The ultrasonic key to clean fuel

Economic concerns are pushing operators to use longer fuel cycles and higher thermal duties. As a result, axial offset anomaly (AOA) has become particularly costly to PWRs. Subcooled nucleate boiling now occurs routinely in the upper spans of the fuel in aggressively driven PWR cores, and corrosion products released from steam generator and primary system surfaces accumulate there preferentially. These deposits form a matrix for accumulating anomalous levels of boron, either by precipitating insoluble boron compounds or by adsorbing boric acid. Such boron hideout negatively skews the power profile beyond core design predictions, decreasing shutdown margin and posing challenges for the operating crews. Conventional remedies for this situation are extremely costly: either power derating during severe AOA cycles or purchasing additional fuel to reduce core thermal duty.

In addition to contributing to the cause of AOA, excessive core deposition may result in fuel failures in both PWRs and BWRs. Further, iron- and nickel-based fuel deposits become activated, and subsequent re-release of these activated corrosion products increases the primary system dose rates when they get redistributed throughout the primary circuit during power transients, startup, and shutdown.

FUEL CLEANING

The Electric Power Research Institute (EPRI) developed an advanced fuel cleaning cleaning system, as part of its Robust Fuel Program, in an effort to address these problems, especially the mitigation of AOA. The idea of fuel cleaning for AOA mitigation is to remove corrosion product deposits from the second-cycle fuel before reloading it into the core. Lacking this initial deposit of corrosion products, boron hideout is impeded, and the risk of AOA developing early in the next cycle on aggressively driven reload fuel is eliminated. In addition, fuel cleaning removes the possibility of redistributing deposits from reload fuel to feed fuel, postponing the hideout of boron and onset of AOA for the hottest feed fuel assemblies.

There is growing evidence that the fuel deposits formed in high duty cores are less effectively removed by conventional shutdown chemistry evolutions than in cores that are not subject to AOA. Particulate inventory can be more prevalent in the circuit in these PWRs, and is potentially more troublesome with respect to core performance. Routine fuel cleaning can provide an effective alternate method for limiting the total corrosion product inventory for plants operating high thermal duty cores over multiple cycles.

The idea of cleaning reload fuel is to eliminate a large fraction of the activated particulate material from the primary system, to prevent its redistribution onto ex-core surfaces. Instead, the corrosion products are captured on membrane filters, which are safely stored in the spent fuel pool or storage pit. Thus the quantity of ex-core corrosion products that can cause increased personnel dose exposure in subsequent outages is significantly reduced. Expected ALARA radiation management benefits include:

For PWRs:

• Reduction of particulate bursts on shutdown.

• Lower primary system dose rate.

• Less contamination in reactor cavity.

• Less uncontrolled release during fuel handling.

For BWRs:

• Less corrosion product in under-vessel drains, shutdown and clean-up heat exchangers.

• Less activity on control rod drive (CRD) filters, reducing exposures associated with routine CRD rebuilding.

• Improved fuel pool clarity and reduced dose rate at the fuel pool surface.

FULL SCALE DEMONSTRATION

EPRI’s ultrasonic fuel cleaning technology has been under development since late 1997. The method is based on novel ultrasonic technology that allows the inner rods to be cleaned without the need to disassemble the fuel assembly. The general arrangement of the ultrasonic cleaner and waste collection module, as it would be located in the spent fuel pool, is shown in the diagram (left).

The fuel bundle to be cleaned is inserted into a chamber similar to a cell of the spent fuel rack. The chamber is surrounded by high-powered, ultrasonic transducers. The cleaning energy output and spatial arrangement of these transducers is such that the ultrasonic energy fills the space within the cleaning chamber uniformly. In effect, the ultrasonic energy is able to “see around” solid obstructions so that the inner rods of the fuel assembly receive enough energy to be cleaned effectively, without subjecting the outer rods to so much energy that there could be danger of displacements that threaten cladding or fuel pellet integrity.

Pumps continuously draw pool water through the cleaning chamber and into a filtration system attached to the cleaning fixture. The deposits released from the fuel are suspended in the water as it flows through the chamber, and the flowing water sweeps the particles into the filtration system for collection and ultimate disposal.

This fuel cleaning technology was developed by EPRI and its contractors, Dominion Engineering and Centec XXI. The method was first successfully demonstrated with the assistance of AmerenUE in April 1999, and again in August 1999 on spent fuel at the Callaway PWR, through a programme jointly sponsored by the EPRI Robust Fuel Programme and the utility.

As part of this demonstration programme, the cleaning system was fully qualified for use in the spent fuel pool. It was determined that the ultrasonic energy absorbed by the fuel during the cleaning process was benign relative to the flow-induced vibrational and translational stresses endured by the fuel in normal operation. Thus the cleaned fuel was deemed safe to be returned to service as reload fuel in subsequent cycles.

The effectiveness of the cleaning process is strikingly illustrated in a photo-montage (right) of an actual Callaway fuel bundle before cleaning and after just ten minutes in the ultrasonic cleaner. The after-cleaning image, the photo on the right, illustrates the removal of corrosion product deposits from most of the fuel surfaces exposed to the ultrasonic energy. Based on laboratory measurements, the bundle’s inner fuel rods are expected to be clean as well, similar to the exterior rods shown in the photographs.

In October 1999, AmerenUE cleaned an additional sixteen assemblies during Callaway’s tenth refueling outage. These cleaned assemblies were returned to the reactor as re-load fuel for the Cycle 11 core. They are currently in service and are being monitored for signs of anomalous power depression or any adverse consequences resulting from the cleaning process. To date (June 2000), none have been seen for the cleaned bundles.

The utility anticipates significant benefits to offset the expense of routine fuel cleaning at Callaway. Specifically, fuel cleaning is expected to reduce the occurrence of AOA, or to significantly delay its onset such that unacceptable shutdown margin deterioration will not occur, and to eliminate the risk of power derates associated with severe AOA late in cycle life. Additional benefits may be realised if thermal duty can be recovered in future core designs, thereby reducing the number of feed fuel assemblies purchased for each new core. The benefits from reduced personnel exposure are more difficult to quantify, but out-of-core radiation fields may well decrease as repeated fuel cleaning depletes the total inventory of corrosion products in the primary circuit.

Analysis of the net benefit resulting from fuel cleaning will be on-going as the nuclear industry accumulates more experience with cores containing cleaned fuel.

CALLAWAY’S PLANS

As PWRs are driven to higher thermal duty cycles, the corrosion products on fuel are more resistant to removal via simple expedients such as shutdown chemistry excursions and pH control. Corrosion product removal via routine fuel cleaning offers an alternate method for disposing of corrosion products in the primary system. Following the successful plant demonstration, Callaway currently plans to clean nearly all of their reload assemblies for Cycle 12 next spring. Other utilities are also evaluating fuel cleaning at their PWRs that are susceptible to AOA.

Fuel cleaning also shows promise for BWRs. For these plants, reducing the corrosion product inventory in the primary system translates into reduced ex-core radiation fields and improved ALARA performance. The possibility of cleaning and reusing once-burned fuel for BWRs with heavy fuel deposit loadings that might otherwise force premature discharge represents an additional potential benefit. In light of the successful performance with PWR fuel, laboratory and mock-up feasibility studies have been completed to demonstrate the viability of this ultrasonic fuel cleaning technology for removing deposits typical of BWR fuel.