The optical in situ monitoring presented is a completely new method for monitoring emissions from nuclear facilities. Figure 1. Schematic representation of the CRDS instrument for radiocarbon detection. The main path of the QCL laser is shown with the continuous blue line, while the dashed blue line represents the path for the laser wavelength calibration using a reference etalon.
Figure 2. Sample-processing unit. The sample flow direction is controlled by solenoid valves represented by three connected triangles. They enable selecting whether the catalytic converter is used or bypassed and alternating between the two CO 2 traps. Figure 3. Allan deviation of the C14 measurement. The blue line is the Allan deviation for a 14 CO 2 measurement of a 3. Figure 4. Two absorption spectra recorded from the nuclear power plant stack.
The spectra were recorded on September 25th a and 26th b. The ring-down data, shown in blue, are smoothed with a moving average filter with a window size of The red lines represent the fitted sum of Voigt profiles, and the corresponding residuals are shown below.
A clear difference in the intensity of the 14 CO 2 peak at In a , the mole fraction of C14 was 1. The N 2 O line at Figure 5. Continuous measurement of radiocarbon activity concentrations over time at Loviisa NPP. The period inside the black dashed square contains the C14 activity concentrations measured from CO 2 only without the catalytic conversion of hydrocarbons.
The measured data points and fit results XLSX. Such files may be downloaded by article for research use if there is a public use license linked to the relevant article, that license may permit other uses.
The authors wish to acknowledge Fortum Power and Heat Oy for offering the location for the field campaign as the owner and operator of the Loviisa nuclear power plant and participating to the funding of the campaign. This work has received funding from the Euratom research and training programme under grant agreement no.
More by Johannes Lehmuskoski. More by Hannu Vasama. More by Jouni Hokkinen. More by Katja Heiskanen. More by Matti Reinikainen. Box 23, Loviisa, Finland.
More by Satu Rautio. More by Guillaume Genoud. Cite this: Anal. Published by American Chemical Society. Article Views Altmetric -. Abstract High Resolution Image.
Nuclear power plays an important role in mitigating climate change before renewable zero-emission energy sources are more widely used. The core technology is fundamentally free of greenhouse gas emissions and enables continuous, high-capacity energy production. Nuclear power plants NPPs are constantly monitored to ensure minimum impact on the environment, and most of the radionuclides arising from NPPs are already efficiently measured. However, emissions of gaseous beta-emitters, such as radiocarbon C14 and tritium, are still challenging to monitor as a suitable method for their automatic on-line detection is lacking.
In particular, radiocarbon emissions require monitoring because airborne radiocarbon can accumulate in photosynthesising organisms, such as plants used as human food. Currently, radiocarbon monitoring is mostly based on liquid scintillation counting LSC or accelerator mass spectrometry AMS.
They both require prolonged sample collection, complex sample preparation, and labor-intensive analysis work, especially when analyzing gaseous samples. LSC is used in the nuclear industry, but the method suffers from overlapping scintillation peaks of other radionuclides, which therefore need to be separated chemically beforehand. Hence, there is a need for new technologies to ensure more efficient monitoring of radioactive gaseous emissions.
Radiocarbon is produced naturally at a constant rate in the upper parts of the atmosphere in the 14 N n,p 14 C reaction by the interaction of atmospheric nitrogen with thermal neutrons produced by cosmic rays. Their pathways in an NPP depend on the structure of the power plant, but typically, they are evacuated through the NPP stacks. The released C14 can be assimilated by living organisms outside the facilities, and therefore, C14 emissions must be monitored.
In particular, CO 2 is absorbed by all photosynthesizing organisms and is therefore a high risk for the environment, while CH 4 is mainly a byproduct of organic activity and can be exploited only by specialized methanotrophic bacteria and archaea. C14 is present in high concentrations in many types of waste, such as spent ion-exchange resins, reactor structures, and moderator graphite.
In many countries, monitoring of radiocarbon emissions from NPPs is required by nuclear safety regulations. However, none of the current detection methods can provide automated in situ monitoring nor can provide measurements with a good time resolution.
The use of optical methods offers several advantages in terms of size, cost, and usability over the current state of the art.
In particular, high sensitivity can be achieved when using cavity-enhanced spectroscopy methods, such as cavity ring-down spectroscopy CRDS.
In this work, we present the novel use of CRDS for continuous in situ monitoring of radiocarbon stack emissions from a nuclear power plant. Besides radioactive emission monitoring, radiocarbon is of interest in other fields. Radiocarbon content is an indicator of the origin and age of a carbon-containing material, having completely decayed in fossil carbon, while biogenic carbon contains the natural abundance of 1.
Therefore, radiocarbon is commonly used to date historical artifacts. Moreover, determining the radiocarbon content is an ideal solution for verification of biofraction in combusted materials that are mixed from multiple sources. This allows for a better understanding of the contribution of carbon of the fossil origin to climate change and can be used to develop more advanced climate models.
Eventually, atmospheric radiocarbon monitoring can be used as a tool for authorities to identify the producers of fossil emissions and enforce international climate agreements. Another significant application benefitting from the development of C14 detection is pharmacology, where C14 labeling of a drug molecule enables tracing its metabolic routes in the human body. Laser spectroscopy relies on detecting the light absorption of the species of interest at a specific wavelength.
In CRDS, the absorption path length is increased by placing the gas sample in an optical cavity formed by two high-reflectivity mirrors, resulting in a high sensitivity. The light of a narrow-line-width laser is coupled between the two mirrors, resulting in light intensity buildup inside the cavity. After the light intensity reaches a set threshold, the light source is switched off, and the light in the cavity decays exponentially.
In an empty cavity, the decay time, also known as the ring-down time, depends only on the light losses of the cavity. Additional losses due to the light absorption of a sample gas decreases the decay time. The measurement is independent of intensity fluctuations of the laser source as the exponential decay of light is fitted to determine the ring-down time. Its lasing wavelength is tuned between The laser beam is collimated by an aspheric lens and guided through two Faraday optical isolators of 30 dB isolation each to mode-matching optics and then to the cavity.
The optical isolators minimize the optical feedback from the cavity back to the laser. Two isolators were used as the isolation from a single isolator was not sufficient.
The laser TEM 00 mode is matched to the cavity mode using two concave gold-coated mirrors. The cavity is formed by two high-reflectivity ZnSe mirrors with dielectric coating and a reflectivity of The mirrors are situated 38 cm apart from each other.
The 0. The cavity is insulated with polyurethane foam, and its temperature is actively stabilized with a temperature controller regulating four Peltier elements. The FPGA acquisition card also sends a trigger signal to the laser driver, when light intensity in the cavity reaches a set threshold level. This rapidly offsets the QCL to another wavelength and thus stops the light coupling into the cavity, which in turn initiates the light-intensity decay, that is, the ring-down event.
The offset step was experimentally adjusted to minimize the exponential fit residual and the rate of out-filtered ring-down events.
The ring-down events recorded by the FPGA card are automatically processed and fitted with an exponential function to extract the ring-down time using LabVIEW-based software. The acquisition software automatically filters out exponential fits with non-flat residuals resulting from higher-order cavity mode coupling and other noise sources. A scroll pump is used to evacuate the cavity, whose pressure is monitored with a capacitance manometer.
The measured vacuum ring-down time is 3. To record a spectrum, the QCL wavelength is scanned over the wavelength range of interest by ramping the laser driving current with a sawtooth waveform at a frequency of 40 Hz, while the QCL temperature is kept constant. A germanium etalon is used to calibrate the non-linear relationship between driving current and laser wavelength.
It is thus necessary to slowly scan the cavity length with the mirror on the piezo-controlled platform to increase the wavelength resolution of the measurement. The electronics, data acquisition, power supplies, and pump are positioned in two levels beneath the optical board and cooled down by two fans flowing air through the rack.
A computer-aided design of the rack assembly is presented in the Supporting Information. High Resolution Image. The atmospheric CO 2 concentration is about parts per million ppm , and similar levels are measured in NPP stacks.
To reach the highest sensitivity in radiocarbon detection with CRDS, CO 2 needs to be first captured and purified from the sample air as the targeted 14 CO 2 concentrations are too low to be measured directly at the atmospheric CO 2 concentration.
Therefore, an on-line automated sample-processing unit was coupled to the CRDS instrument. Two parallel CO 2 traps were made of aluminum cylinders and filled with the resin. The traps are heated resistively, while active cooling is achieved with heat sinks and fans. The two traps can trap sample air alternately and the sample flow is controlled by solenoid valves as shown in Figure 2. A 45 min trapping time was used, after which the CRDS cavity and the trap are connected and pumped to vacuum before releasing CO 2 by heating the trap.
The CO 2 releasing procedure from the end of the trapping until the C14 measurement starts takes 20 min. The C14 measurement is followed by pumping down the cavity to a 2 mbar pressure for a measurement of CO 2 concentration in the cavity.
In total, the two measurements take 10—15 min, after which the cavity is pumped to vacuum before the next measurement. Meanwhile, the heated trap is flushed with sample air to ensure that it is purged from all the trapped CO 2. The trap is then actively cooled down back to room temperature with a fan before the next trapping sequence.
To capture all the radiocarbon in the form of CO 2 , the sample air is guided before the traps through a palladium catalyst, which was prepared as described in refs 25 and The palladium catalyst converts CH 4 and possible other hydrocarbons into CO 2.
The measurement of C14 in the form of CO 2 only is performed by bypassing the catalyst. Downstream the catalyst, a Vaisala GMP carbon dioxide sensor is used to obtain the total concentration of the carbon species.
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