variation of core temperature with heating schedules in mtr operation

7/29/2019 Variation of Core Temperature With Heating Schedules in MTR Operation

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Variation of Core temperature with Heating schedules inMTR Operation

Ladola Y S, S Chowdhury, Vivek Sharma, S B RoyUranium Extraction Division, BARC, Mumbai-400094

email-id: [email protected]

Abstract: Magnesio-thermic Reduction (MTR) of Uranium tetra Fluoride (UF4) is one of

the important industrial methods for producing nuclear pure uranium metal in massive form.

Nuclear grade natural Uranium (U) metal ingots are produced regularly in UED, BARC

following MTR route for fuelling research reactors in BARC. MTR is a batch type operation and

batch size has been scaled up over the years from 80 kg to 500kg in phases. 500kg process has

been recently designed and established at UED, BARC.

This is a bomb type reaction and is represented by

UF4 +2Mg =U +2MgF2 (H0298 = 83.5 Kcal/gm mole) [1]

Small excess of magnesium is required to improve yield. This thermite type reduction is carried

out in a closed reaction vessel, known as MTR reactor, lined with magnesium fluoride powder.

MTR reactors are made of boiler quality steel. Use of MgF2, a reaction by-product, as lining

material reduces the chance of foreign element contamination. Lining of MgF2 not only prevents

direct contact of the molten metal and slag with the reaction vessel but also acts as an insulating

material immediately after firing and holds the hot molten mass for longer period, thereby

facilitating adequate metal-slag separation. For conducting the reduction reaction, stoichiometric

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quantity of UF4 and Mg chips are blended and charged inside the lined reactor. Once the

charging is over, the top surface of charge is covered with fine MgF2 powder and the reactor is

sealed by fixing a lid. This sealed reactor is then heated inside an electric furnace at a predefined

heating schedule for the reduction to take place. The initiation of reaction is called Firing.

MTR reaction mechanism is a complex one. A large number of side reactions as well as parallel

reactions occur during the conversion of UF4 to U. This reaction is exothermic and final

temperature of the molten product mass i.e. U and MgF2 goes up to around 1600-17000C. U

settles down at bottom due to large density difference with slag. When the MTR reaction is

initiated at 25

0

C, reaction heat is not sufficient to melt reaction products (U & MgF2) completely

and an additional heat of 6.8 kcal/per gm mole must be supplied to effect their complete melting

[2]. Preheating is done to supply additional heat. Heat balance calculation shows that the charge

should attain a minimum temperature of 200oC before firing to have molten U and MgF2. Even

this is not sufficient for adequate metal-slag separation. Better metal-slag separation requires

low viscous molten product mass (U and MgF2) and that can be assured by higher temperature of

reaction product. High temperature can be obtained by higher heat input to charge before firing.

Higher core temperature ensures higher heat input. But, there is a limit to heat input to reactant

charge. Temperature of charge can not be increased endlessly before firing as surplus heat may

lead to a violent explosive reaction. Considering these two contradicting demands, understanding

of variation of core temperature with preheating is very important for safe and efficient

production of uranium metal. In this paper we have studied response pattern of core temperature

with different heating schedule in 500kg MTR Batch.

Key words: Magnesiothermic reduction, heat transfer, core temperature and heating schedule.

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1. Introduction:

Uranium (U) metal can be produced in a number of ways. Reduction of Uranium tetra fluoride

(UF4) by magnesium (Mg) or calcium (Ca) has been used for large-scale production of nuclear

grade Uranium metal ingots. When UF4 is reduced under specific conditions, a solid regulus of

material is formed under cover of slag. The initiation of reaction is generally called firing of the

charge. Firing is confirmed by sensing sudden rise in charge temperature which is recorded by

thermocouple inserted inside the top capping of the charge. This temperature is called Core

Temperature. For obtaining massive uranium, the products of the reaction, the uranium and the

slag should be sufficiently fluid and remain so, long enough for the dispersed particles of freshly

produced uranium to come together, coalesce and merge to the primary interface. The heat of

reaction should be enough to obtain such condition even after compensating heat losses to the

containers and its surroundings. Magnesiothermic Reduction (MTR) of UF4 is being practiced in

UED, BARC for production of nuclear grade uranium metal ingot and it has been observed that

to obtain above mentioned condition an optimized preheating schedule is essential.

2. Reaction features:

MTR reaction mechanism is a complex one. A large number of side reactions as well as parallel

reactions also occur during the conversion of UF4 to U.

These reactions are

1) UF4 +2Mg U +2MgF2 (H0298 = 83.5 Kcal/gm mole) [1]

2) Mg+2UF4 MgF2 +2 UF3

3) Mg +UO2F2 UO2+MgF2

4) 2Mg +UO2 2MgO +U

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5) Mg +UO3 MgO +UO2

6) 2Mg +U3O8 2MgO +3UO2

7) 2Mg +O2 2MgO

8) 3Mg +N2 2Mg3N2

9) Mg +H2O MgO +H2

10) Mg +2HF MgF2 +H2

11) 2UF4 +O2 UF6 +UO2F2

12) 9UO2F2 3UF6 +2U3O8 +O2

13)2UF4 +2H2O +O2 2UO2F2 +2HF

14) UO2F2 +H2O 2U3O8 +12HF+O2

15) UF4 +2H2O UO2+4HF

16) N2+3H2 NH3

The reaction between uranium tetraflouride and Magnesium is less exothermic than to be

adequate by itself to realize the fusion of the products. When the MTR reaction is initiated at 25

0C, reaction heat is not sufficient to melt reaction products (U & MgF2) completely and heat

balance calculation shows an additional heat of 6.8 kcal/per gm mole must be supplied to effect

their complete melting [2]. However, the necessary temperature for fusion and separation is

being attended by adding heat to the charge before the reaction initiation. Once the reaction is

initiated, it proceeds to completion spontaneously yielding massive amount of metal. Final

temperature of the product mass i.e. U and MgF2 goes up to around 1600-1700oC. At this

temperature, both U and MgF2 are in molten state. U settles down at bottom due to large density

difference with MgF2.This separation is very important for yield improvement. Due to high

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density difference between charge and product material, after cooling a cavity is formed above

product mass.

3. Materials and methods:

Magnesio-thermic Reduction reaction is carried out in boiler quality reaction vessel called MTR

reactor. The reactor is lined with refractory material i.e. magnesium fluoride (MgF2) to protect

vessel from melting due to the heat of reaction and prevent contamination of the U metal with

the material of the reaction vessel. The blended UF4 and Mg charge is packed in the lined

reactor. Top of the charge is capped with MgF2 powder to protect lid. The lid is then bolted to

the reactor and the sealed reactor is loaded in the furnace for preheating. Preheating is done by

following predefined heating schedule for the initiation and completion of the reaction.

Temperature is measured at different position of reactor using thermocouples as shown in Figure

1. Zone temperature is controlled by PLC controller as per heating schedule. All zone

temperatures and core temperature are continuously recorded till completion of firing. MTR

batch size has been scaled up over the year from 80 kg U to 500 kg U. Furnace has been

changed from bogie type to pit furnace for better utilization of heat input. Based on previous

experience, different heating schedules as shown in Figure 2(a)-2(d) were selected for preheating

of the 500 kg MTR Batches. Core temperature is continuously observed for firing indication.

Firing is indicated by sudden rise of Core temperature as core temperature suddenly increases

due to heat of reaction.

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4. Results and discussions:

With increase in preheating temperature, enthalpy of reactants (UF4 and Mg) changes as shown

in Figure 3 [2]. Therefore preheating is done to supply additional heat to effect complete charge

melting and it has been calculated that whole charge should attain a minimum temperature of

200oC before firing to have molten U and MgF2. However, this is also not sufficient for adequate

metal-slag separation. Better metal-slag separation occurs in low viscous molten mass (U and

MgF2), which can be assured by higher temperature of reaction products. For attaining this

higher temperature higher heat input to charge before firing is necessary. But, there is a limit to

heat input to reactant charge before firing as excess heat may lead to violent reaction which may

cause explosion [3]. Considering these two contradicting demands, understanding of variation of

core temperature with preheating schedule is very important for safe and efficient production of

uranium metal. Since this is a bomb type reaction and triggering can occur anywhere in the

charge, exact measurement of firing temperature is not possible. Moreover, though sufficient

soaking time is provided, temperature distribution in the charge is not uniform [4]. Firing occurs

when any single point of charge reaches at firing temperature and this sets limit on heating rate

as higher rate of heating increases temperature of some portion of charge at firing temperature

level before adequate heating of full charge and leads to premature firing. Low rate of heating

take more time to complete any batch and lead to lower rate of production. Optimum rate of

heating is very important which is ensured by following optimum heating schedule. It has been

observed that increase of core temperature during preheating is an indicator of rate of heat input.

Therefore study of response pattern of core temperature is very important in optimizing the

heating schedules. During present study, response of core temperature with time for different

heating schedules has been observed and is shown in Figure 2(a)-2(d). It has been observed that

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heating schedule 1, 2 and 3 have similar core temperature pattern. Temperature pattern of

heating schedule 4 is initially similar to other heating schedules but it becomes sluggish at the

end. Recovery of different batches with different hating schedule is tabulated in Table 1. Very

good recovery has been obtained in all heating schedules. However both excellent and poor

recoveries have been observed for heating schedule 3 and 2. From available limited data it can

be observed from profile of core temperature that heating schedule 4 have optimum rate of

heating and consistent recovery.

5. Conclusion:

Heating schedule is important for control of rate of heating and overall heat input. As expected,

during preheating variation of core temperature for a particular heating schedule shows similar

pattern with a difference in absolute value within a range of 50 oC. This may be due to

variation of properties of reactants and linning material. Heating schedule 2 and 3; both have one

premature firing with firing temperature 265 oC and 273 oC. Therefore it can be concluded that

for higher recovery ignition point core temperature must be above 300 oC and preferably above

340 oC. From the limited no. of available data it appears that heating schedule 4 have optimum

rate of heating and heat input. However, to establish its effectiveness more trials are needed.

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6. Reference:

1. C D Harrington and A E Ruehle (ed.) Uranium Production Technology.

D. Van Nostrand co. Inc. (1959)

2. Y S Ladola, S Chowdhury, S Sharma and S B Roy Recovery of locked up Uranium in Slag

disc by co-melting in MTR,Trans. IIM, vol. 61, April-June 2008, pp 103-106.

3. A K Mohanty Physico-chemical aspects of metallothermic reduction proceedings of the

symposium on Metallothermic processes in metal and alloy extraction, December 1983, pp

21-41

4. S Soni, S Manna, V Iswaran, S B Roy and P Munshi Modeling of unsteady state heat

transfer in externally heated Magnesiothermic Reduction Reactor in press, November 2008,

IAEA, Austria.

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Table 1: MTR batch details

MTRBatchNo.

Equipment:Furnace,Reactor, Lid

Heating Schedule,total heating Time (hr),Energy consumption

(kW-hr)

Recovery

%

Core Temperatureat time of firing

1 301, G1, G1 H1 34.00 895 88.67 3482 301, G1, G1 H1 31.00 865 89.11 3513 301, G1, G1 H1 27.00 920 91.90 3814 301, G3, G3 H1 29.55 985 90.10 3775 302, G7, G7 H2 36.15 1245 89.54 4586 302, G8, G8 H2 24.30 990 92.23 3957 302, G7, G7 H2 24.40 895 90.14 3458 302, G8, G8 H2 17.25 755 81.55 3159 302, G7, G7 H3 28.50 801 94.34 34810 302, G8, G8 H2 22.10 778 94.34 371

11 302, G7, G7 H3 15.50 775 62.76 26512 302, G8, G7 H2 17.05 670 71.68 27313 302, G8, G8 H4 28.35 905 90.59 36614 302, G8, G8 H4 29.40 925 90.37 395

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TC

TT

TM

TB

Figure 1 Schematic diagram of MTR reactor 1.Red colour: reactor body and lid 2.Black clour:graphite crucible 3. Green colour: UF4 and Mg charge 4. Gray colour: MgF2 linning andcapping) 5. TC: core temperature thermocouple 6. TT: top zone thermocouple 7. TM: middle

zone thermocouple 8. TB is bottom zone thermocouple.

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Figure 2b: Response of Core temperature for heating Schedule 2

Figure 2a: Response of Core temperature for heating Schedule 1

Temperature(oC)

T

emperature(oC)

Heating Schedule 2

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35 40

5

7

0

2

Time (hr)

Heating Schedule 1

0

100

200

300

400500

600

700

0 5 10 15 20 25 30 35 40

1

2

3

4

Time (hr)

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Heating Schedule 3

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35 40

Temperature(oC)

Time (hr)

1

9

Figure 2d: Response of Core temperature for heating Schedule 4

Figure 2c: Response of Core temperature for heating Schedule 3

Heating Schedule 4

0

100

200

300

400

500

600700

0 5 10 15 20 25 30 35 40Time (hr)

Tem

perature(oC)

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15

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0

5

1

1

Entha

lpyofmaterial,Kcal/gmmole

0

5

20

0 Temperature, 0C

UF4 +2Mg

UF4

Mg

100 200 300 400 500

Figure 3: Enthalpy of reactant (UF4 and Mg) at different temperature.

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variation of core temperature with heating schedules in mtr operation

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