Elsevier

Annals of Nuclear Energy

Volume 76, February 2015, Pages 113-124
Annals of Nuclear Energy

Alternative reflectors for a compact sodium-cooled breed-and-burn fast reactor

https://doi.org/10.1016/j.anucene.2014.09.048Get rights and content

Highlights

  • A good neutron reflector is necessary in a small breed-and-burn fast reactor (B&BR) to improve the neutron economy.

  • Impacts of various alternative reflectors on a sodium-cooled B&BR have been investigated from the neutronics perspectives.

  • LME (lead–magnesium eutectic) and PbO are recommended as promising alternative reflector materials for the small B&BR.

Abstract

The breed-and-burn fast reactor (B&BR) is a unique fast reactor concept that offers attractive characteristics in terms of core performance and non-proliferation aspects. The B&BR has the ability to breed its own fuel and use it in situ to achieve an extremely long life. In order to achieve the breed-and-burn condition, the neutron economy should be very good. In this work, a compact sodium-cooled B&BR is investigated from the physics point of view. In a compact size B&BR which usually has a higher neutron leakage, a good neutron reflector is essential to maintain the neutron economy. In the conventional sodium-cooled B&BRs, a steel reflector such as HT-9 is usually adopted as reflector material. In the current work, various alternative reflector materials such as pure lead, lead–bismuth eutectic (LBE), lead–magnesium eutectic (LME), PbO, MgO, Ni, and HT-9 are investigated from the neutronics perspectives. Several characterizations including the reflecting performance, core neutron spectrum, core leakage, power distribution, and sodium coolant void reactivity (CVR) have been performed to understand the behavior of each reflector material. The impact of the coarse lattice reflector configuration on the reflector performance and CVR has also been analyzed. In addition, the impact of the reflector material on the gas expansion module (GEM) worth has been investigated, too. Finally, it was concluded that lead-based reflectors such as LBE, LME, and PbO, are promising alternative reflector candidates for a compact B&BR. The calculations were all done by using a continuous energy Monte Carlo code.

Introduction

The breed-and-burn fast reactor (B&BR), also known as the traveling wave reactor (TWR), is a unique fast reactor concept characterized by its ability to breed its own fuel and simultaneously use the bred fuel in situ to achieve a longer lifetime and a very high fuel burnup. Due to this capability, the B&BR offers a high fuel utilization without any fuel reprocessing and the associated fuel cycle can be quite simpler. It has recently gained more attention and is being investigated in various research entities such as Tokyo Institute of Technology (Sekimoto et al., 2001), TerraPower (Ellis et al., 2010), and University of California, Berkeley (Heidet and Greenspan, 2012). The research groups have performed various conceptual studies of the B&BRs.

The B&BR core considered in this study is sodium-cooled and consists of two axial fuel regions: the initial core and the blanket region. The core configuration is similar to the CANDLE reactor (Sekimoto et al., 2001). The active core, which is the initial core region at the beginning, moves slowly toward the un-burned blanket. During this process, the fertile fuels in the blanket region are slowly converted into fissile fuels. Consequently, the theoretical lifetime of the B&BR core could be proportional to the height of the blanket in the axial direction. The initial core is usually loaded with an LEU (Low-Enriched Uranium) fuel to achieve criticality, and either natural or depleted uranium is used as the blanket fuel. The PWR (Pressurized Water Reactor) spent fuel can also be used as the blanket fuel in this type of reactor. A study for recycling the PWR spent fuel has been performed by the author in a small and compact sodium-cooled 250 MWth B&BR (Hartanto and Kim, 2012a, Hartanto and Kim, 2012b). A similar study of B&BR for recycling the spent fuel has also been performed in a large size 2600 MWth B&BR by using a rather simple homogeneous modeling of the complex core (Kim, 2010).

A small size reactor is considered in this work because it is relatively easier to achieve the passive decay heat removal and the reactor system design can be simplified. The B&BR can provide two unique advantages: a long life core and higher fuel burnup. The long-life fast reactor with a high burnup can be attractive if frequent refueling of small reactors is costly. In particular, if the lifetime of the reactor can be equal to the system lifetime, the advantages of a long-life B&BR core would be maximized.

In a small size B&BR where the neutron leakage is relatively high, a good neutron reflector is very important to maintain the neutron economy so that the reactor can achieve the equilibrium breed-and-burn condition and high fuel burnup. A steel reflector such as HT-9 is usually utilized as the neutron reflector in the sodium-cooled B&BRs. In a previous work (Hartanto and Kim, 2012a, Hartanto and Kim, 2012b), it was shown that a liquid lead can be efficiently used as new reflector material in the sodium-cooled B&BR, although several technical concerns can be raised on the Pb reflector in a sodium-cooled B&BR environment (Hartanto and Kim, 2013). In this work, various alternative reflector materials such as pure Pb, lead–bismuth eutectic (LBE), lead–magnesium eutectic (LME), PbO, MgO, pure Ni are investigated for a small size sodium-cooled B&BR. Previously, an MgO reflector was considered for a sodium-cooled fast reactors (Macdonald and Driscoll, 2010, Yun et al., 2012) to improve the neutron economy via reduced neutron leakage. An MgO reflector can soften the neutron spectrum in the core peripheral region so the fission rate can be enhanced in the boundary layers and the softer neutron spectrum in reflector region results in a reduced leakage. As a consequence, the neutron shielding in the MgO-reflected core can be better than in the traditional HT-9 reflector.

Several characterizations including the reflecting performance, core neutron spectrum, core leakage, power distribution, and sodium coolant void reactivity (CVR) have been performed to understand the impacts of each reflector material and find attractive alternative reflector for the B&BR. In fast reactors, the gas expansion module (GEM) (Waldo et al., 1986) device is often utilized to improve the inherent safety of the reactor. The impact of the reflector materials on the GEM has also been investigated in the current study.

In the characterization and analysis, the continuous energy Monte Carlo code McCARD (Shim and Kim, 2010) is used with the ENDF/B-VII.0 nuclear data library. The McCARD code can be run on parallel computers and it also has a built-in depletion routine, thus it can be used in a stand-alone mode for the core depletion analysis.

The paper is organized as follows. Section 2 provides a brief description of the compact B&BR concept and the alternative reflector materials. The analysis results and discussion are provided in Section 3. Finally, the conclusions are drawn in Section 4.

Section snippets

Compact breed-and-burn reactor concept

The compact sodium-cooled B&BR core configuration devised and considered in this work is shown in Fig. 1, Fig. 2. Table 1 shows the major core design parameters. The core consists of 78 hexagonal fuel assemblies, 78 hexagonal reflector assemblies, and 7 control rod assemblies. The equivalent core radius is about 111.30 cm. The total active core height is 150 cm and the initial LEU core height is 70 cm. The reactor power is set to 250 MWth so that the electrical output should be about 100 MWe by

Reflector-dependent core characterization

Several important core characteristics influenced by the various reflector materials have been investigated in this section, which includes the reflector performance in the term of core lifetime, core conversion ratio, core neutron leakage, neutron spectrum, radial and local pin power distribution, reflector temperature reactivity coefficient, and coolant void reactivity coefficient. These characteristics will provide a better understanding of the physics and performance of each material as a

Conclusions

A physics study of alternative reflector materials has been concluded in order to find a new reflector for a sodium-cooled compact B&BR. Various materials such as Pb, LBE, LME, PbO, MgO, Ni, and HT-9, have been studied as candidate reflectors. The alternative reflectors have been compared in terms of core lifetime, conversion ratio, neutron leakage, neutron spectrum, radial and local pin power distributions, and coolant void coefficient.

In term of the neutron economy, the pure lead and

Acknowledgements

This work was supported by the Long-term Nuclear R&D Program of the Ministry of Science, ICT and Future Planning (MSIP) in Korea (No. NRF-2013M2A8A6035680).

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