Analysis of neutron physics and thermal hydraulics for fuel assembly of small modular reactor loaded with ATFs

Highlights

• Neutron physics and thermal hydraulics calculations for small modular reactor loaded with ATFs.

• k_inf of NuScale SMR is calculated, and it is found to be larger than the traditional reactor.

• NuScale has excellent performance, which can ensure the integrity of fuel under REA.

• Most ATFs can reduce the radial power peak, except U₃Si₂-SiC at BOC, and U₃Si₂-Zr, U₃Si₂-SiC, U₃Si₅-SiC at EOC.

• MFCT is significantly reduced by U₃Si₂-FeCrAl fuel rod under rod ejection accident.

Abstract

The new small modular reactor (SMR) NuScale has been in the approval process for Nuclear Regulatory Commission by its off-site fabrication and assembly and good safety performance. To ensure the safety of the fuel rod the rod ejection accident at the beginning of cycle (BOC) and the end of cycle (EOC), the traditional fuel rod is replaced by accident tolerant fuels (ATFs). Neutron physics and thermal hydraulic analysis need to be performed before the actual application to ensure feasibility. In this paper, neutron physics and thermal hydraulics for SMR loaded with ATFs are analyzed. The results show that the 4% uranium enriched ATF pellets can meet the lifetime requirements, and the application of silicon uranium fuel pellets can significantly reduce maximum fuel centerline temperature (MFCT). Safety margin analysis was also performed for different ATF material failure criteria. The results show that ATF materials can enhance the safety of NuScale reactors.

Introduction

At present, reactors with high thermal power (more than 3000 MW) are used in most nuclear power plants. The cost of construction of the traditional commercial reactor is high and the construction period is long. The melting probability of the core is increased due to high power in severe accidents, and the needs of small power grids cannot be met. Next-generation nuclear power reactors have been developed worldwide under the direction of the International Atomic Energy Agency (IAEA) in the last 20 years. Some researchers in nuclear fields have been working on designing and developing small modular reactors (SMRs). There are mainly four types of SMRs, including light water reactors (LWR), heavy waters reactors (HWR), gas-cooled reactors (GCR) and liquid metal cooled fast reactors (LMFR) (Markou and Genco, 2019). Among them, although with very advanced designs, GCR and LMFR are still under research and lack of solid testing and industrial experience. Therefore, small modular LWRs with relatively mature technology has become the research object of this paper. Table 1 lists the most promising small modular LWRs for near term commercialization according to the SMR outlook report (Alzaben et al., 2019, Bae et al., 2019).

The advantages of SMRs are as follows:

• A power generation system for areas where it is difficult to access or have no infrastructure to transport fuel

• Modular design and construction

• Passive safety

• Long-life cycle and reduced need for refueling

• Proliferation resistance(Vujić et al., 2012)

In the recent decade, SMRs have been attracting worldwide interest due to the abovementioned benefits, and much work on neutronic and thermal hydraulics are carried out.

As to the neutronics, numerical benchmark calculations for a soluble-boron-free SMR assembly have been performed using the WIMS, Serpent and MONK by Syed Bahauddin Alam et al. For evaluating criticality values, data library discrepancies are performed for three candidate nuclear data files: ENDF/B-VII, JEF2.2 and JEF3.1 (Alam et al., 2019b). Xuezhong Li et al. proposed a new 13 × 13 assembly loaded with ATFs: U₃Si₂ – FeCrAl system which applied to long-life marine SMR. The fuel enrichment of this assembly is raised to 13%, prolonging the burnup to up to 95,000 MWD/tU, which means that the reactor can survive for more than 15 years at its rated power density. The preliminary design is well-pleasing and it may be a better choice for future marine SMRs (Li et al., 2019b).

when it comes to the thermal hydraulics, Hyun-SikPark et al. discussed thermal hydraulics tests about the standard design approval of SMART to verify its design bases, design tools, and analysis methodology (Park et al., 2017). A preliminary analysis of the long-term decay heat removal was performed by Marco Santinello et al. for SMR submerged in the sea or an artificial lake by using Relap5-Mod3.3. The behavior of the passive safety systems is simulated for 25 h to certify the safety potentialities of the submerged SMR concept (Santinello and Ricotti, 2019). Min-Gil Kim et al. used the system thermal hydraulic analysis code to track the accident initiator of both large pressurized water reactor (PWR) and SMR. Furthermore, more transient scenarios will be analyzed to understand the physical characteristics of SMR better in the viewpoint of intelligent control system development in the future (Kim and Lee, 2019).

Many attempts on coupling neutronics and thermal hydraulics have been tried on SMRs. The NuScale SMR was simulated using MCNP5/MCNPX 2.7 coupled with RELAP5 codes by A. Sadegh-Noedoost et al. Its fuel depletion and material changes were calculated for 730 days, proving that the core is non-proliferation (Sadegh-Noedoost et al., 2020). Y. Alzaben et al. have been studied REA consequences on a Boron-free SMR core at the BOL, which is performed with coupled neutronics and thermal hydraulics tools, and a high safety margin against fuel and cladding failure was concluded (Alzaben et al., 2019). The neutronic – thermal hydraulic coupling analysis of the fuel channel of a new generation of the small modular PWR – CAREM25 including hexagonal and square fuel lattice has been performed using MCNP – ANSYS CFX by Ali Erfaninia et al. The power peaking factors, the temperature distribution of the fuel and coolant along the hot and average fuel channels and the thermal neutron flux distribution in the throughout the core have been calculated which can be used for further thermal hydraulics studies and safety analyses of the fuel channels (Erfaninia et al., 2017).

Despite SMRs have many advantages, there are still challenges waiting for them. For example, nuclear fuel cycle challenge, spent fuel management (including final disposal), decommissioning, and diffusion issues are involved throughout the fuel cycle (Budnitz et al., 2018).

To improve the safety of SMRs under accidents, ATFs are implemented to replace traditional fuel and cladding. Most ATF pellets have the advantage of greater uranium density and better heat transfer performance (Chen et al., 2019). The advanced ATF cladding is attractive to provide much stronger oxidation resistance and better in-pile behavior under severe accident conditions for giving more coping time (Qiu et al., 2019).

In this paper, based on the existing research of SMRs, the rod ejection accidents (REA) analysis of the NuScale SMR loaded with ATFs, which is an integral pressurized-water reactor designed by NuScale Power, LLC, is performed. Several promising ATFs and claddings, including U₃Si₅, U₃Si₂, Silicon carbide ceramic (SiC) and FeCrAl, are evolved. Section 2 mainly introduced the assembly of NuScale, simulation method and ATFs. The simulation results are shown in Section 3. In the end, discussions and conclusions are presented.