Tritium（3H）

・Tritium is produced by nuclear reactions (14N+n→ 3H+12C, etc.) in the upper atmosphere between nitrogen or oxygen and secondary cosmic rays originating from primary cosmic rays.
(Some are produced by spontaneous nuclear fission of uranium, etc., in the earth's crust. However, the amount is negligible compared to the amount produced in the atmosphere.)

・Release into the environment by past atmospheric nuclear tests (ternary fission of 235U, 238Pu, etc., and reaction of the produced neutrons with N or O)

・Release into the environment from nuclear facilities (nuclear power plants and reprocessing plants) (ternary nuclear fission of the nuclear fuel and nuclear reaction of 10B in the control rods (moderators), and activation of reactor water)

The annual tritium production in the atmosphere is estimated to be 72 PBq (P (PETA) = 1015), most of which is oxidized to tritiated water (HTO: one hydrogen atom replaced by tritium (T)), absorbed into the water cycle, including atmospheric moisture, rainwater, land water, and seawater.

In addition to tritiated water, other chemical forms of tritium include gaseous tritium (HT, CH3T), tissue-free water tritium (TFWT) found in organism samples where tritium has been incorporated, and organically bound tritium (OBT). Organically bound tritium can be classified into two types: exchangeable type, in which tritium is easily exchanged with hydrogen by binding to oxygen or nitrogen molecules, and non-exchangeable type, in which tritium is bound to carbon chains and is not easily exchanged.

Tritium is a radioactive isotope of hydrogen, one of the principal elements constituting the human body. Therefore, the chemical form of tritium at the time of intake—since it determines metabolic behavior—is important for dose assessment.

When tritium enters the body through one of the three pathways—respiration (inhalation), skin (absorption), or ingestion (food and drink)—the chemical forms to be considered differ: moisture in the air and hydrogen gas for respiratory and dermal routes, and water and organically bound tritium for food and drink. In addition, understanding the behavior of tritium within the environmental ecosystem—including agricultural, livestock, and marine products—is also necessary.

To treat water contaminated with artificial radionuclides released in the wake of the Fukushima Daiichi Nuclear Power Plant accident, the Advanced Liquid Processing System (ALPS) is employed to remove 62 nuclides including radioactive cesium. However, ALPS cannot remove tritium.

The distillation method removes foreign substances from water samples. It is used when the target concentration is about > 0.5 Bq/L, depending on the specifications of the analyzer. In the case of an organism sample, freeze-drying is generally used to separate the sample into tissue-free water and dry matter for analysis. Tissue-free water is distilled after wet decomposition (refluxing) of organic matter. For dry matter, water obtained from combustion (combustion-produced water) is collected, and the organic matter in the combustion product is wet-decomposed (refluxed) and then distilled.

This is a method of enriching tritium and is used for detecting samples with low tritium concentration (e.g., seawater). This method involves reducing the volume of distilled water samples by electrolysis and enriching tritium through the isotope effect (water containing protium and deuterium is more easily electrolyzed than water containing tritium). It is possible to enrich tritium 6-8 times. There are primarily two methods employed: electrolysis of alkaline water solutions using metal electrodes and electrolysis using a solid polymer electrolyte (SPE).
In the method using metal electrodes, hydrogen and oxygen are generated together, which can easily form oxyhydrogen gas (a mixture of hydrogen and oxygen in the 2:1 volume ratio) and therefore requires strict safety precautions. It is essential to prevent the generated gases from accumulating indoors by directing them through silicone tubing or similar means to a safe outdoor location for proper ventilation and exhaust.