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Webserver Date: 22-August-2019

Preparation of Enriched Boron and Boron Carbide

 
 
Dr. A. K. Suri and C. Subramanian
Bhabha Atomic Research Centre

One of the basic requirements associated with the development and operation of nuclear reactors is control and containment of neutrons that sustain, and also are produced during fission reactions. Boron is one of the very few elements to possess excellent nuclear properties. The neutron absorption property of boron is mainly due to the presence of Boron-10 (B10), which undergoes the main capture reaction:

 

5B10 +0n1 ? 3Li 7 + 2He4 + 2.6 MeV

 

The cross section of this reaction, which varies from 3850 barns for thermal neutrons, to a few barns for fast neutrons, makes it an excellent candidate for absorbing neutrons in thermal reactors. At higher energies, the cross section of most other elements becomes very small, where as that of B10 decreases monotonically with the energy and at higher neutron energies of >1MeV also B10 exhibits sufficient absorption cross section values. Natural boron that contains 19.8% of B10 atoms, can be enriched up to 99% and hence the use of boron is very attractive in the entire energy spectrum. The products of (n,a) reaction of B10 namely lithium and helium, are stable and non-radio-active isotopes. Boron carbide due to its high boron content, chemical stability and refractory character is extensively used as control rod material.

 

Development of processes and production of materials for neutron absorption required by various atomic energy programmes of India, dates back to mid seventies. Metallurgy Division, BARC developed the production technologies for boron carbide powder and also supplied the materials needed for Tarapur Atomic Power Station (TAPS) control rods and various shielding applications during the construction of the research reactor Dhruva. The developmental work was continued to meet the requirements of boron powder for use in neutron sensor and for applications in defence. The current activities of Materials Processing Division of BARC are concerned with the development of technologies for the preparation of boron and its compounds with enriched B10 for application in fast reactors.

 

Methods for preparation of Boron carbide

 

Boron carbide is usually prepared by carbothermic reduction of its oxide. In the industrial practice, Acheson type furnaces are used, wherein the charge close to the heated graphite rod reacts and the balance charge acts as heat insulator and remains partially reduced. In this type of system, the amount of charge converted to boron carbide is very less in each heat and also loss of boron due to evaporation of its oxides is high. Hence this process is not suitable for preparation of enriched boron carbide.

 

Fig.1 Flow sheet for preparation of boron from boric acid

 

Magnesiothermic reduction of boron oxide in presence of carbon also yields boron carbide but the magnesium content of the carbide is high due to the formation of magnesium boride during the reduction. The reaction products also have to be purified by acid leaching to remove magnesium oxide and excess magnesium. Hence this processes also is found unsuitable for the production of enriched boron carbide.

 

Synthesis of boron carbide from its elements where the purity and carbon content can be controlled, appears to be the best suited method for the preparation of enriched boron carbide for control rod elements.

 

Preparation of Elemental Boron

 

Elemental boron though barely reactive at room temperature, reacts violently with almost all the elements at high temperatures. Due to this high reactivity it is extremely difficult to prepare pure boron. Due to its extreme hardness, pulverization of boron introduces impurities from the crushers and causes a superficial oxidation of the grains. The most suitable method for the production of boron powder is by electrolytic route.

 

Materials Processing Division has developed a flow sheet for the preparation of boron from enriched boric acid and synthesis of boron carbide using this boron.

 

Conversion of Boric Acid to Boro-Potassium Fluoride

 

Conversion of Boric acid to KBF4 is carried out in two steps. In the first step, boric acid granules are dissolved in hydro fluoric acid. After complete dissolution, saturated solution of potassium hydroxide is added to the boron containing liquor to precipitate out KBF4. As these reactions are exothermic in nature, the addition of chemicals should be slow, the solution continuously stirred and required to be cooled to avoid the evaporation of BF3 species and loss of boron. The precipitate is filtered, washed and dried to obtain dry boro-potassium fluoride.

 

 

Electro winning of Boron

 

The electrolytic setup used for the extraction of boron from KBF4 is shown in Fig.2. A retort fabricated from inconel material with a top flange having facilities for introduction of cathode and gas ports forms the basic reactor vessel. A graphite crucible kept inside this vessel holds the molten salt and also acts as anode during electrolysis. Cathode made of mild steel sheet is inserted through the top flange. The molten bath is made up of a mixture of KCl-KF-KBF4 and the over all electrolytic reaction can be written as:

 

4KBF4 + 2KCl ® 4B +6KF +5F2 + Cl2

 

Fluorine evolved at the anode replaces chlorine of KCl and forms KF. The gas evolved at the anode is a mixture of chlorine and fluorine. During electrolysis, a build-up of KF is found to be detrimental, as it results in decrease in conductivity of the electrolyte. To some extent, this effect can be overcome by periodical addition of KCl and KBF4.

 

Procedure

 

Dried salt mixture is taken in the graphite crucible, the system assembled and evacuated using a rotary vacuum pump. After ensuring the removal of moisture from the assembly, the vessel is filled with argon and heated further under a continuous purge of argon gas. After attaining the temperature of electrolysis (800-850° C) a D.C potential of 3.5 to 4.0V is imposed between the cathode and anode. The gases evolved at the anode are sucked out, scrubbed in alkali and then vented out to atmosphere. A Cathodic current density of around 85-125Amps/dm2 is maintained during the course of the electrolysis. After a fixed duration, the cathode is lifted above the molten salt, the salts allowed to drain, cooled within the cell in argon atmosphere and then taken out of the assembly and dipped in water. The experiment is continued with a fresh cathode. Periodic replenishment to the bath is done by the addition of dried KCl and KBF4. Boron content of the bath before shut down is reduced to the minimum by stripping the bath without furt her addition of the salts.

 

Processing of Boron Deposit

 

Boron deposit collected from a number of cathodes is processed together. The deposits are crushed in a ball mill and further ground to finer size by prolonged grinding or by high-energy milling. Ground deposit is washed with water to remove the carried over salts and leached repeatedly in hot hydrochloric acid solution to remove the contaminants picked up during milling operation. After sufficient purification, the powder is washed free of acid, dried by alcohol wash and finally vacuum dried in oven at a temperature of 150° C. Freshly prepared powder is highly pyropheric in nature and hence is required to be stored in closed containers filled with argon gas.

 

A typical sample of boron powder produced by fused salt electrolysis analyses to contain 95-96% of boron, ~l% of both oxygen and carbon and the major metallic impurities being iron and silicon in the range 1000-5000 p.p.m.. In the experiments carried out using boric acid with 40% B10 enrichment, the starting material boric acid and boron produced, were analyzed. Enriched Boron powder prepared in the laboratory has been coated on aluminum and stainless steel assemblies to form part of gamma compensated boron lined neutron ion chambers and the same have been tested to have adequate sensitivity to measure a neutron flux of 104nv.

 

Synthesis of Boron Carbide

 

In the laboratory, process parameters have been optimized for synthesis of boron carbide from its elements. Fine powders of boron and carbon (petroleum coke/graphite) are mixed thoroughly in a planetary mixer to obtain a homogeneous charge. This mixed powder is compacted to form pellets and the pellets are heat treated in a vacuum induction furnace to form boron carbide. Though the formation of boron carbide starts at 1200° C, a higher temperature was found to be essential to obtain a good crystalline product. Boron carbide synthesized by this route has been found to be suitable for use as control rod material.

 

Based on the development work carried out in MPD, BARC, an electrolytic cell with a capacity to produce ~6gms. of boron per hour has been set up at IGCAR, Kalpakkam. Regular operation of this cell will provide the necessary engineering data, operational experience and materials performance needed for the design of production plant.