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Advances in the Development and Testing of Micro-Pocket Fission
M. A. Reichenberger1, T. D. F. George1, R. G. Fronk1, P. B. Ugorowski1, J. A. Geuther1, J. A.
Roberts1, B. W. Montag, T. Ito2, H. B. Vo-Le1, S. R. Stevenson1, D. M. Nichols1, and D. S.
1Semiconductor Materials and Radiological Technologies (S.M.A.R.T.) Laboratory,
Department of Mechanical and Nuclear Engineering, Kansas State University, 3002 Rathbone
Hall, Manhattan, KS 66506 United States of America
2Department of Chemistry, Kansas State University, 213 CBC Building, Manhattan, KS
Corresponding author: email@example.com
Abstract A new method of fabricating micro-pocket fission detectors (MPFDs) has been developed. Deployable
into iron-wire flux wells, MPFDs are neutron sensors capable of real-time, in-core flux measurements. A cyclic
potential sweep method was used to deposit 0.1 ± 0.006 µg of natural U onto one 0.33-mm diameter Pt electrode
on one face of the MPFD chamber. The MPFD was operated in the central thimble and IRIS flux wells of the
KSU TRIGA Mk II nuclear reactor. The MPFD response was observed to be linear with respect to reactor power
with no sign of dead time between 10 kWth and 700 kWth. The IRIS flux well was utilized to observe a $1.50
reactor pulse with a max power peak full width at half maximum of 12 ms.
The need to monitor the neutron flux (n·cm-2·s-1) within the core of a nuclear reactor has
driven research in numerous neutron detection methods. The neutron flux within the core of a
nuclear reactor is indicative of its operational power level; an increase in the neutron flux
indicates an increase in operational power level. In order to gain a complete understanding of
the operation of a nuclear reactor, the neutron flux and power level must be accurately
reported to operators. Presently, solutions exist to monitor the neutron flux via radiation
monitors that are kept outside of the core. However, this method of flux measurement can is
susceptible to errors due to neutron transport through the materials found between the core
and the detector system. Moreover, information regarding minute variations in flux around the
core, due to fuel burn up or control-rod insertion etc., is entirely lost at distances outside of the
Development and deployment of small, accurate, and robust neutron flux measurement
systems is an important enhancement for advancing nuclear fuel technology. A need exists to
place neutron sensors within the reactor core to provide information on the neutron flux for
both nuclear test reactors, and commercial power reactors . The high-radiation and high-
heat environment found within a nuclear reactor core are not conducive to the operation of
many types of radiation detectors. First, the high neutron flux found within a reactor core,
often on the order of 1014 n·cm-2·s-1 , will either burn up a detector’s neutron conversion
material too quickly, reducing the device’s overall lifetime, or will induce a count rate so high
that the detector becomes experiences significant dead time. Second, the high temperature
present within most reactor cores, often exceeding 300°C , would either destroy many
detector systems (such as scintillators) or render them entirely unusable (such as would be the
case for most semiconductors). Furthermore, in-core sensors must typically be located within
narrow channels within the reactor core (< 1 cm in diameter) . The physical requirements
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for in-core neutron sensors limit material selection and device geometry. Several technologies
exist which are used to measure neutron flux for in-core and near-core environments.
Ionization chambers and fission chambers are commonly used for near-core neutron
measurements . Typical ionization and fission chambers are necessarily large, and are only
capable of monitoring neutrons which have escaped the reactor core. Such devices are
impractical for in-core measurements because of their large size, fragile construction, and
large flux perturbation. Miniature fission chambers are commonly used for in-core neutron
measurements. Miniature fission chambers are typically lined with highly enriched uranium,
and made in a cylindrical geometry . Device dimensions for miniature fission chambers are
usually in the mm to cm range. The burnup of fissile material, and buildup of fission
fragments in sealed miniature fission chambers greatly limits the application of such devices
for extended periods of time. A fission chamber using enriched 235U will decrease in
sensitivity by 10% after an integrated neutron fluence of 1020 n cm2 in a typical power nuclear
reactor . In order to extend stable device lifetime, fertile isotopes can be added to the
neutron-sensitive coating . The buildup of fission fragments in the sealed gas chamber of
typical miniature fission chambers also produces a ‘memory effect’, where the radioactive
decay of fission fragments in the detection gas produces a residual current, reducing device
accuracy . Sub-miniature fission chambers utilize highly-enriched (97% 235U) uranium
coatings upon the cathode wall of small, sealed, proportional gas detectors . Current-mode
operation is required for in-core operation of sub-miniature fission chambers due to the high
neutron sensitivity of the fissile coating. The stable device lifetime also suffers from the high
sensitivity, limiting the effective use of such detectors to fluences < 2 × 1021 n cm-2 . Such
devices are impractical for use in critical mock-ups, high performance material test reactors
(MTRs) and transient test reactors because of low-fluence design and large flux perturbation
when installed in-core or near-core.
Alternatively, iron or gold activation analysis can be used to determine the neutron fluence
within a reactor core during an operational period. However the neutron fluence is not as
useful for experiments in high-performance reactors, transient test reactors, and critical mock-
ups, which distinctly benefit from real-time flux measurement. Finally, self-powered neutron
detectors (SPND) incorporate neutron-sensitive materials that decay by beta or gamma-ray
emission. The simplest versions of SPNDs rely on the direct measurement of the beta decay
current following a neutron absorption . In contrast to typical fission chambers, SPN
detectors are very small, and require no applied bias. However, the output current from SPN
detectors is very small and suffers a time delay due to the nuclear decay .
2. Theory and Prior Research
Previous simulations have shown that a power density profile of a research reactor can be
reconstructed using in-core neutron flux measurements . Large prototypes (3-mm diameter
electrode) using 235U coatings were successfully tested in the neutron beam port of the KSU
TRIGA Mk II research reactor . The small size of MPFDs present advantages for in-core
neutron monitoring . The high specific ionization of fission fragments within the small (< 1
mm thick) gas pocket compared to gamma rays, high-energy electrons, and beta particles
yields low background gamma ray sensitivity . Pulse height discrimination can therefore be
used when operating in pulse mode to separate the signal from non-neutron interactions. The
small mass of neutron-conversion material present in the gas pocket results in an effective
neutron sensitivity as low as 10-8 interactions per flux unit (depending on the energy-
dependent flux profile). This low sensitivity results in minimal flux perturbation in the
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detector region, producing an accurate flux measurement. Limiting flux perturbation due to
the measurement device is particularly important for research and test reactors, where spatial
flux profiles may be less understood than in large reactors .
Prototype MPFDs have been constructed and tested [4, 5, 7]. All of the prototype devices have
demonstrated exceptional gamma-ray discrimination and have been used to detect neutrons in
fluxes varying from 1.6 × 106 n·cm-2·s-1  to 2.4 × 1013 n·cm-2·s-1 . Large prototypes (3-
mm diameter) using enriched 235U neutron-conversion material coatings suffered from
significant dead time at elevated fluxes . Enhancements to the detector chamber design and
fabrication process were necessary to reliably produce MPFDs capable of deployment into test
nuclear reactors . Furthermore, recent progress electrodepositing natural U and Th onto
small, circular Pt electrodes has advanced the capability to produce MPFDs with very thin
neutron-sensitive coatings . The cyclic potential sweep method described elsewhere 
was used to fabricate the MPFD tested in previous iterations , and was improved as part of
the present work.
3. Advances in MPFD Design and Fabrication
The present work was initiated with four primary objectives: Multi-nodal, in-core deployment
via iron-wire flux well, neutron flux measurement, extended sensor operational lifetime, and
reliable reproducibility. Multi-nodal measurement of the neutron flux is necessary to
reconstruct the spatially varying power profile for the research reactor primarily through the
measurement of the thermal-neutron flux. Large prototypes have been previously built and
tested in specialized flux wells at the KSU TRIGA Mk II research reactor [4, 5, 7]. However,
FIG. 1. Multi-wire MPFD assembly
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deployment into an iron-wire flux well is intended to enable sensor deployment and
calibration at many other research reactor facilities in the future, leading to the reduction of
overall device dimensions. While previous prototypes and other in-core flux sensors utilize
enriched 235U, the present work sought to develop an in-core sensor with an extended
operational lifetime in an active research reactor. Finally, the electrodeposition process was
enhanced to improve reliability of the neutron-reactive coating process and enable the
reproduction of sensors with similar characteristics.
The design parameters were set such that the detector chamber would fit within an iron wire
flux well, thus modifications to previous designs were necessary. In order to facilitate a much
smaller detector chamber (total volume < 1 mm3), a multi-wire electrode configuration was
chosen over the parallel-plate electrodes used in previous iterations . The parallel wires run
through an ionization chamber which is created by placing an Al2O3 spacer between two
Al2O3 disks (Fig. 1). The present MPFD chamber measured 2 mm × 1 mm × 1.5 mm with an
active chamber measuring 0.33 mm (diameter) × 0.5 mm. The anode and cathode wires were
then run to an electronic feedthrough at the top of the flux-wire well and into the charge-
sensitive preamplifier. The entire flux-wire well was back-filled with Ar ionization gas which
is allowed to flow into the ionization chamber by the loose-stacked fabrication of the chamber
. A benefit of the design is that multiple pieces can be stacked to build multiple fission
chambers, or several longer fission chambers composed of longer elements, upon a single
string of wire electrodes. One or more surfaces of the disks and spacer can be coated with
The improved electrodeposition techniques described in , were used to deposit 0.1 ± 0.006
µg of natural U onto one 0.33-mm diameter Pt electrode which was evaporated onto the
MPFD disk face, as shown in Fig. 2. The mass of U was then measured using alpha-particle
spectroscopy . After the MPFD node was constructed, the insulated anode and bare cathode
wires were held together using PTFE heat shrink and inserted into the central-thimble flux
well at the KSU TRIGA Mk II nuclear reactor.
FIG. 2. Uranium electrodeposited MPFD disk.
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4. In-Core Reactor Testing
Both low- and high-frequency noise were present from the detector assembly during initial
testing stages. The 5-m long signal wires connecting the detector chamber to the electronic
feedthrough at the top of the flux well acted as radio antennas. Significant effort was made to
reduce radio-frequency (RF) noise by grounding the flux well, adding a tinned-copper
overbraid to all signal cables after the electronic feedthrough, and shielding the preamplifier
boxes. Furthermore, a band RF filter was used after the preamplifier to isolate the neutron-
induced signal from the remaining noise. A shaping amplifier with a gain of 90 was used to
amplify the signal for counting. A 4-µs shaping time was used in order to integrate out
oscillatory mechanical pump noise that was permeating into the signal. The lower-level
discriminator (LLD) was set by operating the detector within the reactor core while at 0-kWth
reactor power. The TRIGA Mk II nuclear reactor was then operated at 100 kWth. The
resulting pulses from the ORTEC 142PC preamplifier, ORTEC 2022 shaping amplifier, and
Canberra 2031 single channel analyzer are shown in Fig. 3.
After identifying neutron-induced pulses at a reactor power of 100 kWth, the detector was
tested at different reactor powers in order to determine the linearity of the MPFD response
TABLE I. MPFD measurements in the central thimble flux well of the KSU TRIGA Mk II
FIG. 3. Neutron-induced MPFD signals after each of 3 stages of processing:
Preamplifier, shaping amplifier, and single-channel analyser.
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rate. Three one-minute measurements were taken at each of the following reactor power
levels: 10 kWth, 100 kWth, 300 kWth, 400 kWth, 500 kWth, 600 kWth, and 700 kWth. The
response of the MPFD is summarized in Table 1, and depicted in Fig. 4.
The response rate of the MPFD was sufficient to measure reactor power transients. Both
positive and negative reactivity insertions were executed to observe increasing and decreasing
power transients. In order to observe increasing power transients, the reactor operators were
asked to establish a 40-second period of increasing power from 1 kWth and allow the reactor
to stabilize by temperature feedback. The negative temperature coefficient for the KSU
TRIGA Mk II research reactor allows for natural stabilization of power after a small reactivity
insertion. Reactivity insertions yielding 20-second and 10-second periods were also executed.
Each reactivity insertion resulted in a power overshoot which decreased to a stable power as
shown in Fig. 5.
FIG. 4. Dynamic range testing of a MPFD in the central thimble of the KSU TRIGA Mk II
research reactor. The response of the detector was linear with reactor power.
FIG. 5. MPFD response to power transients of 40, 20, and 10 seconds. A 5
second moving average was used to smooth variations in the scalar response.
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Decreasing power transients were observed by control rod insertions of known negative
reactivity. After reaching an elevated operating power of 700 kWth, negative reactivity
insertions of -$0.10, -$0.20, -$0.40, -$0.80 (-70, -140, -280, -560 pcm respectively) and a
SCRAM were executed. After each subsequent negative reactivity insertion, the reactor was
brought back to 700 kWth (with the exception of the -$0.20 insertion which was a
combination of two sequential -$0.10 insertions). After the -$0.80 insertion, the reactor was
brought to full power (all rods out) and a SCRAM was executed. The detector response
tracked the reactor power with each negative reactivity insertion, as shown in Fig. 6.
Reactor pulsing is one unique advantage of in-core neutron detector testing at the KSU
TRIGA Mk II nuclear reactor facility. During a reactor pulse, the pneumatically-operated
control rod (pulse rod) is ejected from the core when the reactor is critical at low power.
Following the ejection of the pulse rod, the positive reactivity insertion (greater than $1.00,
FIG 6. MPFD response to negative reactivity insertions of -$0.10, -$0.40, and -
$0.80 followed by a reactor SCRAM. A 5 second moving average was used to
smooth variations in the scalar response.
FIG.7. MPFD response to a $1.50 reactor pulse measured from the IRIS flux
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~700 pcm for the KSU TRIGA Mk II nuclear reactor) creates a prompt supercritical excursion
where power increases from 10 W to nearly 1 GW in approximately 10 ms. The negative
temperature coefficient for the TRIGA nuclear reactor then compensates for the reactivity
insertion in the subsequent 10 ms as the reactor returns to a normal power level. During the
reactor pulse, flux levels in the central thimble can exceed 1017 n·cm-2·s-1. Flux levels in the
IRIS flux well had shown to be ~100× lower than the central thimble. Therefore, reactor pulse
testing was conducted in the IRIS where maximum pulse flux was expected to reach 1015
n·cm-2·s-1. The MPFD response during a $1.50 reactor pulse is shown in Fig. 7. The full width
at half maximum (FWHM) for the pulse measured by the MPFD was 12 ms while the reactor
operator staff reported a FWHM of 10 ms from the near-core compensated fission chamber.
5. Final Remarks - Conclusions
Stackable MPFDs using dual-wire electrodes have been designed, built, and successfully
demonstrated as in-core neutron sensors. Advances in the MPFD fabrication process have
enabled these devices to be deployed into iron-wire flux wells. The detection chamber (< 0.05
mm3) successfully operated with a linear detector response in the KSU TRIGA Mk II research
reactor at power levels from 10 kWth to 700 kWth with no signs of detector dead time. Both
increasing and decreasing power transients were observed using the MPFD in the central
thimble flux well, while a $1.50 pulse was observed using the MPFD deployed in the IRIS
flux well. Further advancements in the fabrication and deployment of MPFDs will enhance
the data acquisition capabilities for high neutron flux environments.
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