Real-Time Actinide Monitoring with Tensioned Metastable Fluid Detectors for Enhanced Safeguards and Security in Spent Nuclear Fuel Chemical Reprocessing Facilities

Introduction

It has previously been demonstrated that tensioned metastable fluid detectors (TMFDs) offer a variety of unique mechanisms for detecting alpha and neutron radiation while remaining selectively insensitive to gamma photons and beta decay [1,2]. This paper will discuss some of the capabilities of TMFD systems and how they can be utilized for enhancing safeguards and security in spent nuclear fuel (SNF) chemical reprocessing facilities. 

The TMFD technology is based on the principle of radiation seeded cavity nucleation and growth to audible-visible levels in tensioned metastable state liquids.  When a nuclear particle interaction occurs in a sufficiently tensioned liquid, rapid vaporization results in a visible bubble and an audible mechanical shock [3]. The phenomenon is similar to bubble chambers and superheated droplet detectors but in this case the metastability in the liquid is created with tension (sub-vacuum pressures at room temperature) instead of thermal superheat.

Two particular TMFD device configurations have been developed for application to a variety at radiation detection scenarios.  The first is the centrifugally tensioned metastable fluid detector (CTMFD), which uses the centrifugal force principle - created in a specialized rotating enclosure to create the tension needed to detect nuclear particles.  By suitably tailoring the liquid's metastable state, the as-desired sensitivity to energy and signature of radiation particle can be controlled. The second device is the acoustically tensioned metastable fluid detector (ATMFD), which uses resonance-mode focused acoustic waves in the liquid (much like in a laser cavity) to induce oscillatory compression-tension states.  Like for the CTMFD, the ATMFD also offers the capability for energy and particle sensitivity selection based by tailoring of the micro-second timed spatially-variant acoustic wave fields [4].

Applications of TMFD Technology to Spent Nuclear Fuel Chemical Reprocessing Plants

The application of TMFDs for providing real-time monitoring of neutron and alpha radiation emitting special nuclear materials (SNMs) like U, Pu, Np and Cm is discussed in sub-sections that follow:

SNM alpha spectroscopy

The TMFD systems have been found capable of radiation energy discrimination capabilities.  Unlike many traditional radiation detectors which rely on electrical signal height or pulse shape discrimination, the TMFD uses the inherently unique mechanics of its detection technique to offer transformational spectroscopic capability.  Various isotopes of SNMs such as Pu-239, Pu-238, Am-241, and Cm-242 emit alpha radiation at distinct energies, the detection of which provides for the tell-tale sign for their presence (or diversion). The detection mechanism in the TMFD is threshold-based meaning under chosen conditions particles depositing at or above the selected energy are detected while others are not – while also remaining unfettered by the presence of “nuisance” radiation such as beta-gamma emission.  Straightforwardly, the amount of mechanical tension or negative pressure (Pneg) that a liquid is placed will determine the specific energy of alpha radiation particles it is sensitive to. 

Detection of alpha decay from actinides and spectroscopy has been systematically demonstrated in the CTMFD by dissolving known quantities of specific actinides into the detector fluid and recording the detector response with Pneg.  Calibration has been done with several NIST certified single isotope transuranic actinide solutions [5].  After doing these calibrations, it was not only found that the difference in energy between the actinide elements was easily detectable but also that the difference in energy between individual isotopes of the same element could be detected.

Several applications of CTMFD based alpha monitoring and spectroscopy in a UREX or PUREX SNF chemical reprocessing facility are under study using a build-in protocol.  First, since Uranium exhibits the lowest energy alphas of the actinides in the UREX process, it is easily ignored in scenarios where there is no interest in the Uranium concentration.  Also, the ability to ignore uranium alphas could allow for detection of unwanted transuranic extraction during the UREX phase.

Since the Cm isotopes emit the highest energy (~6MeV) alphas amongst the actinides of interest, they are the first to detect.  By setting the CTMFD to be sensitive to only Cm, the unwanted extraction of Cm prior to the TRUEX process can be monitored in real-time.  Very small quantities of actinides can be detected in the CTMFD.  The CTMFD has been shown to be sensitive to quantities below 1 part per million for Uranium-238 and below 10 parts per quadrillion for Pu-238.

Another key application of CTMFD based alpha spectroscopy is for Pu isotopes monitoring.  Since the alpha energies of Pu are higher than that of Np, the quantity of Pu in the NPEX product stream can be determined.  Plutonium would also be detectable in the UREX and CCD-PEG products if it were present therein, unexpectedly.  Some experimentation has also been done to measure the ratio of Pu-238 to Pu-239.  Detection of mixtures of Pu-238:Pu-239 in ratios of 1:1, 2:1, and 3:1 have been successfully conducted.

Active Detection of Fissionable/Fissile Material

In many scenarios, it may be desirable to monitor the location of fissionable/fissile materials in a reprocessing facility without removing samples.  Active interrogation offers non-destructive assay if the interrogating radiation can be discriminated.  The ATMFD's energy discrimination capabilities allow for lower energy particles from an active interrogation source to be ignored while detecting higher energy fission neutrons if fissionable material is present.  Simulations have been conducted using the Monte Carlo nuclear particle transport code MCNP-PoliMi to predict the utility of the ATMFD in active detection of fissionable material in a reprocessing facility. 

The ATMFD has been modeled in MCNP-PoliMi along with a representative 5cm diameter pipe filed with the UREX feed liquid from a PWR with 3% enriched fuel with 33GW-day/MTU burnup [6]. The scenario was modeled as one of many example applications of this technique.  Two active interrogation sources were considered.  The first was a 2.45 MeV D-D fusion neutron source and the other a 60 keV neutron source [7].  For active interrogation to work, the ATMFD must be set to avoid detection of the source particles.  This is accomplished by lowering the acoustic drive power so that the detector is not sensitive to the source neutrons. 

The simulations offer insight that using a modest intensity interrogating neutron source of, ~ 10⁸ neutrons per second, that fissionable material in a reprocessing facility should be detectable [6].  For example, if the interrogating neutron source is a D-D accelerator emitting 2.45 MeV neutrons, the TMFD will be sensitive only to fission-induced neutrons above 2.45 MeV.  The same scenario utilizing a 60 keV source would allow for detection of only fissile (vs fissionable) material which could then be used to monitor the movement of potential weapons grade SNMs with specificity, while ignoring many of the other actinides which may not be of interest.        

Directional Neutron Monitoring

Past research work by our group has shown that the ATMFD is not only capable of detecting the presence of neutrons while remaining completely insensitive to intense gamma fields (>10²³ g/cc/sec) [2] but also be capable of detecting directionality of incident neutrons from SNMs [1].  Directional neutron detectors offer vastly superior background suppression when compared to traditional neutron detectors (e.g. He³, BF₃), and the ability to image the neutron source directly.  An ability which would be particularly advantageous in both passive interrogation scenarios where one needs to discriminate neutrons originating from a single targeted location and active interrogation scenarios where one needs to discriminate interrogating neutrons (or photons) from neutrons resulting from the fission of the targeted material.  Assessments have been conducted utilizing MCNP-PoliMi to investigate the possibility of employing the direction-position sensing capabilities of the ATMFD system to indicate the presence and location of neutron emitting isotopes in various stages of SNF fuel chemical reprocessing plant.

The full paper and presentation will provide details of theoretical and experimental assessments.

References

1. Archambault, B. A., J. A. Webster, J. R. Lapinskas, T. F. Grimes, R. P. Taleyarkhan and A. Eghlima, "Transformational Nuclear Sensors - Real-Time Monitoring of WMDs, Risk Assessment & Response," IEEE 978-1-4244-6056-5/10, pp.421-427, 2010.

2. Sansone, A., S. Zielinski, J. A. Webster, J. Lapinskas, R. P. Taleyarkhan and R. C. Block, "Gamma-Blind Nuclear Particle-Induced Bubble Formation in Tensioned Metastable Fluids," Transactions of the American Nuclear Society, Vol. 104, pp. 1033, Hollywood, Florida, June 26-30, 2011.

3. Taleyarkhan, R. P., C. D. West, and J. Cho, "Energetics of Nano-to-Macro Scale Triggered Metastable Fluids," Oak Ridge National Laboratory Report ORNL/TM-2022/233, 2022.

4. Lapinskas, J., et al., "Towards leap-ahead advances in radiation detection," Proceedings of the 16th International Conference on Nuclear Engineering (ICONE-16), Orlando, Florida, 2008.

5. Lapinskas, J., S. Zielinski, J. A. Webster, R. P. Taleyarkhan, S. McDeavitt, Y. Xu, "Tension metastable fluid detection systems for special nuclear material detection and monitoring," Nuclear Engineering and Design, 240, pp. 2866-2871, 2010.

6. Webster, J. A., R. P. Taleyarkhan, "Tensioned Metastable Fluid Detectors in Nuclear Security for Active Interrogation of Special Nuclear Materials - Part B," World Journal of Nuclear Science and Technology, Vol. I, pp. 66-76, 2011.

7. Hagmann, C. A., et al., "Active Detection of Shielded SNM With 60-keV Neutrons," IEEE Transactions on Nuclear Science. Vol. 56, pp. 1215-1217, 2009.