Masakatsu Saeki

Effects of pyrolytic carbon structure on diffusivity of tritium

Abstract The release behavior and diffusivity of tritium were studied as a function of the degree of the preferred orientation utilizing pyrolytic carbons deposited on alumina substrates. The release spectra of tritium during a constant rate annealing shifted to a higher temperature with an increase in the extent of anisotropy of the carbon structure. The amounts of tritium retained in a sample after annealing depended strongly on the anisotropy factor. Furthermore, a great discrepancy was observed between the diffusion coefficients obtained from isotropic and from anisotropic pyrolytic carbons. These experimental results were attributed to an anisotropic diffusion of tritium in carbon materials, that is, tritium atoms or ions diffuse mainly between basal planes.

Sorption of gaseous tritium on the surface of type 316 stainless steel

Abstract The sorption of gaseous tritium on the type 316 stainless steel was studied. The stainless steel was first contacted with gaseous tritium, and then the remaining tritium was evacuated. During a gradual etching from the surface by an acid solution, the tritium was released as HTO with a fraction of HT. They were radioassayed separately. The HTO mostly originates from the tritium present on the outer-most surface and about 90% of it could be released easily into water. However, the rest is sorbed tightly and remained in the surface layer. A fraction of the sorbed-tritium will diffuse atomically through the surface layer into the bulk of stainless steel and is released as HT by etching. The activation energy of the diffusion was determined as 32.8 kJ/mol.

A thermal desorption study of the surface interaction between tritium and type 316 stainless steel

The thermal desorption of tritium from a type 316 stainless steel exposed to gaseous tritium (HT-gas) was studied. In the desorption spectra of tritium, four distinct peaks appeared at about 430 (peak HT-1), 540 (peak HTO), 750 (peak HT-II) and 970 K (peak HT-III). The activation energies for the peaks HT-I, HTO and HT-II were determined as 20 ± 4, 27 ±7 and 55 ±9 kJ/mol, respectively. It is presumed that the tritium leading to the peak HTO forms tritoxyl ion (OT−) by combining with surface O2− ion or by the isotopic exchange with the protium of surface OH− ion on the sorption process. The OT ion combines with the OH− ion or the dissolved protium into HTO at the desorption process. The tritium leading to both peaks HT-II and HT-III is sorbed and dissociates into atomic species which diffuse into the bulk, while that leading to the peak HT-I is very weakly held on the topmost surface of stainless steel in the form of a molecule or HT+ ion.

Study on Sorption of Tritium on Various Material Surfaces and Its Application to Decontamination of Tritium-Sorbing Materials

Behavior of sorption of tritium on surfaces of stainless steel, aluminum and borosilicate glass has been studied using gaseous tritium with high specific activity and the newly obtained knowledge has been applied to chemical and thermal decontamination of tritium-sorbing materials. The behavior depends on a character of surface of each material. The total amount of sorbed-tritium is evaluated a function of time of exposure to gaseous tritium. The depth-profile of sorbed-tritium is obtained by chemical etching method. From these results, practical samples of stainless steel were decontaminated. The vacuum-bakeout for 24 hours at 773 K and dissolution by dilute (5%) HCl for 2 hours enable the authors to obtain the decontamination factor of 200.

Chemical decontamination of the tritium-sorbing surface of Type 316 stainless steel

The chemical decontamination of Type 316 stainless steel surfaces exposed to gaseous tritium has been examined by using a technique of combined thermal desorption and chemical treatment with corrosive solutions. With the finding that the tritium sorbed at the various sites on the stainless steel surface can selectively be eliminated by treatment with a particular corrosive solution, the distributions of sites and their properties have been elucidated. On the basis of the results, it has been demonstrated that a two-step chemical treatment, i.e., the dipping in dilute HCl for the whole surface corrosion and then in a CuSO4-H2SO4 solution for intergranular corrosion, leads to an almost complete removal of tritium sorbed on the stainless steel surface with minimized dissolution of the bulk. It is also shown that the non-oxidizing acids like HC1 accelerate the sorbed tritium to release in the form of HT, while the oxidizing acids like HNO3 allow it in the form of HTO.

Distributions of radionuclides on and in spent nuclear fuel claddings of pressurized water reactors

Abstract Radioactivities in Zircaloy-4 cladding of PWR spent nuclear fuel have been examined as a function of fuel burnup in the region of about 7000–40000 MWd/t. The fission products 137 Cs and 106 Ru in the cladding increased linearly with fuel burnup, whereas the radionuclides 134 Cs and 154 Eu, which were produced by neutron capture of fission products, increased with the second power of fuel burnup. Tritium and the activation products of alloy constituents and impurities increased in proportion with fuel burnup. The gross activities of α emitters on the inner and outer surfaces of the cladding increased with 3.1 and 1.3 powers of fuel burnup, respectively. The distribution of radionuclides in the cladding has been examined by radiochemical analysis combined with stepwise etching from the inner surface of the cladding. More than 98% of fission products were present within a 10 μm depth from the inner surface of the cladding, while activation products such as 60 Co were distributed homogeneously in the interior of the cladding except on the outer surface. The distribution of tritium tended to become heterogeneous with increasing fuel burnup.

Effect of surface treatments on the sorption of tritium on type-316 stainless steel

Abstract The effect of surface treatments on the interaction between tritium and type-316 stainless steel was studied by a thermal desorption technique. The surface treatments caused changes in intensity for all of the four desorption peaks of tritium. The chemical passivation resulted in a decrease of the peak at 540 K (peak HTO) as well as the peak at 430 K (peak HT-I). The vacuum bake-out at 773 K led to a decrease of the peak at 750 K (peak HT-II) and an increase of the peak at 970 K (peak HT-III). The sputter-etching resulted in an increase of the peaks HTO and HT-I. These changes in the behavior of thermal desorption of tritium can primarily be attributed to the variation of the surface composition. It is suggested that the peaks HTO and HT-I are given by desorption of the tritium of which sorption is related to iron on the surface, while the peaks HT-II and HT-III are given by desorption of the tritium which is trapped on the sites of carbide and nickel, respectively, in the surface layer.

Diffusivity of tritium in Li-Al alloys

Abstract The diffusion coefficients of tritium in Li-Al alloys were measured. The observed diffusion coefficients were expressed by the following equations: D = 8.5 × 10 −5 exp[ −43.9± 14.8(kJ/mol)/RT] (cm 2 /s ), D = 9.3 × 10 −5 exp[ −48.5± 15.0(kJ/mol)/RT] (cm 2 /s ), and D = 5.3 × 10 −5 exp[ −62.7± 13.8(kJ/mol)/RT] (cm 2 /s ) for 0.02, 0.26 and 1.12% Li-Al alloys, respectively. Results showed that the diffusion coefficients as well as the apparent activation energy for diffusion depend upon the lithium concentration in the alloy.

Observation of 235U NMR in the Antiferromagnetic State of UO 2

The 235 U ( I =7/2) NMR for magnetic uranium sites with 5f-electrons has been first observed in the antiferromagnetic state of UO 2 . The hyperfine field is estimated to be 252.3 ± 0.5 T. Well articulated quadrupole splittings have also been observed with | e 2 q Q (3 cos 2 θ-1)/ h | = 392 ±11 MHz. This indicates the presence of an electric field gradient produced by the Γ 5 -ground state for 5f-electrons in the magnetic ordering.

Influence of radiation damage on diffusivity of tritium in graphite

Abstract The diffusivity of tritium in graphite has been investigated as a function of radiation damage. For the same graphite, it was confirmed that the diffusivity of tritium decreased as the fluence of neutrons received by specimens increased. Furthermore, it was found that the diffusivity of tritium in graphite crystal decreased with expansion of the lattice spacing of graphite, even among different graphites.