Design of Spallation Target for TEF-T

To evaluate the feasibility of a designed beam window of TEF target, numerical analysis with a three-dimensional (3D) model was performed. The analysis was done by considering the current density and shape of the incident proton beam to the target and the thermal fluid behavior of Pb-Bi around the beam window as a function of flow rate and inlet temperature. The thickness of the beam window is also considered from 2 to 3 mm. After the temperature distribution analysis, structural strength of the beam window is determined to evaluate soundness of the target. A concave shape beam window was used for this analysis. The prototype design of the beam window for TEF target system is shown in Fig. 8.2.

The material of the beam window would be a type 316 stainless steel. The concave section in the center part of the target was connected to the convex section in the terminal part, and then it was connected to the straight tube. A straight tube part has coaxially arranged annular and tube-type channels. The inner diameters of the outside tube and inside tube were set to 150 and 105 mm, respectively. The total length of the analysis region was 600 mm, which corresponds to an effective target depth for the 400 MeV proton. An irradiation sample holder, which was installed in the inner tube, holds eight irradiation specimens in the horizontal direction. The size of each specimen was 40 x 145 x 2 mm. The rectification lattice having the aperture of the plural squares type was installed at the front end of the sample holder. A slit 2 mm in width was arranged along the side of the rectification lattice to cool the sample holder by flowing Pb-Bi.

The thermal-fluid behavior of the target was analyzed by the STAR-CD. The quarter-part model was set to tetra metric type and the divided face was set to a reflected image condition. At first, Pb-Bi flowed through the annular region and joined in the center of the beam window, and then, turned over and flowed in the inner tube after having passed a rectification lattice and an irradiation sample. In a default condition, flow rate at the inlet of annulus region was set to 1 l/s, and this was equivalent to the flow velocity of 0.125 m/s. Because the Pb-Bi flow forms a

Fig. 8.2 Prototype LBE spallation target for TEF-T

image43complicated turbulent flow, the standard k-є model for high Re number type was used for a turbulence model. Heat deposition distribution by the primary proton beam, which was calculated by a hadronic cascade code PHITS [8], was used for the analysis. The internal pressure to the inside of the beam window was set to

0. 3 MPa in consideration of the flowing Pb-Bi and the cover gas. On the outer side of the beam window and the border of the atmosphere, release of the radiant heat was considered. Embrittlement of the structural materials by irradiation was not considered.

The analyses were performed by changing flow rates from 1 to 4 l/s. In each case, a dead region was commonly formed in the center of the inside of the beam window. The maximum velocity of Pb-Bi was confirmed at the rectification lattice part and was approximately 1.2 m/s in the case of the inlet flow rate of 1 l/s. When the inlet flow rate increased to 4 l/s, the maximum velocity in the target reached

4.8 m/s, which is too high to apply to the Pb-Bi target. The maximum temperature is 544 °C in the case of a 3-mm-thick window. The peak temperature can be decreased to 477 °C in the case of 2-mm-thick window. The temperature differences between outside and inside at the center of the window were 65 and 37 °C in the case of the 3-mm-thick window and the 2-mm-thick window, respectively. From these results, it was determined that a condition of 2 mm was desirable.

Based on the results provided by STAR-CD, analysis to verify the feasibility of the beam window was performed by ABAQUS code. The operating conditions for the first stage of material irradiation in TEF were decided by a result of the analysis on each condition. The temperature and thermal stress for the steady state were estimated using ABAQUS code, the computational code for the finite-element method. In the ABAQUS code, only a beam window was modeled as the cylinder-slab geometry. From the analysis result for the 2-mm-thick window, the stress strength reached the maximum value of 190 MPa on the outer surface of the beam window. When the maximum temperature of the beam window is adopted to 470 °C from the result of STAR-CD, maximum stress is lower than the tolerance level of the materials for fast reactor, and hence, the feasibility of a designed beam window was confirmed.

8.4 Conclusion

To perform the design study for the transmutation system of long-lived nuclides, the construction of TEF, which consists of two buildings, TEF-T and TEF-P, is proposed under the J-PARC Project. According to the current construction sched­ule, TEF-T will be built at the first phase and TEF-P will be constructed at the latter phase. Licensing procedures for TEF-P construction will be processed simulta­neously with TEF-T construction.

TEF-T is a facility to prepare the database for engineering design of an ADS using a 400 MeV-250 kW proton beam and the Pb-Bi spallation target. The purposes of TEF-T are R&D for the structural strength of the beam window, which is irradiated by both high-energy protons and neutrons, compatibility of the structural material with flowing liquid Pb-Bi, and operation of the high-power spallation target. Several kinds of target head can be installed according to the experimental requirement. It was shown that the reference case of injected proton beam condition (400 MeV-250 kW and 20 pA/cm2 of beam current density) was applicable to the TEF-T target. Further studies to improve irradiation performance are under way.

TEF-P is a critical assembly, which can accept the 400 MeV-10 W proton beam for the spallation neutron source. The purposes of TEF-P are the experimental validation of the data and method to predict neutronics of the fast subcritical system with spallation neutron source, demonstration of the controllability of a subcritical system driven by an accelerator, and basic research of reactor physics for transmu­tation of MA and LLFP. The distinguishing points of the TEF-P in comparison with existing experimental facilities can be summarized as follows: (1) both the high — energy proton beam and the nuclear fuel are available, (2) the maximum neutron source intensity of about 1012 n/s is strong enough to perform precise measurements even in the deep subcritical state (e. g., keff = 0.90) and is low enough to easily access the assembly after the irradiation, (3) a wide range of pulse width (1 ns-

0. 5 ms) is available by the laser charge exchange technique, (4) MA and LLFP can be used as a shape of foil, sample, and fuel by installing an appropriate shielding and remote handling devices.

Along with the design study of the TEF, R&D for the components required for TEF, such as the laser charge exchange technique to extract a very low power proton beam, test manufacturing of MA fuel-handling devices, and operation of lead-bismuth test loops are under way. From the experimental results of the laser charge exchange technique, beam extraction in the magnetic field is successfully demonstrated. Mockup of the coolant simulator block and remote handling mech­anism for pin-type fuel loading has been done. An effective method to remove polonium with a standard stainless mesh filter was established through the hot experiments. Significant improvement of analysis accuracy of actual ADS was expected by critical experiment with MA fuel at TEF-P.

When the target of TEF-T operates with a full power beam, a fast neutron spectrum field is formed around the target and it is possible to apply multipurpose usage. Various research plans have been proposed, and layout of the experimental hall surrounding the target is under way. Basic physics application such as mea­surements of nuclear reaction data is considered as one of the major purposes. We called for a preliminary letter of intent to encourage the project. Requests for multi­purpose usage will be taken into account in the facility design of TEF.

Open Access This chapter is distributed under the terms of the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

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