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1. Advanced Heavy Water Reactor Amit Thakur Reactor Physics Design Division Bhabha Atomic Research Centre, INDIA

2. Design objectives of AHWR The Advanced Heavy Water Reactor (AHWR) is a unique reactor designed in India for the large scale commercial utilisation of thorium and integrated technological demonstration of the thorium cycle Design objectives / Challenges • Maximise the power from thorium • Maximise the in-situ burning of 233U • Negative power coefficient • Heat removal through Natural Circulation • Uniform coolant flow ; flat radial power distribution , short active core height • Low power density • Bottom peaked axial power/flux distribution • Fuel cycle aspects • Self-sustainance in 233U • Plutonium as make-up fuel ; minimise the Pu inventory and consumption • Maximise the burnup

3. Physics design criteria • Maximize power from thorium • Core averaged negative void coefficient of reactivity at operating conditions. • The discharge burnup of the fuel should be greater than 30 GWd/Te. • Minimize consumption of plutonium. • Initial plutonium inventory should be as low as possible. • The system should be self-sustaining in 233U. • The total thermal power to the coolant should be 920 MW.

4. Thermal hydraulic considerations of uniform coolant flow • Local peaking factor has to kept within the TH margins • Suitable enrichments • Better TH margins - Higher MCHFR • Relatively bottom peaked power distribution • Axially graded fuel pins (Outer pins)

5. Major disadvantages Reactor safety Low delayed neutron fraction eff Reactor operation Photoneutron production ( start-up) Radiation problem --Association of U232 with U233 Reactor control problem due to Pa233 Reprocessing issues Dissolution of thorium fuel is more difficult than uranium fuel, due to its high material stability Major Advantages Availability Improved nuclear characteristics Improved technological performance compared to UO2 Stable crystal structure Higher thermal conductivity High melting point lower thermal expansion coeff. Improved fuel behaviour Dimensional stability at high burn-up Reduced generation of actinides in Th-U-233 cycle Properties of thorium

6. a b Ac-228 Ra-228 Th-232 6.7 y 1.4e+10 y b 6 hrs a a a a Rn-220 Po-216 U-232 Th-228 Ra-224 1.91 y 3.64 d 56 s 69 y a 0.15 s Pb-212 Tl-208 a b b 33.7 % 3.1 m 10.6 hrs Bi-212 Pb-208 2.6 MeV  a b 60.6 m 0.7-1.8 MeV  66.3% 0.3s Po-212 Thorium decay chain

7. Thorium burnup chain

8. Comparison of fertile species 238U and 232Th n ,    232Th 233Th 233Pa 233U 1.4 x 1010 y 7.37 barns 22.3 min 1500 barns 1.59 x 105 y c 47 b ; f 530 b 27 days 20 barns Fertile Fissile n ,   238U 239U  239Np 239Pu 2.4 x 104 y c 270 b ; f 752 b 4.5 x 109 y 2.7 barns 23.5 min 22 barns 2.36 days 32 barns Fissile Fertile

9. Neutronic properties of some fissile and fertile isotopes

10. Comparison of eta for different fissile isotopes 233U, 235U and 239Pu

11. AHWR D5 lattice D5 lattice consisting of 54 fuel pins placed in three arrays of 12, 18 and 24 pins in each array and a central displacer region Fuel composition : Ring 1 – (Th,U)MOX - 3.0 % 233U (12 pins) Ring 2 – (Th,U)MOX - 3.75 % 233U (18 pins) Ring 3 – (Th,Pu)MOX – 4.0% Pu (24 pins) (Lower) 2.5% Pu (24 pins) (Upper) 3.25 % Pu (average) Displacer region – (Zircalloy-2 rod) Pu composition : Discharge composition of Pu from PHWR at 6700 MWd/T Core average discharge burnup : 34000 MWd/T Displacer rod Cross section of AHWR D5 cluster

12. Plutonium burning in AHWR • Placing the Pu pins in the outer ring where it sees a significant thermal flux is responsible of Pu burning • Initially the plutonium isotopes contribute about 46 % of the total absorptions and this reduces to • 15 % at the end of the cycle. • 239Pu absorptions • reduces by about 90 % • at discharge due to its depletion. • In the initial core cluster, the burnups achievable are relatively low and hence lower burning Plutonium burning in the composite cluster Discharge composition of the Pu fuel 238Pu 239Pu 240Pu 241Pu 242Pu Initial (PHWR) 68.79 24.6 5.26 1.35 Composite cluster 1.84 2.28 37.33 22.31 36.24

13. Uranium : In-situ generation and burning • (Th,Pu) MOX pins placed in well • thermalised spectrum to maximise • 233U production • Self-sustenance in 233U can be achieved by proper neutron spectrum • Fuel utilisation has to be traded off with TH parameters and safety parameters like negative void reactivity • At core average burnup, the power from Th-233U is about 65- 70 % Cluster averaged U compositions 232U 23U 234U 235U 236U Composite cluster 0.16 81.61 14.80 2.68 0.35

14. Salient features of the core design • Boiling light water cooled • Heavy water moderated • Fuel cycle based on (Th,U) and (Th,Pu) • Axially graded fuel for bottom peaked flux distribution for better TH characteristics • on power fuelling • Two independent fast acting S/D systems • Total No. of channels - 513 • No. of fuel channels - 452 • Fuelling rate (annual) - 82 (ch) • Average dis. burnup - 34000 MWd/t Core layout of AHWR equilibrium core

15. Optimised core power distribution

16. SAFETY FEATURES OF AHWR • The main aim of AHWR design is to make the void reactivity –ve (in order to gain extra degree of safety) • To achieve this it is a nightmare for the designer but a delight for the operator. • Different approaches for making the void reactivity negative. 1) Addition of some slowly burnable absorber in the displacer region of the fuel cluster. 2) Decrease the lattice pitch.

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