- Email: [email protected]
Multiple sample irradiator for pool reactors
NUCLEAR INSTRUMENTS
A N D M E T H O D S 53 (I967) 355-356; © N O R T H - H O L L A N D
PUBLISHING
CO.
M U L T I P L E S A M P L E IRRADIATOR FOR P O O L REACTORS* D. A. K U R T Z
Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania, U.S.A. and W. W. MILLER
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, U.S.A. Received 25 April 1967 Multiple samples can be irradiated simultaneously in pool reactors by insertion in a rotating foam plastic disc devised by the authors. Data supporting uniform irradiations is presented.
One frequently meets the problem of subjecting a group of similar samples to an identical neutron irradiation for activation analysis purposes using a small research pool reactor. A disc shaped sample holder, carrying fifteen samples and a flux monitor in its periphery and rotating in a plane parallel to a reactor core face, has proved to be an effective solution to this problem. When the required irradiation times are short ( < 5 min), a pneumatic tube system can be used. This procedure requires a calibration of the neutron flux gradient along the length of the pneumatic "rabbit", and one flux monitor for each group of two to four samples. When longer times are necessary the procedure becomes tedious and also expensive because of radiation damage to the moulded polyethylene rabbits. Some reactors are equipped for carrying out longer sample irradiations with a rotating specimen rack which handles many samples, carrying each sample around the cylindrical reactor core. Our reactor is not so equipped. It is a modified "Triga" pool type reactor, unreflected, with the fuel elements arranged in a hexagonal array. The available face of the active lattice (five fuel elements) is 38 by 20 cm. Over this area the neutron MAGNETS
flux will vary by a factor of two, and is attenuated in the water shield perpendicular to the core face with a relaxation length of about 3 cm. A solution to the problem of using the region near the core face as described to obtain exposures of each of a set of samples to the same integrated neutron flux is to rotate the set in such a manner that each member sweeps the same path through the region, rotating at a period that is short compared to the time variations in the neutron flux and to the length of the irradiation. Development of a device to carry out this procedure is beset with several restrictions. The device must operate at a depth of about 7 m in water. The sample carrier must not itself be activated to a degree that prevents its being brought to the surface for loading and unloading, and it must also be either an inexpensive, oneuse item, or not suffer significant radiation damage. We find that expanded polystyrene insulating board secured from a local builders' supply under the trade name " D y f o a m " satisfies the requirements for the construction of a rotating sample holder for the expo* Work carried under sponsorship of Department of Health, Education and Welfare, PHS Grant GM-9487-04. ATTACHMENT WIRES
NYLON BEARINGS
[ _./t
A
c A. A l t a c h m e n t to m o u n t i n g B. M o t o r
housing
C. E x t e n s i o n D Dyfoam
(I rpm
plate motor}
portion
s a m p l e holder w i t h
hub
Fig. I. Complete multisample irradiation device "Lazy Mary".
355
356
D. A. KURTZ AND W. W. MILLER
sure. of groups of samples to the flux at the face of the core of our reactor. This material.acquires a relatively low induced activity on irradiation, and is sufficiently stable dimensionally to withstand exposures of 5 x 10 x6 nvt thermal. The device as developed is shown in fig. 1. The expanded polystyrene disc is readily cut from the board with a knife or with a "cookie cutter" made from a one-gallon can. Holes for the insertion of the samples are cut with a cork borer using a wooden guide to assure perpendicular placement. The holes are # equally spaced on the circumference of a 16 cm dia. circle. For use, the loaded disc is fastened to the rotating hub and the whole assemble is brought to the core so that the disc is the necessary distance from and approximately parallel to the core face. The reactor is TABLE I (I) (2) (3a) Disc Integrated count, Integrated count, position net net 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 tY
v'x
60275 60072 60 131 60 195 60 228 leaked, discarded 60 287 60616 60 658 60 775 61 225 60 690 60 593 60 336 61 267 60817 60 544 373 (0.62%) 246 (0.41%)
(3b)
X/X
51 238 52 843 53 544 53 649 54 091 53 567 53 329 53 427 53 912 53 878 53 956 53 408 53 060
0.956 + 0.005 0.986 + 0.005
53 045 52 177
0.989 + 0.005 0.972 + 0.005
boron 53 610 (Xs_ta) 313 (0.53~,) 231 (0.43~0)
brought up to power for the necessary time and shut down. The disc alone, with its samples, is removed from the hub and brought to the surface. The very active hub, with its 24Na from the 27Al(n,ct) reaction, and the Cu and M n activities from impurities in the aluminum, remains under water. To determine the degree of uniformity of irradiation obtainable with this device, a set of 16 ampules containing weighed aliquots of a dilute solution of sodium oxalate was prepared, double encapsulated in polyethylene ampules (Polycaps), and irradiated as described for two hours. The induced activities of these samples were determined with a scintillation detectormultichannel analyzer, integrating the 1.37 MeV peak. The data, after normalizing for variations in sample weight and decay before measurement are listed in column 2 of table 1. The standard deviation of the set of measurements, 0.62%, is made up of the contribution from the statistics of counting, 0.41°/0, plus a second factor which includes all other contributions to "the error in the experimental procedure, including the variation in the integrated flux to each sample. This allows us to say that the variation of irradiation of the members of the set is not greater than 0.5%. To determine the degree that the absorption of neutrons by one sample might affect the activation of neighboring samples in this geometry, the experiment was repeated. The sample in position 16 was replaced with one containing a solution of 94 mg of boric acid, a quantity that will absorb about one third of the neutrons traversing the sample. The data for this set is in column 3a of the table. The mean of the set has been calculated from members 3-13, excluding the first and second nearest neighbors. In column 3b are the ratios of the values of these neighbors to the mean, and the error of this ratio. It can be seen that the flux to the nearest neighbors is decreased by about 4°/0, while the second nearest neighbor is not sensibly affected.
A N D M E T H O D S 53 (I967) 355-356; © N O R T H - H O L L A N D
PUBLISHING
CO.
M U L T I P L E S A M P L E IRRADIATOR FOR P O O L REACTORS* D. A. K U R T Z
Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania, U.S.A. and W. W. MILLER
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania, U.S.A. Received 25 April 1967 Multiple samples can be irradiated simultaneously in pool reactors by insertion in a rotating foam plastic disc devised by the authors. Data supporting uniform irradiations is presented.
One frequently meets the problem of subjecting a group of similar samples to an identical neutron irradiation for activation analysis purposes using a small research pool reactor. A disc shaped sample holder, carrying fifteen samples and a flux monitor in its periphery and rotating in a plane parallel to a reactor core face, has proved to be an effective solution to this problem. When the required irradiation times are short ( < 5 min), a pneumatic tube system can be used. This procedure requires a calibration of the neutron flux gradient along the length of the pneumatic "rabbit", and one flux monitor for each group of two to four samples. When longer times are necessary the procedure becomes tedious and also expensive because of radiation damage to the moulded polyethylene rabbits. Some reactors are equipped for carrying out longer sample irradiations with a rotating specimen rack which handles many samples, carrying each sample around the cylindrical reactor core. Our reactor is not so equipped. It is a modified "Triga" pool type reactor, unreflected, with the fuel elements arranged in a hexagonal array. The available face of the active lattice (five fuel elements) is 38 by 20 cm. Over this area the neutron MAGNETS
flux will vary by a factor of two, and is attenuated in the water shield perpendicular to the core face with a relaxation length of about 3 cm. A solution to the problem of using the region near the core face as described to obtain exposures of each of a set of samples to the same integrated neutron flux is to rotate the set in such a manner that each member sweeps the same path through the region, rotating at a period that is short compared to the time variations in the neutron flux and to the length of the irradiation. Development of a device to carry out this procedure is beset with several restrictions. The device must operate at a depth of about 7 m in water. The sample carrier must not itself be activated to a degree that prevents its being brought to the surface for loading and unloading, and it must also be either an inexpensive, oneuse item, or not suffer significant radiation damage. We find that expanded polystyrene insulating board secured from a local builders' supply under the trade name " D y f o a m " satisfies the requirements for the construction of a rotating sample holder for the expo* Work carried under sponsorship of Department of Health, Education and Welfare, PHS Grant GM-9487-04. ATTACHMENT WIRES
NYLON BEARINGS
[ _./t
A
c A. A l t a c h m e n t to m o u n t i n g B. M o t o r
housing
C. E x t e n s i o n D Dyfoam
(I rpm
plate motor}
portion
s a m p l e holder w i t h
hub
Fig. I. Complete multisample irradiation device "Lazy Mary".
355
356
D. A. KURTZ AND W. W. MILLER
sure. of groups of samples to the flux at the face of the core of our reactor. This material.acquires a relatively low induced activity on irradiation, and is sufficiently stable dimensionally to withstand exposures of 5 x 10 x6 nvt thermal. The device as developed is shown in fig. 1. The expanded polystyrene disc is readily cut from the board with a knife or with a "cookie cutter" made from a one-gallon can. Holes for the insertion of the samples are cut with a cork borer using a wooden guide to assure perpendicular placement. The holes are # equally spaced on the circumference of a 16 cm dia. circle. For use, the loaded disc is fastened to the rotating hub and the whole assemble is brought to the core so that the disc is the necessary distance from and approximately parallel to the core face. The reactor is TABLE I (I) (2) (3a) Disc Integrated count, Integrated count, position net net 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 tY
v'x
60275 60072 60 131 60 195 60 228 leaked, discarded 60 287 60616 60 658 60 775 61 225 60 690 60 593 60 336 61 267 60817 60 544 373 (0.62%) 246 (0.41%)
(3b)
X/X
51 238 52 843 53 544 53 649 54 091 53 567 53 329 53 427 53 912 53 878 53 956 53 408 53 060
0.956 + 0.005 0.986 + 0.005
53 045 52 177
0.989 + 0.005 0.972 + 0.005
boron 53 610 (Xs_ta) 313 (0.53~,) 231 (0.43~0)
brought up to power for the necessary time and shut down. The disc alone, with its samples, is removed from the hub and brought to the surface. The very active hub, with its 24Na from the 27Al(n,ct) reaction, and the Cu and M n activities from impurities in the aluminum, remains under water. To determine the degree of uniformity of irradiation obtainable with this device, a set of 16 ampules containing weighed aliquots of a dilute solution of sodium oxalate was prepared, double encapsulated in polyethylene ampules (Polycaps), and irradiated as described for two hours. The induced activities of these samples were determined with a scintillation detectormultichannel analyzer, integrating the 1.37 MeV peak. The data, after normalizing for variations in sample weight and decay before measurement are listed in column 2 of table 1. The standard deviation of the set of measurements, 0.62%, is made up of the contribution from the statistics of counting, 0.41°/0, plus a second factor which includes all other contributions to "the error in the experimental procedure, including the variation in the integrated flux to each sample. This allows us to say that the variation of irradiation of the members of the set is not greater than 0.5%. To determine the degree that the absorption of neutrons by one sample might affect the activation of neighboring samples in this geometry, the experiment was repeated. The sample in position 16 was replaced with one containing a solution of 94 mg of boric acid, a quantity that will absorb about one third of the neutrons traversing the sample. The data for this set is in column 3a of the table. The mean of the set has been calculated from members 3-13, excluding the first and second nearest neighbors. In column 3b are the ratios of the values of these neighbors to the mean, and the error of this ratio. It can be seen that the flux to the nearest neighbors is decreased by about 4°/0, while the second nearest neighbor is not sensibly affected.