**4.1 Conditions for estimates of 99Mo production rates**

The JMTR core arrangement is shown in Fig. 2. The capsule of the 99Mo 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× 1018 n/(m2· s) and 2.6× 1018 n/(m2· 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 (NH4)6Mo7O24·4H2O and K2MoO4 solutions.

99Mo production rates are estimated based on the following conditions in addition to the conditions of Table 2:


Fig. 2. JMTR core arrangement

Unirradiation and γ-ray irradiation tests were carried out by using the selected two aqueous molybdate solutions (aqueous (NH4)6Mo7O24·4H2O and K2MoO4 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

1. The compatibility between the two static aqueous molybdate solutions and stainless

2. The two solutions are chemically stable and have smooth circulation under unirradiation

3. The ratios of hydrogen in the gases generated by the radiolysis of the two solutions are

However, the pH of the aqueous (NH4)6Mo7O24·4H2O solution needs to be adjusted from weak alkaline to weak acid for the prevention of precipitation. This is a disadvantage as one

At present, the aqueous K2MoO4 solution, which has no pH adjustment and has higher molybdenum content than that of the aqueous (NH4)6Mo7O24·4H2O solution, is investigated

The estimates of 99Mo production rates by the solution irradiation method are shown in cases using aqueous (NH4)6Mo7O24·4H2O and K2MoO4 solutions as irradiation targets,

The JMTR core arrangement is shown in Fig. 2. The capsule of the 99Mo 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× 1018 n/(m2· s) and 2.6× 1018 n/(m2· 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 (NH4)6Mo7O24·4H2O and K2MoO4 solutions. 99Mo production rates are estimated based on the following conditions in addition to the

• The generation and reduction of 99Mo by the neutron capture reaction of 98Mo (n, γ)

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

99Mo and 99Mo (n, γ) 100Mo and the radioactive decay of 99Mo are evaluated.

**4. Estimates of 99Mo production rates by solution irradiation method** 

4. The effect of γ heating on the two solutions is the same level as that on pure water.

investigated under γ-ray irradiation. As a result, the following were found:

steel is very well under unirradiation and γ-ray irradiation.

5. The integrity of PZC is maintained under γ-ray irradiation.

**3.2.2 Unirradiation and γ-ray irradiation tests** 

and γ-ray irradiation.

higher than that of pure water.

of the candidates for the irradiation target.

as the first candidate of the irradiation target.

assuming the 99Mo production in JMTR.

conditions of Table 2:

**4.1 Conditions for estimates of 99Mo production rates** 

• 99Mo doesn't exist in the initial stage of the calculation.

• The circulation of the two irradiation targets is not considered.


Table 2. Conditions of capsule and two irradiation targets of aqueous (NH4)6Mo7O24· 4H2O and K2MoO4 solutions

### **4.2 Basic equations for estimates of 99Mo production rates**

The disintegration rates of 98Mo (isotopic ratio: 24.138%) and 99Mo are shown as following equations:

$$\frac{dN\_{98}}{dt} = -\phi \sigma\_{98} N\_{98} \tag{1}$$

Development of 99Mo Production Technology with Solution Irradiation Method 331

Using the specific 99Mo generation of 0.286 TBq/g-Mo, 99Mo production rates are estimated.

Here, the dilution effect by the unirradiated aqueous molybdate solution and the decay time of 99Mo 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 99Mo production system is about 2,500 cm3 and that the time from the post-irradiation to the shipment is one day. After one day, 99Mo decays to 0.78 times. Time from the irradiation to the shipment is one week. The 99Mo production rates

The 99Mo production rate in the case using the aqueous K2MoO4 solution is about twice compared with that in the case using the aqueous (NH4)6Mo7O24・4H2O solution. It is a distinct advantage of the aqueous K2MoO4 solution. However, in order to aim to provide 100% of the 99Mo (5,000 Ci/w) imported into Japan and to increase the production rate,

In the 99Mo 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 cm3, 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

An aqueous K2MoO4 solution, which was the first candidate of the irradiation target, was

used in the test. The purity of K2MoO4 used in the test was over 98%.

(99Mo shipping activity in the case using the aqueous (NH4)6Mo7O24・4H2O solution)

**5. Compatibility test between flowing aqueous molybdate solution and** 

(99Mo shipping activity in the case using the aqueous K2MoO4 solution)

In the case using the aqueous (NH4)6Mo7O24・4H2O solution as the irradiation target,

(99Mo production in the case using the aqueous (NH4)6Mo7O24・4H2O solution)

In the case using the aqueous K2MoO4 solution as the irradiation target,

(99Mo production in the case using the aqueous K2MoO4 solution)

= 372.8 g × 0.286 TBq/g-Mo = 106.6 TBq = 2,881.9 Ci

= 702.7 g × 0.286 TBq/g-Mo = 201.0 TBq = 5,431.7 Ci

at the shipment are estimated as follows:

= 2,881.9 Ci × 1,663/2,500 × 0.78 = 1,495.3 Ci/w

= 5,431.7 Ci × 1,663/2,500 × 0.78 = 2818.3 Ci/w

**structural material** 

solution was investigated.

some ideas such as the concentration of 98Mo are needed.

$$\frac{dN\_{99}}{dt} = -\left(\mathcal{A} + \phi \sigma\_{99}\right) N\_{99} + \phi \sigma\_{98} N\_{98} \tag{2}$$

The solutions of the equations (1) and (2) are as follows:

$$N\_{\rm ss}(t) = N\_{\rm ss}(0) \exp\left(-\phi \sigma\_{\rm ss} t\right) \tag{3}$$

$$N\_{sp}(t) = \frac{\phi \sigma\_{gs}}{\lambda + \phi \left(\sigma\_{gs} - \sigma\_{gs}\right)} N\_{gs}(0) \left[ \exp\left(-\phi \sigma\_{gs} t\right) - \exp\left[-\left(\lambda + \phi \sigma\_{gs}\right)t\right] \right] \tag{4}$$

where *N*98 and *N*99 are the atom number densities of 98Mo and 99Mo (n/cm3), *t* is time (s), φ is neutron flux (n/(cm2s)), *σ*98 and *σ*99 are the capture cross section of 98Mo and 99Mo (cm2), and *λ* is the decay constant of 99Mo (1/s). When the neutron flux, the capture cross section, the decay constant and the time are given for the equations (3) and (4), 99Mo generation rate per unit volume can be calculated depending on the time.

The specific activity of the generated 99Mo is calculated from the following equation:

$$-\frac{dN\_{gg}}{dt} = \frac{\mathcal{W} \times \mathbf{4.17} \times 10^{-3}}{AT} \tag{5}$$

where *W* is the mass of 99Mo (g), *A* is the atomic mass number of 99Mo, and *T* is the half-life of 99Mo (s).

#### **4.3 Estimated results of 99Mo production rates**

The relationship between the irradiation time and the calculated specific 99Mo generation (generated 99Mo activity per 1 g of molybdenum) is shown in Fig. 3. When the irradiation time is 6 days (144 h), the specific 99Mo generation is 0.286 TBq/g-Mo as shown in Fig. 3.

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

Using the specific 99Mo generation of 0.286 TBq/g-Mo, 99Mo production rates are estimated. In the case using the aqueous (NH4)6Mo7O24・4H2O solution as the irradiation target,

(99Mo production in the case using the aqueous (NH4)6Mo7O24・4H2O solution)

= 372.8 g × 0.286 TBq/g-Mo = 106.6 TBq = 2,881.9 Ci

330 Nuclear Reactors

*dN N N*

*NtN* ( )*t* <sup>98</sup> <sup>98</sup> <sup>98</sup> = exp)0()( −

( ) *tN <sup>N</sup>* [ ] () ( ) *<sup>t</sup>* { }*<sup>t</sup>* <sup>98</sup> <sup>98</sup> <sup>99</sup>

neutron flux (n/(cm2s)), *σ*98 and *σ*99 are the capture cross section of 98Mo and 99Mo (cm2), and *λ* is the decay constant of 99Mo (1/s). When the neutron flux, the capture cross section, the decay constant and the time are given for the equations (3) and (4), 99Mo generation rate per

where *N*98 and *N*99 are the atom number densities of 98Mo and 99Mo (n/cm3), *t* is time (s),

The specific activity of the generated 99Mo is calculated from the following equation:

*dt*

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

*W*

*Nd* <sup>23</sup>

where *W* is the mass of 99Mo (g), *A* is the atomic mass number of 99Mo, and *T* is the half-life

The relationship between the irradiation time and the calculated specific 99Mo generation (generated 99Mo activity per 1 g of molybdenum) is shown in Fig. 3. When the irradiation time is 6 days (144 h), the specific 99Mo generation is 0.286 TBq/g-Mo as shown in Fig. 3.

+−−−

*AT*

0 50 100 150 200 250

Irradiation time (h)

=− + + λ φσ

99 99 98 98

 φσ

φσ

exp

−+ <sup>=</sup> (4)

λ φσ

<sup>99</sup> ×× 1017.4 =− (5)

φσ

(2)

(3)

φis

( ) <sup>99</sup>

*dt*

99 98 98 <sup>99</sup> )( exp)0(

σσφλ

unit volume can be calculated depending on the time.

**4.3 Estimated results of 99Mo production rates** 

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

Specific 99Mo generation (TBq/g-Mo)

of 99Mo (s).

φσ

The solutions of the equations (1) and (2) are as follows:

In the case using the aqueous K2MoO4 solution as the irradiation target,

(99Mo production in the case using the aqueous K2MoO4 solution)

= 702.7 g × 0.286 TBq/g-Mo = 201.0 TBq = 5,431.7 Ci

Here, the dilution effect by the unirradiated aqueous molybdate solution and the decay time of 99Mo 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 99Mo production system is about 2,500 cm3 and that the time from the post-irradiation to the shipment is one day. After one day, 99Mo decays to 0.78 times. Time from the irradiation to the shipment is one week. The 99Mo production rates at the shipment are estimated as follows:

(99Mo shipping activity in the case using the aqueous (NH4)6Mo7O24・4H2O solution)

= 2,881.9 Ci × 1,663/2,500 × 0.78 = 1,495.3 Ci/w

(99Mo shipping activity in the case using the aqueous K2MoO4 solution)

= 5,431.7 Ci × 1,663/2,500 × 0.78 = 2818.3 Ci/w

The 99Mo production rate in the case using the aqueous K2MoO4 solution is about twice compared with that in the case using the aqueous (NH4)6Mo7O24・4H2O solution. It is a distinct advantage of the aqueous K2MoO4 solution. However, in order to aim to provide 100% of the 99Mo (5,000 Ci/w) imported into Japan and to increase the production rate, some ideas such as the concentration of 98Mo are needed.
