Continuous Irradiation of Thorium in Equilibrium core of Standard PHWRs

Transcription

1 Continuous Irradiation of Thorium in Equilibrium core of Standard PHWRs ABSTRACT G. Raja Rao, R. Padhy and M. V. Parikh Reactor Physics section, Kakrapar Atomic Power Station, NPCIL mvparikh@npcil.co.in Earlier Initial flux flattening in fresh core was achieved by loading 35 thorium bundles mostly in the central part of the core. It s not only helped in operating the reactor close to rated power from the beginning of operation but also generated U33 inventory to some extent. However, the demand for U33 is increasing continuously especially that for Advanced Heavy Water Reactor (AHWR) core. It has been proposed that Thorium bundles may be continuously irradiated in Standard PHWRs in equilibrium core configuration to generate U33 on regular basis. In this paper a scheme of loading Thorium bundles is proposed which has been evaluated by using KAPS operating data. The effect of this loading scheme on operational parameters like allowed operating power and reactivity load has been studied and ensured to remain within the acceptable limit. Safety parameters like Void Coefficient, Power defect, worth of shut down devices etc and the kinetic parameters are also evaluated and demonstrated to be equivalent or better than the original all Natural Uranium core. INTRODUCTION In early 22 MWe PHWR cores depleted bundles (656 for Rawatbhata and Kalpakkam units, 384 for Kalpakkam, Narora and Narora) were loaded in the central zone of the reactors to flatten the flux so that a higher global power can be extracted from the core right from beginning. In KAPS, for the first time in standard 22 MWe PHWRs flux flattening was achieved by loading 35 Thoria bundles 1 (instead of depleted bundles) scattered all though the core. After successful loading of 35 thorium bundles in KAPS reactor, Thorium bundles were also loaded in subsequent reactors i.e KAPS, RAPS-3&4, Kaiga1&2 and RAPS after En-mass feeder replacement for initial flux flattening. During the operation from initial core to equilibrium core (~ 6 ), these bundles were discharged from the core and there after no more Thoria bundles were loaded. Considering the requirement of U33 for AHWR, a study was carried out for continuous irradiation of thorium bundles in few selected channels of equilibrium core of standard PHWRs. In order to arrive the loading scheme to obtain U33 on regular basis following objectives were fixed based on the operational and production of U33 requirements : Reduction in Allowed operating power should be less than 2 % FP Reactivity load should be less than 2 mk Burn up of Thoria bundles should be more than 15 n/mb so that the production of U33 from these bundles shall be more than 1 gram per bundle. The proposed scheme was evolved using actual operation data of KAPS core from 7 to 23 and the fuel management code TRIVENI Peripheral channels were selected for thorium loading at 2 nd and 3 rd string location to reduce the

2 Diff-pow(%FP) reactivity load. Further the softer neutron spectrum in the peripheral region helps in achieving cleaner U33. It was established that the proposed loading scheme of Thorium Bundles into the core do not make any significant effect on the following safety parameters. Worth of shutdown system Void Coefficient and Power defect Kinetic parameters like delayed neutron fraction and prompt neutron life time.. Evolution of Proposed Thorium Loading Scheme The refueling data of KAPS from 7 to 23 was used. 12 peripheral channels having the lowest Equilibrium channel power viz. A-8, A3, B-6, B5, C-4, C7, D-3, D8, R-3, R8, T-6 and T5 were identified for loading Thorium bundles. These channels were refueled with Thorium (Th) bundles at string position 2 and 3 and rest with Natural Uranium (NU) bundles as and when they were refueled during the actual KAPS operation. These channels were refueled again with another 2 Thorium bundles loaded at 2 nd and 3 rd location when they were next refueled. By this process now these channels will have 4 Thorium bundles i.e. at 2 nd, 3 rd, 1 th and 11 th location. At the time of third refueling again 2 Thorium bundles are loaded at 2 nd and 3 rd location maintaining 4 Thorium bundles in the channel and 2 Thorium bundles are discharged. Thus from 3 rd refueling onwards 2 thorium bundles from the selected channels will be regularly discharged. The reduction in allowed operating power with respect to the original NU core for the proposed Thorium loading core is shown in figure and the reactivity load due to such loading along with number of Thoria bundles in the core is shown in figure respectively. Figure Restriction on allowed power (%FP) due to Th bundles loaded in KAPS# Diff-Allo-pow(%FP)

3 DIFF-RHO(mk) No.of Th-bundles Figure 2 Reactivity load due to Th bundles loaded in Kaps# Diff-RHO(mk) no.of Th bundles in the core 5 It can be seen from the above figures that both the criteria viz. reduction in Allowed operating power and Reactivity load are satisfying the desired criteria. Residence time of each Thorium bundle, discharge burn up and the U33 build up during this period is given in table. Table Th-loaded Th-discharged Residence time Discharge -bup CH-ID U33 (gm) () (n/mb) A8(1) A8(11) A13(1) A13(11) B6(1) B6(11) B15(1) B15(11) C4(1) C4(11) C17(1) C17(11) D3(1) D3(11) D18(1) D18(11) R3(1) R3(11) R18(1) R18(11) T6(1) T6(11) T15(1) T15(11)

4 DIFF_RHO(mk) No. Of Th bundles Diff-pow(%FP) It can be seen from the above table that for the normal residence time (as per KAPS operating data) the Burn up of Thoria bundles is less and hence the production of U33 is lower than the desired value. However, the burn up and U33 production can be easily increased by suitably adjusting the residence time. To assess the overall effect of increasing the residence time, arbitrarily for all the 12 channels the residence time was made almost 1.5 times and the simulation was carried out. The reduction in allowed operating power for this case is shown in figure -3 and the reactivity load along with number of Thoria bundles in the core is shown in figure-4 respectively. Figure 3 Restriction on allowed power (%FP) due to Th-bundles loaded in Kaps# Diff-allo-pow(%FP)] Figure 4 Reactivity load due to Th-bundles loaded in KAPS# Diff-RHO(mk) # Th bundles in the core

5 It can be seen from the above figures that both the reduction in Allowed operating power and Reactivity load are still satisfying the desired criteria even though the residence time of Thoria loaded channels is increased. The revised Residence time of each Thorium bundle, discharge burn up and the U33 build up during this period is given in table. Table Th-loaded Th-discharged Residence Dischrgebup(n/Mb) (gm) U33 CH-ID time() A8(1) A8(11) A13(1) A13(11) B6(1) B6(11) B15(1) B15(11) C4(1) C4(11) C17(1) C17(11) D3(1) D3(11) D18(1) D18(11) R3(1) R3(11) R18(1) R18(11) T6(1) T6(11) T15(1) T15(11) It can be seen that the desired amount of U33 can be obtained by adjusting the residence time suitably. The burn up of Thorium bundles may be considered as the simplest criteria for deciding the refueling of the Thoria loaded channels. The worth of Shutdown systems for the above proposed loading scheme of Thorium bundles is estimated and given in Table-3. No. of Thorium bundles in the core Table -3 13PSS+SR (mk) 3SSS+SR (mk) All NU NU + Th All NU NU + Th

6 The variation of kinetic parameters like effective delay neutron fraction, generation time and safety parameters like power defect and void coefficient of reactivity are analyzed using REFGAIN program 3 and given in Table-4. Table -4 β eff GEN_TIME (Sec) POW_DEF(mk) VOID(mk) All NU NU + Th All NU NU + Th All NU NU + Th All NU NU + Th E E E E E-3 5.5E E E E-3 5.4E E E E-3 5.5E E E It can be seen from Table-3 and Table-4 that the proposed loading scheme of Thorium bundles into 12 channels does not make any effect on the safety parameters. Conclusion: The proposed scheme of Thorium irradiation in 12 peripheral channels in equilibrium core of 22 MWe PHWRs can be followed to generate U33 on regular basis. The reactivity load of about 2 mk and the operating reactor power restriction of about 2% FP can be taken care through proper fuel management. Safety parameters like Void Coefficient, Power defect, worth of shut down devices etc and the kinetic parameters are equivalent to the original all Natural Uranium core. By following this loading scheme U33 can be obtain from the 4 th year of Thoria bundle loading in the core and the expected U33 production may be of the order of about 2 kg / year at 9 % capacity factor. Reference: 1. Kamala Balakrishnan and Anil Kakodkar, Optimization of the initial fuel loading of the Indian PHWRs with thorium bundles for achieving full power, Ann of nuclear energy, Vol 21, No. 1, R. Srivenkatesan et al., A New TRIVENI version with Explicit Simulation of Xenon and Rhodium. Th. PD Note No Nov5, M.P.S.Fernando et.al.,refgain_manual. NPCIL