IRRADIATION TARGET FOR RADIOISOTOPE PRODUCTION, METHOD FOR PREPARING AND USE OF THE IRRADIATION TARGET

Abstract

The invention provides a sintered rare earth metal oxide target for producing a radioisotope in an instrumentation tube of a nuclear power reactor, wherein the sintered target has a density of at least 90 percent of the theoretical density, and wherein the sintered target contains chromium in an amount of from 500 to 2000 μg/g, and Mg and/or Ca in an amount of from 1000 to 6000 μg/g. The sintered target is prepared by providing a rare earth metal oxide powder, blending the rare earth metal oxide powder with chromium oxide, dry granulating and consolidating the powder in a mold to form a spheroidal green body, and sintering the green body in solid phase to form a spheroidal ytterbia target.

Claims

1.-28. (canceled)

29. A sintered rare earth metal oxide target for producing a radioisotope in an instrumentation tube of a nuclear power reactor, wherein the target comprises chromium in an amount of from 500 to 2000 μg/g, and Mg and/or Ca in an amount of from 1000 to 6000 μg/g.

30. The target according to claim 29 consisting of the rare earth metal oxide doped with chromium in an amount of from 1000 to 6000 μg/g, Mg and/or Ca in an amount of from 1000 to 6000 μg/g, aluminum in an amount of between 500 and 8000 μg/g, and unavoidable impurities.

31. The target according to claim 29 having a density of at least 90 percent of the theoretical density.

32. The target according to claim 29 wherein the rare earth metal oxide is represented by the general formula R.sub.2O.sub.3 wherein R is a rare earth metal selected from the group consisting of Nd, Sm, Y, Dy, Ho, Er, Tm, Yb and Lu.

33. The target according to claim 32 wherein the rare earth metal is Sm, Y, Ho or Yb.

34. The target according to claim 32, wherein the rare earth metal is monoisotopic.

35. The target according to claim 29, comprising Mg in an amount of between 1000 and 6000 μg/g.

36. The target according to claim 29, comprising aluminum in an amount of between 500 and 8000 μg/g.

37. The target according to claim 29, having a density of at least 92 percent of the theoretical density.

38. The target according to claim 29, having a porosity of less than 10%.

39. The target according to claim 29, comprising pores having a size less than 100 μm.

40. The target according to claim 29, having an average grain size of 35 μm or more.

41. The target according to claim 29, wherein the target is speroidal and has a diameter in a range of from 1 to 5 mm.

42. The target according to claim 29, wherein the target is resistant to a pneumatic transport pressure of 10 bar and/or an impact velocity of 10 m/s.

43. A method according to preparing an irradiation target according to claim 29, comprising the steps of: providing a powder blend consisting of a rare earth metal oxide, chromium oxide and a binder wherein chromium oxide is present in the powder blend in an amount of from 1000 to 3000 μg/g; pre-consolidating the powder blend to form granules having a grain size of less than 500 μm, and consolidating the granulated powder blend to form a green body; or pelletizing the powder blend by agglomeration in a rotating drum or on a rotating disc to form a green body; and placing the green body on a support comprising Mg and/or Ca and sintering at a temperature of at least 1700° C. to form a sintered rare earth oxide target having a sintered density of at least 90% of the theoretical density.

44. The method according to claim 43, wherein the powder of the rare earth metal oxide has a purity of greater than 99%.

45. The method according to claim 43 wherein the binder is a metal salt of a fatty acid.

46. The method according to claim 43, wherein the binder is added to the powder blend in an amount of between 0.01 to 0.1 weight percent.

47. The method according to claim 43, wherein the powder blend is pre-consolidated using a pressing force in a range between 10 and 50 kN to form a pre-consolidated slug or pellet.

48. The method according to claim 47, wherein the pre-consolidated slug or pellet is milled and sieved to form the granules.

49. The method according to claim 43, wherein further binder is added to the granules in an amount of between 5 and 10 weight percent.

50. The method according to claim 43, wherein the granules are compression molded by hydraulic pressing at a pressing force in a range from 0.1 to 10 kN.

51. The method according to claim 43, wherein the green body is sintered in a reducing atmosphere comprising hydrogen and an inert gas.

52. The method according to claim 43, wherein the total amount of Ca and/or Mg in the sintered target is not greater than 6000 μg/g and/or the total amount of aluminum is not greater than 8000 μg/g.

53. A method for producing radioisotopes wherein the sintered rare earth metal oxide target according to claim 29 is inserted in an instrumentation tube of a commercial nuclear power reactor and exposed to neutron flux when in energy producing operation.

54. The method according to claim 53 wherein the commercial nuclear power reactor comprises a system for generating radioisotopes in an operating nuclear reactor vessel comprising an irradiation target drive subsystem having means to produce a pressurized gaseous fluid that interacts with the sintered rare earth metal oxide target to drive the target from a target storage subsystem into the instrumentation tube, and from the instrumentation tube into a removal subsystem after irradiation.

55. The method according to claim 53 comprising inserting the sintered rare earth metal oxide target in an instrumentation tube extending into a reactor core by means of pressurized air and exposing the sintered targets to neutron flux encountered in the nuclear reactor when operating, for a predetermined period of time, so that the sintered target converts to a radioisotope, and removing the sintered target and produced radioisotope from the instrumentation tube.

56. The method according to claim 53 wherein the rare earth metal oxide is ytterbia and the radioisotope is Lu-177.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0074] Preparation of Sintered Ytterbia Targets

[0075] A sintered ytterbia target was produced by providing an ytterbia powder, blending the ytterbia powder with chromium oxide, dry granulating and consolidating the powder in a mold to form a spherical green body, and sintering the green body in solid phase to form a spherical ytterbia target.

[0076] The starting ytterbia powder was monoisotopic Yb-176 oxide and had a purity of greater than 99%/TREO, with the following specification being used:

TABLE-US-00001 Yb.sub.2O.sub.3/TREO (% min.) 99.9 TREO (% min.) 99 Loss On Ignition (% max.) 1 Rare Earth Impurities % max. Tb.sub.4O.sub.7/TREO 0.001 Dy.sub.2O.sub.3/TREO 0.001 Ho.sub.2O.sub.3/TREO 0.001 Er.sub.2O.sub.3/TREO 0.01 Tm.sub.2O.sub.3/TREO 0.01 Lu.sub.2O.sub.3/TREO 0.001 Y.sub.2O.sub.3/TREO 0.001 Non-Rare Earth Impurities % max. Fe.sub.2O.sub.3 0.001 SiO.sub.2 0.01 CaO 0.01 Cl.sup.− 0.03 NiO 0.001 ZnO 0.001 PbO 0.001

[0077] 2 g of the ytterbia powder were blended with 3 μg chromium oxide and 0.1 g aluminum distearate. Optical analysis showed that the ytterbia powder had an average grain size of about 10 μm.

[0078] The powder blend was thoroughly mixed and pre-consolidated in a tablet press using a pressing force of 30 kN to form pre-consolidated slugs or pellets. The slugs were milled and sieved to form granules having a maximum particle size of 425 μm. The granules were blended with 0.13 g of aluminum distearate as a lubricant or binder, and compression molded to form spherical green bodies using a pressing force of 1 kN.

[0079] The spherical green bodies were placed on a support made of magnesium oxide and subjected to a degreasing step for removing organic binder components by keeping the green bodies at 500° C. for 0.5 hours. Thereafter, the green bodies were heated to a sintering temperature of 1760° C. at a heating rate of 5K/min, and kept at the sintering temperature for 6 hours under atmospheric pressure using a sintering atmosphere consisting of argon and hydrogen.

[0080] Analysis of the Sintered Ytterbia Targets

[0081] The ytterbia targets obtained by the above process had a spherical shape and an average diameter of about 1.7 mm as measured using a micrometer screw gauge.

[0082] The density of the ytterbia targets was 8.594 g/cm.sup.3 as measured by hydrostatic weighing. Therefore, the ytterbia targets had a sintering density of 93.72% of the theoretical density.

[0083] Further, one of the sintered spherical ytterbia targets was ground down to the center of the target and analyzed by optical microscopy at 50-fold and 100-fold magnification. Software-assisted evaluation of the micrographs showed that the maximum pore size was 67 μm, and that the total porosity was 4.4%. The average grain size of the sintered ytterbia was about 40 μm.

[0084] The metal content of the sintered ytterbia was measured by inductively coupled plasma mass spectrometry (ICP-MS). The ytterbia targets had a chromium content of 1040 μg/g (ppm), an aluminum content of 5730 g/g, and a magnesium content 4380 μg/g, each referring to the metal atom content.

[0085] Stability Tests

[0086] Conditions in the reactor core of a commercial nuclear reactor include high pressure and temperatures above 300° C. Moreover, the sintered ytterbia targets must be able to withstand transport conditions in the instrumentation tubes of the nuclear reactor. Conservative calculations show that inserting the sintered targets in the instrumentation tube, transporting the targets to the nuclear reactor core and harvesting the irradiated targets from the instrumentation tubes will involve at least four impacts at a transporting pressure of 10 bar and/or an impact velocity of 10 m/s.

[0087] 25 sintered ytterbia targets were inserted into a laboratory scale ball measuring system of a nuclear reactor, and shot five times through the system using pressurized air at a pressure of 10 bar thereby creating a total of ten impacts. The targets were then visually inspected to determine any damages. Thereafter, the sintered targets were stored at 350° C. for two weeks, and again subjected to a transport through the ball measuring system thereby creating another two impacts.

[0088] All of the sintered ytterbia targets survived the stability test without any damage.

[0089] Ytterbia-176 is considered to be useful for producing the radioisotope Lu-177 which has applications in medical imaging and cancer therapy, but which cannot be stored over a long period of time due to its short half-life of about 6.7 days. Yb-176 is converted into Lu-177 according to the following reaction:

.sup.176Yb(n,γ) .sup.177Yb(−,β) .sup.177Lu.

[0090] The test results indicate that the sintered ytterbia targets obtained by the method of the present invention are useful precursors for the production of Lu-177 in the instrumentation tubes of a nuclear reactor during energy producing operation.

[0091] Similar reactions are known to the person skilled in the art for the production of other radioisotopes from various rare earth oxide precursors.