Dpa Values Research Articles

Damage Calculation for First Wall Submitted to High Neutron Flux in a Tokamak

ABSTRACTThe displacement per atom (dpa) has been a specific issue to evaluate the damage in the first wall of the Tokamak. Two different first wall materials were evaluated. In this study, MCNP5 code was used to obtain the neutron flux, the energy deposition and the main reaction rates, on the inboard and outboard first wall. The damage calculations were performed by the SPECTER code using the neutronic parameters obtained by MCNP5. The Tokamak reactor modeled has similar dimensions to the ITER. Tungsten and beryllium alloys were simulated on the outboard first wall. The results indicate which material has a higher resistance to be damage and dpa values for the analyzed material.

Evaluation of the reactor pressure vessel steels by positron annihilation

This paper presents a comparison of commercially used German and Russian reactor pressure vessel steels from the positron annihilation spectroscopy (PAS) point of view, having in mind knowledge obtained also from other techniques from the last decades. The second generation of Russian RPV steels seems to be fully comparable with German steels and their quality allows prolongation of NPP operating lifetime over projected 40years. The embrittlement of CrMoV steels is relatively low due to effect of higher temperature which implies partial in situ annealing of primary microstructural point defects and therefore delays the degradation processes caused by neutron irradiation.PAS techniques can be effectively applied for evaluation of microstructural changes caused by extreme external loads (characterized by high dpa values) by proton implantation, with aim to simulate irradiation and for the evaluation of the effectiveness of post-irradiation thermal treatments. We used our actual and previous results, collected during last 20years from measurements of different RPV-steels in “as received”, irradiated and post-irradiation annealed state and compare them with the aim to contribute to general knowledge based on experimental PAS data.Actual results from irradiated German and Russian steels confirmed that no large voids or vacancy clusters were formed at defined irradiation conditions stated according to the real operational conditions at nuclear power plants. This indicate the fact that vacancy type defects bear hardly any responsibility for radiation-induced hardening and embrittlement of reactor pressure vessel steels and does not affect significantly the long-term operation of nuclear power plants from safety point of view.

Calculation of the damage and the H and He productions for 14.1-MeV neutrons

The interactions of 14.1-MeV neutrons from nuclear fusion reactions with a pfc (plasma facing component) are analyzed using the MCNPX/SPECTER code system. The dpa (displacement per atom) values in the three-layered pfc of graphite, Cu, and SS316L based on the KSTAR (Korea Superconductor Tokamak Advanced Reactor) device are calculated by the SPECTER code. The production rates of hydrogen and helium gas are calculated for an analysis of the nuclear damage effect. The serious irradiation condition of the ITER (International Thermonuclear Experimental Reactor), a neutron flux of 3.5 × 1013 neutrons/cm2·sec, was applied. The dpa values during oneyear operation were estimated as 0.6 dpa for graphite, 4.6 dpa for Cu and 2.2 dpa for SS316L. And the production rates of H and He atom for SS316L were estimated to be 40 appm/dpa and 11 appm/dpa, respectively.

The production of amorphous regions in carbon nanotubes by 140 keV He ion irradiation

The production of amorphous regions in carbon nanotubes irradiated with 140keV He ions was studied using Raman spectroscopy and transmission electron microscopy (TEM). Intensity ratios of Raman D to G bands (ID/IG) initially increase and then decrease as a function of ion fluence at all investigated irradiation temperatures (room temperature, 200 and 400°C). The critical ion fluences corresponding to the maximum in ID/IG ratios increase with increasing irradiation temperature because of the enhanced defect annealing. The displacement per atom (dpa) values, consistent with a maximum in ID/IG ratios, are determined to be 0.15dpa at room temperature and 200°C, and 0.3dpa at 400°C. TEM examination of all irradiated specimens supports Raman results indicating that the maximum in ID/IG correlates to the formation of amorphous regions. The study shows that after formation of amorphous regions at high fluences, ID/IG ratio can be no longer used to measure amorphous/graphitic content in CNTs.

DPA calculation for the D-D nuclear fusion reaction in the KSTAR PFC

The interactions of fusion reaction products such as 2.45-MeV and 14.06-MeV neutrons and 3.05-MeV protons with the KSTAR PFC (plasma facing component) are analyzed using Monte Carlo codes. The dpa (displacement per atom) values in the three-layered PFC of graphite, Cu, and SS316L are calculated, and the depth profile was analyzed for different-type secondary particles. The PHITS code was used for this study. The deposited energy was also calculated for analysis of the nuclear heating effect. The serious irradiation condition of the International Thermonuclear Experimental Reactor (ITER) parameter, a neutron flux of 3.5 × 1013 neutrons/cm2·sec, was applied. The dpa values during one year operation were estimated as 0.75 dpa for graphite, 4.57 dpa for Cu, and 2.69 dpa for SS316L.

The expected radiation damage of CSNS target

The radiation damage to the tungsten target and its SS316 vessel for Chinese Spallation Neutron Source (CSNS) has been estimated with a Monte-Carlo simulation code MCNPX2.5.0. We compare the effects on the radiation damage due to two different proton beam profiles: a uniform distribution and a Gaussian distribution. We also discuss the dependence of the radiation damage estimation on different physics models. The results show the peak displacement productions in vessel and the fourth target plate are 2.5 and 5.5dpa/y, respectively, under a Gaussian proton beam. The peak helium productions in the vessel and the fourth target are 305 and 353appm/y, respectively, under the same proton beam. Based on these results and the allowable dpa values we have estimated the lifetime of the tungsten target and its vessel.

Effects of ion irradiation on the hardness properties of graphites and C/C composites by indentation tests

Small blocks of graphite and C/C composite materials were argon ion irradiated. The load–depth curves were obtained from the micro indentation tests before and after ion irradiation. For the C/C composite material, the indentation test was also performed perpendicular to the depth direction of ion irradiation and the distribution of the hardness properties was examined in the depth direction. As a result, hardness properties of the carbon materials were able to be expressed as a function of dpa values.

DPA calculation for Japanese spallation neutron source

A neutron generating target and surrounding components, such as moderators and proton-beam windows, of intense pulsed spallation sources suffer serious radiation damage. In order to construct a maintenance scenario of the Japanese Spallation Neutron Source, the life estimation of those components due to the radiation damage becomes indispensable. For this purpose, we calculated DPA values of the important components, based on the calculation of displacement cross-sections using PHITS and NJOY codes with the LA150 library. Maximum DPA values at the end of 5000 MW h are 3.9 for the target vessel, 2.8 for the reflector and moderator vessels, and 0.4 for the proton-beam window. We also discuss the effect of the proton-beam profile and proton-energy dependence on the DPA values. Briefly, we showed the calculation result on the helium and hydrogen production.

Electron spin resonance measurement of irradiation defects in vitreous silica irradiated with neutrons and ion beams

The electron spin resonance (ESR) measurement of irradiation defects in ion beam irradiated vitreous silica was performed. The spin densities of E ′ centers, non-bridging oxygen hole centers (NBOHCs), and peroxy radicals (PORs) were measured as a function of the ion beam intensity, fluence, and post-irradiation isochronal annealing temperature. The results of the ion beam irradiation were compared with those of neutron irradiation, and both results were consistently explained by considering the dpa values of irradiation. Based on the proposed mechanism and mass balance, the concentration of the other defects such as oxygen deficiency centers (ODCs) were evaluated.

Analysis of the monoclinic–tetragonal phase transition of zirconia under irradiation

Zirconia produced by the oxidation of zirconium alloys in nuclear reactors exhibits a phase transition under ionic irradiation, simulating a neutron irradiation. To understand the mechanism responsible for this irradiation driven phase transition, different kinds of projectiles were used to irradiate pure monoclinic zirconia samples. The evolution of these irradiated samples as a function of dpa has been studied using grazing X-ray diffraction. The Rietveld method has been applied on collected X-ray diffraction diagrams to study the phase produced under irradiation and the kinetics of its formation. Even at high dpa values, only the monoclinic and tetragonal phases were used to simulate X-ray diffraction diagrams. No amorphisation of zirconia was observed. The evolution of unit cells and short range strains in both phases under irradiation leads us to think that the irradiation driven transition is martensitic. Supposing that the inelastic stopping power in sub-cascades is responsible for the irradiation driven phase transition, we propose a model based on the Landau–Ginzburg effective hamiltonian to explain both the m→ t transition observed under irradiation and the t→ m transition measured during isochronal annealing after irradiation.

Investigation of the flattened fissile fuel enrichment possibility with a (D, T) driven hybrid blanket cooled by flibe (Li 2BeF 4)

This work investigated the possibility of the fuel production with flattened cumulative fissile fuel enrichment (CFFE) at the hybrid reactor, to be cooled by the flibe (Li 2BeF 4) and fueled by UO 2 with a LWR fuel rod and CANDU fuel rod diameters, LWR spent fuel and CANDU spent fuel, with an original fuel rod diameter during the operation period of 5 years. For that purpose, the new fuel zone structure is provided by means of the ratio of the flibe to fuel per fuel row in the fuel zone being varied. Neutronic performance of the (D, T) driven hybrid blanket with this fuel zone is followed by a plant factor of 75% under a first wall load of 5 MW/m 2. The fuel row numbers are selected as 10. For all fuels, the possibility of the fuel production having almost the same CFFE in all fuel rows of the fuel zone of the hybrid blanket is possible by a deviation of 2%. Moreover, the fissile fuel production capability of the suggested blanket increased considerably. However, tritium breeding ratios and the displacement-per-atoms (dpa) values in the first wall and clad material are almost not affected by this blanket structure, the energy production decreased slightly. At the end of the operation period of 5 years, the CFFE value reached ≈8.5, ≈9.3, ≈8.4 and ≈8.2% for UO 2 with LWR rods, LWR spent fuel, UO 2 with CANDU rods and CANDU spent fuel, respectively. The remaining fuel from hybrid blankets with CANDU and LWR spent fuels have enough safety from the viewpoint of the plutonium non-proliferation since the isotropic percentage of 240Pu in the produced plutonium is higher than 7%. However, other cases with UO 2 fuel can reach sufficient safety after an operation period of 30 months.

Effect of ?-Irradiation on Superconductivity in Polycrystalline YBa2Cu3O7-?

A bulk polycrystalline sample of YBa_{2}Cu_{3}O_{7-\delta} (\delta \approx 0.1) has been irradiated by \gamma-rays with ^{60}Co source. Non-monotonic behavior of T_c (defined as the temperature at which normal resistance is halved) with increasing irradiation dose \Phi (up to about 220 MR) is observed: T_c decreases at low doses (\Phi \leq 50 MR) from initial value (\approx 93 K) by about 2 K and then rises, forming minimum. At highest doses (\Phi \geq 120 MR) T_c goes down again. The temperature width, \delta T_c, of resistive transition increases rather sharp with dose below 75 MR and somewhat drops at higher dose. We believe that this effect is revealed for the first time at \gamma-irradiation of high-T_c superconductor. The cross section for the displacement of lattice atoms in YBCO by \gamma-rays due to the Compton process were calculated, and possible dpa values were estimated. The results obtained are discussed taking into account that the sample is granular superconductor and, hence, the observed variations of superconducting properties should be connected primarily with the influence of \gamma-rays on intergrain Josephson coupling.

Investigation on the zirconia phase transition under irradiation

Zirconia, ZrO 2, produced by the oxidation of zirconium alloys in nuclear reactors, possesses a high stability under neutron irradiation. No amorphisation of yttrium-stabilised zirconia has been observed even at high dpa values (≈100 dpa). In pure monoclinic zirconia, a phase transition monoclinic → cubic (tetragonal) induced by irradiation has already been observed. The aim of this work is to study in detail the mechanism responsible for this transition. For that purpose, different kinds of irradiations with electrons (to study point defects) and low energetic ions (to study clusters due to collision cascades) have been performed on zirconia samples. A local probe (Raman spectroscopy) and a non-local probe (grazing X-ray diffraction) have been used to characterise the phase formed during irradiation, which is clearly the tetragonal phase. For the ionic implantation, the grazing X-ray diffraction permits to separate effects due to the ballistic collisions and the implantation peak. Using this method, it was possible to show that the profile of the tetragonal phase was only linked to the dpa profile. This result associated to the results obtained by the Raman spectroscopy (broadening of Raman peaks) shows that the phase transition may be induced by clusters formed near the collision cascades.

Neutronic Calculations for a Magnetic Fusion Energy Reactor with Liquid Protection for the First Wall

Liquids may be used between the magnetic confined fusion plasma and the first wall of the plasma chamber to reduce the material damage through displacements per atom (dpa) and helium gas production. This could extend the lifetime of the first wall in a magnetic fusion energy (MFE) reactor to a plant lifetime of ~30 yr.Neutronic calculations are carried out in S16P3 approximation for a typical HYLIFE-II blanket geometry, an inertial fusion energy (IFE) reactor design. This provides a comparison of the damage data between compressed and uncompressed targets, for IFE and MFE applications, respectively, by using Flibe (Li2BeF4), natural lithium, and Li17Pb83 eutectic as both coolant and wall protection. In the consideration of mainline design criteria, including sufficient tritium breeding ratio (TBR = 1.1), material protection (dpa < 100 and He < 500 parts per million by atom in 30 yr of operation), and shallow burial index, coolant zone thickness values are found to be 60 cm for Flibe, 171 cm for natural lithium, and 158 cm for Li17Pb83 with Type 304 stainless steel (SS-304) as structural material.Material damage investigations are extended to structural materials made of SiC and graphite for the same blanket to obtain waste material suitable for shallow burial after decommissioning of the power plant.The dpa values and helium production rates in graphite are comparable to those in SS-304. However, they are higher in SiC than in SS-304 and graphite.The average neutron heating density in the external 1.6-mm-thick SS-304 shell of the investigated blanket beyond the SiO2 insulation foam decreases rapidly with increasing thickness of the Flibe coolant. With DR = 60 and 80 cm, it becomes only 594 and 95 μW/cm3, respectively. The design limit for heat generation density in superconducting coils for magnetic fusion is 80 μW/cm3. A very important result of this work is that a blanket with liquid-curtain protection would not require extra shielding for superconducting coils around the fusion plasma chamber. This could result in an important simplification of the design.

Stone Fragility - Measurement of Stone Mineral Content by Dual Photon Absorptiometry

We measured the mineral content of urinary tract stones by dual photon absorptiometry, which is widely used for the analysis of bone mineral content, and compared the values of the stones by dual photon absorptiometry (DPA values) with the results of an in vitro fracture study as well as those of an in vivo extracorporeal shock wave lithotripsy treatment study. The results of a preliminary experiment showed that the DPA values of 20 urinary tract stones reflected actual stone mineral content. As a result of the in vitro fracture study, the DPA value calculated by volume of a struvite stone, which was the most easily disintegrated, was the lowest (0.53 g/cm3). The DPA values of calcium oxalate monohydrate and apatite stones, which poorly disintegrated, were the highest (0.98, 1.01 g/cm3). The DPA value of calcium oxalate dihydrate, which moderately disintegrated, was 0.86 g/cm3. By the in vivo extracorporeal shock wave lithotripsy treatment study, the total DPA values of stones measured before extracorporeal shock wave lithotripsy treatment in 12 patients were 0.73 +/- 0.34 g in successful cases and 1.92 +/- 0.43 g in unsuccessful cases with a significant difference between the two (p less than 0.05). These results showed that the measurement of stone mineral content by dual photon absorptiometry was useful in predicting the fragility of stones against shock waves before performing extracorporeal shock wave lithotripsy treatment for patients with urinary tract stones.

In-situ high-voltage Electron Microscopy of defect ordering under high-dose irradiation

High-dose irradiation of metals employing dpa values (= displacements per atom) substantially larger than unity may lead to ordered arrangements of defects at temperatures well below the temperature of void formation. We report on a systematic study of defect-ordering phenomena in three fcc metals with markedly different stacking-fault energies, viz. Cu, Ag, and Ni.5N-pure foils of Cu, Ag and Ni were prepared for in-situ electron microscopy in the conventional way. They were irradiated in an AEI-EM7 high-voltage electron microscope (HVEM) with lMeV electrons (e-) at various temperatures (Tirr) between 20 and 500K (Ag and Cu) or between 300 and 600K (Ni). The electron flux density and the total dose, Φ, were typically 7×l023e-m-2s-1 and 1x1027e-m-2, respectively. The development of the defect patter was investigated in the HVEM during the irradiation.In all three metals a temperature regime has been found in which visible defects are regularly arranged along &lt;100&gt; directions after a minimum dose of ∼1026 e-m-2 (Fig.l).

The Influence of Irradiation-Induced Defects on the Superconductivity of YBa2Cu3O7

Thin superconducting films of YBa2Cu3O7 have been irradiated at 293 K and 77 K with H, He and Ar ions. Independent on the mass and the energy of the incident ions the Tc reduction below 4.2 K occurred in the region between 2×10-3 and 1.6×10-2 displacements per atom (dpa). For dpa values greater than 4×10-3 (0.2 eV/atom) a strong increase of the resistance was noted indicating a metallic to insulating transformation. The orthorhombic to tetragonal phase transformation occurred between 0.01 and 0.03 dpa while the transformation into the amorphous phase was observed at about 0.1 dpa after He irradiation at 77 K. The damage mechanisms which are responsible for all this changes are discussed.

Heavy-ion and electron irradiation effects in vitreous silica

Amorphous silica samples have been bombarded with Ni +6 ions of 46.5 MeV to produce a large number of atomic displacements up to about 1 dpa. Other samples with the same impurity content have been irradiated with electrons of 1.5 MeV. Optical absorption spectra induced with these treatments, in the range 190 to 2000 nm, have been compared, focussing the attention on the two prominent bands at 215 and 245 nm observed in the heavy-ion irradiated samples. While the band at 215 nm, identified as the E′ 1 coulour centre, is present after irradiations with both types of particles, the so-called B 2 band at 245 nm is totally absent after electron bombardment and therefore it can be attributed to direct atomic displacements. Conversely, the concentrations of E′ 1 does not correlate with dpa values; the origin of these centres must therefore be sought in other processes, such as ionization and strained bonds.