Laser-Induced Breakdown Spectroscopy (LIBS) is a powerful tool transforming industries worldwide.
With a flash of light, it reveals the essence of any material, no preparation required. In the metallurgy industry, LIBS unravels the composition of molten metals. In pharmaceuticals, it ensures drug purity. It’s a watchdog in waste management, a prospector in mining, and a guardian of our food supply.
But LIBS is more than just a tool. Scientists have pushed its limits, creating supercharged versions like Double-Pulsed LIBS (DP-LIBS), Nanosecond LIBS (ns-LIBS), and Femtosecond LIBS (fs-LIBS). They’ve harnessed sparks, microwaves, and filaments of light to shatter its limits.
What is LIBS?
- LIBS is an analytical technique that uses a laser to create a miniature plasma explosion within a material.
- The plasma emits light that reveals the elemental composition (fingerprint) of the sample.
Key characteristics
- High accuracy and speed
- Non-destructive
- Minimal to no sample preparation required
Applications
- Materials science
- Environmental analysis
- Nuclear technology:
– Analysing used nuclear fuel without disassembly
– Monitoring nuclear reactor components for wear or chemical changes to enhance safety and efficiency
How LIBS is revolutionising nuclear technology
The power of LIBS-based analysis methodologies is unlocking new possibilities within the nuclear community. Picture this: a technique with the potential to transform how we characterize nuclear-relevant and radiological materials. It’s no wonder that interest is surging, with review after review spotlighting its growing importance.
But don’t just take our word for it. A deep dive into the Web of Science database, targeting keywords like ‘laser-induced breakdown spectroscopy’ and ‘LIBS’ alongside terms like ‘nuclear fuel’, ‘nuclear fusion’, and ‘nuclear nonproliferation’, turned up more than 200 articles.
This flood of research underscores the pivotal role LIBS is set to play in tackling the nuclear field’s biggest challenges and seizing its most promising opportunities.
The nuclear fuel cycle explained
The nuclear fuel cycle is the progression of nuclear fuel through various stages, from creation to disposal. It begins with mining and milling uranium ore to extract uranium oxide (U3O8). Enrichment increases the concentration of fissile U-235, and fuel fabrication forms the enriched uranium into fuel rods. These rods are loaded into a nuclear reactor, where fission generates electricity. After several years, the now spent nuclear fuel is removed. Reprocessing separates usable material (uranium and plutonium) from high-level waste. The usable material can be recycled as mixed oxide (MOX) fuel. Eventually, all radioactive waste is vitrified (encased in glass) and placed in secure storage facilities. The ultimate goal is deep geological disposal, where waste will remain for thousands of years. Each stage has environmental and proliferation concerns, necessitating careful management and regulation.
Used nuclear fuel, also known as spent nuclear fuel, is nuclear fuel that has been irradiated to the point where it is no longer useful in a nuclear reactor. It is highly radioactive and thermally hot. The fuel is typically made up of small pellets of uranium dioxide arranged in long rods. After removal from the reactor, the spent fuel is stored in pools of water or in dry cask storage to cool and shield its emissions. Due to its extreme radioactivity, managing spent nuclear fuel is a significant challenge for nuclear power generation. Various methods are being explored for its final disposal, including deep geological repositories.
LIBS in the nuclear fuel cycle
In the nuclear fuel cycle, LIBS offers several advantages. Firstly, it enables real-time monitoring of nuclear processes, such as fuel fabrication or waste reprocessing, allowing for tighter process control and improved safety. Secondly, LIBS can analyse samples directly, eliminating the need for time-consuming and potentially hazardous sample preparation.
For example, studies have demonstrated its potential for analysing the composition of nuclear fuels, detecting contaminants in reprocessing streams, and characterising radioactive waste. LIBS has also shown promise for monitoring the degradation of nuclear materials under irradiation.
Key challenges
The technique is sensitive to matrix effects, where the composition of the sample influences the analytical signal. Additionally, the detection of certain isotopes, such as plutonium, can be challenging due to spectral interference.
Challenges remain in its accessibility for widespread implementation in the nuclear fuel cycle. One of the primary barriers is the complexity and cost of LIBS instrumentation. Commercial LIBS systems are often expensive, limiting their availability to well-funded research institutions or large industrial facilities. Additionally, the operation and maintenance of LIBS systems require specialised expertise, creating a hurdle for smaller laboratories or facilities with limited resources.
Another challenge is the need for robustness and reliability in nuclear environments. LIBS systems must be able to withstand the harsh conditions often present in nuclear facilities, including radiation fields, high temperatures, and corrosive materials. Developing LIBS instrumentation that can maintain its performance over long periods in such environments is a significant technical challenge.
Data analysis is another aspect where accessibility can be an issue. The interpretation of LIBS spectra requires advanced statistical models and machine learning algorithms to correct for matrix effects and interferences. Access to this specialised software and the expertise to apply it can be a barrier for some organisations.
Past and current research paths
Front zone of NFC
LIBS has several key applications in the nuclear fuel cycle (NFC). Early on, it was used for direct uranium enrichment assays in UF6 gas, eliminating the need to send samples to labs for mass spectrometry. This field analysis is highly beneficial for safeguards and plant operation. Recent studies have achieved high accuracy using LIBS, with no need for calibration samples.
After enrichment and fuel fabrication, the NFC generates power through nuclear fission. Traditional reactors use oxide fuel in water-cooled systems. LIBS is useful for monitoring these reactors because it can be delivered via fibre, keeping equipment away from radiation. However, the fibres can degrade due to radiation, causing absorption issues. Researchers have studied this effect and ways to mitigate it.
Many LIBS studies have focused on detecting trace elements in reactor steels. For example, copper (Cu) content can indicate the alloy type and potential for radiation damage. LIBS systems have been developed with fibres up to 100m long for this purpose. Other elements like chromium (Cr), manganese (Mn), and nickel (Ni) are also monitored for signs of structural failure. Affordable laser options are being explored to make these systems more viable.
LIBS is also used to detect light elements like hydrogen (H) isotopes, which are important in fuel cladding and reactor coolants. Researchers have used LIBS to measure these isotopes in zircaloy cladding and in heavy water reactor coolants. Similar techniques have been applied to measure boron (B) and lithium (Li) isotopes, which act as neutron absorbers.
Overall, LIBS offers many advantages for nuclear fuel cycle monitoring, including speed, direct sampling, and the ability to detect nearly any element. Its fibre-based nature makes it ideal for harsh reactor environments. Ongoing research addresses challenges like radiation-induced fibre damage and seeks to make the technology more affordable and widely applicable.
Mid zone reactor monitoring
Advanced reactors, including High-Temperature Gas-cooled Reactors (HTGRs), Molten Salt Reactors (MSRs), and Liquid Sodium Fast Reactors (LSFRs), are being developed for better efficiency and safety. LIBS supports this research.
HTGRs use helium (He) coolant with fuel pins or pebble fuel and graphite moderators. A concern is fission gas leakage into the coolant. LIBS can detect trace xenon (Xe) in He with a 0.2 μmol/mol limit. LIBS also monitors graphite degradation by measuring carbon in the coolant. It maps pebble fuel elements for quality assurance.
LSFRs use liquid sodium (Na) as the coolant due to its heat transport properties and inability to moderate neutrons. LIBS monitors Xe, krypton (Kr), and He from failed fuel pins with 22 parts per billion (ppb), 40 ppb, and two parts per million (ppm) limits. It measures Lead (Pb) and Indium (In) in liquid Na with six ppm and five ppm limits. Laser Ablation-Laser Induced Fluorescence (LA-LIF) approaches are used for increased sensitivity. LIBS monitors Na aerosols to identify leakages.
MSRs use molten salts as both coolant and fuel. This eliminates traditional fuel concerns but presents challenges like corrosion and salt monitoring. LIBS quantifies salt-induced corrosion on materials. It maps salt species permeation into graphite. LIBS monitors the MSR off-gas stream in real-time, detecting aerosolised species, noble gases, and daughters. It evaluates off-gas treatment components like Xe and Kr captured by filters. LIBS is proposed to monitor the reactor salt. A Partial Least Squares-Artificial Neural Network (PLS-ANN) model monitors Uranium (U) in salt with adequate precision. LIBS analyses molten salt isotopes, which are important for the neutron economy. It measures hydrogen isotopes for radiological significance and uranium enrichment for safeguards. Radiation effects on LIBS measurements of noble gases and salts have been investigated.
Back zone of NFC
The back end of the nuclear fuel cycle (NFC) deals with what happens to nuclear fuel after it’s been used in a reactor. The fuel is put in a pool of water to cool down and then moved to dry storage. Eventually, it’s sent to be reprocessed so the still-useful parts can be removed and used again. LIBS is a technique that’s been used to make sure the fuel is stored and reprocessed safely.
LIBS is useful because it can measure many elements at once without having to dissolve the samples or take them out of the hot cells where they’re stored. This makes it safer for the equipment. One study used LIBS to detect nine fission products in fake fuel and several in nuclear waste.
A project used a robot with a LIBS system to check the storage containers for salt that could cause corrosion. They were able to measure how much chlorine was present. Later, they made the system work with a robot that could move around the container to take measurements.
There are two ways to reprocess the fuel: aqueous and pyroprocessing. Both remove fission products and recover the unused fuel. Both involve dangerous environments and harsh materials, so LIBS is useful. Much research has focused on using LIBS to measure lanthanide fission products. LIBS spectra of these have many lines, giving options but causing interference. Many studies have looked for key peaks with minimal interference. Some have worked on estimating transition probabilities to allow for calibration-free measurements. For aqueous reprocessing, directly measuring liquids is important. One study used a liquid jet to measure zirconium in solutions with detection limits of 4 mg/L. The liquid jet fixed issues like splashing and changing liquid surface location.
The most common method in aqueous separation is the PUREX (Plutonium Uranium Reduction EXtraction) process, which involves mixing the dissolved fuel with organic solvents to extract plutonium and uranium. These can then be fabricated into new reactor fuel or used in nuclear weapons. Reprocessing reduces the volume and radioactivity of waste that must be stored, but it’s highly controversial due to proliferation risks and high costs. Countries like France and Japan reprocess commercial reactor fuel, while the United States currently does not due to nonproliferation concerns.
Pyroprocessing is an alternative nuclear reprocessing method that uses high-temperature electrochemical reactions instead of aqueous solutions. This approach is being developed to treat fuel from advanced reactor designs. In a pyroprocessor, the spent fuel is dissolved in a molten salt bath, and an electric current is used to selectively separate the different components. The primary advantage is enhanced proliferation resistance, as the process makes it harder to isolate pure plutonium. However, pyroprocessing is still in the experimental stage and faces technical challenges before it can be commercially deployed. It’s being researched as a potential way to close the nuclear fuel cycle for future reactor systems.
LIBS has been used in hot cells to analyse radioactive materials. One method measured a Cs sample from a distance. Another study developed two LIBS systems for hot cells; one used a laser port, and the other used an umbilical connection. They found that UVFS optics resisted radiation damage better than BK7 glass. A similar LIBS probe was installed at the Korea Atomic Energy Research Institute. A robotic arm with LIBS and Raman capabilities was also developed for hot cell use.
About 40% of LIBS studies have been focused on monitoring salts in pyroprocessing. Most work has used frozen salts due to the challenges of analysing corrosive molten salts. Many studies froze salt samples for testing. One quantified cerium (Ce) and gadolinium (Gd) in frozen LiCl-KCl but had issues with salt dust. Another used a glovebox to quantify samarium (Sm), uranium (U), Gd, and magnesium (Mg) in eutectic lithium chloride-potassium chloride (LiCl-KCl). They successfully quantified samples using a method combining LIBS and electrochemical data. A study used an Ar-filled holder to quantify praseodymium (Pr), holmium (Ho), and erbium (Er) in LiCl-KCl. LIBS recently quantified oxygen in frozen salt samples.
Few studies have analysed molten salts. One found that molten salt gave the most repeatable data but lower signal intensity. Recent work improved the experimental system and models. They used a glovebox, acoustic sensors, and a cover gas to prevent dust accumulation. These improvements allowed the measurement of Ce in molten LiCl-KCl. Machine learning created a model robust to liquid-level variation. strontium (Sr) and molybdenum (Mo) in LiCl-KCl were monitored using a machine learning algorithm trained on defocused spectra.
An aerosol system was also being developed for online pyroprocessing monitoring. It used a nebuliser to prevent clogging and analysed the aerosol stream. The system measured Ce and U in molten LiCl-KCl. Its ability to extract samples and analyse the aerosol elsewhere makes it suitable for industrial pyroprocessing.
