The Girdler sulfide (GS) process, also known as the Geib-Spevack (GS) process, is an industrial production method for making heavy water (deuterium oxide), an important component of many nuclear reactors because it acts as a neutron moderator. It takes its name from Karl-Hermann Geib and Jerome S. Spevack, who independently invented it in the early 1940s, or the Girdler company, which built the first American plant using the process.

The method is an isotopic exchange process between H₂S and H₂O ("light" water), that produces heavy water over several steps. It is a highly energy intensive process. Normal water contains 155 parts per million of deuterium, relative to hydrogen.

Until its closure in 1997, the Bruce Heavy Water Plant in Ontario (located close to the Bruce Nuclear Generating Station) was the world's largest heavy water production plant, with a capacity of 700 tonnes per year. It used the Girdler sulfide process to produce heavy water, and required 340,000 tonnes of feed water to produce one tonne of heavy water.

Presently, India has seven heavy water production plants. The first of these to use the Girdler process is located at Rawatbhata near Kota, Rajasthan. .

The process

Each of a number of steps consists of two sieve tray columns. One column is maintained at 30°C and is called the cold tower and the other at 130°C and is called the hot tower. Deuterium extraction is done based on the difference in separation between 30°C and 130°C.

Hydrogen sulfide gas is circulated in a closed loop between the cold tower and the hot tower. Demineralised and deaerated water is fed to the cold tower where deuterium migration preferentially takes place from the hydrogen sulfide gas to the liquid water. This "enriched" water from the cold tower is fed to the hot tower where deuterium transfer takes place from the liquid water to the hydrogen sulfide gas. An appropriate "cascade" setup accomplishes enrichment.

Normally in this process, water is enriched to 15% to 20% deuterium. Further enrichment to "reactor-grade" heavy water (>99% deuterium) is done in a vacuum distillation unit.

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