Looking beyond natural uranium alone as fuel material, several strategies/approaches are under consideration for use in PHWRs. Use of reprocessed material will reduce volume of spent fuel material and disposable fuel waste. Consequently this will reduce overall fuel cycle costs. Short length fuel bundles and on-power refueling provision in PHWRs provides flexibility to use variety of fuel loading patterns and different fuel types and consequently permits optimum use of fuel in the reactor. Following paragraphs cover the alternative fuel designs and core loading concepts in use or under consideration for use in Indian PHWRs.
Thorium
As part of Indian long term fuel cycle strategy of using thorium, irradiation of thorium is planned present power reactors to gather some experience. In the 220 MWe PHWRs, 35 Thorium bundles have been used for flux flattening in the initial core such that the reactor can be operated at rated full power in the initial phase. These bundles are distributed throughout the core in different bundle locations, both in the high power and low power channels. This loading was `successfully demonstrated in KAPS-1 and subsequently adopted in the initial reactor loading of KAPS-2, KAIGA-1 & 2 and RAPP 2,3&4. So far 232ThO2 bundles have been successfully irradiated in different reactors. The thorium dioxide fuel bundle fabrication and irradiation has provided valuable experience.
It is now planned to irradiate thoria bundles to higher burnups with suitable modification in design. It is also planned to take up loading a few thorium bundles regularly during equilibrium reactor operation.
MOX-7
It is also proposed to load MOX fuel in one of the existing PHWRs. For this purpose, MOX-7 bundle design has been evolved, which is a 19-element cluster, with inner seven elements having MOX pellets consisting of 0.4 wt % Plutonium dioxide ( about 70% fissile) mixed in natural uranium dioxide and outer 12 elements having only natural uranium dioxide pellets.
Based on detailed studies, an optimised loading pattern and refuelling scheme has been evolved for loading the bundles in an existing operating reactor. The scheme evolved is to load MOX-7 bundles in outer burnup zone and retain natural UO2 fuel bundles in inner burnup zone. The present natural uranium core will be converted gradually to a mixed MOX - natural UO2 core in a span of about 3 years. The core average discharge burnup in equilibrium core increases to around 9000 MWd/TeHE with this scheme. Due to this the fuelling rate comes down by 25%.
Initially trial irradiation of 50 number of MOX-7 bundles in one of the KAPS reactors is being taken up this year. Regulatory review has been completed and permission has been obtained for this purpose. Special bundle transport package and storage racks have been developed such that subcriticality is assured. The 50 fuel bundles are currently under fabrication and loading of these in the reactor would commence by this year end.
The different advanced fuel cycles relevant to PHWRs were reviewed during the nineties by a Committee appointed by DAE (Ref.2). The committee is also of the view that recycling of plutonium in PHWRs could provide an elegant way of dealing with the available spent fuel inventories. The major fuel cycle cost is back end cost and is typically 45-65%. Hence the fuel cycle which reduces back end cost by having higher burnup tends to be cheaper. The other high contributor to the energy costs is fuel fabrication. For, the MOX fuel, the back end cost is less compared to Nat. U cycle due to higher burnup. However the MOX fuel fabrication cost is 6 times that of Nat. U. In view of this the fuelling cost for MOX can be made competitive only by going to high burn-ups.
Depleted Uranium
In earlier years, RAPS, MAPS and NAPS reactors were loaded with 384 to 550 depleted uranium fuel bundles as a part of initial core fuel loading for the purpose of flux flattening. Recently, schemes have been worked out for fresh PHWR cores to maximize the use of depleted uranium whereby 40% of the fuel bundles can be of depleted uranium with U235 content of around 0.6%. The fresh core of MAPS-2, after the en-masse coolant channel replacement, was loaded in this fashion, effecting significant savings in natural uranium requirement.
Similar reactor physics studies have been carried out for use of large of number depleted Uranium bundles as a part of initial fuel loading in 540 MWe Reactors coming up at Tarapur. Fuel loading in first of these units will be taken up around mid 2004. The loading scheme consists of loading of depleted uranium bundles with different uranium 235 contents.
Theoretical studies, to use depleted uranium in combination with natural uranium for regular refueling in some of the current operating 220 MWe PHWRs has been completed.
Use of depleted uranium results in significant savings in available natural uranium reserves. Assuming that the depleted material is free, the depleted uranium fuel bundle cost consists of fabrication cost and other levies and it works out to be 50% of Nat U bundle cost.
Schemes are also worked out to load slightly enriched uranium (SEU) fuel bundles in PHWRs with 0.85% U235. This gives maximum energy output per kg of natural U processed.
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