Controlled fusion has a potential to produce electricity in an environmentally benign way, meeting the needs of a growing world population. In fusion reaction the deuterium fuel can be separated from regular water, while tritium has to be manufactured - "bred". The lithium-containing "blanket" surrounding the reactor core absorbs neutrons from the fusion reaction and lithium is transformed into tritium and helium. The helium-cooled pebble-bed (HCPB in EU) and the water-cooled ceramic breeder (in Japan) concepts have been designed for the tritium blanket in fusion reactor. Ternary lithium oxides such as Li2TiO3 and Li4SiO4 are regarded as candidate breeders in solid blanket concepts.

Generally tritium generated by neutron irradiation exists as -OT- or T-, then migrates to the surface, and gradually desorbs as hydrogen molecular form (HT, T2) or water form (HTO, T2O) by recombination process or by an exchange reaction with an H2 molecule in the sweep gas. Both the migration of tritium to the surface and the release of tritium on the surface are affected by radiation defects. Therefore understanding on the interactions between tritium and radiation defects is needed.

The objective of the present research is to clarify the existence states and release behaviors of hydrogen isotopes affected by radiation defects in ternary Li-oxides (LixMOy). For this purpose, IR absorption analysis and thermal desorption spectroscopy (TDS) were combined with the ion implantation technique. Since introduction of tritium enough to be detected by IR absorption analysis is difficult, the deuterium ion irradiation is applied, which can introduce not only hydrogen isotopes (deuterium), but also radiation defects (such as Li vacancies, M vacancies and F centers) simultaneously. Corresponding to -OT- or T- induced by neutron irradiation, -OD- and D- on/near the surface can be formed in deuterium irradiation. Various -OD- affected by proximate defects can be identified by FT-IR, and possible existence of non-O-D states such as D- and release behavior of deuterium can be analyzed by TDS.

Powder and single crystals of ternary Lithium oxides (LixMOy) including LiAlO2, Li2TiO3, Li2SiO3, Li4SiO4, LiNbO3 and LiTaO3 were used as samples. The experimental system consists of FT-IR (Mattson, Infinity Gold) with an MCT detector, a mirror for the diffuse reflectance method, a Quadruple Mass Spectrometer (QMS), an ion gun, a vacuum chamber, a heating unit, a Faraday cup, and a cover to prevent any ion irradiation of the molybdenum sample holder.After the pretreatment, 3keV D2+ irradiation was performed, and in situ IR absorption analysis was conducted during the ion irradiation.. The ion implantation depth was about 50 nm according to calculation by SRIM2003 code. Some powder samples were irradiated at a flux of 4~16×10(16) D2+ m(-2) s(-1) up to the fluence level of 0.3~5×10(21) D2+ m(-2). The irradiation was directed towards the single crystal sample surface at an angle of 45°.

The release behavior of deuterium was detected by QMS during the heating process with a rate of 20 K min-1 up to 800 K after ion irradiation. The amount of deuterium released as different chemical forms were estimated based on the fundamental vacuum relationship of mass flow. For single crystals in-situ analysis was also applied during the heating after ion irradiation. All of FT-IR and TDS results of experimented LixMOy samples are summarized in Table.1.

Deuterium irradiated into ternary lithium oxides exist as multiple O-D states and non-O-D states. Multiple O-D states can be indicated by different O-D vibration peaks observed in IR spectra. It is considered that there are three possible states of D+ in LixMOy after D2+ irradiation, namely sub.D+Li, sub.D+M and int.D+. All of D+ (whether sub.D+ or int.D+) are attracted by neighboring oxygen ions to form the O-D states of sub.D+ or int.D+. In addition, the isolated or hydrogen bonded -OD- exist on the surface (termed as surface -OD-). Multiple O-D vibration peaks have been observed in D2+-irradiated lithium ternary oxides, corresponding to the above multiple O-D states. Their fractions are dependent on the sample shape (single crystal or powder), the irradiation fluence and the temperature of sample during irradiation.

It is observed that the O-D vibration peak corresponding to sub.D+M has higher frequency. It is considered that the production probability of M vacancy can be decreased with atomic weight of M increasing, due to the evaluation on that by a simplified binary collision approximation (BCA) model. Therefore the fractions of different O-D states can be affected by M, which is reflected by the average frequency (termed as v(O-D)) of all O-D vibration peaks weighted by weighted by each proportion. It is considered that v(O-D) decreases with the increasing atomic number of M in LixMOy. Based on a multiple states model it seems that LixMOy with a heavier atom as M might be good choice from the respect in the diffusion of hydrogen isotopes.

The increase of O-D with desorption of deuterium during the heating after irradiation indicates the existence of non-O-D states, which has been observed in LiAlO2 single crystal. The unpaired electron state is considered to be a trapping site for deuterium as non-O-D states.

Deuterium irradiated into LixMOy (Li2TiO3, Li2SiO3, Li4SiO4,LiTaO3, LiAlO2 powder samples and LiAlO2, LiTaO3, LiNbO3 single crystal samples) is released as four chemical forms: HD, D2, HDO and D2O. These release chemical forms can be divided into two groups, D2/HD (termed as non-condensative forms) and D2O/HDO (termed as condensative forms). In non-condensative forms, whether powder or single crystal, HD and D2 can be desorbed at the same temperature. In condensative forms, desorption temperatures of HDO and D2O are not always the same, for example desorption temperature of D2O is higher that of HDO in LiTaO3 and Li4SiO4 powder. In Li2SiO3, Li4SiO4 and LiTaO3 powder, desorption temperature of non-condensative forms is higher than that of condensative forms. However it is contrary in Li2TiO3 and LiAlO2 powder. It reflects that desorption behaviors of deuterium are dependent on material.

HDO desorption is considered to be mainly due to the recombination reaction between -OD- and surface -OH- induced by the adsorption of water vapor on the sample. Desorption of non-condensative forms (D2/HD) should be correlated to unpaired electron state indicated by the ESR signal peak. D2O desorption is affected by the combination of HDO desorption and D2 desorption. Deuterium desorbed as D2 achieves the saturation at certain irradiation fluence, which suggests that concentration of unpaired electron state induced by irradiation is limited in the sample. It can be seen that D2 is the predominant chemical form of deuterium (the fraction of deuterium released as D2 in all forms > 60%) from the LixMOy single crystal. Although HDO can almost not be detected in irradiated single crystal samples, it is one of the main chemical forms of deuterium released from irradiated powder samples (the fraction of deuterium released as HDO in all forms >30%). It reflects the influence of the sample shape that desorption behaviors of deuterium are different between powder and single crystal. Considering that the main difference between powder and single crystal is the area of surface, the fraction of different release chemical forms should be dependent on the surface condition of sample. Desorption behavior of deuterium is corresponding to the existence states of deuterium. O-D state with higher vibration frequency has a higher annihilation temperature, also corresponds to deuterium desorbed at a higher temperature

Finally, three factors appear to govern the release behavior of deuterium from ternary lithium oxides. The first is the corresponding vibration frequency of O-D state. It is considered that an O-D state with a higher vibration frequency has a higher annihilation temperature, also corresponds to deuterium desorption at a higher temperature. The second is the surface -OH- induced by the adsorption of H2O vapor on the sample. In water forms (HDO and D2O), deuterium desorbed as D2O is much less than that as HDO. HDO desorption is considered to be mainly due to the recombination reaction between -OD- and surface -OH-. D2O desorption is partly dependent on the HDO desorption. The third is unpaired electron state induced by irradiation damage. Deuterium desorbed as hydrogen molecular forms (D2/HD) is correlated to unpaired electron state indicated by the ESR signal peak.

The complexity observed in desorption behavior of deuterium from LixMOy was considered to result from the combination influence of these three factors. The fractions of deuterium desorbed as different chemical forms are dependent on concentrations of O-D state, surface -OH- and unpaired electron state. The difference in desorption temperatures of deuterium as different chemical forms should be correlated to the potential energies of O-D state, surface -OH- and unpaired electron state.

Based on the multiple states model, it is considered that the existence states of deuterium in the bulk have no influence on the release chemical forms of deuterium. The fractions of different release chemical forms should be mainly dependent on the concentrations of deuterium as different existence states on the surface not in the bulk. Therefore the above conclusions on desorption behavior of deuterium from D2+ irradiated ternary lithium oxides can be partly applied for that of tritium after neutron irradiation.

核融合エネルギーは、資源問題が少ないこと、高レベル放射性廃棄物の発生がないこと、およびプルトニウムなどの核不拡散上の問題が少ないことのために将来のエネルギー源の候補と考えられている。核融合炉が実現されるためには、プラズマ炉心性能の向上とともに、発電ブランケットの研究開発が必須である。固体ブランケットではリチウム酸化物がトリチウム増殖材料として考えられている。ブランケットでは中性子との反応によって生成されたトリチウムを滞留時間を少なくして回収する必要がる。このとき、中性子の照射によって様々な欠陥が生じ、トリチウムの存在状態や放出速度はこれら欠陥との相互作用によって影響される。本研究の目的は、様々な三元系リチウム酸化物について、D2+の照射後の重水素の存在状態と加熱放出挙動の測定から、欠陥存在下での水素同位体の存在状態、放出挙動を明らかにすることを目的とする。また、様々な三元系リチウム酸化物について、存在状態と放出挙動を体系的に明らかにする試みを行うことも目的としている。

本論分は7章で構成されている。

第1章は研究の背景と目的が書かれている。

第2章は実験方法について説明している。この研究で用いた実験手法は3keVD2+を試料に照射した後、O-Dの存在状態を拡散反射型(粉末試料)あるいは透過型(単結晶試料)FTIRで観察することと、照射後加熱放出(TDS)による化学種(HD,D2,HDO,D2O)毎の放出速度の測定である。実験した三元系リチウム酸化物は、LiAlO2, Li2TiO3, Li2SiO3, Li4SiO4, LiNbO3およびLiTaO3である。

第3章は三元系リチウム酸化物における多種類O-D存在状態について記述している。LiAlO2についてFTIRでの観察によると3種類のO-D(ODα, ODβ、ODγ)が観察された。それらは、それぞれ、表面OD、Li空孔あるいはAl空孔に影響されたOD、および格子間に存在するD+によるODと帰属されている。さらに、Li2TiO3, LiTaO3のFTIRによるOD観察結果についても説明している。また、結晶構造が同じである、LiNbO3とLiTaO3のFTIR結果の比較から、観察されたODβに相当する2つのピークのうち高波数側の振動はNbあるいはTa空孔に影響されたODであることが明らかにされた。

第4章は三元系リチウム酸化物からの重水素放出挙動について記述している。一般にHD,D2という分子状と、HDO,D2Oという水蒸気状の化学形に分けられることが分かり、単結晶の場合には、分子状のものの割合が大きいことが示されている。また、LiAlO2, Li2TiO3では水蒸気の化学形での放出が分子状での放出に比してより高温で観察されるのに対して、Li4SiO4, Li2SiO3では分子状での放出が水蒸気よりも高温で生じるという観察結果が報告されている。

第5章は存在状態と放出挙動との相関について記述している。まず、重水素の放出挙動は表面に吸着されたことによって生じるOHに影響されることおよびHDOの放出は表面におけるOHとODの再結合反応によることが示されている。また、D2, HDの放出温度はESRピークの消滅温度と強い相関があるという興味深い結果が示されている。さらに、加熱放出時における、ODβのFTIR吸収強度の増加から、ODとしてでなく別の化学形(D0,D-)での存在が示されている。また、加熱時のFTIR吸収の変化とTDS結果を詳細に比較検討することによってODとして存在する重水素の一部はD2として放出するという結論も示されている。また、放出される重水素の様々な化学形は主に、酸素イオンや不対電子の存在などという表面化学環境に主に起因することを示している。

第6章においては、実験で使用した様々な三元系リチウム酸化物(LixMyOz)について重水素の存在状態や放出挙動を体系的に説明することの第1歩の検討結果が示されている。ここでは、Li置換型ODの振動数と存在割合、放出化学形の割合(分子形、水蒸気形)および放出温度(分子形、水蒸気形)と、Mの原子番号、Mの電荷数、Mの電気陰性度及び酸素濃度との相関について調べている。この相関性の研究は今後、三元系リチウム酸化物を体系的に研究していく際に有効であることが示されている。

第7章は結論である。

以上、本研究は、核融合炉固体増殖材料である三元系リチウム酸化物についてトリチウム回収挙動解明の基礎として高エネルギーで照射した重水素の存在状態と放出挙動について多くの有用な知見を与えており、システム量子工学、特に核融合工学への貢献は小さくない。

よって本論分は博士(工学)の学位請求論文として合格と認められる。