Development of 99 Mo Production Technology with Solution Irradiation Method

Technetium-99m (99mTc, half-life: 6.01 hours) is the world’s most widely used radiopharmaceutical for exams of cancer, bowel disease, brain faculty and so on, and it is used for more than twenty million exams per year in the world and more than one million exams per year in Japan. The demand for 99mTc is continuously growing up year by year. The features of 99mTc as the radiopharmaceutical are as follows:

In the conventional solid irradiation method, solid targets including natural molybdenum such as MoO 3 pellets are irradiated with neutrons in a testing reactor, and 99 Mo is produced by the 98 Mo (n, γ) 99 Mo reaction.The post-irradiation process is only dissolution of the irradiated solid targets with an alkaline solution, and only a small amount of radioactive waste is generated in the process compared with the (n, f) method.The 99 Mo production cost of this method or the (n, γ) method is only 0.83 US$/37 GBq (Boyd, 1997).
However, the (n, γ) method has the disadvantage of producing 99 Mo with a low-level specific activity of 37-74 GBq/g-Mo and therefore the method has not had practical application in earnest.In order to utilize 99 Mo with the low-level specific activity, a highperformance adsorbent for (n, γ) 99 Mo is needed.The Japan Atomic Energy Research Institute (the present organization: JAEA) and KAKEN Inc. had developed the highperformance molybdenum adsorbent of Poly-Zirconium Compound (PZC) in 1995 (Hasegawa et al., 1996) and improved PZC (Hasegawa et al., 1999), and then the practical application of the (n, γ) method is just in sight.The molybdenum adsorbent performance of PZC is over 100 times compared with the conventional molybdenum adsorbent of alumina. 99Mo production in JMTR will start by using the solid irradiation method.JMTR aims to provide 99 Mo of 37 TBq/w (1,000 Ci/w), and it will cover about 20% of the 99 Mo imported into Japan (Inaba et al., 2011).

Solution irradiation method
In the new solution irradiation method, a solution target including natural molybdenum such as an aqueous solution of a molybdenum compound (aqueous molybdenum solution) is irradiated with neutrons in a testing reactor, and 99 Mo is produced by the 98 Mo (n, γ) 99 Mo reaction.This new method is the improved type of the solid irradiation method, and it is possible to enhance the 99 Mo production compared with the solid irradiation method.The solution irradiation method has the following advantages compared with the solid irradiation method: 1.It is easy to increase the irradiated volume by using a capsule with larger volume than that of a rabbit.The rabbit is a small sized (150 mm length) capsule (Inaba et al., 2011).2. The separation and dissolution processes after the irradiation are not necessary because the irradiation target is an aqueous solution.3. The amount of generated radioactive waste is smaller than that of the solid irradiation method.In this new method, efficient and low-cost 99 Mo production compared with the conventional 99 Mo production can be realized by using the (n, γ) reaction and PZC.This new method aims to provide 100% of the 99 Mo imported into Japan.

Overview of solution irradiation method
3.1 Structure of 99 Mo production system with solution irradiation method The schematic diagram of the 99 Mo production system with the solution irradiation method is shown in Fig. 1.The system consists of an irradiation system, a supply and circulation system, and a collection and subdivision system.In the irradiation system, an aqueous molybdenum solution in a capsule installed in a reactor core is irradiated with neutrons under static or circulation condition, and 99 Mo is generated.In the supply and circulation system, the solution is supplied to the capsule through pipes and is circulated by a circulator in irradiation operation.A gas disposal device and a heat exchanger are installed in order to take measures against the radiolysis gas and heat generated from the solution by irradiation.
The system is designed so as to minimize unirradiated solution.In the collection and subdivision system, after the solution including the generated 99 Mo is collected from the capsule through pipes, it is treated so as to be products such as PZC-99 Mo columns or 99 Mo transport containers.The detailed design of the 99 Mo production system is carried out based on the results of future investigations and tests.

Progress of the development made thus far
The most important element of the solution irradiation method is the aqueous molybdenum solution as the irradiation target.The solution with a high concentration near the saturation Mo Production Technology with Solution Irradiation Method 327 is used for efficient 99 Mo production, and the solution always is in contact with the structural materials of the capsule and the pipes in the 99 Mo production system under irradiation.Aqueous molybdate solutions are promising candidates for the irradiation target.The effect of the solutions on metals such as the structural materials has been researched, and molybdates are known as corrosion inhibitors (Kurosawa & Fukushima, 1987;Lu et al, 1989;McCune et al, 1982;Saremi et al, 2006).However, the behavior of aqueous molybdenum solutions including the aqueous molybdate solutions under such the conditions is not well understood.Therefore, the following subjects about the fundamental characteristics of the solutions should be investigated: 1. Selection of the aqueous molybdenum solutions as candidates for the irradiation target 2. Compatibility between the solutions and the structural materials 3. Chemical stability of the solutions 4. Effect of γ ray and neutron irradiation on the solutions such as the radiolysis, the γ heating and the activation by-products.
The some subjects described above had already investigated (Inaba et al., 2009), and the progress of the development made thus far is explained as below:

Selection of candidates for irradiation target
The selection of candidates for the irradiation target was carried out.The conditions required for the irradiation target solution are as follows: 1.The irradiation target solution has the high molybdenum content for the efficient production of 99 Mo. 2. Few activation by-products are generated by target solution irradiation for the prevention of radioactive contamination.3. The solution has good compatibility with the structural materials of the capsule and the pipes for the prevention of corrosion.4. The solution is chemically stable and has no generation of precipitation for the prevention of an obstruction to the solution's flow.
Based on the conditions (1) and ( 2), two aqueous molybdate solutions (aqueous ammonium molybdate and potassium molybdate solutions) were selected as the candidates for the irradiation target among the aqueous solutions of general molybdenum compounds.
The activation by-product of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O is only 92m Nb.The activation by-products of K 2 MoO 4 are 42 K and 92m Nb.The γ-ray energy emitted from 42 K is high.However, by using PZC, it is possible to remove 42 K and 92m Nb from the two aqueous molybdate solutions irradiated with neutrons.
The conditions (3) and ( 4) were confirmed by tests with the two solutions. www.intechopen.com

Unirradiation and γ-ray irradiation tests
Unirradiation and γ-ray irradiation tests were carried out by using the selected two aqueous molybdate solutions (aqueous (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and K 2 MoO 4 solutions), and compatibility between the two solutions and the structural materials of stainless steel and aluminum, the chemical stability, the circulation characteristics, the radiolysis and the γ heating of the two solutions were investigated.In addition, the integrity of PZC was investigated under γ-ray irradiation.As a result, the following were found: 1.The compatibility between the two static aqueous molybdate solutions and stainless steel is very well under unirradiation and γ-ray irradiation.2. The two solutions are chemically stable and have smooth circulation under unirradiation and γ-ray irradiation.3. The ratios of hydrogen in the gases generated by the radiolysis of the two solutions are higher than that of pure water.4. The effect of γ heating on the two solutions is the same level as that on pure water.5.The integrity of PZC is maintained under γ-ray irradiation.
However, the pH of the aqueous (NH 4 ) 6 Mo 7 O 24 ·4H 2 O solution needs to be adjusted from weak alkaline to weak acid for the prevention of precipitation.This is a disadvantage as one of the candidates for the irradiation target.
At present, the aqueous K 2 MoO 4 solution, which has no pH adjustment and has higher molybdenum content than that of the aqueous (NH 4 ) 6 Mo 7 O 24 ·4H 2 O solution, is investigated as the first candidate of the irradiation target.

Estimates of 99 Mo production rates by solution irradiation method
The estimates of 99 Mo production rates by the solution irradiation method are shown in cases using aqueous (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and K 2 MoO 4 solutions as irradiation targets, assuming the 99 Mo production in JMTR.

Conditions for estimates of 99 Mo production rates
The JMTR core arrangement is shown in Fig. 2. The capsule of the 99 Mo production system with the solution irradiation method is installed into the irradiation hole, M-9 with maximum and average thermal neutron fluxes of 3.5 × 10 18 n/(m 2 • s) and 2.6 × 10 18 n/(m 2 • s) (Department of JMTR Project, 1994).The capsule consists of inner and outer tubes, and an aqueous molybdate solution is irradiated with neutrons in the inner tube to prevent the solution from leaking into the reactor coolant.Table 2 shows the conditions of the capsule and the two irradiation targets of the aqueous (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and K 2 MoO 4 solutions. 99Mo production rates are estimated based on the following conditions in addition to the conditions of Table 2:

•
The generation and reduction of 99 Mo by the neutron capture reaction of 98 Mo (n, γ) 99 Mo and 99 Mo (n, γ) 100 Mo and the radioactive decay of 99 Mo are evaluated.• 99 Mo doesn't exist in the initial stage of the calculation.

•
The decay of neutron flux due to the inner and outer tubes of the capsule is considered.

•
The circulation of the two irradiation targets is not considered.The solutions of the equations ( 1) and ( 2) are as follows: ( ) where N 98 and N 99 are the atom number densities of 98 Mo and 99 Mo (n/cm 3 ), t is time (s), φ is neutron flux (n/(cm 2 •s)), σ 98 and σ 99 are the capture cross section of 98 Mo and 99 Mo (cm 2 ), and λ is the decay constant of 99 Mo (1/s).When the neutron flux, the capture cross section, the decay constant and the time are given for the equations ( 3) and ( 4), 99 Mo generation rate per unit volume can be calculated depending on the time.
The specific activity of the generated 99 Mo is calculated from the following equation: where W is the mass of 99 Mo (g), A is the atomic mass number of 99 Mo, and T is the half-life of 99 Mo (s).

Estimated results of 99 Mo production rates
The relationship between the irradiation time and the calculated specific 99 Mo generation (generated 99 Mo activity per 1 g of molybdenum) is shown in Fig. 3.When the irradiation time is 6 days (144 h), the specific 99 Mo generation is 0.286 TBq/g-Mo as shown in Fig. 3. Here, the dilution effect by the unirradiated aqueous molybdate solution and the decay time of 99 Mo from the generation to the shipment are considered.It is assumed that the volume of the aqueous molybdate solution in the capsule and the pipes in the irradiation system and the supply and circulation system of the 99 Mo production system is about 2,500 cm 3 and that the time from the post-irradiation to the shipment is one day.After one day, 99 Mo decays to 0.78 times.Time from the irradiation to the shipment is one week.The 99 Mo production rates at the shipment are estimated as follows: ( 99 Mo shipping activity in the case using the aqueous (NH 4 ) 6 Mo 7 O 24 ･4H 2 O solution) = 2,881.9Ci × 1,663/2,500 × 0.78 = 1,495.3Ci/w ( 99 Mo shipping activity in the case using the aqueous K 2 MoO 4 solution) = 5,431.7Ci × 1,663/2,500 × 0.78 = 2818.3Ci/w The 99 Mo production rate in the case using the aqueous K 2 MoO 4 solution is about twice compared with that in the case using the aqueous (NH 4 ) 6 Mo 7 O 24 ･4H 2 O solution.It is a distinct advantage of the aqueous K 2 MoO 4 solution.However, in order to aim to provide 100% of the 99 Mo (5,000 Ci/w) imported into Japan and to increase the production rate, some ideas such as the concentration of 98 Mo are needed.

Compatibility test between flowing aqueous molybdate solution and structural material
In the 99 Mo production system with the solution irradiation method, a flowing target solution with a high concentration is in contact with the structural material of the capsule and the pipes, and then it is important to investigate compatibility between the flowing target solution and the structural material.In the previous tests (Inaba et al., 2009), the circulating solution test was carried out under γ-ray irradiation.However, the SUS304 specimen used in the test was only immersed in the bottom of an irradiation container with a volume of 2,000 cm 3 , and the specimen had no influence of the circulating solution flow, and then the compatibility was not cleared.Therefore, the compatibility test between the flowing target solution and the structural material was carried out, and the corrosivity of the flowing target solution for the structural material as well as the chemical stability of the solution was investigated.
An aqueous K 2 MoO 4 solution, which was the first candidate of the irradiation target, was used in the test.The purity of K 2 MoO 4 used in the test was over 98%.

Test apparatus
Fig. 4 shows the schematic diagram of a test apparatus, which was used in order to investigate compatibility between a flowing aqueous K 2 MoO 4 solution and a structural material and the chemical stability of the solution.The test apparatus consists of a immersion container for immersing specimens under flow, a glass storage tank with a volume of about 700 cm 3 , a thermocouple inside the storage tank for solution temperature measurement, a feed pump to circulate the solution, a flowmeter, Teflon tubes with an inner diameter of 7.5 mm to connect each component, two syringes, which were used for depressurization, solution supply, air purge and solution sampling, a data logger to collect temperature data and to monitor the temperature and so on.Some components such as the immersion container and the storage tank were installed into a heating chamber to heat the solution.

Fig. 4. Schematic diagram of test apparatus for compatibility test
The immersion container consists of a glass outer tube with an outer diameter of 22 mm and a height of 62.5 mm and a Teflon inner holder with an inner diameter of 13 mm and a height of 60 mm, and two specimens (specimen 1 and 2) were fixed in the center of the container by the holder as shown in Fig. 5 and they were arranged one above the other in the container.The storage tank was located upstream of the immersion container to keep the solution temperature constant and to prevent the solution from pulsating by the feed pump.In addition to the storage tank, a looped long Teflon tube connected between the pump and the storage tank was used to keep the solution temperature in the heating chamber.The total length of the circulation route of the solution was about 6.8 m, and the total quantity of circulating solution was about 300 cm 3 except the volume of the storage tank.

Test method and conditions
The compatibility test was carried out by using the test apparatus with a closed loop shown in Fig. 4.After the specimens were set in the immersion container, aqueous K 2 MoO 4 solution was injected in the closed loop, and the solution was circulated at a constant flow rate.The flow rate was set at about 120 cm 3 /min, considering the flow velocity assumed in an actual 99 Mo production system.The concentration of the solution was adjusted to about 90% of the saturation for the prevention of crystallization, and the temperature of the solution was maintained at about 80˚C for the prevention of boiling.As the specimens immersed in the solution, stainless steel SUS304 was used based on the results of the previous immersion tests (Inaba et al., 2009).SUS304 has been used as the structural material of capsules and pipes in JMTR.The size of the specimens was 10 W ×30 L ×1.5 T mm.Table 3 shows the chemical composition of a SUS304 specimen.The total immersion time of the specimens was 112.7 days, and the immersion time under flow was 84.5 days out of a total of 112.7 days.The total immersion time was longer than the immersion time under flow because the feed pump was temporarily stopped by the planned blackouts and the pump troubles.
During the test, at regular intervals, the specimen 1 was taken from the immersion container, and the specimen's weight was measured after washing by pure water and drying, and its surface state was observed.In addition, the aqueous solution was sampled from the closed loop by using one of the syringes, and the pH and molybdenum concentration of the solution were measured, and the solution state was observed.
After the test, the specimen 1 and 2 were taken from the immersion container, and the specimens' weight was measured, and their surface states were observed.In addition, the aqueous solution was sampled from the closed loop, and the pH and molybdenum concentration of the solution were measured, and the solution state was observed.3. Chemical composition of SUS304 specimen

Results and discussions
The average temperature and flow rate of the aqueous K 2 MoO 4 solution used in the test were 81˚C for a total immersion time of 112.7 days and 123 cm 3 /min for a total immersion time under flow of 85.5 days respectively.

Corrosivity of flowing aqueous K 2 MoO 4 solution for SUS304
The surface states of the two SUS304 specimens before and after the immersion in the flowing aqueous K 2 MoO 4 solution for a total of 84.5 days are shown in the Fig. 6, and the relationships between the immersion time and corrosion rates of the specimens are shown in Fig. 7.The corrosion rates were estimated by the following equation: The equation ( 6) shows the wastage thickness per unit time.In the visual observation and comparison of the two specimens' surfaces before and after the compatibility test, whereas streamlined patterns, partly slight tarnish and the partly slight loss of metallic luster were found on the surfaces, obvious corrosion such as corrosion products was not found.The corrosion rate of the specimen 1 increased temporarily to 0.10 mm/y in the initial stage of the test (an immersion time of 21 days) and decreased finally to 0.02 mm/y.On the other hand, the corrosion rate of the specimen 2 was 0 mm/y at the beginning and end of the test.
There was no change in the state of the specimen 1 surface in the initial stage of the test, and the temporary increase of the specimen 1 corrosion rate might be affected by taking out from the immersion container.For the confirmation of the detailed surface states, the specimen 2 as the representative of the two specimens were observed and analyzed with an inverted materials microscope and a field emission Electron Probe Micro Analyzer (EPMA).Fig. 8 shows the inverted materials microscope photograph of the specimen 2 surface.The black lines and dots in Fig. 8 are preexistent scratches and hollows.Tarnish is recognized on the surface.Fig. 9 shows the Scanning Electron Microscope (SEM) photograph of the specimen 2 cross-section surface taken with the EPMA, and Fig. 10 shows the color map of the specimen 2 cross-section surface analyzed with the EPMA.The cross-section surface was prepared by cutting the center of the specimen 2, mounting in a resin and polishing.A thin coating layer, which is thought to be the cause of the tarnish, is found on the surface as shown in Fig. 9.To see Fig. 10, K and Mo, which are the main components of K 2 MoO 4 , are not detected and a relativelyhigh level of Si is detected on the surface.After the test, the corrosion of the glass outer tube in the immersion container was found, and then it is considered that the main component of : 81ºC (average) Conc.
: 57 wt% (initial stage) Flow rate : 123 cm 3 /min (average) • Specimen 1 ○ Specimen 2 www.intechopen.comthe coating layer is Si eluted from the tube.This Si coating layer might inhibit the corrosion of the specimens.In any case, the progress of the corrosion was not observed in the SUS304 specimens, and SUS304 has good compatibility with a flowing aqueous K 2 MoO 4 solution.During the test term, the aqueous K 2 MoO 4 solution was chemically stable, and the precipitation or the deposit was not generated in the solution.Then the molybdenum concentration of the solution was almost constant before and after the test, and the concentrations before and after the test were 396.2 mg/mℓ and 384.0 mg/mℓ respectively.The concentrations were measured with an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES).The pH of the solution was also almost constant at pH9.5-9.7.

Fig. 1 .
Fig. 1.Schematic diagram of 99 Mo production system with solution irradiation method

Fig. 3 .
Fig. 3. Relationship between irradiation time and specific 99 Mo generation

Fig. 6 .
Fig. 6.Surface states of SUS304 specimens before and after compatibility test

Fig. 7 .Fig. 8 .
Fig. 7. Relationships between immersion time and corrosion rates of SUS304 specimens immersed in flowing aqueous K 2 MoO 4 solution for 84.5 days

Fig. 9 .
Fig. 9. SEM photograph of specimen 2 cross-section surface immersed in flowing aqueous K 2 MoO 4 solution for 84.5 days

Table 1 .
Comparison between three 99 Mo production methods www.intechopen.com