|
内容記述 |
Self-cooled Li and Flibe blankets with V-alloy structure are attractive <br />concepts and considered for the LHD-type helical fusion reactor design<br /> FFHR. Maximum thickness allowed for blanket is 1.2 m in FFHR. Blanket<br /> functions such as heat removal due to 14 MeV D-T fusion neutrons, breeding<br /> of tritium fuel and radiation shielding must be fulfilled within this thickness. <br /> To verify blanket functions, detailed neutronics investigations are <br />required. One of the key options for Li and Flibe blankets is the use of<br />neutron multiplier Be because, in spite of potential benefit of Be from<br /> neutronics aspects, there are some issues specific to Be such as irradiation<br /> effects. Thus it is necessary to evaluate quantitatively the impact of Be on<br /> tritium breeding ratio (TBR), radiation shielding and radio activation. For Li <br />blanket, it is necessary to apply an electrical insulating coating e.g. Er<small>2</small>O<small>3</small>, to<br />reduce the magnet-hydro dynamic (MHD) pressure drop when Li flows in a<br />strong magnetic field. However, neutronics investigation on the effect of<br /> Er<small>2</small>O<small>3</small> is scarce. <br /> To provide the reactor design with viable data, the neutronics analysis <br />procedure including transport and activation codes and nuclear libraries<br /> applied to liquid blankets need to be examined and, if necessary, improved.<br />For this purpose, benchmark studies with systematic comparison of<br /> calculations and experiments are necessary. <br /> The objectives of the present study are: <br /> To investigate Li and Flibe blankets for FFHR by neutronics analysis<br />with focus on the effect of Be and Er2O3 coating on TBR, shielding and<br />activation. <br /> To examine or improve the neutronics calculation procedure for<br />application to liquid blankets by comparison with activation experiments<br />using D-T neutrons.<br /> In the neutronics assessment of FFHR, 3 dimension Monte Carlo code<br />MCNP-4C with JENDL3.2 pointwise nuclear data file and FISPACTL2001<br />code with EAF-2001 file in 175 energy groups were used for neutron<br />transport and activation calculations, respectively. The results obtained are<br />qualitatively assumed as follows: Use of Be can significantly improve TBR<br />for both Li/V-alloy and Flibe/V-alloy blankets. For the Li/V-alloy blanket with<br />Be, the shielding property can be greatly improved maintaining the adequate<br />TBR. On the other hand, the shielding property of the Flibe/V-alloy blankets<br />with and without external Be is comparable. The quantitative estimate of<br />the effects of Er<small>2</small>O<small>3</small> coating showed that the activation of the coating could<br />influence, the long-term activation property of the structural components. <br />However, recycle of materials for the structural components is still feasible<br />with Er<small>2</small>O<small>3</small> coating in 10μm thickness. With 1μm thickness of the coating,<br />the activation level of the blanket is close to the hands-on recycling limit. <br /> For the purpose of verifying or improving the neutronics calculation <br />procedure, especially activation analysis of liquid blankets, a series of <br />irradiation experiments were performed using Fusion Neutronics Source<br />(FNS) at Japan Atomic Energy Agency (JAEA), which is well-suited to<br /> neutronics study relevant to a D-T fueled reactor. The specimens of<br />V-4Cr-4Ti, Er and Teflon in 10 mm×10 mm×0.03-0.1 mm were prepared for<br />studying the activation of V-alloy structure, MHD coating of Er<small>2</small>O<small>3</small>, and F in<br />molten salt Flibe, respectively. In addition to the assembly for direct D-T<br />neutron irradiation, Be, Li and Li/Be mock-ups were assembled to examine<br />the dependence of the activation on neutron spectrum expected in fusion<br />blankets. <br /> The activities of specimens measured with a high purity Ge detector<br />were compared with the calculations using FISPACT-2001 codes and <br />EAF-2001 file. The neutron flux calculated by MCNP-4C was used as input<br />file in FISPACTL-2001 calculation. In this study, the continuous-energy<br />cross-section data were also used in activation calculation in the limited<br />cases in addition to the data of EAF-2001 in 175 and 315 energy groups. <br /> Comparison of calculation (175 groups) to experiment (C/E) shows that <br />the values of C/E for most of radioactive nuclides (<sup>18</sup>F, <sup>48</sup>Sc, <sup>161</sup>Er, etc)<br />produced by reactions with high-energy threshold lie in the range of 0.8-1.2.<br />A significant underestimation for <sup>168</sup>Ho (C/E 0.62) was found in D-T neutron<br />irradiation case. It could be caused by the effects of coarse energy grouping<br />near 14 MeV because <sup>168</sup>Er(n,p)<sup>168</sup> Ho has a threshold energy close to 14 MeV<br />and its cross-section rises steeply around 14 MeV. In fact, the C/E value of<br /> <sup>168</sup>Ho calculated with continuous-energy cross-section approached unity. <br /> The comparisons of activation for <sup>171</sup>Er and <sup>52</sup>V, both of which are the<br /> products of (n, Υ) reactions, were performed with activation data in 175, 315<br />energy groups and continuous data. For <sup>52</sup>V, the major contribution is the<br />reaction with thermal neutrons. Using 315 group cross sections, C/E values<br />of <sup>52</sup>V show a clear trend approaching to unity relative to those using 175 <br />energy groups case because of the finer group structures in thermal neutrons<br />range. The results also agree well with the results using the <br />continuous-energy cross section. The reaction <sup>170</sup>Er(n, Υ)<sup>171</sup>Er has a huge<br />resonance peak at 95 keV. Large overestimations for <sup>171</sup>Er were observed<br />with group data due to the coarse grouping in the resonance region. Use of<br />continuous-energy cross-section provided much better agreement with<br /> experiment for Be mock-up, where the neutron flux is almost constant with<br />energy in the resonance range. On the other hand, for the case of Be/Li<br /> mock-up, where the flux rapidly increases with energy in the resonance<br /> range, large discrepancy (C/E 〜1.17) was observed even with continuous<br />cross-section. This result clearly indicates the necessity for re-evaluation of<br /> the cross-section data of <sup>170</sup>Er(n, Υ)<sup>171</sup>Er reaction. This issue was disclosed<br />though the comparative examination of C/E in various neutron spectra, and<br />thus unique achievement of this study. <br /> Conclusions of the present study are: <br />A) Detailed neutronics characterization of Li and Flibe blankets using<br />V-alloy structure for FFHR has, for the first time, been carried out: <br /> A-1) Advantage of the use of Be is quantified. With Be, TBR design margin<br /> for Li and Flibe blankes could be improved. For Li blanket with Be, the<br /> shielding design margin could also be improved with adequate TBR. The<br /> results provide information necessary for characterizing trade-off of using<br /> Be in Li and Flibe blankets. <br /> A-2)The Er<sub>2</sub>O<sub>3</sub> coating induces the long-term radioactivity of Li blanket. <br /> However, recycle of structural materials is still feasible with Er<small>2</small>O<small>3</small> coating<br /> in 10μm thickness. Thinner Er<sub>2</sub>O<sub>3</sub> coating is necessary for realizing <br /> hands-on recycling.<br />B) Experimental studies of activation characteristics for materials in Li and <br />Flibe blankets have been carried out using a D-T neutron source, especially<br /> for the first time for the Er<small>2</small>O<small>3</small> coating: <br /> B-1) For the threshold reactions, where the threshold energy is close 14 MeV <br /> and its cross-section rises steeply around 14 MeV, continuous-energy <br /> cross-section data is needed.<br /> Be2) For the (n, Υ) reaction with high cross-section at thermal neutrons<br /> region, finer grouping is necessary.<br /> B-3) Use of continuous-energy cross-section is necessary for (n, Υ) reaction<br /> with dominant resonance peak in medium energy ranges.<br /> B-4) The cross section data of <sup>170</sup>Er(n, Υ)<sup>171</sup>Er needs re-evaluation. |
要旨・審査要旨 (358.4 kB)
