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DOI : 10.22661/AAPPSBL.2014.24.5.13
A Study on Accelerator-driven Transmutation System in JAEA
TOSHINOBU SASA
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A Study on Accelerator-driven Transmutation System in JAEA

TOSHINOBU SASA
J-PARC CENTER, JAPAN ATOMIC ENERGY AGENCY

After the accident at Fukushima-Daiichi nuclear power plant, reduction of radioactive wastes received much interest from the public in Japan. To reduce the burden of radiological nuclides contained in spent nuclear fuel, Partitioning-Transmutation (P-T) technology is noted as an option for nuclear waste management in the new national strategic energy policy. By using P-T technology, radiological hazards can be reduced by less than 1/100 and the area required for a waste disposal site can be decreased. As for the system for waste transmutation, an accelerator-driven system is desirable as a dedicated transmutor. To perform basic studies for an accelerator-driven system, the Japan Atomic Energy Agency has promoted the design of the Transmutation Experimental Facility within the framework of the J-PARC project. A lead-bismuth spallation target, which is bombarded with 400 MeV - 250 kW protons, and a low-power subcritical reactor will be installed in the facility. Construction will start within a few years after the national review of P-T technology.

Keywords: accelerator-driven system, partitioning, transmutation, J-PARC, radiological waste, minor actinide, Transmutation Experimental Facility.

INTRODUCTION

Due to the Great East Japan Earthquake and the ensuing tsunami, the Fukushima-Daiichi Nuclear Power Plant was seriously damaged and many nearby residents were forced to be evacuated. After the accident, the Science Council of Japan recommended the Atomic Energy Commission of Japan to prioritize research and development (R&D) to reduce the radiological burden of high level wastes (HLW). In 2014, the cabinet of Japan decided on a new strategic energy policy enhancing R&D to reduce the burden of radioactive waste in spent nuclear fuels by using a fast reactor and/or accelerators.

The Japan Atomic Energy Agency (JAEA) is proceeding with R&D to reduce the radiological hazard of HLWs by Partitioning and Transmutation (P-T) technology [1]. For the transmutation of radioactive nuclides in HLW, the management of minor actinides (MA) is significantly important. By using P-T technology, MA can be transmuted effectively by using the accelerator-driven system (ADS), which combines a high intensity proton accelerator and a fast subcritical core. It is also important that the P-T using ADS be compatible with various power generation cycle scenarios, because ADS can be established independently from the power reactor fuel cycle.

To realize the ADS, the innovative nuclear system, there are many issues to be solved. Within the framework of the J-PARC project, JAEA plans to construct a Transmutation Experimental Facility (TEF) to study MA transmutation by both MA-loaded fast reactors and ADS [2]. The TEF is located at the end of the LINAC, which is also an important component to be developed for future ADS, and shares the proton beam with other experimental facilities in J-PARC. R&D for important technologies required to build the TEF are also performed, such as application methods of MA bearing fuel in the critical/subcritical assembly, spallation product removal methods especially for polonium, and so on. The objectives of the TEF, the latest design concepts, and key technologies to construct a TEF are described.

BENEFITS OF P-T INTRODUCTION

To introduce the P-T cycle, a double stratum concept was proposed by the JAEA. In the double-strata fuel cycle, the power generation cycle including the Fast Breeder Reactor is the first stratum and the ADS is installed as the second stratum cycle. In the first stratum, uranium and plutonium are recycled and MA is partitioned off to be sent to the second stratum for transmutation. At the reprocessing and partitioning plant in the first stratum, fission products (FP) are separated into three groups: the Sr-Cs, the platinum group metals (PGM: ruthenium, rhodium and palladium) and the other elements including lanthanides. Strontium and cesium are calcined, and stored to decrease their decay heat. The separated PGM may be utilized as a catalyst. Iodine is separated in a reprocessing plant, and disposed of as low-level long-lived waste or transmuted in ADS. The other waste elements from the first stratum are supposed to be vitrified with higher density (~35 wt.%) than is conventional (~15 wt.%) because of the low heat generation and the extraction of PGM, which is undesirable for the glass melting process.

In the second stratum, MA-based fuel is loaded into the ADS to maximize the transmutation amount of MA and spent MA fuel is also reprocessed by a dry reprocessing process. Since the wastes from the second stratum are relatively low, because the actinide's inventory is smaller than that of the power generation cycle, the wastes from the second stratum give a limited effect on the whole system of waste management.

The conventional reprocessing for spent fuel from light water reactors forms about 3.4 pieces of vitrified waste packages per 1 TWh of electricity generation, and therefore the area of geological repository for 1 TWh is about 150 m2. If the MA is recovered and recycled, the required space per one vitrified waste package will be kept at 44 m2, and therefore the area per unit of electricity generation can be reduced by about 40%. In addition, if MA is recycled and Sr-Cs is cooled separately for about a hundred years, the total repository area will be reduced to 1/4 of the non-P-T case because the decay heat from the Sr-Cs is decreased significantly. If the Sr-Cs is stored and cooled for a much longer period, for example about 300 years, the heat generation from the Sr-Cs is no longer influential to the repository structure, and hence it can be disposed of in a very compact manner [3].

JAEA PROPOSED ADS

The JAEA's reference design for ADS is a tank-type subcritical reactor, where lead-bismuth eutectic (LBE) alloy is used as both the primary coolant and the spallation target, as shown in Fig. 1. The spallation target region is located at a central part of the subcritical core. In the target region, LBE flows from the core bottom along to the dedicated wrapper tube and to the flow guide tube. About a 1.5 GeV - 30 MW proton beam is supplied from the linear accelerator to operate the ADS. The rated power, which is controlled by adjusting the injected proton beam power from the accelerator, is 800 MWth. Therefore, the maximum neutron multiplication factor during the whole burn-up cycle was set at 0.97.

A tank-type system was adopted to eliminate the necessity of heavy-weighted primary piping. All primary components, including two primary pumps and four steam generators, are set up in the reactor vessel. The heat generated in the target and the core is removed by forced convection of the primary LBE, and transferred through the steam generators to a secondary water/steam system for power conversion. The inlet and outlet coolant temperatures were set to 300 and 407 ºC, respectively, which are low enough to prevent material corrosion by LBE.

Fig. 1: ADS for transmutation of MA proposed by JAEA.

Nitride fuel was selected as the fuel for dedicated MA transmutation, which is suitable for high MA concentration and reprocessing for ADS. To minimize the burn-up reactivity change and the power peaking, the fuel region was divided into several zones for the different fuel compositions. About 2,500 kg of MA is loaded in the core and 10% of the MA can be transmuted annually. It means that 4 to 5 ADS units are required to support the current Japanese nuclear power reactors.

TRANSMUTATION EXPERIMENTAL FACILITY

As shown in Fig.2, TEF consists of two individual buildings: the ADS Target Test Facility (TEF-T) [4] and the Transmutation Physics Experimental Facility (TEF-P) [5].

Fig. 2: Transmutation Experimental Facility.

The two buildings are connected by a beam transport line with a low power beam extraction mechanism using a laser beam. TEF-T is planned as a material irradiation facility which can accept a maximum 400 MeV-250 kW proton beam on a LBE spallation target. It also has the possibility of being used for various research purposes such as measurement of the reaction cross sections of MA and structural materials, basic science studies and so on. TEF-P is a facility with a critical/subcritical assembly to study the neutronic performance and the controllability of ADS. Using these two facilities, basic physical properties of the subcritical system and the engineering tests of the spallation target will be studied. R&D for several important technologies required to build the facilities are also performed, such as the laser charge exchange technique to extract a very low power beam (less than 10 W) for reactor physics experiments, a remote handling method to load MA bearing fuel into the critical/subcritical assembly, the spallation and activation product removal method especially for the polonium, and so on.

Outline for TEF-T

The main purpose of TEF-T is to obtain the data to evaluate the actual lifetime of the beam window. TEF-T mainly consists of a spallation target, a LBE cooling circuit, and hot cells to handle the spent target and irradiation test pieces.

A high power spallation target, which will be mainly used for material irradiation of candidate materials for a beam window of full-scale ADS, is an essential issue to realize a TEF-T. To set up the beam parameters, future ADS concepts will be taken into account. In the reference case of the target, the proton beam current density of 20 μA/cm2, which equals the maximum beam current density of the JAEA-proposed 800 MWth ADS, was selected. The material of the irradiation target would be a type 316 stainless steel for temperatures below 450 ºC and a T91 steel for higher operation temperatures. The irradiation performance of the reference case was evaluated at around 8 DPA/yr by 400 MeV-250 kW protons of irradiation. This value is about 20% of the DPA considered in the beam window of the JAEA-ADS. Further optimization of the target design to increase the DPA is underway.

When LBE is irradiated by high energy protons or neutrons, polonium isotopes will be accumulated and they should be carefully controlled. The removal method of polonium was studied for the design of the exhaust circuit of TEF-T. An equilibrium vaporization test of polonium from liquid Pb-Bi was performed and equilibrium vaporization characteristics were measured by the transpiration method with LBE, which was irradiated at the JAEA/JMTR [6]. It was shown that at a low temperature of around 450 ºC, which is considered a standard operational condition of TEF-T and future ADS, most accumulated polonium remained in LBE as a chemical compound with Pb or Bi which is much harder to evaporate than elemental polonium. Another experiment to recover evaporated polonium in the exhaust circuit was performed [7]. LBE samples were irradiated at the JAEA/JRR-4 and were heated in a special vacuum vessel up to 690 ºC. By adopting the multi-layered filter, which consists of the stainless steel meshes with two different finenesses, escaping polonium can be decreased to 1/400.

Outline of TEF-P

Several neutronic experiments for ADS have been performed in both Europe [8, 9] and Japan. In Japan, subcritical experiments were performed at the Fast Critical Assembly (FCA) of the JAEA with a 252Cf neutron source and a DT neutron source. Subcritical experiments with a thermal subcritical core driven by 100 MeV protons are being performed at Kyoto University Research Reactor Institute. There have been, however, no subcritical experiments combined with a spallation source installed inside the subcritical fast-neutron core. The purposes for building the TEF-P are (1) to study reactor physics aspects of the subcritical core driven by a spallation source, (2) to demonstrate the controllability of the subcritical core including the power control by the proton beam power adjustment, and (3) to investigate the transmutation performance of the subcritical core using a certain amount of MA and long-lived FP (LLFP).

TEF-P was designed with reference to the FCA, the horizontal table-split type critical assembly with a rectangular lattice matrix, to utilize the operation experiences and existing experimental data of the FCA. In this concept, the plate-type fuel for FCA with various simulation materials such as lead and sodium for coolants, tungsten for the solid target, ZrH for the moderator, B4C for the absorber, and AlN for the simulating nitride fuel, can be commonly used by the TEF-P. The proton beam will be introduced horizontally at the center of the assembly and various kinds of spallation targets can be installed at various axial positions off the radial center of the subcritical core.

In the experiment with a proton beam, the effective multiplication factor of the assembly will be kept less at than 0.98. One proton with the energy of 400 MeV produces tens of neutrons by the spallation reaction with a heavy metal target such as lead. The 10 W proton beam corresponds to the source strength of 1012 neutrons/sec, and has enough strength to measure the neutronic characteristics. From the viewpoint of the accuracy of neutronic analyses for subcritical systems, it is desirable to make the core critical in order to ensure the quality of the experimental data of the subcriticality and the reactivity worthwhile. So, the subcritical core will be made to have a critical condition when the proton beam is suppressed.

As for the transmutation characteristics of MA and LLFP, fission chambers and activation foils are used to measure the transmutation rates. The cross section data of MA and LLFP for high energy regions (up to several hundred MeVs) can be measured by the Time of Flight (TOF) technique with a proton beam of about a 1ns pulse width which can be delivered by a special beam extraction device using an Nd:YAG laser source [10]. Several kinds of MA and LLFP samples are also being prepared to measure their reactivity value, which is important for the integral validation of the cross section data.

One of the main purposes of TEF-P is to perform integral experiments using MA because the present accuracy of nuclear data is not sufficient for the ADS design [11]. To improve the accuracy of the nuclear data especially for MA, both the differential experiments and the integral experiments are necessary, even though the integral experiments on MA are more difficult than those on the major actinides. The effectiveness of MA-loaded experiments with a certain amount of MA was discussed [12]. By using a certain amount of MA, which is on the order of a kg, typical improvement can be obtained.

SUMMARY

The JAEA has been promoting various types of R&D on P-T fuel cycles including a dedicated ADS transmutor. As for the basic experimental studies necessary to establish an innovative system like the ADS, a plan to build a Transmutation Experimental Facility has been proposed within the framework of the J-PARC project. The design optimization of TEF-T to improve irradiation performance to simulate ADS operation conditions, including R&D for polonium management, was carried out. The effectiveness of TEF-P experiments using a certain amount of MA was assessed quantitatively. The national review of the TEF construction is underway and construction will be decided within a few years.

References

[1] T. Sasa, et al., "Research and development on accelerator-driven transmutation system at JAERI", Nuclear Engineering and Design 230, 209-222 (2004).
[2] K. Tsujimoto, et al., "Feasibility of Lead-Bismuth-Cooled Accelerator-Driven System for Minor-Actinide Transmutation", Nucl. Tech., 161, 315-328 (2008).
[3] H. Oigawa et al., "Current status and future plan of research and development on partitioning and transmutation technology in Japan", Proc. 11th OECD/NEA Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation", San Francisco, USA (CD-ROM, 2012).
[4] T. Sasa, et al., "Conceptual Study of Transmutation Experimental Facility (2) Study on ADS Target Test Facility" (in Japanese), JAERI-Tech 2005-021 (2005).
[5] H. Oigawa, et al., "Conceptual Design of Transmutation Experimental Facility", Proc. Global2001, Paris, France (CD-ROM, 2001).
[6] S. Ohno, et al., "Equilibrium Evaporation Behavior of Polonium and Its Homologue Tellurium in Liquid Lead-Bismuth Eutectic", Journal of Nuclear Science and Technology, 43:11, 1359-1369 (2006).
[7] T. Obara, Y. Yamazawa, T. Sasa "Polonium decontamination performance of stainless steel mesh filter for lead alloy-cooled reactors", Progress in Nuclear Energy, 53, 1056-1060 (2011).
[8] R. SOULE, et al., "Neutronic Studies in Support of the ADS: The Muse Experiments in the MASURCA Facility", Nuclear Science and Engineering, 148, 124-152 (2004).
[9] W. UYTTENHOVE, et al., "The neutronic design of a critical lead reflected zero-power reference core for on-line subcriticality measurements in Accelerator Driven Systems", Annuals of Nuclear Energy, Vol. 38, No. 7, 1519-1526 (2011).
[10] T. TOMISAWA, et al., "Investigation of Photo Neutralization Efficiency of High Intensity H- Beam with Nd:YAG Laser in J-PARC", Proc. of 7th European Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators (DIPAC 2005), 275-277 (2005).
[11] T. Sugawara et al., "Analytical Validation of Uncertainty in Reactor Physics Parameters for Nuclear Transmutation Systems, " J. Nucl. Sci. Technol., 47(6), 521-530 (2010).
[12] T. SUGAWARA, et al., "SND2006-V. 10-1: Design of MA-loaded Core Experiments using J-PARC", Proc. of 2006 Symposium on Nuclear Data, Jan. 25-26, 2007, (2007) [CD-ROM].

 

Toshinobu Sasa is a sub-leader of Transmutation Section, J-PARC Center, Japan Atomic Energ Agency. After receiving a D. Eng from the Tokyo Institute of Technology, he worked at the Japan Atomic Energy Research Institute, a former Institute of JAEA. His research fields are nuclear reactor engineering and spallation target technology.

 
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