent iron. Tubes of Zircaloy-4 coated with this gettering alloy are available from SAES Getters S.p.A. as SAES alloy type ST123. The gettering alloy has a porous structure, and consequently, has a high surface area and a high rate of absorption for both oxygen and hydrogen. In addition, the gettering alloy has a high coefficient of diffusion for both hydrogen and oxygen. Thus, the oxygen absorbed by the gettering alloy diffuses from the surface of the getter material 9 into the bulk of the getter material 9 instead of forming a hydrogen absorption-inhibiting oxide film on the surface of the getter material 9. Access by hydrogen to the zirconium-containing substrate when not coated with the gettering alloy is normally greatly inhibited by the formation of tightly adherent chemical films of the oxide and nitride of zirconium. The coating 17 of the gettering alloy on the surface of the getter material 9 allows the hydrogen to gain access to the zirconium-containing substrate 15. This provides a higher density of hydrogen loading than would the gettering alloy alone, because of the higher density of zirconium in the zirconium-containing substrate than in the porous gettering alloy, and because of the higher chemical affinity of the zirconium-containing substrate for hydrogen in comparison with the gettering alloy. The chemical thermodynamic properties of the zirconium-containing substrate predominate, and at high hydrogen loading, these properties are more advantageous than those of the gettering alloy. For instance, the getter material of the invention will operate at a temperature that is typical of the secondary liquid metal coolant in the secondary coolant loop of a liquid metal nuclear reactor after the secondary coolant has been cooled y passage through the second heat exchanger. As a liquid metal, such as liquid sodium, flows through the inlet 5 into the housing 3 and through the getter material 9, and is discharged from the housing 3 through the outlet 7, hydrogen and oxygen impurities are removed from the liquid metal. The distribution of absorbed hydrogen and oxygen decreases exponentially from the inlet 5 along the length of the getter trap 1. If sufficiently large amounts of oxygen are absorbed, then a proportion of the upstream getter material 9 could have reduced effectiveness and capacity for simultaneous hydrogen absorption. Thus, these considerations can be taken into account in designing the getter trap 1, with an upstream section of getter material 9 being in effect sacrificed for oxygen removal. The absorption of hydrogen by the gettering alloy is reversible by heating the getter material 9 to a temperature on the order of 700.degree. C. The absorption of oxygen by the gettering alloy is not reversible. Thus, the design of a getter trap 1 must take into consideration the life expectancy of the getter material 9 due to oxygen absorption, based upon the environment in which the particular getter trap 1 is to be used. The getter trap 1 of the invention can include a second getter material 21 comprising pellets 27 of the gettering alloy. Such a second getter material 21 is available from SAES Getters S.p.A. as SAES alloy type ST172. Thus, in a second embodiment of the invention, illustrated in FIG. 4, the housing 3 of the getter trap 1A includes a first section 19 in which a second getter material 21 is randomly disposed. The getter material 9 is randomly disposed in a second section 23, located downstream of the first section 19. The first section 19 and second section 23 of the housing 3 are preferably separated by means, such as a baffle plate 25. The upstream, second getter material 21 operates primarily as an oxygen getter, enabling the getter material 9, located downstream of the second getter material 21, to operate primarily as a hydrogen getter without interference from oxygen being absorbed by the getter material 9. Alternatively, in a third embodiment of the invention, illustrated in FIG. 5, in getter trap 1B, the second getter material 21 is randomly disposed in a second housing 29, located directly upstream of the housing 3 in which the getter material 9 is randomly disposed. The second housing 29 includes an inlet 31 and an outlet 33. The second getter material 21 is held in place within the second housing 29 by a pair of baffle plates 35. A connection 37 secures the outlet 33 of the second housing 29 and the inlet 5 of the first housing 3 togeter. Thus, the liquid metal containing hydrogen and oxygen impurities flows into the getter trap 1B through the inlet 31 of the second housing 29, flows through the second getter material 21, and out of the second housing 29 through the outlet 33. The liquid metal then flows through the inlet 5, flows through the getter material 9, and out of the getter trap 1B through the outlet 7 of the housing 3. These embodiments of the getter trap are expected to be particularly useful in the secondary coolant loop of a liquid metal cooled nuclear reactor, where the oxygen load is small compared to hydrogen load arising from corrosion of the steam-containing third loop. The getter trap 1 of the invention is particularly useful in a liquid metal cooled nuclear reactor system 39 (FIG. 6). The system 39 includes a primary coolant loop 41 wherein the primary coolant, a liquid metal such as liquid sodium, is heated as it passes through the nuclear core of the liquid metal nuclear reactor 43. The primary liquid metal coolant is then cooled by passing through a first heat exchanger 45, and is directed back to the reactor 43 by a pump 47. Liquid metal coolant, circulating in a secondary coolant loop 49, absorbs thermal energy from the primary liquid metal coolant through indirect heat exchange in the first heat exchanger 45. The heated secondary liquid metal coolant then passes through a second heat exchanger 51 wherein the thermal energy from the secondary liquid metal coolant is used to heat water flowing in a third loop 53 to produce steam, which, in turn, is used to drive a steam driven device (not shown), such as an electrical generator. The secondary liquid metal coolant is then directed from the second heat exchanger 51 back to the first heat exchanger 45 by a pump 55. The secondary liquid metal coolant often contains undesired hydrogen from the water used to operate the steam driven device, and oxygen from other sources. Thus, the secondary coolant loop 49 includes a getter trap 1, preferably in the embodiments illustrated in FIGS. 4 and 5, of the invention downstream of the second heat exchanger 51 to eliminate the undesired hydrogen and oxygen from the secondary liquid metal coolant. Flow to the getter trap 1 is controlled by a valve 57. It is easily understood that the getter trap of the invention can be used in the primary coolant loop in situations where the primary coolant includes hydrogen and oxygen contaminants. In the primary coolant loop, containing undesired hydrogen and oxygen, the getter trap 1' would be included, following a valve 57', preferably in the line between the pump 47 and the liquid metal reactor 43. The insensitivity of the getter trap to the temperature of operation would also, alternatively, allow the same to be placed in the hot inlet line to the heat exchanger 45 of the primary coolant loop 41, or in the hot inlet line to the heat exchanger 51 of the secondary coolant loop 49.