Analyzing Corrosion Residues in Cooling Water Systems of Nuclear Power Plants with Energy Dispersive X-ray Fluorescence (EDXRF)

Early detection of corrosion of the metal alloy components in cooling systems is a key safety consideration in nuclear power plants, particularly boiling and pressurized water reactors.

The presence of corrosion residues in cooling water can provide a useful early indication of a potential safety issue, but the direct determination of these residues in cooling water is not achievable due to very low concentrations.

Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Filters must be placed in the cooling system to collect corrosion residue over a period of time, with a volume of 100 L of cooling water generally passing through the filter.

Two filter types are typically used:

• Cellulose-based "normal" filters may be installed in the secondary circuit of pressurized water reactors (PWR)
• Ion exchange filters may be installed in the primary circuit in boiling water reactors (BWR), as well as in PWR

These filters are able to collect a number of elements of interest, including:

• Cations: Fe, Co, Cr, Cu, Mn, Ni, Zn, Pb, Ca, Mg, Na and Cs.
• Anions: I, Cl and Br.

These filters are typically analyzed using ICP-OES, meaning that they must be inserted into solution prior to analysis. This method requires the use of dangerous chemicals and around half a day’s analysis time, including the preparation of samples.

Methods

Sample Preparation

Sample preparation is not required when performing EDXRF analysis - the filter is placed into the instrument directly, with the scan requiring less than 10 minutes for each filter.

EDXRF analysis is non-destructive, ensuring the sample is maintained for future reference. Instrument calibration is straightforward, and it offers excellent long-term stability.

Analysis Using the ARL QUANT'X EDXRF Spectrometer

The ARL™ QUANT'X from Thermo Scientific™ is a direct-excitation benchtop EDXRF spectrometer that features a total of nine filters and a 50 w X-ray tube (Rh or Ag target).

It facilitates the accurate analysis of fluorine to uranium, offering an analytical range of ppm to high percentage levels.

The ARL™ QUANT'X is suitable for analysis of all types of samples, including solids, powders, liquids, filters and thin layers. It can perform analysis in air atmosphere, vacuum or helium.

The novel silicon drift detector (SDD) features a high-transmission window, enabling accurate detection of the different elements deposited on the filter. The SDD is cooled via the Peltier effect, with its large effective area of 30 mm², enabling three times faster analysis than older detector types.

Figure 1. ARL QUANT'X EDXRF ANALYZER. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The ARL™ QUANT'X is able to detect a low amount of corrosion material on the filter – even when this is distributed in a non-homogenous manner – and is suitable for use with filters with a diameter of 47 mm.

Its use of direct excitation ensures that all primary radiation is employed in the excitation of the sample, while its new SDD detector provides excellent detection efficiency for all elements present. Finally, its large area of excitement ensures representative analysis of the filter.

Quantitative Analysis – Reference Materials

Reference materials may be prepared in the laboratory or acquired from third-party suppliers.

Cellulose-based filters are prepared by passing a watery suspension of solid metal oxides through the filters, while ion exchange filters are prepared by passing a solution, including soluble metals, through the membranes.

Results

Iron Determination

The results presented here outline the determination of iron deposits that have been extracted from the secondary water-cooling system of a PWR. These deposits were present on cellulose filters.

Figure 2 highlights the calibration of Micromatter™ filters at three different concentrations, including one white filter. Figure 3 shows the validation of this analysis using ICP-OES (Figure 3).

Figure 2. Determination of Fe -Calibration. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Figure 3. Determination of Fe -Validation. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Ion Exchange Filters

The results presented here show the use of the ARL™ QUANT’X EDXRF to analyze filters containing Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, U and Cd. These were analyzed using direct excitation and employing a combination of primary filters, SDD detector and a 50 w X-ray tube with Rh anode.

The analysis method has been optimized by combining the optimal analysis conditions for each section of the periodic table of elements. Measurement times varied between 60 seconds and 240 seconds per condition, depending on the load on the filter.

The total load of material found on an ion exchange filter is typically low, meaning that a standard linear calibration curve is able to provide an R² > 0.999 while offering excellent repeatability and reproducibility.

The filters in the example presented here contain Br, resulting in K lines which interfere with U L lines.

Table 1. Conditions. Source: Thermo Fisher Scientific – Materials & Structural Analysis

Figure 4. Filter spectra with different loads of material. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Detection Limits

The detection limits featured below have been calculated using a white filter.

Table 2. Detection limits in ng/cm² with 60 seconds of analysis per condition. Source: Thermo Fisher Scientific – Materials & Structural Analysis

* Br Kbinterferes with U La hence the higher MDL

Conclusion

EDXRF analysis offers an ideal, robust tool for the monitoring of corrosion in nuclear industry cooling systems via the analysis of water filters.

EDXRF analysis is non-destructive, and no sample preparation is necessary. Both calibration and analysis are rapid and straightforward, and because the whole filter is analyzed, problems associated with sampling inhomogeneities are avoided.

This approach also offers excellent detection limits and short analysis times. Since the SDD detector is cooled by the Peltier effect, this approach can also be used to detect radioactive elements.

Acknowledgments

Produced from materials originally authored by Pascal Lemberge from Thermo Fisher Scientific.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.