Hydrogen Transportation Behavior and its Control on the Water-Metal Boundary Using Pure Tritium

为了探讨水-金属界面处氢从金属向水套迁移的详细机理，在423 K温度下，用含氚气体的纯铁管道（4）（10 mm × 75 mm^1 × 1 mm ^t）进行了一系列氚渗透实验（1 kPa，7.4GBq，纯度>90%），监测水夹套中氚的化学形态，用He或1%H_2的He平衡气体加压至0.8MPa，通过离子室吹扫，水起泡器和氧化催化剂床。对纯铁管试样表面进行初步机械抛光。在0. 8 MPa-He气氛中，在423 K下生长20小时以上的氧化层样品。通过SEM、XRD等手段对试样表面进行分析，氧化层由内部的细小部分和外部的多孔部分组成，氧化层为磁铁矿（Fe_3O_4）而非赤铁矿（Fe_2O_3）。在此条件下，氧化物内部细小部分的生长速度为0.06~0.07 μm/h。主要结果如下1.在本实验条件下，纯铁对水套的渗透量约为计算值的1/5.即使表面氧化层（微细多孔的磁铁矿）生长，氚对水的渗透率也没有显著变化.而当磁铁矿层生长到> 1.4μm时，氚的HT分数开始为30%，然后急剧下降到约1/50。这表明了HT分数的减少与氧化层生长之间的关系.在423 K、0.8MPa条件下进行He平衡吹扫时，纯He和1%H_2之间的溶氢量差异对氚的渗透形态和渗透量没有明显影响，说明纯铁中的氚是通过磁铁矿表面羟基物种的同位素交换反应迁移到水中的。另一方面，如果表面羟基密度小，则氚在表面复合后会以氢气的形式渗透到水中。

In order to discuss detailed mechanism of hydrogen transferred from metal to water jacket at the water-metal boundary, a series of tritium permeation experiment has been performed at 423 K, using pure iron piping (4)10mm x 75mm^1 x 1mm^t) contained tritium gas (1 kPa, 7.4 GBq, >90% purity) under monitoring chemical species of tritium in the water jacket, which pressurized to 0.8MPa by He or 1%H_2 in He balance gases, by purging through ion chambers, water bubblers and oxidation catalyst bed. Surface of pure iron tube sample was mechanically polished initially. Also, samples with oxide layer, which grew in the water of 0.8MPa-He at 423K for more than 20 hours, were prepared. Sample surface was analyzed by SEM, XRD etc. The oxide layer consisted of inner fine part and outer porous part, and was magnetite (Fe_3O_4) not hematite (Fe_2O_3). The inner fine part of oxide grew 0.06~0.07 μm/h in the above condition. The main results are as follows1. The permeation amount through pure iron to water jacket was about 1/5 of the calculated value in this experimental condition.2. Even if surface oxide layer (fine and porous magnetite) grew, tritium permeation rate to water was not changed drastically.3. However, HT fraction of permeated tritium was 30% initially, then, it decreased drastically to about 1/50 after > 1.4μm of fine magnetite layer growth. It suggested the relation between the HT fraction decrease and oxide layer growth.4. Permeated species and amounts were not affected clearly by dissolved hydrogen amount difference between pure He and 1% H_2 in He balance purging under 0.8 MPa at 423 K.From the above results, it suggests that the tritium in pure iron would transfer to water by isotope exchange reaction through surface hydroxyl species of magnetite. On the other hand, if surface hydroxyl density is small, the tritium would permeate to water as hydrogen gas after surface recombination.