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FOUNDER’FOUNDER’FOUNDER’FOUNDER’S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015S DAY SPECIAL ISSUE 2015

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DEVELOPMENT AND PRODUCTION OF 10B ENRICHED BORON CARBIDE (B4C) PELLETS FOR CONTROL ROD APPLICATION IN

PFBR

J.K.Sonber, T.S.R.Ch.Murthy*, K. Sairam, R. D Bedse, R.C. Hubli# and J.K. Chakravartty

Materials Group

Dr. R.C. HubliDr. R.C. HubliDr. R.C. HubliDr. R.C. Hubli, Materials Group, Materials Group, Materials Group, Materials Group and his and his and his and his teamteamteamteam received the DAE Group received the DAE Group received the DAE Group received the DAE Group Achievement Award for the year 2013Achievement Award for the year 2013Achievement Award for the year 2013Achievement Award for the year 2013

Abstract This paper presents a brief account of the programme for the process development and production of 10B enriched boron carbide pellets for control rod application in PFBR. At Materials Group, BARC a processing scheme has been developed for the preparation of enriched boron carbide pellets through powder metallurgy route. Using this process, 11,180 pellets of required specification for PFBR was manufactured and supplied to BHAVINI for control and diverse safety rod (CSR and DSR) applications. Introduction Boron carbide (B4C) enriched with 10B isotope has been selected for control rod application in Prototype Fast Breeder Reactor (PFBR), which is being constructed at Kalpakkam. Neutron absorption property of boron is due mainly to the presence of 10B, which undergoes the main capture reaction [1-3]

5B10 +0n

1 → 3Li 7 + 2He4 + 2.6 MeV

The cross section of this reaction varies from 3850 barns for thermal neutrons to a few barns for fast neutrons. At higher energies, the cross section of most other elements becomes very small, where as that of 10B decreases monotonically with the energy and even at higher neutron energies of >1MeV, 10B exhibits sufficient absorption cross section values. Natural boron that contains 19.8% of 10B atoms can be enriched up to 99% and hence the use of boron is very attractive in the entire neutron energy spectrum [2-5]. The products of (n,α) reaction of 10B namely helium and lithium are stable and non-radioactive isotopes. Boron carbide is extensively used for control rod and neutron shielding applications in nuclear reactors. High boron content, high melting point, high temperature strength, resistance to radiation damage, chemical inertness and low density are the key properties which make boron carbide suitable for control rod application in fast reactors [1-9]. The major issue in boron carbide is processing difficulties. Due to refractory nature and brittleness, only powder metallurgy route is used for fabrication of solid shapes. Sintering of boron carbide is extremely difficult due to its high melting point (2450oC), strong covalent bonding and low intrinsic self-diffusivity. At lower temperatures, surface diffusion and evaporation - condensation mechanism are favoured which result in mass transfer without densification. At higher temperature, exaggerated grain growth

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takes place which results in poor mechanical propertiesissues, dense monolithic Btemperatures. This process is called hot pressing. Development of Processing The pellet fabrication has primarily two steps, synthesis of boron carbide powder and fabrication of dense pellets of Bcarbide is presented in Fig.1. Before starting the production ostudies were carried out on synthesis and densification of boron carbide. Effect of process parameters on product quality was investigated and all the process parameters were optimized for fabrication of pellets with desir

Fig. 1: Process flow sheet for fabrication of enriched boron carbide pellets

For synthesis of B4C, elemental synthesis route was selected as it involves minimum loss of boron and also gives better control over boron and petroleum coke were taken as starting materials. Boron and carbon were wet mixed in planetary ball mill using alcohol as medium. The mixed powder was dried in vacuum oven and then converted to green vacuum induction furnace in the temperature range of 1800synthesize B4C. The synthesized Bsubjected to energy intensive grinding operations to obtain micron sized particles.



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takes place which results in poor mechanical properties [1-10]. In order to overcome these B4C can be prepared by applying external pressure

s. This process is called hot pressing.

Scheme

The pellet fabrication has primarily two steps, synthesis of boron carbide powder and fabrication of dense pellets of B4C by hot pressing. Flow sheet for fabrication of enriched boron carbide is presented in Fig.1. Before starting the production of enriched boron carbide pellets, studies were carried out on synthesis and densification of boron carbide. Effect of process parameters on product quality was investigated and all the process parameters were optimized for fabrication of pellets with desired specification.

Process flow sheet for fabrication of enriched boron carbide pellets

C, elemental synthesis route was selected as it involves minimum loss of boron and also gives better control over purity & homogeneity of B4C

boron and petroleum coke were taken as starting materials. Boron and carbon were wet mixed in planetary ball mill using alcohol as medium. The mixed powder was dried in vacuum oven and then converted to green pellets by cold pressing. These green pellets were heated in vacuum induction furnace in the temperature range of 1800-1900o

The synthesized B4C was in the form of loosely sintered mass, which was sive grinding operations to obtain micron sized particles.

In order to overcome these external pressure at high

The pellet fabrication has primarily two steps, synthesis of boron carbide powder and Flow sheet for fabrication of enriched boron

f enriched boron carbide pellets, studies were carried out on synthesis and densification of boron carbide. Effect of process parameters on product quality was investigated and all the process parameters were optimized

Process flow sheet for fabrication of enriched boron carbide pellets

C, elemental synthesis route was selected as it involves minimum loss of powder. Amorphous

boron and petroleum coke were taken as starting materials. Boron and carbon were wet mixed in planetary ball mill using alcohol as medium. The mixed powder was dried in vacuum oven

pellets by cold pressing. These green pellets were heated in oC under vacuum to

was in the form of loosely sintered mass, which was sive grinding operations to obtain micron sized particles.



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Effect of process parameters for synthesis and grinding, like temperature, time, charge composition, rpm were studied and the optimized processing parameters were established in order to get the required powder specifications.For fabrication of pellets, the synthesized B4C powder was filled in a multi-cavity graphite die and hot pressed in vacuum at high temperature (~1950oC). The required density of pellets was achieved by optimising the hot pressing temperature, pressure, holding time, particle size and length to diameter ratio of pellet. The final pellets are machined for finishing and chamfering as per the given specifications. More scientific details on synthesis and consolidation of boron carbide can be found elsewhere [10]. After optimizing the processing parameters, few experiments were conducted to ascertain the reproducibility and it was found that the processing route has good reproducibility. The creation of appropriate infrastructure and establishment of extended laboratory scale production and processing facilities and testing of the methodologies for making boron carbide pellets, have been key factors in asserting indigenous capabilities in the department for the production and supply of control rods for PFBR. The developmental work to establish the process flow sheet and operating parameters for synthesis of boron carbide and consolidation to high density shapes was completed first using natural boron. After achieving the required material purity, density and properties, trial production work has been initiated with 10 B enriched material. Production and supply of enriched boron carbide pellets for PFBR The developed processing scheme was used for production of 11,180 pellets of enriched boron carbide and loaded in the first core of PFBR. There are two types of control rods in PFBR (1) Control safety rods (CSR) and (2) Diverse safety rods (DSR). There was a requirement of 7355 pellets for CSR and 3821 pellets for DSR. Enriched boron was received from Heavy Water Plant, Manuguru. Boron carbide powder was synthesized using batch size of 1 Kg. During hot pressing, 62 pellets were made in one batch using multi-cavity graphite die. For quality control, the following checks were carried after every unit operation to account for the purity of the charge and contamination picked up during grinding operations. • Chemical analysis for all the specified elements and compounds at different stages. • Particle size & its distribution and specific surface area of boron, carbon and B4C powders. • Monitor the change in weight in each step (synthesis, grinding, drying, and densification). • Phase identification by XRD (to be carried out after synthesis of B4C) Required machining, grinding and chamfering of the pellets were carried out by Centre for Design and Manufacture (CDM), BARC. Inspection was carried out for all the finished pellets by dimensional and visual inspection as per specification in supplied drawings in the presence of QA-CDM, QA-NFC and QA-BHAVINI. Visual Inspection (VI) of all the pellets has been carried out using 5x magnifier to check the presence of (a) end chips, (b) circumferential chips and (c) cracks as per ASTM-C-751-07. The pellets were critically examined and found to be within the acceptable maximum permissible values. Pellets have been verified for perpendicularity (Orthogonality) using V-block & Dial indicator set-up. The measured values (0.13 mm) were within the permissible value. Density of the samples was found to be within the acceptable limits. Chamfer angle was measured with “Profile Projector”. Pellet stack length

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was measured with “digital Vernier Calipers”. calibration validity. Visually acceptable pellets have been used to make the stackrequired stack length was achieved by selecting the appropriate pellets combination without need to cut/grind the last pellet. were prepared for CSR applicationsEach pellet stack length has been packed in speciallyin specially designed lockers in the stack was designated with a code number. Each box BHAVINI seal & signed by QA team members from NFC and BHAVINI.major facilities used for synthesishow the photograph of enriched boron carbide pellets and shipping of DSR stacks.

Fig. 2: Induction furnace for synthesis ofcarbide

Fig. 4: Enriched boron carbide pellets for PFBR



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measured with “digital Vernier Calipers”. All the measuring instruments are Visually acceptable pellets have been used to make the stack

achieved by selecting the appropriate pellets combination without the last pellet. A total of 171 nos. of acceptable pellet stack lengths (

SR applications and 60 stack lengths (1010 mm) were prepared for DSR. Each pellet stack length has been packed in specially designed thermocol containersin specially designed lockers in the presence of BHAVINI & NFC Quality Control Team

designated with a code number. Each box was labeled with pellet Stack number, BHAVINI seal & signed by QA team members from NFC and BHAVINI.major facilities used for synthesis and consolidation of enriched B4C respectively. Fig.4&5 show the photograph of enriched boron carbide pellets and shipping of DSR stacks.

for synthesis of powders Fig. 3: Hot Press for fabrication of enriched

pellets

Enriched boron carbide pellets for PFBR Fig. 5: Shipping of DSR stacks

All the measuring instruments are having Visually acceptable pellets have been used to make the stacks. The

achieved by selecting the appropriate pellets combination without the able pellet stack lengths (710 mm)

and 60 stack lengths (1010 mm) were prepared for DSR. designed thermocol containers and sealed

Quality Control Team. Each labeled with pellet Stack number,

BHAVINI seal & signed by QA team members from NFC and BHAVINI. Fig. 2&3 show the C respectively. Fig.4&5

show the photograph of enriched boron carbide pellets and shipping of DSR stacks.

Hot Press for fabrication of enriched boron

Shipping of DSR stacks



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Summary Enriched Boron carbide pellets were fabricated by powder metallurgy route, which involves the following intermediate steps:

• Preparation of B4C from elements with controlled composition. Starting powder chemistry, particle size and morphology have been tuned in order to get the desired quality.

• Micron size powders of B4C without any significant contamination from grinding media were prepared by energy intensive grinding operations.

• Dense enriched boron carbide pellets were fabrication by hot pressing without using any sinter additives.

• Density, hardness, fractures toughness, compressive strength, flexural strength, thermal conductivity, coefficient of thermal expansion properties were evaluated for finished pellets.

• Microstructural characterization (XRD, SEM-EDS) were carried also out.

Acknowledgements The authors would like to acknowledge the contributions of Dr. A.K. Suri, Dr. N.Krishnamurthy, Shri R.K.Fotedar and Shri C. Subramanian during the initial stages of this developmental activity.

References

[1] T S R Ch Murthy and A Mukherjee; “Densification of Boron Carbide and Titanium Diboride”, Metals Materials and Processes 18[2] 151-158 (2006)

[2] A.K. Suri, C. Subramanian, J. K. Sonber and T. S. R. Ch. Murthy; “Synthesis and consolidation of boron carbide: A Review”; (Full critical review) International Materials Review, 55[1],4-40 (2010).

[3] C Subramanian, A. K. Suri and T. S. R. Ch. Murthy, “Development of Boron –based Materials for Nuclear Applications”, (Technology Development Article) BARC Newsletter, 313, 14-22 (2010)

[4] C Subramanian., T.K. Roy, T.S.R.Ch.Murthy, P. Sengupta, G. B. Kale, M. V. Krishnaiah, A. K. Suri, “Effect of Zirconia addition on Pressureless Sintering of Boron Carbide”; Ceram. Int. 34[6] 1543-1549 (2008)

[5] J.K.Sonber, P.K.Limaye, T.S.R.Ch.Murthy, K. Sairam, A Nagaraj, N.L. Soni, R.J. Patel and J.K.Chakravartty. “Tribological properties of Boron Carbide in sliding against WC ball”, Int. J. Refract. Met. Hard Mater.51, 110-117 (2015)

[6] K. Sairam,T. S. R. Ch. Murthy, J. K. Sonber, C. Subramanian, R. C. Hubli, A. K. Suri; “Mechanical properties of HfB2 reinforced B4C matrix ceramics processed by in situ reaction of B4C, HfO2 and CNT”; Book Title: Materials Challenges and Testing for Manufacturing, Mobility, Biomedical Applications and Climate, Chapter: Reliable manufacturing with advanced Materials, Publisher: Springer



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International Publishing, Editors: Udomkichdecha, W, Böllinghaus, Th, Manonukul, A, Lexow, J., pp.87-96 DOI: 10.1007/978-3-319-11340-1_9 (2014).

[7] K. Sairam, J.K. Sonber, T.S.R.Ch. Murthy, C. Subramanian, R.K. Fotedar, P. Nanekar and R.C.Hubli, “Influence of Spark Plasma Sintering Parameters on Densification and Mechanical properties of Boron Carbide”, Int. J. Refract. Met. Hard Mater. 42, 185-192 (2014)

[8] Shankar Kumar, K. Sairam, J.K. Sonber, T.S.R.Ch. Murthy, Vidhyasagar Reddy, G.V. S. Nageswara Rao, and T. Srinivasa Rao; “Hot-Pressing of MoSi2 reinforced B4C Composites”, Ceram. Int. 40,16099-16105 (2014)

[9] K. Sairam, J.K.Sonber, T.S.R.Ch.Murthy, C.Subramanian, R.C. Hubli and A.K.Suri, “Development of B4C-HfB2 Composites by Reaction Hot Pressing” Int. J. Refract. Met. Hard Mater. 35, 32-40 (2012)

[10] J. K. Sonber, T. S. R. Ch. Murthy, C.Subramanian, R.K.Fotedar, R. C. Hubli and A. K. Suri; “Synthesis, densification and characterization of Boron Carbide”, Trans. Ind. Cerm. Soc. 72[2], 100-107 (2013).

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