Elsevier

Annals of Nuclear Energy

Volume 35, Issue 7, July 2008, Pages 1264-1273
Annals of Nuclear Energy

A comprehensive study on neutronics of a lead–bismuth eutectic cooled accelerator-driven sub-critical system for long-lived fission product transmutation

https://doi.org/10.1016/j.anucene.2007.12.007Get rights and content

Abstract

The aim of this study is to investigate the high-level waste (HLW) transmutation and fissile breeding potentials of a lead–bismuth eutectic (LBE) cooled accelerator-driven system (ADS) for the various configurations (the target radius, RT = 10–50 cm and the radial thickness of the sub-critical core, δSC = 50–80 cm) and for the various fuel compositions (the fuel volume fraction, VFF = 10%, 12%, 15% and 20% and the fissile fraction, FF = 10–24%) under sub-critical condition. The long-lived fission products (LLFPs: 99Tc, 129I and 135Cs nuclides) and the uranium mono carbide (UC) ceramic fuel are considered as the HLW and the fissile fuel, respectively. The neutronic calculations have been performed per the incident proton (1000 MeV) with the high-energy Monte Carlo code MCNPX in coupled neutron and proton mode using the LA150 library. The numerical results bring out that the case of RT = 30 cm, δSC = 80 cm, VFF = 10% and FF = 23% is the optimum configuration and fuel composition, from the energy gain point of view, and has a high neutronic performance for an effective LLFP transmutation and fissile breeding.

Introduction

High-level wastes (HLWs) are the highly radioactive materials produced as a byproduct of the reactions that occur inside nuclear commercial reactors. They mainly includes minor actinides (MAs: isotopes of Np, Am and Cm) and long-lived fission products (LLFPs: 129I, 99Tc, 135Cs, etc.). Most countries’ preferred option for isolation of nuclear waste from the public and the environment is to bury it underground in a deep geological repository. However, the solutions to this problem are still debatable both technically and ethically because of their very long half lives (thousands of years to millions of years). Therefore, the HLW management has become one of the top priority issues with the rise of development and utilization of nuclear energy, and has been investigated increasingly since the latter years of the 20th century.

An alternative innovative approach is to burn and/or transmute HLW by using high-energetic neutron source. Nuclear transmutation can be defined as the transformation of one isotope into another isotope by changing its nuclear structure. In order to transmute efficiently HLW, the high intensity neutron source is needed, such as: fusion-driven transmuter (FDT) and accelerator-driven system (ADS). The concept of ADS combines a particle accelerator with a sub-critical core. The sub-criticality enables that a fuel with a large fraction of fuel and/or minor actinides can be loaded into the sub-critical system without any reactivity induced safety problems, which would not be the case in a critical system and that the system can be controlled safely and easily because the sub-critical systems need an external source of neutrons to start the nuclear reactions. The basic process in an ADS is nuclear transmutation, and in general, an ADS consists of three parts: (1) accelerator, (2) spallation neutron target (SNT) and (3) sub-critical core (often called blanket) which surrounds the SNT. A high intensity continuous wave proton beam with an energy of around 1 GeV and a current of several tens milliamperes is injected into a target of heavy metal. This results in a spallation reaction that emits neutrons. The spallation process is a nuclear reaction where high-energy particles hit target nuclei of heavy elements. The main purpose of the high power spallation target in an ADS is to provide the primary neutron flux for driving the fission process in the surrounding sub-critical core. The number of spallation neutrons per incident proton depends on the beam energy and on the mass of the target nuclei. Due to their high atomic number, heavy metals such as lead, mercury, uranium, tungsten, tantalum, or eutectics such as lead–bismuth are the most appropriate choices for the target material. Lead–bismuth eutectic (LBE) is today the reference target material for ADS applications. Both lead and lead–bismuth exhibit very low neutron capture making them good candidates from a neutronic standpoint.

Good neutron economy is crucial for the ADS since it determines the power and consequently the cost of the accelerator. The sub-critical systems need an external source of neutrons to start the nuclear reactions. These external neutrons are supplied by the SNT bombarded with high-energy protons coming from accelerator which in turn generates a large number of neutrons via the spallation reactions. These neutrons are primary neutrons, and they can consequently be multiplied in the sub-critical core containing fissile fuels and/or MAs. Thus, the neutron economy is enhanced by further multiplying the spallation neutrons in a sub-critical medium.

A great number of works on the ADSs and on their neutronics have emerged in the scientific literature. Most of them have been concerned with specific design concepts. Nifenecker et al. (2001) have presented a comprehensive review paper on basic neutronics of the ADSs by examining tens of studies. Gokhale et al. (2006) also reviewed on energy production and waste transmutation in the ADSs by analyzing three international CDROM databases, (INIS, INSPEC and Chemical Abstracts). The aim of these reviews is to provide the reader with a basic understanding of sub-critical reactors and discuss some practical examples.

As explained above, the main objective of an ADS is to transmute long-lived radioactive waste in order to reduce the amount and radiotoxicity of radioactive wastes, as well as energy production. Lawrence (1953) first proposed to transmute thorium into 233U using fast neutrons from spallation of a high-energy proton beam. Rubbia et al. (1995) showed that an accelerator could directly drive a sub-critical power reactor and that the neutron spectrum from a lead moderator would sweep the capture resonances of transuranic isotopes and consume long-lived wastes. In the last decade, various types of accelerator-driven transmutation technologies have been studied to produce energy and transmute radioactive wastes (Abanades and Perez-Navarro, 2007, Adam et al., 2007, Artisyuk et al., 1999, Brolly and Vertes, 2005, Haeck et al., 2006, Kerdraon et al., 2003, Mukaiyama et al., 2002, Nishihara and Takano, 2002, Park et al., 2002, Saito et al., 2006, Seltborg and Wallenius, 2006, Takano et al., 2000, Takizuka et al., 2002, Tsujimoto et al., 2000, Tsujimoto et al., 2004, Tucek et al., 2007, Wade et al., 2002, Wallenius and Eriksson, 2005, Westlen and Wallenius, 2006, Westmeier et al., 2005, Yang, 2002, Yang et al., 2004).

In our previous studies on the nuclear fuel transmutation (Yapıcı, 2003a, Yapıcı, 2003b, Yapıcı, 2003c, Yapıcı et al., 2004, Yapıcı et al., 2006a, Yapıcı et al., 2006b), the HLW transmutation potentials of various FDT configurations were investigated. Furthermore, the neutronic limits were analyzed for various infinite spallation neutron target mediums driven by energetic protons (Yapıcı et al., 2007). The objective of this present study is to investigate the LLFP transmutation and fissile breeding potentials of a LBE cooled ADS for various configurations and fuel compositions.

Section snippets

Accelerator-driven sub-critical system

This study presents the transmutation and fissile breeding potentials of a typical cylindrical accelerator-driven sub-critical system loaded with the uranium mono carbide (UC) ceramic fuel and LLFP assemblies. The LLFP nuclides (99Tc, 129I and 135Cs), discharged from high burn-up (33 GWd/tHM) PWR-MOX spent fuel represented as MOX22 (standard) in NEA (2000), are selected as the HLW. It is assumed that these nuclides are in metallic form and isotopically separated. The isotopic fractions and

Calculational procedure

The neutronic calculations have been performed per the incident proton (1000 MeV) with the high-energy Monte Carlo code MCNPX (Waters, 2002) in coupled neutron and proton mode using the LA150 library (Chadwick et al., 1999). This library consists of evaluated reaction cross-sections and emission spectra up to 150 MeV for incident neutrons and protons, for over 40 target isotopes important in the SNTs, structural materials, and shielding. The intranuclear cascade of spallation reactions is

Transmutation reactions

Two transmutation reactions are important for the nuclear waste management: (1) neutron capture and (2) fission. Fig. 2 shows the neutron capture and subsequent decay reactions of the considered LLFP nuclides. As is apparent in the figure, the 99Tc, 129I and 135Cs isotopes capture a neutron to become the short-lived isotopes, which decay into stable 100Ru, 130Xe and 136Ba nuclides, respectively. Effective transmutation of these nuclides will require local moderation of neutrons because they

Conclusions

The transmutation and fissile breeding potentials of a typical cylindrical ADS have been investigated. In order to be able to obtain the optimum transmutation and energy production, the various design configurations and fuel compositions have been examined. The main conclusions derived from these analyses are cited briefly below:

  • The PN can reaches 1044 n/p in the case of VFF = 10%, δSC = 80 cm and FF = 24%.

  • The LPR is in the range of 6.8% and 11.4%.

  • The optimum energy production can be supplied in the

Acknowledgement

This work is supported by the Research Fund of the Erciyes University, Project No. FBT-06-83.

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