Integral Study On The Reaction Of D-T Neutron With Thorium And

According to the conceptual design of fusion-fission hybrid reactor, natural uranium or thorium could be used as the fuel cladding. The feasibility of the physical design of hybrid reactor with natural uranium and thorium is decided by the accuracy of the simulation methods and nuclear cross sections. There only thorium isotope in nature is232Th, which will be converted into fissile233U when it absorb a neutron and after passing through the two-stage decay, generated233U arise fission to form uranium thorium cycle. The proliferation of233U is related with neutron multiplication reactions of thorium, the fission reaction rate and the (n,2n) reaction rate of232Th is an important parameter to reflect the fission and multiplication reaction in the physics design of thorium uranium cycle. The reliability of cross section data of these relevant reactions must be tested through neutronic integral experiments. Resolved natural uranium simulation device is the best device to check the physical design of fusion-fission hybrid reactor cladding at present, it’s important to measure the capture rate of238U in the device since many parameters in the physical design of a reactor are sensitive to the capture rate of238U.In order to verify the accuracy of the evaluated cross sections of thorium and uranium, a series of integral experiments were carried out by using a D-T neutron source and neutron activation method. A thorium oxide sample device based on a depleted uranium sphere and a series of thorium oxide/depleted uranium assembled devices were established for checking the evaluated cross sections of thorium fission reaction and (n,2n) reaction, and232Th(n, f) reaction rate and232Th(n,2n) reaction rate along the axial centerline of those cylinders were obtained. The238U capture rate in some channels of the resolved natural uranium simulation device was measured. Experimental results were compared with these simulation results by using the MCNP5program with some commom evaluated data bases.Thorium fission reaction rate was obtained by measuring the gamma rays emmitd from85mKr which was produced by thorium fission. Combined with the measured151.16keV y-ray count and the calculated neutron spectrum in thorium samples and the yield of85mKr from thorium fission, the thorium fission rate with an uncertainty between5.3%and5.6%along the axial centerline of those cylinders were obtained after the self-absorption correction factor and the detect efficiency were determined. These integral experiments in depleted uranium sphere, three ThO2cylinders, two ThO2cylinders and a depleted uranium cylinder, three ThO2cylinders and a depleted uranium cylinder were simulated by using MCNP5program with ENDF/B-VI.6and ENDF/B-Ⅶ.0libraries, ENDF/B-Ⅶ.0thorium data produce significantly better fission rates (average2.5%,2.9%,2.1%,1.8%) relative to ENDF/B-Ⅵ.6data, the calculations/experimental results (C/E) with ENDF/B-VII.O library of the four sets of devices were0.98,0.88,0.79and0.78in average.By measuring the84.2keV y-ray emitted from231Th to study the232Th(n,2n) reaction rate.232Th(n,2n) reaction rate was obtained by measuring the84.2keV y-ray emitted from activated thorium samples and the determination of self-absorption correction factor and detect effeciency with an experimental uncertainty of6.7%-6.9%. In the four devices, ENDF/B-Ⅶ.0calculation results were higher than ENDF/B-VI.6with an average of about9.8%,9.6%,9.6%,9.7%, and shows a great discrepancy between the two libraries. The calculations/experimental results (C/E) with ENDF/B-Ⅶ.0library of the four sets of devices were1.08,0.96,0.95and0.89in average.The238U(n, y) reaction rate in natural uranium device was researched by measuring the277.6keV characteristic y-ray emitted from239Np in circular natural uranium foils with an experimental uncertainty of4.4%-4.5%. The C/E with ENDF/B-VI.6and ENDF/B-Ⅶ.0libraries are0.91and0.925in average respectively. The D ion beam drift on the target could be neglected through the MCNP5simulation. Analysis shows that the great discrepancy is mainly due to the ratio of resonance region to all neutrons is large and the capture cross section of238U in resonance region should be improved.