Nuclear Fuels

PWR fuels :

Pressurized water reactor (PWR) fuel elements are made of uranium oxide pellets sheathed in Zircaloy tubes of about 1 cm diameter. These fuel elements are arranged in 14x14 or 17x17 formation and are about 4 meters in length. The fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. The fuel bundles are usually enriched . The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement. The Zircoloy tubes are pressurized with helium to try to minimize pellet cladding interaction (PCI) which can lead to fuel rod failure.

CANDU fuel :

The fuel bundles are about a half meter long and 10 cm dia. They consist of sintered (UO2) pellets in Zirconium alloy tubes, welded to Zirconium alloy end plates. Each bundle is roughly 20 kg, and a typical core loading is on the order of 4500-6500 bundles, depending on the design. The bundle has typically have 37 identical fuel pins radially arranged used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (four inches) in diameter, 0.5 m (20 inches long) and weighs about 20 kg (44 lbs) and replaces 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to their more efficient heavy water moderator), however, some newer concepts call for low enrichment to help reduce the size of the reactors.

BWR Fuel:
It is similar to PWR fuel except that it is canned to prevent density changes near the fuel as the same can affect the nuclear reactions and thermal hydraulics of the reactor.The no of fuel pin per assembly is of the order of 90's varying to design to design. The no of assemblies depend on the size of the core.

Magnox Fuel :The Metallic fuel is used In Magnox reactor which are gas cooled reactors operating in UK. The size varied from 50MWEe to ~500MWe. They were the precursors of the Advanced Gas cooled reactor. Unenriched Uranium is cladded with an alloy of Mg-Al and other metals in small amounts. The main disadvantage of this fuel is limit on max fuel pin temp hence the efficiency of the plant and reactivity of Magnesium with water prevents long term under water storage.

TRISO Fuel: It consists of a fuel kernel composed of uranium oxide (sometimes Uranium carbide or UCO), coated with four layers of three isotropic materials.

1. The first layer is a porous buffer layer made of carbon.
2. The second layer pyrolytic carbon (PyC).
3. The third ceramic layer of Siilicon Carbide retains the fission products and gives the TRISO particle structural integrity.
4. The outer layer of is of PyC.

TRISO fuel particles are designed not to crack at temperatures beyond 1600°C(due to differential thermal expansion or released fission gas pressure). They can contain the fuel in the worst accident scenario in a properly designed reactor. Two such reactor designs are pebble bed modular reactor (PBMR), in which thousands of TRISO fuel particles are dispersed into graphite pebbles, and a prismatic-block gas cooled reactorin which the TRISO fuel particles are fabricated into compacts and placed in a graphite block matrix. Both of these reactor designs are high-temperature gas-cooled reactors (HTGR), which is a type of very high temperature reactors (VHTR).

Types of Stainless Steels

Austenitic Steels: This comprises the 300 series of stainless steels and accounts for over 70% of total stainless steel production. Nickel is added to stabilise the austenite structure of iron, manganese can also be added to preserve the austenitic structure but at a lesser cost. Austenitic steels contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
Superaustenitic stainless steels are produced by adding higher amounts of manganese.
High Molybdenum causes greater resistance to chloride pitting and crevice corrosion if it has content (>6%).Higher nickel content ensures better resistance to stress-corrosion cracking over the 300 series. The 300 Series—austenitic chromium-nickel alloys:

Type 304—the most common grade of steel; the classic 18/8 stainless steel
Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability. Better wear resistance and fatigue strength than 304.
Type 302—same corrosion resistance as 304, with slightly higher strength due to additional carbon.
Type 303—easier machining version of 304 via addition of sulfur and phosphorus. Type 304—the most common grade; the classic 18/8 stainless steel.
Type 309— better temperature resistance than 304
Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; Alloy addition of molybdenum prevents chloride attack and crevice corossion. 316 steel is used in the manufacture and handling of food and pharmaceutical products where it is often required in order to minimize metallic contamination. It is also known as "marine grade" stainless steel due to its increased resistance to chloride corrosion compared to type 304. SS316 is often used for building nuclear reprocessing plants. Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L.
Type 321 :similar to 304 but lower risk of weld decay due to addition of titanium.
Type 347:with addition of niobium for desensitization during welding.

Steel Gr	Composition
303	17-19 Cr, 8-10 Ni, 0.15 C, 2.0 Mn, 1.0 Si, 0.20 P, 0.15 S min, 0.60 Mo (optional)
304	18-20 Cr, 8-10.50 Ni, 0.08 C, 2.0 Mn, 0.75 Si, 0.045 P, 0.030 S, 0.10 N
304L	18-20 Cr, 8-12 Ni, 0.03 C, 2.0 Mn, 0.75 Si, 0.045 P, 0.030 S, 0.10 N
316	16-18 Cr, 10-14 Ni, 0.08 C, 2.0 Mn, 0.75 Si, 0.045 P, 0.030 S, 2.0-3.0 Mo, 0.10 N
316L	16-18 Cr, 10-14 Ni, 0.03 C, 2.0 Mn, 0.75 Si, 0.045 P, 0.030 S, 2.0-3.0 Mo, 0.10 N
309	22-24 Cr, 12-15 Ni, 0.20 C, 2.0 Mn, 1.0 Si, 0.045 P, 0.030 S
347	17-19 Cr, 9-13 Ni, 0.08 C, 2.0Mn, 0.75 Si, 0.045 P, 0.030 S (Nb +Ta, 10 xC min,1 max)
420	12-14 Cr, 0.15 C min, 1.0 Mn, 1.0 Si, 0.040 P, 0.030 S
440A	16-18 Cr, 0.60-0.75 C, 1.0 Mn, 1.0 Si, 0.040 P, 0.030 S, 0.75 Mo

MOX fuels and Alternate Fuel Cycles

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.

Indian PHWR Fuel

The fuel bundles of PHWRs India are short cylindrical assemblies. Each coolant channel has 12 or 13 such fuel bundles.
220 MWe Reactor fuel : 19-element fuel bundle design. Stack of cylindrical sintered natural uranium dioxide fuel pellets, inside a zircaloy fuel tube and sealed at both ends by end plugs. The fuel elements are arranged in concentric rings and are assembled together by welding them to an end plate on each side to form a bundle. The bundle length is 495 mm and the weight is 16 Kgs. The spacers and bearing pads are attached to fuel elements by spot welding. The inside surface of fuel sheath is graphite coated to decrease the fuel element failure rate due to power ramps.
540 MWe and 700 MWe Reactor fuel: 37 element fuel bundle design is an extension of the closed packed 19 element fuel bundle. One more ring of elements has been added. All the elements are of a small diameter of 13 mm. The bundle has been designed to generate a bundle power of about 1 MW.

On power bi-directional fuelling is done using two fuelling machines, one at either end of the coolant channel. Pressure tubes containing a string of short length fuel bundles and the on-power refueling permit flexibility in choosing fuel designs and in-core fuel management parameters to maximize fuel utilization. A defective fuel can also be identified and removed from the reactor while it is in operation.

The unit energy cost distribution for PHWRs:

The fuel consumption cost:- 18% to 25% of unit energy cost.

The Capital cost:- typically 27%, .

The operation and maintenancecost:-typically10%

The heavy water costs :- Typically 32% .

Paradox:
PHWRS are optimally designed from neutron economy considerations, uses natural uranium fuel economically in comparison to other types in terms of extracting maximum energy per gram of natural uranium . Still the fuel consumption cost is high due to high cost of indigenous raw meterial. MOX fuels