By taking advantage of the appropriate critical pressure, high density, stability, and sluggishness of CO₂ near its critical point, the use of supercritical CO₂(S-CO₂) as a coolant in the direct cycle reactor is evaluated (Ahn et al., 2015; Wu et al., 2020). The S-CO₂–cooled reactor, as a promising innovative reactor, has the advantages of simple system, high thermal efficiency, small volume, and light weight, and it represents an important development direction for nuclear energy innovation and development (Michael, 2004).

Safety and economics are the main objectives of the S-CO₂–cooled reactor. A new fuel concept is referred to as accident-tolerant fuels (ATFs) are capable of mitigating the potential consequences of beyond-design-basis accidents (Bragg-Sitton, 2014). The fully ceramic microencapsulated (FCM) fuel (Terrani et al., 2012) has become one among the ATF options which is based on tristructural isotropic (TRISO) (Bragg-Sitton and Carmack, 2015)–coated particles embedded in silicon carbide (SiC) matrix. FCM fuel borrows from the TRISO particle design from the high-temperature gas reactor (HTGR) technology but uses SiC as a matrix material rather than graphite in HTGR. Benefiting from the multiple barriers of TRISO and SiC matrix, FCM fuel has extremely high radioactivity retention capability compared to that of the conventional standard UO₂ fuel. By the advantages of safety, a new fuel assembly based on the FCM fuel is proposed for the S-CO₂–cooled reactor in this study.

In further research, although the FCM fuel contains moderation materials such as SiC, the assembly cannot be sufficiently moderated due to the weak moderating performance of S-CO₂, which affects the neutronics economy seriously. The study on improving the deficiency of weak moderating performance is described in this article.

Unlike the light water moderated or heavy water moderated (Zhang et al.,2019; Zhang et al.,2020) nuclear reactors, the S-CO₂ cooled reactor needs specific moderator rods to improve the slow-down of neutrons. The introduction of moderator rods can effectively improve the moderating performance of S-CO₂–cooled reactor, but it will lead to the deterioration of uniform moderation. To further improve the uniform moderation, moderator rod arrangement and fuel enrichment partition method are studied. Finally, this study shows clearly that a better balance between uniform moderation and sufficient moderation can be obtained in the high-performance S-CO₂–cooled reactor assembly.

Because of stronger neutron moderating capacity, metal hydride has been used as a solid moderator in reactors. Yttrium hydride (YH_1.8) is more suitable to use in the S-CO₂ cooled reactor because it is more stable than zirconium hydride (ZrH_1.6) at high temperature.

The design of YH_1.8 solid moderator rod used in this study is shown in Figure 5. To reduce the release of hydrogen atoms, the solid moderator rod is coated with stainless steel. The YH_1.8 solid moderator rod geometry and composition are given in Table 4.

The moderating method is to insert the YH_1.8 solid moderator rod into part of fuel holds instead of the FCM fuel. The moderating performance of assembly can be changed by using different number of YH_1.8 solid moderator rods. The number of YH_1.8 solid moderator rod analyzed in this study is shown in Table 5, and the arrangements of the YH_1.8 solid moderator rods are shown in Figure 6.

K ∞ by different YH_1.8 solid moderator rod number as fuel burn up varies is shown in Figure 7. The calculation results clearly show that adding YH_1.8 solid moderator rod can significantly enhance moderating performance and then improve the fuel reactivity greatly. K ∞ of H₂O is also described in Figure 6. When the number of YH_1.8 solid moderator rods is greater than 12, the K ∞ of S-CO₂ assembly is significantly larger than that of H₂O assembly. These results show that moderating performance can be improved significantly by inserting the YH_1.8 solid moderator rod. When 12 YH_1.8 solid moderator rods were inserted, the S-CO₂ assembly was moderated sufficiently and the fuel share was reduced by only 22.2%.

The moderating effect of different materials was further studied. Commonly used moderating materials such as graphite and BeO are used for comparison with YH_1.8. K ∞ of different moderating materials as fuel burn up varies are shown in Figure 8. K ∞ of YH_1.8 is significantly larger than that of graphite and BeO. The results show that using YH_1.8 as a moderator is the best choice to improve the moderating performance of S-CO₂ assembly.

Inserting the YH_1.8 solid moderator rods cause uneven moderating in the assembly. In order to get uniform moderating, the arrangement of the moderators must be studied.

The moderating effect and the change of fuel share were considered, and then the number of YH_1.8 solid moderator rods is selected as 12 in this section. The different arrangements of YH_1.8 solid moderator rods are shown in Figure 9 and power uneven factor by different solid moderator arrangements as fuel burn up varies are shown in Figure 10. The arrangement B of solid moderator obtains the lowest uneven factor and becomes the preferred arrangement in this study.

Figure 11 presents the BOL (Begin Of Life) relative power distribution of S-CO₂ assembly using arrangement B. Figure 11 clearly shows that the maximum power is at site A and the minimum power is at site B. In order to further realize the uniform moderating to reduce the power uneven factor, the fuel share of FCM partitioning method is applied in the S-CO₂ assembly design. The fuel share of FCM is reduced from 40 to 35% at site A which have maximum power and the fuel share of FCM is increased from 40 to 45% at site B which have minimum power. In addition, this partitioning method ensures that the fuel weight remains the same in the S-CO₂ assembly. The BOL relative power distribution of S-CO₂ assembly using the partitioning method is shown in Figure 12. The relative power at site A decreases from 1.069 to 0.964, the relative power at site B increases from 0.915 to 1.005, and then the uneven factor decreases from 1.069 to 1.028. Figure 13 presents a power uneven factor by arrangement B with the partitioning method as fuel burn up varies; it shows that the fuel share of the FCM partitioning method can achieve more uniform moderating and power distribution.

As a new type reactor with development potential, the S-CO₂–cooled reactor has many advantages, such as simple system, high thermal efficiency, and small volume. FCM fuel which is a potential option of ATF is capable to mitigate the potential consequences of beyond-design-basis accidents and become the preferred type of fuel for the S-CO₂–cooled reactor.

In this article, a new fuel assembly design concept based on FCM fuel for S-CO₂–cooled reactor is proposed. Then, the design of YH_1.8 solid moderator rod was adopted to improve the moderating performance of the fuel assembly. In order to deal with the moderating non-uniformity caused by the moderator, the arrangement of moderators and the method of fuel share partitioning were well studied, and then the high-performance fuel assembly based on FCM fuel for S-CO₂–cooled reactor is proposed at last. The main conclusions of the study are as follows:

1) The fuel assembly based on the FCM fuel for the S-CO₂–cooled reactor was proposed because of the safety advantage. Due to the weak moderating performance of S-CO₂, the neutrons in this assembly cannot be sufficiently moderated and affects the neutronics economy seriously.