Monitoring for Actinide Neutron Emissions From Spent Nuclear Fuel Under Extreme Gamma Fields Using Centrifugally Tensioned Metastable Fluid Detector Sensor Technology

Conventional neutron sensor systems [1,2] to detect the diversion of transuranic elements in SNF have a well-known deficiency in their lack of ability to provide real-time assessments. Material accountability using such techniques [3–8] for neutron signal monitoring are either too low in neutron detection efficiency (e.g., ≪ 1% for fission chambers) or others like moderated proportional/ionization chambers like He-3 detectors can quickly become saturated at >0.1 Gy/h (10 Rad/h) fields within the extreme γ/β environment expected from SNF, or require time-consuming, off-site laboratory analysis. It is well-known that considerable uncertainty exists for actinide content (esp., Pu-239, U-235, Cm-242, etc.) in SNF discharged from a conventional 3 GWt light water reactor (LWR) [9]. From a safeguards-security standpoint (i.e., diversion potential), it is important to assess the actinide content in SNF that is either intended for storage or reprocessing. The challenge in monitoring neutron emissions from SNF (MTU/SF) at the intensity of ∼10⁶ n/s is complicated by the relatively huge (∼10¹⁹ γ/s) gamma radiation emissions, which can create radiation exposures in the 100 Gy/h (10⁴ Rad/h) range.

Our past studies have revealed that the tensioned metastable fluid detector (TMFD) sensor technology presents a promising candidate for addressing such and related problems [9–24]. Table 1 summarizes the salient features of the TMFD architecture, including references to past publications supporting the claimed capabilities. Concerning the present application for actinide neutron emission-based identification, it is noted that CTMFDs can offer up to 80% (intrinsic neutron detection efficiency) alongside offering spectroscopic information with potential 100% gamma-blind operability even under 150 Gy/h (15,000 Rad/h) accelerator photon fields. In contrast to conventional sensor systems, TMFDs do not rely on components such as pre-amplifiers, high voltage sources, and PMTs for light flash or charge collection and extensive shock-sensitive electronic trains.

For completeness, we first provide a brief overview of the TMFD technology, and then present studies and results about qualifying CTMFDs for front-end SNF monitoring of actinide from their characteristic neutron emission signals.

It is not well-known that fluids like solids can also be placed under tensioned (below vacuum) negative pressure (Pneg) states—progressively weakening the intermolecular bonds to the point at which a femtoscale incoming ionizing particle (e.g., MeV neutron interaction) can rupture the bonds resulting in a cavitation detection event (CDE); indeed, a CDE starting at the nm scale, which quickly within microseconds can amazingly grow to ordinary human visible-audible-recordable scales. A variety of methods to cause Pneg states in ordinary inorganic and organic fluids have been developed as candidates for TMFDs [10,18]. One such method which was used for the present study is the CTMFD shown schematically in Fig. 1(a). Figure 1(b) provides a pictorial view of the CTMFD system used for this study, including key components.

At its core, the typical CTMFD comprises a diamond-shaped glass enclosure filled with sensing fluid (Fig. 1(a)), which is connected to a variable-speed remotely controlled drive motor. The diamond-shaped glass piece is spun about a central axis. As the glass piece rotates, centrifugal forces stretch the fluid along the centerline, weakening the intermolecular bonds holding the fluid together and placing the fluid in the central volume in as-desired tensioned metastable negative pressure (Pneg) states. An incident neutron strike onto the atoms of the sensing fluid creates high (∼10⁶ MeV/m) linear energy transfer (dE/dx) recoil ions, which deposit their energy within nanometers and under the right nucleation conditions, which can lead to a fast-growing CDE. In contrast, low dE/dx radiation (e.g., ∼1 MeV/cm gamma-beta radiation) deposits radiation energy over much larger length scales such that nucleation of a CDE is physically impossible for Pneg states used in TMFDs. This aspect allows the TMFD sensor to offer 100% gamma-beta radiation blindness while offering 80–99% intrinsic efficiency for detecting neutrons, alpha, and fission fragments [9,20].

where $ρ$ is the fluid density, $r$ is the meniscus separation, $f$ is the rotational frequency, and $Pamb$ is the ambient pressure. For a given “$ρ$” and “$r$⁠,” the $Pneg$ can be controlled by manipulating the rotational frequency (f), allowing for selective energy discrimination. The CTMFD control software accounts for dynamic temperature compensation.

Figure 2(a) summarizes intrinsic neutron detection efficiency for CTMFDs (on a sensitive volume normalized basis) versus conventional fast and thermal neutron detectors. The used for this study comprised a 40 cm³ sensitive volume (SV) and utilized the inert (0,0,0 on the NFPA scale) organic liquid DFP with the molecular formula C₅H₂F₁₀. As noted, the efficiency gains can range from over 75 versus moderated BF₃ detectors to over 1000× over superheated drop detectors. Figure 2(b) presents evidence for the unique ability for CTMFDs to tailor the intrinsic detection efficiency over 10⁴× by varying the degree of tension Pneg states—approaching the theoretical 100% interaction rates.

We have analytically shown [9] that information on key actinide content in SNF can be derived from external monitoring of the characteristic neutron intensity and energy spectra emissions. Since alpha and beta radiation are readily shielded within the SNF structure, the penetrating background radiation impacting the externally positioned neutron sensor next to an SNF will be dominated by gamma photons—both for intensity and energy spectrum-based effects. The photon energy comes into importance because of possible photoneutron production. Since the sensing fluid in DFP includes “H” atoms for which a 1:6000 are expected to be “D” atoms for which the photoneutron energy threshold is 2.2 MeV, any incoming photon of energy (Eγ) above 2.2 MeV can be expected to result in photoneutron-induced counts in the CTMFD—constituting a contamination signal, which must be accounted for as an additional technical challenge, beyond the expected impact of absorbed dose on production of significant radiolysis gases in DFP that may lead to spurious signals as well.

Since the gamma radiation emission is strongly dependent on decay of constituent fission products in SNF, and to narrow the scope of the challenges involved, it was decided to focus on the state of legacy SNF inventory built up in the U.S.A. This legacy SNF is dominated by fuel assemblies that have experienced a 30-y cooldown period.

Figure 3(a) shows the expected photon intensity and energy distribution estimated using the well-established ORIGEN code [25]. As noted, the highest intensity (∼10¹⁵ γ/s) occurs toward the front end of the spectrum, with a steep drop in photon intensity once the photon energy exceeds ∼1.5 MeV. A prominent peak is observed at ∼0.6 MeV due mostly from the decay of Cs-137, a beta emitter with a half-life of 30.1 years. Figure 3(b) displays the neutron output from 30-y cooldown SNF with most neutron emissions derived from spontaneous fission (SF) with an emission intensity of about 10⁷ n/s.

These results indicate that the gamma-to-neutron intensity can be as high as: ∼10¹¹:1 (for Eγ < 1.5 MeV), ∼10:1 (for Eγ∼ 2–3 MeV), and ∼ 1:1 (Eγ > 6 MeV). Consequently, it is clear that for the impact of radiolysis and radiation dose-related negative effect on CTMFD performance, the absorbed dose effect will be dominated by Eγ < 1.5 MeV gamma photons. In contrast, the potential for photoneutron contamination effects needs to be assessed for possible impact from Eγ ∼ 3 MeV and ∼ 7 MeV photons, respectively.

From a practical perspective, for standoff monitoring of neutron emissions from SNF (with a length span of ∼3.3 m (13–14 ft), a reasonable field of view coupled with conservatism would need a standoff of about 1 m—with the CTMFD positioned midway between the length of a typical LWR SNF. While CTMFDs with SV ranging from 1 mL to 80 mL have been developed, for this study a CTMFD with SV = 15 mL was deployed; such a CTMFD reaches an intrinsic detection efficiency of ∼30% at Pneg > 10 bar.

Calculations were conducted to assess the situation for the gamma dose/exposure rate which indicated ∼ 150 Gy/h (∼15 kRad/h) expected dose rate. At a 1 m standoff, and a neutron source strength of ∼10⁶ n/s, the neutron rate entering the CTMFD (SV = 15 mL) can be readily estimated (assuming 4π reduction with distance) to be = ∼10⁶. 10/(4π.10⁴) = ∼10² n/s. At this neutron rate, the 15 mL CTMFD would readily detect the incoming neutrons at a rate exceeding 50–100 cps (Note: The cosmic background count rate in the 15 cm³ CTMFD operating at a 7 bar Pneg state is ∼ 0.02 cps). For determining the neutron energy spectrum requires more time since this requires the acquisition of detection rates for at least 3–4 Pneg states [18]—estimated to require 15–30 min. We conservatively set the goal to be 1 h.

Based on the aforementioned discussion and analyses, the challenge goal performance metrics for CTMFDs were set as follows:

• Metric 1: The CTMFD should function without spurious signals when subject to Eγ < 1.5 MeV for an accumulated dose < 150 Gy (15 kRad)—this metric includes spurious signals from radiolysis of DFP sensing fluid, and performance degradation of key electronic components used for CTMFD control.

• Metric 2: The CTMFD should remain non-affected by high energy gammas, with Eγ in the 3–7 MeV range when placed in a photon-to-neutron field of 100:1 (for Eγ < 3 MeV) and 1:1 (for Eγ> 6 MeV).

This section presents details on experiments and theoretical analyses to address the aforementioned two challenge metrics

The effect of high-rate gamma dose on the CTMD system was done using Purdue University's Nordion GammaCell 220^TM Co-60 irradiator. Figure 4(a) shows an outside view of the irradiator, which is loaded internally with vertically positioned Co-60 source pencil-shaped elements surrounding the internal irradiation cavity for placement of samples. Figure 4(b) shows the relative distribution of gamma flux, the rate is strongest next to the Co-60 source, and is lower in the center. The CTMFD samples were placed and irradiated in the center of the irradiator, as depicted schematically in Fig. 4(c).

Two formulations of CTMFD-sensing fluids were employed, the first was DFP only (which is sensitive only to fast > 0.1 MeV neutrons), and the second was a borated–DFP mixture (sensitive to 0.02 eV–to–MeV range neutrons). Unless otherwise noted, the sensing fluids were deployed in a sealed CTMFD without any prior degassing; under this condition, the first ∼15–20 detection events are caused by self-degassing of the predissolved air–after which the CTMFD is ready for detecting external neutrons. Prior degassing is conducted by acoustic agitation—a procedure we have developed that is readily accomplished within 1–2 min. Once predegassed, the CTMFD top stem (shown in Fig. 1(a)) is sealed shut to avoid air infiltration before deployment in the assemblage shown in Fig. 1(b).

Before and after gamma dose accumulation to a prescribed level, the CTMFD was examined for the possible impact of radiolysis gas buildup, which may lead to spurious neutron detection signals. Neutron detection signals were monitored under (i) normal background neutron conditions (from cosmic radiation and from neutron sources located in cabinets); and, (ii) from a certified Am-Be isotope neutron source emitting about 2 × 10⁴ n/s at a 1 m standoff from the CTMFD.

Results of the investigations are shown in Fig. 5 through Fig. 7.

Figure 5 shows detection results at Pneg = 0.7 MPa (7 bar) without predegassing—for 0 Gy and 150 Gy irradiation doses, respectively in the presence of a ∼2 × 10⁴ n/s emission rate Am-Be isotope neutron source positioned 1 m away. As seen, the first 15–20 detection events occur at random during self-degassing. Still, after that, the count rate stabilizes to about 11 cpm for both 0 Gy and 150 Gy cases, respectively. This provides proof for having met Metric 1 requirements.

Figure 6(a) presents detection results at Pneg = 0.7 MPa (7 bar) without predegassing (Option 1)—for 0 Gy and 450 Gy irradiation doses, respectively, but in a normal room neutron background condition. Considering the room background radiation is small, for 0 kGy (no irradiation) the count rate is < 1 cpm and is reached readily after the first few detection events. However, unlike for the case with 150 Gy gamma dose, at 450 Gy absorbed dose, there initially occurs a relatively large (20× higher than background) spurious count rate (cpm) signal, which clearly indicates the impact of significant radiolysis gas buildup, accompanied with simultaneous degassing as well since the count rate is seen to drop with each subsequent detection event (during which entrained gas is ejected out from the DFP)—the count rate gradually approaches the background count rate seen for the case without any prior irradiation. This result provides evidence for the need to examine the effect of predegassing (i.e., removal of radiolysis-caused gas pockets).

Figure 6(b) presents the analogous results for Fig. 6(a), but now with predegassing (Option 2) of the DFP sensing fluid after receiving 450 Gy (4.5 kRad) gamma dose. The results are remarkable in that we now obtain a 100% removal of the radiolysis gas buildup effect of the 450 Gy (45 kRad) irradiation. In reality, for SNF, simultaneous neutron monitoring under radiolysis would continually purge any radiolysis gas buildup; however, this aspect needs to be confirmed in an integral test environment involving a 10¹⁰:1 gamma:neutron source rate environment.

Figure 7 presents exciting results on the null effect of irradiation with 150–750 Gy gamma doses on borated-DFP filled CTMFDs, and without requiring any predegassing (Option 1). Results are striking in that the borated formulation evidently changes the radiochemistry nature of the CTMFD sensing fluid to permit us to 100% overcome the impact of radiolysis. The count rates obtained with and without an external neutron source are within 1 SD error of the cases with and without gamma dose levels even as high as 750 Gy. Importantly, this result is achieved without any resort to intentional predegassing. Therefore, it appears reasonable to claim that Metric 1 is met and exceeded by 3x—when the sensing fluid is borated DFP. The interesting combined response to irradiation-induced chemical changes and detection of interactions with neutrons await further investigation.

This aspect has been studied at Purdue University in the past [12]—the results are summarized herein for completeness. The CTMFD system (Fig. 1(b)) includes three key sensors: two IR sensors, one for monitoring the rotational speed and the other for monitoring the temperature of the CTMFD's sensing fluid, and one thermocouple (TC) for monitoring the incoming air and integrated with the IR temperature sensor for enabling dynamic temperature compensation. These three sensors were placed inside Purdue University's gamma irradiation chamber and connected to an external computer to monitor the output of the sensors over time. The sensors, each tested separately, were irradiated at a rate of 150 Gy/h for a total dose of ∼2750 Gy. Results are reproduced in Fig. 8 from the original data of Ref. [12].

The IR bubble detection sensor shines an IR beam through the central bulk of the fluid and is a key to detecting radiation-induced cavitation events. The signal from this sensor is read as a value from 0 (no signal) to 1024 (max signal). From experience, the as-built CTMFD functions if the signal value is over 100. Figure 8(a) shows the drop of the IR beam signal with increased dose, indicating functionality retention through an accumulated dose of over 2750 Gy (275 kRad), which is ∼18× greater than needed for Metric 1. While not as high for survivability, the other two temperature-related sensors indicate functionality for < 250 Gy conditions, as seen in Fig. 8(b), and are readily and economically replaceable (∼$1–10).

From Fig. 3, it is noted that the more influential higher energy photons (which could lead to photoneutron contamination) exponentially decrease with increasing photon energy, with a reduction of 10⁵ photons/sec/MTU from 1 MeV to 3 MeV and another 10² times reduction from 3 MeV to 7 MeV. Energetic photons above 2.2 MeV are above the (γ,n) threshold for de-uterium, a naturally occurring isotope of hydrogen within the DFP sensing fluid. The relative potential impact of 7 MeV gamma photons was assessed in an independent study at PNNL and is presented in a companion paper [26]. In tandem, confirmatory experiments for gaging potential contamination at a lower yet significant gamma energy of 3 MeV (which is above the photoneutron threshold for D atoms) were conducted at Purdue University.

The Purdue-based confirmatory experiment (Fig. 9(a)) used a 14 MeV D-T accelerator to target neutrons incident on a 9.1 kg (20 lb) NaCl target to activate Na atoms to produce the expected rate of photons at 3.1 MeV for verification of any interference from photoneutron contamination as may occur next to a SNF in a hot cell. Figure 9(b) shows a schematic of the GEANT code model. The expected photon emission rate from SNF at 3 MeV is 10⁵ times reduced compared to 1 MeV intensities. To achieve the desired conditions, 14 MeV neutrons at a rate of ∼1.7 × 10⁷ n/s were utilized. The D-T neutron generator head was enclosed within a NaCl-filled box and run for 15 min to produce ³⁷S via the ³⁷Cl(n,p)³⁷S reaction, with a cross section of 24 mb at 14 MeV.³⁷S beta decays to ³⁷Cl with a 5.05-min half-life, simultaneously emitting a 3.1 MeV gamma and consequently creating a photon environment above the 2.2 MeV photoneutron threshold. The resulting potential for photoneutron-based contamination was adjudicated by comparing the CTMFD count rates resulting from neutron interactions from fission (using a Cf-252 source of suitable emission rate at the appropriate standoff). The following sections discuss estimations for 3 MeV photon emission rate resulting from activation by 14 MeV neutrons and experimental results for combined effects of fission neutron detection with and without the 3 MeV photon source present.

where $σ$ represents the microscopic neutron absorption cross section, $ϕ$ represents the incident neutron flux (n/cm²/s), $N(0)$ represents the initial atom population, $λ$ represents the decay constant (1/s), $t$ represents the elapsed run time of the D-T neutron generator, and the subscripts 1 and 2 represent the target atom (³⁷Cl) and the product atom (³⁷S), respectively. The estimated ³⁷S population, shown in Fig. 10(a), quickly reaches an equilibrium state within a few minutes and peaks slightly above 10⁶ atoms, after which the D-T generator is turned off. Though not shown in the figure, the ³⁷S population would decay with the characteristic 5.05-min half-life.

The 3 MeV photon emission rate from SNF is ∼10⁸ s⁻¹ versus ∼10^7–8n/s emanating from spontaneous fission. We simulated the relative intensities of 3 MeV photons versus neutrons from SF entering the CTMFD, so as to ensure the ratio of gammas to neutrons is > 10, as is discussed later.

A second, independent theoretical analysis of the expected ³⁷S population (and hence, for 3 MeV gamma photon generation) was performed with the well-known GEANT4 software toolkit, a Monte Carlo-type platform for simulating particles through matter. The simulated setup is shown in Fig. 9(b).

The GEANT4 software toolkit tracks the ³⁷Cl(n,p)³⁷S reaction within the NaCl target, giving an estimate to the ³⁷S population over time. This serves as a confirmatory analysis to the expected 3.1 MeV photon emission rate deduced previously via the Bateman equations. The two analyses both yielded expected ³⁷S populations of approximately $4×106$ atoms, thus validating one another and serving as independent cross-checks. Once the D-T accelerator is shut off after 900 s of irradiation, the ³⁷S population decays with a 5.05-min half-life shown in Fig. 10(b). The associated 3.1 MeV photon emission rate commensurate with the 4 × 10^{6 37}S atoms is thus ∼9109 Bq. Assuming as shown in Fig. 9, the standoff from the CTMFD is ∼11.5 cm (4.5″) and a 1/r² variation of flux, the 3.1 MeV gamma photon emission rate entering the CTMFD is estimated to be ∼ 56 γ/s. In reality, the number of activation gammas produced from the 14 MeV neutrons can be expected to be significantly higher—albeit, of lower energy below the de-uterium photoneutron threshold of 2.2 MeV. The expected photoneutron production rate from ∼ 56 γ/s interactions with D atoms in DFP is discussed later in the last two subsections.

Figure 9(a) shows the experimental setup, including the CTMFD and position of the Cf-252 source of SF neutrons. DFP was used as the sensing fluid. A LaBr₃(Ce) scintillation-based detection system (operating voltage = 470 V; Coarse gain = 8; Fine gain = 1.2) was used for spectroscopic monitoring of gamma-ray signals from the NaCl target. The ²⁵²Cf SF neutron source offered an emission rate of 10⁴ n/s and as positioned, created the desired neutron to 3 MeV gamma emission rates of ∼10×.

As seen in Fig. 9(a), the 15 mL DFP-filled CTMFD was placed adjacent to the middle of the NaCl target, with the central cavity of the bulb ∼0.115 m (4.5″) from the target's outer edge. The LaBr₃(Ce) scintillation detector was placed adjacent to the CTMFD with the scintillation crystal ∼0.114 m (4.5″) from the target's outer edge. The D-T head was buried ∼0.1 m (4″) within the NaCl target – 0.114 m (∼4.5″) from the target base and ∼0.076 m (3″) from the closest edge. The ²⁵²Cf spontaneous fission neutron source was placed ∼0.3 m (12″) from the central cavity of the CTMFD bulb and diametrically opposite the LaBr₃(Ce) detector. The experiment station was elevated at ∼0.76 m (30″) above the floor.

The LaBr₃(Ce) scintillation detector was calibrated using a ∼3000 Bq (0.08 μCi) ⁶⁰Co check source, and further verified with a ∼25,000 Bq (0.65 μCi) ¹³⁷Cs check source results, which are shown in Fig. 10(c). After calibration, a background measurement was taken with both detectors, immediately followed by a second measurement with a ²⁵²Cf source for which the results are presented in Fig. 10(d). Once both measurements were completed, the two detectors were stopped. The D-T generator was run 900 s with an average 4π neutron output of $∼1.7×107$ n/s. After 900 s, the D-T generator was shut off, and a third measurement was taken with the ²⁵²Cf source present to demonstrate the CTMFD's ability to measure the SF neutrons at the same rate as before, despite the presence of 3+ MeV photons emanating from the activated NaCl target.

For all three measurements, the sum counts of the LaBr₃(Ce) detector were totaled, including accounting for the 3.1 MeV photon peak expected from the ³⁷S decay. The counts, shown in Table 2, show a 10× higher count for the activated NaCl with the ²⁵²Cf source present, versus with the ²⁵²Cf source without the activated NaCl target. The average excess count rate is ∼66 γ/s recorded by the LaBr detector. While this rough estimation does not consider the details re: detector efficiency and contributions from lower energy activation gammas (from 14 MeV interactions), it is of the same order of magnitude as that estimated from the aforementioned GEANT code-based calculated estimate of ∼56 γ/s.

As noted from Table 2, the gamma photon population measured by the LaBr detector is 10× higher for the case when the 14 MeV neutron-activated target is included – versus for the photon count rate with the Cf-252 source alone. A theoretical estimate for the photoneutron production rate from 3.1 MeV photons interacting with D atoms in the CTMFD's DFP (C₅H₂F₁₂) sensing fluid was performed. Assuming DFP density = 1600 kg/m³, 1:6000 ratio of D:H atoms, 2.5 mb photoneutron cross section, and a 16 mL sensitive volume, the expected (barring unforeseen effects) photoneutron production rate is calculated to be negligibly small at ∼10⁻⁷n/s. The confirmatory response of the CTMFD to the Cf-252 source neutrons with and without the 3.1 MeV-activated gammas is summarized in Table 3, and in Fig. 11, respectively.

The CTMFD detection rate history for incoming SF neutrons from Cf-252 over time is shown in Fig. 11 for the cases with and without 3 MeV photons. Despite the 3+ MeV photon presence, no difference was observed in the time-averaged CTMFD count rate when monitoring the ²⁵²Cf source with and without the activated NaCl as summarized in Table 3. Thus, the 3.1 MeV photons observed via the LaBr₃(Ce) scintillation detector did not constitute an observable hindrance on the CTMFD's detection capability via photoneutron interference. These results complement the companion results obtained independently in a separate study using 7 MeV-activated photons by researchers at PNNL [26]. As such, it appears that photoneutron-induced contamination remains negligible when using CTMFDs for examining SNF assemblies for their actinide inventory based on their unique neutron emission signature, even when in an intense gamma background.

From Fig. 3, we have noted that the actual 3 MeV gamma emission rate from SNF is ∼10⁸ γ/s versus a neutron emission rate of ∼10^7-8n/s from spontaneous emission. At a 1-m standoff, the expected photoneutron production rate in the 16 mL CTMFD can be calculated to be ∼0.3 n/s; however, this value is insignificant in relation to the ∼10^4–5 higher neutron emission rate from SNF fission neutron rate.

This paper presents a study on assessing the CTMFD sensor technology for its ability to externally monitor for spontaneous and random (α,n) neutrons while remaining blind to background gamma radiation from 30-y cooldown SNF assemblies.

A staged approach was undertaken with ORIGEN code-based estimations of the gamma and neutron energy spectra and relative intensities. Two key challenge metrics were developed to estimate the feasibility of using CTMFDs in the expected neutron-to-gamma environment at a 1-m standoff from a typical LWR SNF assembly.

The first metric focused on the impact of < 1.5 MeV photon dose on radiolysis and potential malfunction of CTMFD electronic components for enabling neutron spectroscopy within 1 h of irradiation leading to ∼150 Gy (15 kRad) absorbed dose. Metric 1 was successfully met and exceeded.

The second metric focused on the possible impact of photoneutrons resulting from gamma photons with Eγ = 3 MeV interacting with D atoms in CTMFDs, in emission rate levels proportion to expected gamma-to-neutron radiation encountered by a CTMFD positioned at 1 m standoff from 30-y cooldown SNF. A 14 MeV neutron DT accelerator system was used with a NaCl source to produce 3.1 MeV activation gamma rays targeted onto a CTMFD with and without external neutrons from a Cf-252 source. While the Cf-252 neutron source was readily detected, the 3.1 MeV photons did not produce any noticeable difference nor impact. Therefore, Metric 2 is also deemed to be successfully met.

Numerous Professor Taleyarkhan's research group students, past and present, are duly acknowledged for their valuable contributions leading to the development of the TMFD sensor technology as also collaborators at various external institutions, including Oak Ridge National Laboratory, PNNL, and Savannah River National Laboratory. Past student J. A. Webster obtained the results on gamma dose on CTMFD electronics as part of his Ph.D dissertation (credited as Ref.12). The research work presented in this paper represents the work of Purdue University co-authors with project coordination for the separate complementary study at PNNL by A. Hagen of PNNL (credited as Ref. 26). The research at Purdue University was conceived, directed, and coordinated by the corresponding author, Professor R. P. Taleyarkhan of Purdue University. The timely support and assistance from Purdue University's REMS organization are duly acknowledged and appreciated.

• Department of Energy (U.S.DOE) – Nuclear Safety R&D Program Office with Patrick Frias as DOE-HQ Program Manager, via PNNL (Grant No. 1400090).

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

Bq =

Becquerel

cc =

cubic centimeter (=mL)

C =

Celsius

Cf =

Californium

Ci =

Curie (= 3.7 × 10¹⁰ Bq)

CPM =

counts per minute

CTMFD =

centrifugally tensioned metastable fluid detector

eV =

electron volt

E =

energy

f =

frequency (Hz)

g =

gram

Gy =

Gray

h =

hour

L =

Liter

LaBr =

Lanthanum bromide

LWR =

light water reactor

m =

meter

min =

minute

mL =

milliliter

NaCl =

sodium chloride

NaI =

sodium iodide

n =

neutron

P =

pressure

Pa =

pascal

Pamb =

ambient pressure

Pneg =

negative pressure

PWR =

pressurized water reactor

r =

meniscus radius

Rad =

absorbed dose (=0.01 Gy)

SNF =

spent nuclear fuel

T =

temperature

TMFD =

tensioned metastable fluid detector

α =

alpha particle

β =

beta particle

γ =

gamma photon

σ =

microscopic cross section (barn)