Fuel design evolution

The most significant development for LWR fuel has been a continuous increase in average discharge burnup to about 45MW·d/kgU, with further increases being planned and many plants operating with discharge burnups at around 60MW·d/kgU. Several CANDU programmes in progress involve new fuel cycles that will ultimately lead to burnups in excess of those achieved for natural uranium. Numerous modifications in fuel designs and materials have been made to adapt fuel performance to higher burnup goals or to improve neutron economy and simultaneously reduce failure risk. Some details are given below.

2.2.1. BWRs

In BWR plants, there has been a steady evolution in fuel design, with the early 6 × 6 designs being steadily supplanted by 7 × 7, 8 × 8, 9 × 9 and, since the mid-1990s, 10 × 10 has become the standard except in Japan [2.10, 2.11]. Early failures due to pellet–cladding interaction (PCI) have been largely eliminated since the 1980s and liner fuel is standard.

The main failure mechanism in BWRs today is debris related and the industry is responding by using improved debris filters, coupled with an improved understanding of the mechanisms causing failure. It is common today for grids to be designed so that any small debris passing through the filter will not be trapped in the grid. This challenges grid designs, especially for 10 × 10 fuel, since there is also a drive to increase fuel loading, using larger diameter rods and reduced space in the lattice [2.12].

There continues to be isolated failures due to other mechanisms, such as PCI or corrosion effects. Minor changes in design have occurred and advanced corrosion resistant claddings are being considered. Empirical operating guidelines have been used to avoid the operating conditions which caused a few PCI failures in 2003, but the root cause of these remains uncertain.

2.2.2. PWRs

Since the early 1970s, many improvements have been made to PWR fuels to improve neutron economics and fuel burnup. Early changes led to reductions in fuel rod diameter and increases in the number of rods in a fuel assembly, and the standard design of today typically includes a 17 × 17 array of fuel rods with diameters ranging from 9.1–9.8 mm. The main improvements in the last 15 or so years include a trend to replace stainless steel components in assemblies, particularly grids, with zirconium alloys for neutron efficiency and changes in cladding alloys to improve fuel rod corrosion resistance, initially by optimizing the composition of Zr-4 alloys, and more recently through the introduction of Zr1%Nb alloys such as M5 (Areva) or Zirlo (Westinghouse). Duplex fuel, with a corrosion resistant zirconium alloy surface, is also being used.

Over the past 15 years, the main fuel failure mechanisms have remained debris fretting and grid to rod fretting, and assembly design has evolved to improve resistance to these problems. Debris resistance has been addressed through the introduction of advanced debris filters built into bottom nozzles (e.g. FUELGUARD, TRAPPER, Areva [2.13]) or through additional protective grids ( such as the P-grid from Westinghouse) combined with long solid metal end plugs and protective oxide coating at the lower end of fuel rods. The change to zirconium

FIG. 2.3. Water chemistry changes in US PWRs [2.1].

continually improved to reduce this problem. For example, no grid to rod fretting has yet been experienced with the Areva HTP fuel design.

Recent changes in assembly designs and materials have not led to new fuel failure mechanisms, but there have been new problems with assembly performance that have led to further design modifications. One major issue was the impact of assembly distortion during operation, which caused control rod friction inside the assemblies to increase, leading to occasional incomplete control rod insertion (IRI) during shutdown. The causes included high burnup assembly growth and cross assembly power tilts, and the remedy has been to increase the rigidity of guide tubes through increased wall thickness and detailed changes at the dashpot end of guide tubes, with designs such as MONOBLOC from Areva. Another issue was the failure of screws attaching springs to the top nozzles of some Westinghouse assemblies through stress corrosion cracking, which led to loss of hold down. Improved design of the top nozzle with a more robust method to retain the springs has now eliminated this problem.

The latest PWR fuel designs now offer a wide range of options, providing ease of use and flexibility in operation. Features such as intermediate flow mixing grids to improve thermal performance, integral poisons such as gadolinium to help achieve high burnup, removable top nozzles to facilitate repair and variable enrichment options within the assembly for optimum neutron efficiency are now readily available.

2.2.3. WWERs

WWER plants can be divided into the original version WWER-440 with channelled fuel assemblies containing 126 fuel rods, and the WWER-1000 series, in operation since the mid-1980s using unchannelled fuel (with one exception) and containing 312 fuel rods, both with hexagonal rod matrix and honeycomb type spacer grids. Fuel rods with 9.1 mm diameter Zr1Nb cladding and annular pellets are used in both types. Additional important differences in comparison to Western PWRs include the use of potassium–ammonia water chemistry, and reactivity control in the WWER-440 through special fuel assemblies containing boron steel tubes in the upper part.

In addition to the introduction of ZrNb alloy spacer grids in place of stainless steel (since 1987 in the WWER-440 and the mid 1990s in the WWER-1000), recent developments in WWER fuels comprise the use of advanced Russian Federation Zr1%Nb/Sn/Fe alloys with higher resistance to irradiation induced growth, creep and corrosion for guide tubes and for fuel rod cladding in applications with extended residence time (5–6 years).

During the past 10 years, WWER-1000 fuel assemblies have had similar issues to PWR assemblies, with assembly distortion and bow being a significant problem for both. Two different design solutions have been introduced; the TVS-A assembly includes a cage on the outside to provide structural rigidity, and the TVS-2 design uses thimble tubes of increased thickness, similar to the solution found for PWR assemblies. Both designs also include features such as debris filters and demountable top nozzles. Other changes being introduced to improve fuel burnup include advanced fuel pellet designs in which the central hole is reduced or eliminated to allow increased uranium loading and grid changes to improve thermal performance.

2.2.4. CANDU/PHWRs

CANDU reactors use strings of short fuel bundles (50 cm in length) in horizontal fuel channels and perform on-line refuelling. The fuel bundles have a circular geometry and elements with thin collapsible cladding, generally using graphite coating (CANLUB). No structural components such as spacer grids, support rods or end fittings are required, since the fuel elements themselves and the thin end plates serve as structural components. Bundle types with 19 and 28 elements (15 mm diameter) and with 37 fuel elements (13 mm diameter) are in operation. The 37 element bundle exists in two versions with small differences in end cap profiles and bearing pad positions to account for different fuel handling systems and channel configurations. (In Canada, the Bruce and Darlington reactors have ‘fuel against flow’ loading machines, the other plants have ‘fuel with flow’ machines.)

As the CANDU fuel types are mature products, their main design features have remained essentially unchanged. Nevertheless, extended technology programmes to further improve fuel performance and evaluate the use of advanced fuel (enriched uranium, recovered uranium from PWR rods, and thorium) are being carried out by AECL as well as through national programmes in India, the Republic of Korea, Argentina and Romania. The most recent fuel design is composed of a 43 element bundle with two different rod diameters, called CANFLEX. This design is at an advanced stage of development, being created through a joint AECL/KAERI programme.