Introduction of Radiation Research Center and its Research Activity onNuclear Fusion Reactor Technology

Joint Symposium between Dalat Universityand Osaka Prefecture University

26th October, 2016, Da lat(Vietnam)Hiroto Matsuura, Masafumi Akiyoshi

Figure 1: Photo of Co-60 gamma-ray source and Cherenkov light inthe water pool.

In 1959, Radiation Center of Osaka Prefecture(RCOP) was established with large scale radiation facili-ties for promoting researches and application in the fieldof radiation science and technology. In 1990, RCOP wasmerged in Osaka Prefecture University (OPU) and reorga-nized a few times. After such historical transition, Radia-tion Research Center (RRC) succeeds the radiation facili-ties, the main members of staff, the radiation technology,radiation protection system, and research activity. [1] Ourcobalt 60 gamma-ray irradiation facility consists of 4 hotcaves and water pool with 4-400 TBq Co-60 source. Totalamount of Co-60 is about 1.8 PBq, which is largest amongJapanese university’s facility. Co-60 sources in the waterpool (Fig. 1) can produce flexible irradiation configurationand maximum dose rate of 50 kGy/h. Other irradiationfacilities in RRC are 16 MeV S-band RF linac, 600 keVCockcroft-Walton electrostatic electron accelerator, Twotypes of X-ray source. These facilities are used for radiation process, simulation test for nuclearreactor or space craft devices, biological application. Recently our group developed atmosphericpressure plasma jet with very low energy ions and electrons. Sterilization experiments withthis plasma source and gamma-ray source are compared and inactivation mechanism is studied.RRC has other Radioisotope handling laboratories and nano-fabrication instruments, and itsstaff belong to 5 research groups and promote basic and applied research in very wide researchfield.

Figure 2: ITER and its divertor.

One of our group research is the nuclear fusion reactortechnology. Nuclear fusion is the energy source of our sunand other stars. Many nations such as US, Russia, EU,Japan, have promoted research study to control fusion re-action and make a compact artificial star on our planet.After D-T burning experiments in 1990th, these countriesjoin to build an experimental reactor ITER and start de-sign study of individual demonstration reactors (DEMO).Figure 2 shows the design of ITER, in which its divertoris indicated with a yellow arrow. Divertor is one of mostimportant component to realize burning plasma. It col-lect extra particle gas such as impurity fron chamber wall,helium ash produced in D-T burning, and unburnt hydro-gen isotops, and evacuate them to prevent core plasmadilution or radiation cooling. Unfortunately this functionalso gathers heat load on the divertor component and peakheat flux density would reach to 100MW/m2 (same orderof sun surface) even under normal condition. Beside of radiation problem, this heat handling isthe biggest technological issue to design DEMO.

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Figure 3: Heat flux measurement ofLHD divertor plasma.

One promising method to reduce heat road is the so-called ”detached plasma formation”. If gas pressure isincreased without back flow to main plasma, plasma ionand electron lose their thermal energy and recombine eachother. Many plasma groups make effort to understand thedetachment mechanism and to keep this state long andsafety. Unfortunately, since divertor and plasma are farfrom thermal equilibrium and their temperature is time-dependent, reasonable mathematic model to relate tem-perature data with time-dependent heat flux. Under thecollaboration with National Institute for Fusion Science(NIFS), we developed the step heat pulse decompositionmodel and applied it to the experimental data of LargeHelical Device (LHD). [2] During 3 second discharge, ad-ditional gas puffing make plasma detached. The sensorfor divertor plasma shows reduction of its temperature.Our model succeed to shows heat flux becomes zero evenbefore discharge termination for the first time.

Figure 4: DEGAS 2 simulationgeometry for TPDSheet-IV experi-ment.

Japanese researcher group found important process fordetachment. Direct Electron- ion Recombination (EIR)becomes important only for very low electron temperature( < 1 eV ). But Molecular Assisted Recombination, whichinvolves molecular ions, promotes detachment process athigher temperature. Tonegara reported density ratio ofmolecular ion (H+

2 , H+3 ) becomes large during detachment

process in TPDSheet-IV device. We started collaborationwith his group and study behavior of neutral particle andmolecular ion. We use DEGAS 2 neutral transport codedeveloped at Prinston Plasma Laboratory, and have de-veloped geometry model as Fig.4. This enable us to getdetail hydrogen atom/molecule density profile in the com-plicated device with powerful gas injection. [3] Now we aredeveloping reaction model including both neutral particleand molecular ion.

As for technological point, heat transfer in divertor plate and cooling pipe must be maximizedand kept its performance during operational period. Heat conductivity ( thermal diffusivity) is the most important parameter to design divertor cooling. Phonon conductivity is themain process in ceramic such as SiC, and heat transport in tungsten (W) also depends on it.Phonon mean free path in solid is determined by both phonon-phonon scattering and phonon-lattice scattering. Unfortunately the latter increase with amount of defect induced by neutronirradiation. [4] So even if cooling design is possible for ITER, it is still an open question todesign reliable divertor for DEMO or commercial reactor with much larger neutron dose. Soit is important to study the relation between radiation induced defect and thermal diffusivity.Since induce radioactivity by neutron absorb reaction make it difficult to get samples for thesestudy. Moreover, since radiation energy heats sample and phonon-phonon scattering dependson temperature, it is difficult to reproduce the reactor temperature condition. We develop thehe method to estimate of thermal diffusivity and amount of defects during the irradiation. Weapply these method to many samples under wide collaboration research.

[1]S.Okuda et al.: Proc. ISSI 2011, Hefei(China), 101-103.[2]H.Matsuura et al.: Contrib. Plasma Phys. 54 (2014) 285-290.[3]H.Matsuura et al.: Plasma Fusion Res., 6(2011) 2401104.[4]M.Akiyoshi et al.: Journal of Nuclear Materials, 386-388 (2009) 303-306.