**1. Introduction**

Fully ceramic microencapsulated (FCM) fuel, which can improve the accident tolerance of light water reactors (LWRs), receives much attention because of its excellent oxidation resistance, fission retention capability, high thermal conductivity, and irradiation stability [1–4]. FCM was designed to improve the fission retention capacity during accident processes; fission retention capacity may be reflected by the integrity of the silicon carbide (SiC) matrix and layers. FCM is composed of historic tri-isotropic fuel embedded in a fully dense and impermeable SiC matrix. The tristructural-isotopic (TRISO) particle consists of the fuel kernel surrounded by four successive layers [5]: the low-density carbon bu ffer layer, whose function is to slow down and retain the generated fission product, and the inner and outer pyroltytic graphite layers, which surround the silicon carbide (SiC) micro pressure vessel. The function of the pyrolytic carbon is severalfold, including the protection of the kernel from aggressive process gases used during TRISO processing, to protect the SiC shell from energetic fission product recoil damage, and to provide thermal–mechanical stability and toughening for this multilayer system. Historically, TRISO and its fuel forms (graphite matrix compacts in the form of the cylindrical

fuel of a prismatic high-temperature gas-cooled reactor or the spherical pebble of a pebble-bed modular high-temperature gas-cooled reactor) operate at a temperature in excess of 1000 ◦C. As the envisioned LWR application of the FCM fuel is at a less aggressive operating temperature for the TRISO (nominally 350–500 ◦C), it is anticipated that higher burn-ups and significantly improved safety benefits realized by the extraordinary fission product retention may be realized [6].

Lower fissile loading was an obvious shortcoming of FCM fuel compared with uranium dioxide (UO2) pellets. The feasibility of FCM fuel to satisfy the reasonable fuel cycle lengths in LWRs was demonstrated by Sen et al. [7]. The effect of the UO2 kernel size, enrichment, and type on the effective full power days (EFPD) was discussed. The results indicated that uranium nitride (UN) kernel FCM fuel can meet the fissile loading requirement of LWRs by optimizing the size of the kernel and coated layers. The sizes of the UN kernel and buffer layer were 800 μm and 100 μm, respectively, and the fraction of the FCM had to be higher than 44 vol.%. The thermal–mechanical performance of the TRISO with UO2 kernels was simulated by PARFUM, PASTA, and other models [8]. In the CO production model, internal pressure and gap heat transfer were considered. The Recoil and Booth models were used to calculate the fission gas release. The thermal and mechanical performance of the coated layer can be calculated in the above models. The performance of the UN kernel TRISO fuel in the LWR environment was reported, and the influence of temperature and kernel size on internal pressure and the survivability of TRISO fuel was studied [9]. The tangential stress of the inner pyrocarbon (IPyC) and SiC layers was calculated using COMSOL multi-physics software (COMSOL-5.2, MERCURY LEARNING AND INFORMATION LLC, Dulles, Virginia). However, the influence of the SiC matrix on TRISO particle performance was not discussed.

The thermal–mechanical performance of FCM pellets is difficult to simulate because of its complex structure and material properties. The temperature field and thermal conductivity of FCM pellets were studied previously [10–12], but thermal and mechanical coupled simulation results are rare. The temperature of the FCM pellet used in a pressure water reactor (PWR) environment was calculated by COMSOL software in the literature [10]. Both homogeneous and heterogeneous models were used in the report, and the calculated result of the homogeneous models was in good agreemen<sup>t</sup> with the result calculated by RELAP. The mechanical performance of FCM was not studied. Ougouag and co-workers [13] studied the influence of the SiC matrix on the performance of TRISO fuel by adding an SiC matrix on the outer surface of a single TRISO particle model. The results indicate that the thickness of the SiC matrix has an obvious influence on the stress distribution of the SiC and IPyC layers. The structure and coated layer size can affect the stress condition of the FCM. The SiC layer stress increased upon ignoring the outer pyrocarbon (OPyC) layer or increasing the SiC matrix thickness. The stress of the SiC layer with a matrix was much higher than the single particle. Schappel [14] simulated the thermal–mechanical performance of an FCM pellet and TRISO particle separately using ABAQUS software. TRISO particles were subtracted from the FCM pellet to simplify the model and decrease the computation amount. The performance of the coated layers was not detected, and the interaction between the TRISO particle and matrix was not reflected in the literature.

There is no criterion to evaluate the integrity of the FCM pellet, and the integrity of the FCM pellet was not studied in previous work as far as we know. In this paper, the integrity of the FCM pellet was defined according to fission retention capacity. The integrity of the FCM pellet under a PWR environment was discussed by investigating the mechanical performance and failure probability of the SiC matrix and SiC layers. A two-dimensional model was established, while the buffer layer and fuel kernel were subtracted from the FCM pellet. The internal pressure of TRISO particles embedded in different locations of FCM fuel was calculated using a TRISO simulation. The interaction between the TRISO particle and the SiC matrix was investigated, and the thermal–mechanical properties of the coated layers and SiC matrix were discussed. The fission production capacity of the FCM pellet was evaluated by investigating the failure probability of SiC layers and the hoop stress of the SiC matrix.

### **2. Governing Equation and Material Properties**
