Monitoring of the atmosphere for fission products (131mXe, 133mXe, 133Xe, and 135Xe) is performed by various laboratories to detect nuclear explosions. Quantification of 127Xe is not routinely performed by laboratories measuring atmospheric radioxenon because it is not a fission product. 127Xe was recently detected by a ground-based beta-gamma air monitoring system. When measured using beta-gamma coincidence detector systems, such as those in use on the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), 127Xe can interfere with the quantification of fission product radioxenon due to overlap of the 127Xe beta-gamma coincidence signatures with those of fission product radioxenon. This work demonstrates quantification of 127Xe at different laboratories with different measurement techniques. Production and purification of 127Xe was performed by neutron activation of enriched 126Xe. The purified 127Xe was then split between laboratories, and detection and quantification methods were developed. At Idaho National Laboratory, a quantification method involving high purity germanium detectors was devised that included self-attenuation correction. At AWE, a beta-gamma coincidence counting method, as used in support of the IMS, was modified to enable the measurement and analysis of the 127Xe samples. Corrections were made for self-attenuation, which showed a strong xenon volume dependency, for some coincidence signatures. The gas sample activity concentration was used as the comparison metric and it showed excellent agreement between the methods.

The International Monitoring System includes a network of radionuclide detector stations and laboratories operated around the world monitoring for nuclear explosions. The United States Radionuclide Laboratory for radioxenon detection (USL16-NGL) was certified by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization in 2016. Since the certification of the laboratory, an additional set of four radioxenon detectors have been added to the laboratory. These supplementary radioxenon detectors allow for improved throughput for the laboratory and improving the ability to measure short lived radioxenon isotopes. In this paper, we describe the implementation of the additional radioxenon detectors and how they compare to current capabilities. Additionally, we detail implementation procedures to leverage the increased throughput.

The International Monitoring System includes a network of radionuclide detectors operated around the world monitoring for nuclear explosions. A key aspect of the International Monitoring System is the detection of radioxenon with a network of stations and laboratories. Beta-gamma detectors are utilized extensively for the detection of radioxenon, and the beta detection is primarily performed with a plastic scintillator cell. Two areas of improvement for plastic scintillator are the sample carry-over (“memory effect”) and energy resolution. While the scintillator can be coated to remove the memory effect, the energy resolution must be improved with a different detector material. Silicon is the current leading candidate for the future beta cell material due to the much-improved energy resolution compared to plastic scintillators (factor of ~ 3x). PNNL is developing a silicon beta cell for use as a potential modular replacement within Xenon International (a next generation radioxenon detection system currently undergoing acceptance testing for potential inclusion in the International Monitoring System). The beta cell utilizes four different silicon detectors to create an active volume for the radioxenon within an outer gas cell. Since there are four separate beta signals (compared to one for plastic scintillators), data acquisition modifications are required. In this paper, we detail the design, efficiency measurements, and long-term testing of the silicon beta cell and potential improvements in isotopic discrimination.

Molten Salt Reactors (MSRs) are one of six Generation IV reactor designs currently under development around the world. Because of the unique operating conditions of MSRs, which include molten fuel and the continuous removal of gaseous fission products during operation, work was performed to model the production of activation and fission products and analyze the potential impact of emissions on the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Simulations were performed to predict the production of IMS-relevant radionuclides in four MSR designs operating under two scenarios: (1) a sealed reactor with releases only during operational shutdown, and (2) continuous reprocessing or sparging of the fuel salt. From these production estimates the radioxenon and radioiodine signatures were extracted and compared to three current reactor designs (Boiling Water Reactor, Pressurized Water Reactor, High-Power Channel-Type Reactor). In cases where continuous reprocessing of the fuel salt occurred, both the radioxenon and radioiodine signatures were nearly indistinguishable from a nuclear explosion. Estimates were also made of the potential emission rate of radioxenon for three reactor designs and it was found that MSRs have the potential to emit radioxenon isotopes at a rate of 1015−8×1016 Bq/d for 133Xe, which may adversely affect nuclear explosion monitoring, if no abatement is used. An assessment was made of activation products using a candidate fuel salt (FLiBe) mixed with corrosion products for the Thorium Molten Salt Reactor (TMSR-LF1).

Molten salt reactors (MSRs) are gaining support as many countries look for ways to increase power generation and replace aging nuclear energy production facilities. MSRs have inherently safe designs, are scalable in size, can burn transuranic wastes from traditional solid fuel nuclear reactors, can store excess heat in thermal reservoirs for water desalination, and can be used to produce medical isotopes as part of the real-time liquid-fuel recycling process. The ability to remove 135Xe in real time from the fuel improves the power production in an MSR because 135Xe is the most significant neutron-absorbing isotope generated by nuclear fission. Xenon-135, and other radioactive gases, are removed by sparging the fuel with an inert gas while the liquid fuel is recirculated from the reactor inner core through the heat exchangers. Without effective abatement technologies, large amounts of radioactive gas could be released during the sparging process. This work examines the potential impact of radioxenon releases on samplers used by the International Monitoring System (IMS) to detect nuclear explosions. Atmospheric transport simulations from seven hypothetical MSRs on different continents were used to evaluate the holdup time needed before release of radioxenon so IMS samplers would register few detections. Abatement technologies that retain radioxenon isotopes for at least 120 d before their release will be needed to mitigate the impacts from a molten salt breeder reactor used to replace a nuclear power plant. A holdup time of about 150 d is needed to reduce emissions to the average level of current nuclear power plants.

The International Monitoring System includes a network of radionuclide detectors operated around the world monitoring for nuclear explosions. A key aspect of the International Monitoring System is the verification of the results from the stations. The verification comes through calibration of the systems, and additional validation measurements from certified laboratories. In support of the Preparatory Commission of the Comprehensive Nuclear-Test-Ban Treaty Organization, there are 16 radionuclide laboratories around the world (with a subset of those laboratories having radioxenon measurement capabilities). We have developed a laboratory system for processing and measuring the radioxenon archive samples from the International Monitoring System. In this paper we describe the design and operation of the laboratory in support of verification of the Comprehensive Nuclear-Test-Ban Treaty.

Identification of nuclear events rely on accurate measurement of radioxenon releases. Measurement of radioxenon relies on accurately knowing the detection efficiency for each isotope. Four xenon isotopes are of interest: 135Xe, 133Xe, 133mXe, and 131mXe. Each isotope has a unique signature; however, there is overlap between each signature. A series of ratio terms, called interference ratios, helps account for the challenges created by the overlap. The ratio and detection efficiency terms are all determined during the initial detector calibration. Additional spectra are needed for a complete calibration: the radon daughter 214Pb/214Bi, the detector background, and the 137Cs quality control (QC).