2.1 Core description

The SD-TMSR is a 2250-MWt reactor with a thermal neutron spectrum. Figure 1 shows a quarter vertical geometry model of the SD-TMSR including the fuel salt, graphite moderator and reflectors, B₄C neutron protection layer, and Hastelloy N alloy vessel. The main parameters of the SD-TMSR are listed in Table 1. The total fuel salt volume is 52.9 m³, which is distributed in the inner and outer zones in the core, top plenum, bottom plenum, and heat exchanger. Fuel salt with a composition of 70 LiF-17.5 BeF₂-12.5 HNF₄ (mol%) is adopted in the SD-TMSR, where the ⁷Li enrichment and the fuel salt density are 99.995% and 3.33 g/cm³, respectively. The B₄C layer and Hastelloy N alloy vessel have an identical thickness of 10 cm.

Fig. 1.Full-screen View

Cross section of SD-TMSR.

2.2 Calculation tool

The MSR reprocessing sequence (MSR-RS) ^[13-15] is adopted in this work to simulate the iodine transmutation, which is coupled with the criticality analysis module (CSAS6), problem-dependent cross-section processing module (TRITON), and depletion and decay calculation module (ORIGEN-S) in the SCALE6.1 program. A flowchart of the MSR-RS is displayed in Fig. 2. First, the core geometry and molten salt compositions are initialized. Then, a criticality calculation is performed by CSAS6 based on the entire core, and a 238-group ENDF/B-VII cross-section database is used. A one-group cross-section library is generated by the TRITON calculation, which performs problem-dependent cross-section processing followed by a multigroup neutron transport calculation. The burnup calculation is performed by ORIGEN-S with online reprocessing and refueling. During each burnup time step, the molten salt compositions are modified following a reprocessing scheme set by the user and are followed by another CSAS6 calculation step to obtain a new neutron flux and a new k_eff determined by both depletion and refueling. Then, a new TRITON input file is produced for the next-step burnup calculation. The cycle calculation is performed iteratively until the cycle time reaches the value set by the user. The MSR-RS was verified in our previous work ^[12-16].

Fig. 2.Full-screen View

Flowchart of MSR-RS.

In this MSR-RS, the gaseous and noble metallic FPs in the fuel salts are removed online through a helium bubbling system with a constant separation time of 30 s and separation efficiency of 100%. The other soluble FPs are continuously removed, and Pa is extracted online by chemical reprocessing at a reprocessing rate of 5 m³/d and separation efficiency of 100% ^[12]. In addition, ²³²Th and ²³³U are injected online into the fuel salt to maintain the reactor criticality and the total actinide inventory constant for the stability of the molten salt.

3.1 ¹²⁹I transmutation scenarios

It is essential to select a proper chemical compound of iodine to minimize its effects on the fuel salt component of SD-TMSR. Several iodide forms such as ThI₄, UI₄, BeI₂, and LiI have been investigated during the past several decades. Compared with ThI₄, UI₄, and BeI₂, LiI is more appropriate for ¹²⁹I transmutation because it has excellent stability in air and can be dissolved into the molten salt at a large amount ^[17]. Hence, LiI is chosen as sample form and is loaded into the fuel salt to substitute for some LiF in the FLiBe carrier salt. For instance, the fuel salt composition becomes 69 LiF-1 LiI-17.5 BeF₂-12.5 HNF₄ (mol%) if 1.0 mol% of LiI is loaded into the fuel salt.

An analysis of the ¹²⁹I transmutation scenario (named scenario 1) is performed by online feeding ¹²⁹I into the fuel salt to keep the ¹²⁹I amount in the core constant during the SD-TMSR operation. Nevertheless, the online feeding of ¹²⁹I is relatively complex for the SD-TMSR operation because it requires an accurate monitoring of the ¹²⁹I inventory both in the core and online feed during the entire operation. Considering the disadvantages in scenario 1, we propose an alternative ¹²⁹I transmutation (scenario 2) in which ¹²⁹I is initially loaded into fuel salt before the startup of the reactor, and no other ¹²⁹I is fed into the fuel salt during the reactor operation. A large loading of ¹²⁹I may improve the transmutation capacity of ¹²⁹I. However, the physicochemical properties of the fuel salt may be changed when a large amount of LiI is loaded. In addition, more ²³³U is loaded into the fuel salt to maintain the reactor criticality, which is disadvantageous from the standpoint of Th-U fuel breeding. Hence, a proportion of 1% LiI is adopted as the upper limit to minimize the change in the fuel salt component.

3.2 Th-²³³U isobreeding performance

Owing to the larger thermal neutron absorption cross section of ¹²⁹I compared with ¹⁹F, more thermal neutrons in the core are absorbed when ¹⁹F is replaced by ¹²⁹I in the fuel salt, which hardens the neutron spectrum of the SD-TMSR (see Fig. 3). Therefore, more ²³³U loading is required to maintain the reactor criticality when a large amount of iodine is loaded into the fuel salt. Fig. 4 shows the initial ¹²⁹I inventory and the required initial ²³³U loading as a function of the LiI molar proportion under the critical condition (k_eff ≈ 1). When the LiI proportion increases from 0 to 1.0 mol%, the increment of initial ¹²⁹I loading is about 2.7 t, and the corresponding ²³³U mass increases almost linearly by about 0.35 t.

Fig. 3.Full-screen View

(Color online) Comparison of initial neutron spectra for different ¹²⁹I loadings.

Fig. 4.Full-screen View

Initial ²³³U and ¹²⁹I loadings as a function of ¹²⁹I molar proportion under critical condition.

The burnups for the above two ¹²⁹I transmutation scenarios with different LiI loading proportions are simulated by the MSR-RS for up to 60 years. The Th-U breeding performance can be evaluated by the net ²³³U production, which is defined as ^[18,19]

where ²³³U (inject) is the total injected ²³³U amount into the core, which includes the initially loaded and online-fed ²³³U masses; ²³³Pa (extract) is the extracted mass of ²³³Pa from the core; and ²³³U (residue) is the residual mass of ²³³U in the core. A positive value of the net ²³³U production means that more ²³³U can be bred than consumed in the core, while a negative value indicates that the produced ²³³U in the core is insufficient to compensate for the consumed ²³³U, and additional ²³³U from other reactors should be fed into the core to maintain the reactor criticality.

The evolutions of the net ²³³U production and inventories of key nuclides for both scenarios with LiI = 1.0% are presented in Fig. 5. For scenario 1, the net ²³³U production monotonically decreases during the 60 years of operation, while the ²³³U inventory in the core increases gradually with the operation time. In particular, some actinides generated from ²³³U, including nonfissile isotopes (e.g., ²³⁴U) and fissile isotopes (e.g., ²³⁵U), also keep accumulating in the reactor. The generation of new fissile isotopes is insufficient to compensate for the negative reactivity inserted by the accumulation of new nonfissile isotopes during the operation. Hence, additional ²³³U has to be injected into the core to maintain the reactor criticality, which makes the ²³³U amount in the core increase gradually from 1.63 t at startup to 1.81 t at the end of life, as shown in Fig. 5(b). During the first two years, the slight increase of ²³³U amount in the core is primarily caused by the accumulated FPs since no considerably heavier actinides are produced in the core for such a short operation time. To maintain the critical operation of the SD-TMSR for 60 years, the total fed mass of ²³³U is about 42.69 t. Meanwhile, the total extracted mass of ²³³Pa is about 37.89 t, which is 4.80 t smaller than the total fed mass of ²³³U. Hence, according to Eq. (1), the net ²³³U production for scenario 1 is about -4.62 t at the end of life (LiI = 1.0%).

Fig. 5.Full-screen View

(Color online) (a) Net ²³³U production and (b) inventory of nuclides for both scenarios (LiI = 1.0%) during 60 years of operation.

By contrast, the ¹²⁹I inventory in the core for scenario 2 decreases from 2.73 t at startup to 0.59 t at the end of life since no ¹²⁹I is fed online into the core during operation. The decrease of ¹²⁹I inventory in the core induces a positive reactivity that exceeds the total negative reactivity caused by the neutron absorption of accumulated FPs and heavier actinides during the entire operation. Therefore, the ²³³U inventory in the core decreases from 1.63 t at startup to 1.53 t at the end of life. However, the initially loaded ²³³U is insufficient to maintain the critical operation of the core in the depletion process owing to its fission depletion and the accumulation of FPs and heavier actinides. Therefore, ²³³U should be continuously fed into the fuel salt to maintain the reactor criticality during the entire operation. For the 60-year operation, k_eff of the core is kept at ~1.0 by the online feeding of ²³³U and Th. In addition, with a lower total ¹²⁹I inventory in the core, the transmutation rate of ¹²⁹I for scenario 2 in the depletion process is much smaller than that for scenario 1, which indicates that scenario 2 requires less ²³³U to compensate for the negative reactivity by ¹²⁹I than scenario 1. In other words, ²³³U is fueled online into the core to primarily compensate for its fission consumption for scenario 2, which leads to a much smaller amount of externally fed ²³³U fuel than scenario 1. The net ²³³U production for scenario 2 is -0.43 t at the end of life, which is 4.19 t greater than that for scenario 1, as shown in Fig. 5(a).

One can also find from Fig. 5 that the net ²³³U production for both scenarios has significantly different evolution trends and always changes with the operation time. Considering the negative net ²³³U production for both scenarios (LiI = 1.0%) at the end of life, the dependence of net ²³³U production on varying LiI loading is presented in Fig. 6. The loss rate of the net ²³³U production for scenario 2 is much smaller than that for scenario 1 because the total ¹²⁹I loading amount of the core for the former is much smaller than that for the latter. For instance, the total ¹²⁹I loading amount of the core for scenario 2 (LiI = 1.0%) is 2.73 t, which is 3.67 t smaller than that for scenario 1 (LiI = 1.0%) because no ¹²⁹I is fed online into the core for scenario 2 (LiI = 1.0%). It is found that when the initial molar proportions of LiI for both scenarios are maintained within ~0.40% and 0.87%, respectively, the net ²³³U production is equal to 0 t, indicating that a ²³³U isobreeding mode for both scenarios can be achieved for a 60-year operation in the SD-TMSR, as shown in Fig. 6.

Fig. 6Full-screen View

(Color online) Net ²³³U production at end of life as a function of initial loading of LiI for scenarios 1 and 2.

3.3. Transmutation capability of ¹²⁹I

The ¹²⁹I transmutation capability of the SD-TMSR can be evaluated by two important parameters: transmuted mass and fraction. The transmuted fraction is defined as

where M₀ (t) is the injected mass of ¹²⁹I into the core, which includes the initially loaded and online-fed ¹²⁹I masses at operating time t. ΔM (t) is the transmuted mass of ¹²⁹I, which is equal to the difference between the injected mass and the residual mass of ¹²⁹I in the core at operating time t.

Fig. 7 presents the evolutions of the transmuted mass, injected mass, and transmuted fraction of ¹²⁹I for both scenarios (LiI = 1.0%). It is found that the transmuted mass of ¹²⁹I for scenario 2 increases with the operation time, but the transmutation rate decreases with the operation time compared to scenario 1 because of the decrease of ¹²⁹I inventory in the core [see Fig. 5(b)]. Therefore, the transmuted mass of ¹²⁹I for scenario 2 is about 2.14 t at the end of life, which is 1.53 t smaller than that for scenario 1. However, the transmuted fraction of ¹²⁹I for the former is 78.34% at the end of life, which is 20.83% greater than that for the latter.

Fig. 7.Full-screen View

(Color online) Transmuted mass, injected mass, and transmuted fraction of ¹²⁹I for scenarios 1 and 2 (LiI = 1.0%) during 60 years of operation.

To investigate the relationship between the transmutation capability of ¹²⁹I and net ²³³U production, the transmuted mass and fraction of ¹²⁹I at the end of life as a function of the net ²³³U production for both scenarios are presented in Fig. 8. The transmuted mass of ¹²⁹I for scenario 2 is slightly larger than that for scenario 1 under an identical net ²³³U production condition, indicating that scenario 2 is superior to scenario 1. When the SD-TMSR achieves a ²³³U isobreeding mode, the transmuted mass of ¹²⁹I for scenario 2 is about 1.88 t, which is 0.16 t greater than that for scenario 1. In addition, the transmuted fraction for the former is 78.96%, which is also significantly larger than that for the latter (65.50%). The transmuted mass and fraction of ¹²⁹I and the net ²³³U production for both scenarios with different LiI loadings are listed in Table 2 and Table 3, respectively.

Fig. 8Full-screen View

(Color online) Transmuted mass and fraction of ¹²⁹I as a function of net ²³³U production for scenarios 1 and 2 at end of life.

In order to investigate the difference in ¹²⁹I transmutation performance under the ²³³U isobreeding mode between the above two scenarios, the evolutions of the transmuted mass and inventory of ¹²⁹I are displayed in Fig. 9. For scenario 1, the transmuted mass of ¹²⁹I increases linearly with the operation time, which is about 1.72 t at the end of life owing to the constant inventory of ¹²⁹I in the core. This indicates that the transmutation rate of ¹²⁹I can stay almost constant at about 28.67 kg/y. By contrast, the transmuted mass of ¹²⁹I for scenario 2 is about 1.88 t at the end of life, while the transmutation rate declines with the operation time owing to the decrease of ¹²⁹I inventory in the core. Hence, the transmuted mass of ¹²⁹I for scenario 2 is larger than that for scenario 1 during the 60 years of operation. However, most of the ¹²⁹I for scenario 2 is transmuted during the first 30 years and is about 1.28 t, which is much greater than that during the remaining 30 years at 0.6 t. Considering its higher transmutation performance and simpler operation of the reactor, scenario 2 with an initial molar proportion of LiI = 0.87% is recommended for the ¹²⁹I transmutation in the SD-TMSR.

Fig. 9Full-screen View

(Color online) Transmuted and residual mass of ¹²⁹I for scenario 1 (LiI = 0.40%) and scenario 2 (LiI = 0.87%) during 60 years of operation.

The transmutation capability of ¹²⁹I in the SD-TMSR is also compared with other reactors in Table 4. The supportive factor is defined as the ratio of the transmuted mass of ¹²⁹I in the different reactors to the yield of ¹²⁹I in a traditional 1000-MWe PWR under normalized thermal output and operation time ^[20]. The transmutation rate, supportive factor, and transmuted fraction of the SD-TMSR are 13.92 g/(MWt·y), 8.19, and 78.96%, respectively, which are significantly larger than those of the TMSR. The SD-TMSR also exhibits an excellent ¹²⁹I transmutation performance compared with the PWR and the fast reactor. Although the transmutation rate and supportive factor of the SD-TMSR are much smaller than those of the ADS, the transmuted fraction of the former is significantly larger than that of the latter since the solid ¹²⁹I target in a solid-fuel reactor cannot undergo long-term operation owing to its mechanism and irradiation performance.