1 Introduction

In the past fifty years, the size of nuclear reactors has grown from 60 MWe to more than 1600 MWe in order to make full use of economy of scale^[1]. However, because large-size nuclear reactors usually require high capital investment and rely heavily on the infrastructure of reactor sites, this has motivated to develop small modular nuclear reactors (SMRs) based on different reactor technologies^[2,3,4], especially for developing countries and remote areas off main grids. The SMRs can be fabricated in modularity and transported to sites by rail, barge, truck, etc. After a long operation (e.g., 5–10 years), the SMRs can be brought back to factories for refueling or directly replaced by new ones. Moreover, the 5–10 SMRs in a site may become a nuclear power plant (NPP) with a comparable power level to large NPPs.

Over the last 30 years, the inherent safety of small modular HTRs (high temperature gas-cooled reactors) has been validated directly by experiments ^[5,6,7]. Our previous paper^[8] presents the neutronic designs of a 20 MWth long-life block-type HTR, called as U-Battery, which can be commercialized in the near future. For the 20 MWth U-Battery, the reference reactor core configuration adopts 148(=37×4) fuel blocks developed for the GT-MHR project^[9], a 29-cm-thick graphite side reflector, and 50-cm-thick top and bottom graphite reflectors. Although the outer diameter of the reactor pressure vessel (RPV) of the U-Battery is limited to 3.7 m based on the maximum size of road transport, the weight of the whole reactor core of the 20 MWth U-Battery is a quite large, decreasing the flexibility for road transport of the U-Battery. In order to strengthen the transportability of the U-Battery, two designs for a 5 MWth U-Battery are proposed in this paper. The first design described in Section 2 uses a RPV with 3.7-m diameter in order to make full use of neutron economy, while the second described in Section 3 uses a RPV with 1.8-m diameter to minimize the reactor core of the U-Battery.

2 A 5 MWth U-Battery with 3.7 m RPV in diameter

This section describes a U-Battery reactor core with thermal power of 5 MWth. Because the thermal- hydraulic design of the 20 MWth U-Battery leads to the volume-averaged temperatures of the 727^oC reactor core, and 227^oC top, 727^oC bottom and 527^oC side reflectors^[10,11], the nuclear design of the 5 MWth reactor core adopted these temperatures as the reference temperature of the mixtures. All calculations are implemented by TRITON module in SCALE 6^[12].

At the 3.7-m outer diameter of the RPV, Fig.1 shows the eight reactor core configurations of the 5 MWth U-Battery. Layout 37×1 has a very small height of the reactor core and thus RPV, which has in total 37 fuel blocks in the reactor core. Layouts 19×2 and 18×2 have 38 and 36 fuel blocks, respectively. Layout 6×2 is modeled in order to compare with the smaller reactor core designs with very thin side reflectors in the next section. Analyzed in the previous paper^[8], the scatter and annular core configurations of 20 MWth U-Battery have better neutronic performance because of a larger mass ratio of graphite to uranium and thus better neutron moderation. Two scatter and one annular core configurations with 24(12×2) fuel blocks are shown in Fig.1 (IV), (V) and (VI), respectively.

The neutronic calculations for the eight reactor core configurations are shown in Table 1.

Fig.1Full-screen View

Reactor core configurations of 5 MWth U-Battery with 3.7-m RPV in diameter (The top reflectors have been removed in the reactors for illustrating the reactor core).

The second column shows the names of reactor core configurations. The third and fourth columns are the effective multiplication factors at beginning of life (BOL) and end of life (EOL), respectively. If the effective multiplication factor of a certain configuration is less than unity, the data in the bracket is the possible maximum fuel lifetime for the specific configuration; otherwise this data is given after operation time of 10 years. The fifth column shows the uranium total mass in the reactor core, and the sixth column shows the fuel cost. All eight reactor core configurations use 20% enriched ²³⁵U.

Case 2 consists of 38 fuel blocks and reaches a fuel lifetime of 10 effective full power years (EFPYs), with resulting k_eff at EOL of 1.028. Case 3 consists of 36 fuel blocks, and the k_eff is 1.045 at EOL. Although the number of fuel blocks of Layout 18×2 is less than that of Layout 19×2, the k_eff of the former is larger than the latter because of better neutron moderation. Although the reactor core of Layout 37×1 consists of 37 fuel blocks, which is larger than the number of fuel blocks of Layout 18×2, it is not able to reach a 10-EFPY fuel lifetime, and the maximum possible fuel lifetime is 7.0 EFPYs, because of the 29-cm-thin side reflector and a large neutron leakage. If the number of fuel blocks reduces to 24(12×2), Cases 5 and 6 just reach a 10-EFPY fuel lifetime, while Case 4 fails. This means that the reactor core configuration is of importance, even though the number of fuel blocks is identical. Lumping fuel blocks increase the resonance escape probability. For Layout 12×2B, the 24 fuel columns are divided into three groups. Each group consists of 4 fuel columns (each containing two fuel blocks), which is surrounded by graphite. For this configuration, neutrons escaping from a particular group of fuel blocks are easily moderated in the surrounding graphite blocks, before they have an interaction with the uranium in another group of fuel blocks. Layouts 9×2 and 6×2 are not able to achieve a 10-EFPY fuel lifetime because the number of fuel blocks is too small. The needed uranium mass of Layout 12× 2B decreases by 36.8%, compared with the uranium mass of Layout 19×2.

Since the effective multiplication factors of both Layouts 19×2 and 18×2 are larger than unity at 10 EFPYs, there are three ways to improve the economic performance of the two configurations. The first is to extend the fuel lifetime of the U-Battery until the k_eff of the reactor is unity. The second is to lower further the fuel costs by decreasing the ²³⁵U enrichment, while keeping the fuel kernel size (R_k) and the packing fraction (PF) of the TRISO particles constant. Cases 1 and 4 in Table 2 are the results of this way. It shows that the enrichment of ²³⁵ U reduces by 2% for Layouts 19×2; and for 18×2, 3.2%, thus their fuel cost reduces by 9% and 15.4%, respectively.

The third way of increasing the economic performance of Layouts 19×2 and 18×2 is to decrease the inventory of uranium in the reactor core by changing the geometric parameters of the TRISO particles, i.e., fuel kernel radius and PF, while keeping the enrichment of the ²³⁵U constant. Cases 2 and 3 as well as Cases 5 and 6 in Table 2 are its results for Layouts 19×2 and 18×2, respectively. Comparison of Case 2 with Case 3 shows that reducing the fuel kernel radius R_k and PF of TRISO particles is equivalent from neutronic point of view, because the fuel costs of both cases are almost identical. From the viewpoint of mechanical stress of fuel kernels, keeping the kernel radius of 0.25 mm is positive, so it is recommended to keep the fuel kernel radius constant and reduce the PF of TRISO particles. If so, the fuel costs of Layouts 19×2 and 18×2 decrease further by 20.6% and 21.1%, respectively. For Layout 12×2B, reducing the fuel kernel radius is better than reducing the PF of TRISO particles, but their difference is rather small.

3 A 5 MWth U-Battery with 1.8 m RPV in diameter

In order to further decrease the reactor weight for better transportability of the U-Battery, reducing the diameter of RPV is effective because this can effectively reduce the reactor core weight of RPV. The inner diameter of RPV is fixed to 1.8 m, because it is the same as the inner diameter of flasks for the transportation of PWR or BWR spent fuel assembles, and the transportation experience of the flasks over the world can be utilized for the transportation of the U-Battery. For the RPV with the 1.8-m inner diameter, the 6 fuel columns are the possible maximum number in the reactor core, and the number of fuel blocks in the axial direction and the material of the side reflector are two key design parameters as shown in Fig.2. If the height of the reactor core is limited to about 4 m, the 4 fuel blocks in the axial direction of the reactor core are the maximum value.

In terms of 1.8-m RPV indiameter, the results of the nuclear design are shown in Tables 3 and 4 for the U-Battery with graphite and BeO side reflectors, respectively. The second column shows the reactor core configuration of the U-Battery. The third and fourth columns are the parameters of the side reflector, i.e., material and maximum thickness. The fifth column shows the maximum fuel lifetime for each reactor core configuration, and the sixth column shows the total reactor mass, including the fuel blocks, central, top, bottom and side reflectors, barrel (assumed 5 cm) and RPV (assumed 10 cm). Because the reactor core is very small and the side reflector is very thin, the barrel and RPV were modeled for all Cases in this section to include their reflection effect.

Fig.2Full-screen View

Reactor core configuration of the U-Battery with 1.8-m RPV in diameter.

4 Conclusion

The reactor core configurations of a transportable 5 MWth HTR with 5–10 effective full power years (EFPYs), called U-Battery, have been investigated by the TRITON module in Scale 6. In order to emphasize its transportability of the 5 MWth U-Battery, the diameter of RPV is limited to 3.7 m and 1.8 m.

For the 3.7-m RPV in diameter, the reactor core with 24 fuel blocks loaded 20% enriched ²³⁵U reaches a fuel lifetime of 10 EFPYs for the 5 MWth U-Battery. Comparisons of the different reactor core configurations with 24 fuel blocks show that Layout 12×2B, a scattering core, achieves the lowest fuel cost. Moreover, reducing the fuel inventory of the reactor core by decreasing the diameter of the fuel kernels and the packing fraction of the TRISO particles decreases the fuel cost of Layout 12×2B by 11%.

When decreasing the RPV diameter from 3.7 m to 1.8 m, it is possible to achieve a fuel lifetime of 4 EFPYs for the 5 MWth U-Battery with a graphite side reflector. If nuclear graphite is replaced by BeO, Layouts 6×3 and 6×4 with the 25-cm-thick BeO side reflector achieve 7 EFPYs and 10 EFPYs, respectively, for the 5 MWth U-Battery. The 20-cm-thick BeO side reflector is sufficiently thick from neutronic point of view. Moreover, it provides more space for the side thermal insulation to protect the barrel and RPV. Future work will focus on the design of the reactivity control system of Layouts 6×3 and 6×4 with 20-cm- thick BeO side reflector, and on the coupled neutronic /thermal-hydraulic evaluation of the different designs.