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RESEARCH ACCOUNT

Developments in sodium technology R. D. Kale and M. Rajan*

Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

Sodium, because of its good heat transfer and nuclear properties, is used as a coolant in fast reactors. It is also used largely as a reducing agent in pharmaceuti- cal, perfumery and general chemical industries. Its affinity to react with air and water is a strong dis- advantage. However, this is fully understood and the design of engineering systems takes care of this aspect. With several experimental and test facilities esta- blished over the years in this country and abroad, ‘sodium technology’ has reached a level of maturity. The design of sodium systems considering all the physical and chemical properties and the development work carried out in this country are broadly covered in this article.

FAST breeder reactors are energy systems which breed more fissionable material than they consume while pro- ducing power. These reactors are attractive where avail- able natural uranium reserves are modest or limited; for example, in the case of India. Plutonium produced in the usual (thermal) nuclear reactors is ideally suited as the fuel material for use in a fast reactor as it possesses high- est fission neutron yield (2.87) under fast neutron- induced fission process. Also, it makes available more than one neutron for conversion of U238 (more than 99% constituent of natural uranium) into Pu239.

Since a fast reactor does not have a neutron moderator such as heavy water or light water, the reactor core has to be compact resulting in high volumetric power density. For example, the power produced per unit cubic metre core of a fast reactor plant is 550 MWt compared to a meagre 8 MWt for a heavy water-moderated reactor plant of the same capacity (600 MWe). Thus it is imperative to use efficient heat-transfer fluid as coolant, which should also possess favourable nuclear characteristics of low neutron moderation/absorption. Liquid metals, and among them liquid sodium, meet almost all the requirements of a fast-reactor coolant with its high thermal conductivity, reasonable specific heat, low neutron moderation and absorption, and high boiling point, giving a large operat- ing temperature range at near atmospheric pressure. Thus sodium has been adopted universally in about 20 fast reactors that have accumulated over 280 reactor years operating experience. The high chemical activity of sodium, including its violent reaction with water has been

*For correspondence. (e-mail: rajanm@igcar.ernet.in)

a matter of concern, but the technology has been mas- tered to circumvent the same. This article examines vari- ous developments in sodium technology resulting in its handling as a conventional engineering heat-transfer medium. The article also briefly covers sodium produc- tion and other uses before proceeding to describe special characteristics of sodium posing design challenges for equipment/components of fast reactors. An overview of sodium technology evolution in India is also covered.

The fast reactor

A fast reactor is one which does not use a moderator to slow down the neutrons produced during fission and the fuel used is fissioned directly by high energy or ‘fast’ neutrons emitted during the fission process. The reactor uses a fairly high concentration of fissionable isotope,

235

239

239

either U

tage when used as a fuel since the number of neutrons produced in Pu fission is high (approximately three), making sufficient number of excess neutrons to produce

238

more Pu from U

core. The high power generated in the compact core of the fast reactor necessitates the use of a liquid metal like sodium as the reactor coolant. Figure 1 shows the sche- matic flowsheet of a fast reactor plant.

1

Pure sodium is a soft, silvery-white metal with a faint pink colour when freshly cut. It melts at 98°C and boils at 883°C (under atmospheric pressure). Its atomic num- ber is 11 and mass number is 23. The atomic structure of sodium represents a single electron in its outer orbit, accounting for its strong chemical activity.

Sodium is one of the six most abundant elements in nature, representing about 2–6% of the crust mass at the earth’s surface. Because of its strong chemical reactivity, sodium is found only as chemical compounds among which sodium chloride is most widely distributed. The only manufacturing process for production of metallic sodium presently used on industrial scale is the electroly- sis of pure sodium chloride in a mixture of calcium and barium chlorides at about 600°C. (The other two chlo- rides help in lowering the melting temperature of pure sodium chloride from about 900 to 600°C). The process is also called ‘fused salt electrolysis’ and is substantially

or Pu

. However, Pu

has a distinct advan-

used as a blanket surrounding the fuel

Sodium – occurrence, production and applications

668

CURRENT SCIENCE, VOL. 86, NO. 5, 10 MARCH 2004

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 Supercritical Fluid Extraction 668.pdf Page 001
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