Taiyo Nippon Sanso Corporation has constructed, being the first efforts in Japan, a recycling system for reusing depleted heavy water through re-enrichment. This development makes it possible to use domestically re-enriched heavy water in addition to imported heavy water, which accounts for a large portion of domestic use. On this occasion, we interviewed Takehiro Igarashi (hereinafter, Igarashi), SI Development Section Chief in the Cryogenic Separation Development Division and Yuta Tsuchihashi (hereinafter, Tsuchihashi) in the Synthesis Development Section of the Tsukuba Chemical Synthesis Development Division, who have been involved in the development, about the background and difficulties in the development as well as future prospects.

　　　　　　　　　　　　　　　　
　　　　　　　　　　　　　　　　　　　　　　　　　　　　Mr. Igarashi (left) and Mr. Tsuchihashi

Wide Applications – also used in protein analysis
Q: Before asking about the background of the development, what is heavy water in the first place?
Igarashi: Simply put, heavy water is a water molecule (H2O) whose constituent hydrogen atoms (H) have been replaced by heavy hydrogen (deuterium). Deuterium is a non-radioactive stable isotope having double the mass of common hydrogen. Deuterium exists naturally in a proportion of about 0.015%, although this varies depending on the object to be measured, sites/latitudes, and so forth, and is thus an extremely rare substance. Most “heavy water” typically sold as a reagent has a deuterium concentration of 95% or more and is produced by enriching a small amount of deuterium using a large quantity of energy. For this reason, heavy water is traded at a much higher price than ordinary water.
Q: What applications does heavy water have?
Tsuchihashi: Although you might be unfamiliar with it, heavy water is used as cooling water in a nuclear reactor of the type called a heavy-water reactor. Moreover, heavy water has come to be used in various fields, for example, in research applications, such as solvents for NMR analysis, and amino acids for compositional analysis of proteins/DNA, and recently for industrial applications as well, such as semiconductor materials. Our company produces/sells “deuterated ammonia,” produced using heavy water as a raw material, and other diverse deuterium-labeled compounds using heavy water as a raw material.
Uncertainty about Import-Dependent Supply – Motivation for the Development
Q: We will now get to the main point. What made you think of recycling used heavy water through re-enrichment?
Igarashi: We had a long-standing idea of re-enriching used heavy water. Meanwhile, under the supply and demand situation in other countries, the export policies of producer nations, the international state of affairs and the geopolitical situation, the uncertainty regarding the stable procurement of heavy water has arisen in recent years. Since the domestic demand for heavy water relies heavily on imports, it is anticipated that stable procurement could be difficult in the future. Accordingly, to ensure the stable procurement of heavy water, we started the development of technology that enables the reuse of depleted heavy water as a resource which had previously been discarded.
Q: Please tell us about the work flow from use to the re-enrichment of heavy water, which has just been developed.
Tsuchihashi: I will first explain the production process for a deuterium-labeled compound using heavy water as a raw material. When producing a deuterium-labeled compound, we conduct a deuteration reaction to replace hydrogen atoms with deuterium atoms. In this reaction, the greater the efficiency is, the higher the deuterium concentration is in heavy water used as a raw material. Accordingly, the heavy water needs to be changed into new one when the concentration of heavy water decreases as the reaction proceeds. Conventionally, used heavy water with low deuterium concentration was discarded.
　　　　　　　　　　
　　　　　　　　　　　　　　　　　　　　　　　　　　Flow Chart after Use to Re-use of Heavy Water

Igarashi: Such used heavy water, which was previously discarded, is re-enriched and reused this time as a raw material. First, used heavy water is purified by removing contaminants and other impurities and is subjected to a quality inspection. After inspection, the used heavy water is loaded into the self-developed re-enrichment apparatus that adopts distillation and is enriched. This step undergoes enrichment to a deuterium concentration of 99 atom% or more. Subsequently, the heavy water meeting in-house criteria in analysis/inspection of the concentration, contaminants, and so forth, becomes ready for reuse. Through enrichment, water of a further reduced deuterium concentration is produced simultaneously and is processed as waste water. By the development of this cycle, part of the heavy water for deuterium-labeled compounds can be procured in-house. At the same time, the amount of waste heavy water is expected to decrease by half as compared to the previous amount. Further, it is also possible to re-enrich again, following the same steps, re-enriched heavy water after use.

　　　　　　　　　　　　　　　　　　　　　
　　　　　　　　　　　　　　　　　　　　　　　　　　Discussion in front of the Re-enrichment Apparatus

Engineering also by Our Own Efforts
Q: What was the greatest difficulty in the development? I would like to ask both of you.
Tsuchihashi: I was in charge of the step of purifying used heavy water where removing impurities from used heavy water was a challenge. Used heavy water, although a single term, varies considerably in the impurity content, the state of contamination and so forth, depending on how it was used. Consequently, we experienced enormous difficulties to standardize a method to remove impurities/contaminants under inconsistent conditions. Although we achieved standardization, a more efficient method is still under study.
Igarashi: Broadly, I experienced difficulties in two areas: 1) instrument design and 2) stabilization. Our knowledge of "air separation" was highly beneficial to investigate instrument design, but there were many unknowns about the heavy water. We ourselves conducted verification as necessary while searching for relevant literature, etc., to design optimal instruments. Moreover, since such instruments had special mechanisms compared to our other apparatuses, we ourselves had to engineer them. After engineering, we also had difficulties since the enriching performance was not readily stabilized as designed. However, we sought advice from many people and resolved the issues one at a time. We were very relieved when stabilization was actually achieved.

Q: What are your thoughts on future development?
Tsuchihashi: I think that demand for deuterium-labeled compounds will increase in the future. To catch up with the demand, we will make efforts to further improve the re-enrichment technology from the aspect of purification.
Igarashi: In the current situation of import-dependent supply, we will fully research, for example, how many companies have the need to, or interest in, the re-enrichment of heavy water and look to the future expansion of our business. Further, we would like to develop the heavy water-related business as one of our core businesses, which follows oxygen-18 in our stable isotope business.