SINBAD ABSTRACT NEA-1517/92
Polyethylene‐Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements
1. Name of Experiment:
------------------
Polyethylene‐Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements (~1987)
2. Purpose and Phenomena Tested:
----------------------------
These benchmark data were collected by measuring a 4.5 kg 94%‐239Pu plutonium
metal spherereflected by spherical polyethylene shells. Six different
configurations of the plutonium sphere were measured: bare, and reflected
by polyethylene 1.27, 2.54, 3.81, 7.62, and 15.24 cm thick. The neutron
and photon emissions from this source were measured using three
instruments: a high‐efficiency highpurity germanium (HPGe) gamma spectrometer,
a gross neutron counter using moderated helium‐3,and a neutron multiplicity
counter using moderated helium‐3. The gamma spectrometry data consist of
gamma spectra with 32,768 channels and a maximum photon energy of 11.8 MeV.
The gross neutron counting data consist of neutron count rates measured with
two different configurations of detector moderation. The neutron multiplicity
data consist of time‐stamped list mode detection events acquired with one
microsecond time resolution.
These data provide observations of the total neutron production rate, the
neutron multiplicity distribution, and the gamma spectrum of plutonium metal
with neutron multiplication between approximately 4 and 17.
3. Description of the Source and Experimental Configuration:
--------------------------------------------------------
The plutonium source for this series of experiments was the BeRP Ball, a
4.5-kg sphere of α-phase, weapons-grade plutonium fabricated by Los Alamos
National Laboratory (LANL) in October 1980.1Figure 1 The sphere is clad in
stainless steel.
The BeRP ball was cast and machined to a mean radius of 3.7938 cm. The
theoretical density of α-phase plutonium metal is 19.655 g/cm3. However, the
measured mass of the plutonium sphere was 4483.884 g, and the volume of the
sphere was 228.72 cm3 (based on the mean diameter). Consequently, the
calculated density of the plutonium sphere is 19.604 g/cm3. LANL documentation
describing the assembly of the BeRP ball indicates the plutonium sphere was
partially immersed in Freon to shrink it just prior to its final assembly in
the steel cladding. The dimensions of the plutonium sphere following this
Freon bath were not recorded by LANL. As a result, the actual density of the
plutonium sphere may be higher than its calculated density if the Freon bath
permanently reduced the volume of the sphere by changing the grain structure
of the metal.
The BeRP ball was encased in a cladding of stainless steel 304 that is nominally
0.0305 cm thick. The nominal composition of the steel cladding is listed in
Table 1. Its nominal density is 7.62 g/cm3. As shown in Figure 1, the cladding
was constructed from two hemishells, each with a nominal inside radius of 3.8278
cm and a nominal outside radius of 3.8583 cm. Each hemishell also had a 4.3764-cm
radius, 0.0457-cm thick flange. When the plutonium sphere was assembled in the
cladding, the two hemishells were electron-beam-welded together at this flange.
Because the outside of the radius of the plutonium sphere is smaller than the inside
radius of the cladding by 0.0340 cm, there is a gap between the plutonium sphere and
the cladding that is nominally 0.0680 cm at its widest point.
The polyethylene reflectors were constructed as a series of five nesting spherical
shells. Each individual shell was constructed of two mating hemishells. Figure 3 is a
photograph of the plutonium source in the nested hemishells.
The mating surfaces of the hemishells were stepped to eliminate any streaming path.
Each shell was supported by its own aluminum support stand. The stands were designed to
keep the center of the plutonium source 8.3 inches above the work surface
4. Measurement System and Uncertainties:
------------------------------------
Gross neutron counting, neutron multiplicity counting, and gamma spectrometry
measurements were performed on the plutonium source in six configurations:
• Bare
• Reflected by 0.5 inch of HDPE
• Reflected by 1.0 inch of HDPE
• Reflected by 1.5 inches of HDPE
• Reflected by 3.0 inches of HDPE
• Reflected by 6.0 inches of HDPE
Gross neutron counting, neutron multiplicity counting, and gamma spectrometry
measurements were collected for each of the preceding reflected configurations
of the plutonium source.
In addition to the calibration and benchmark measurements described in preceding
sections, several other auxiliary measurements were performed. These measurements
were conducted to characterize
• the effect of the polyethylene reflectors’ temperature on the response of the
neutron multiplicity counter.
• the effect of the neutron multiplicity counter’s large moderator on the response
of the gross neutron counter.
• the differences between the reflectors constructed by SNL and reflectors
constructed at LANL.
• the response of the instruments to the californium neutron calibration source in
the polyethylene reflectors.
Gamma spectra were acquired using an Ortec DigiDart multichannel analyzer (MCA). The
MCA was controlled using custom software developed by LANL. All spectra were saved
in Ortec’s floating point “.chn” format. Gamma spectra were collected for 10-minute,
20-minute, and 60-minute dwell times. For a given reflected configuration of the
plutonium source, typically all gamma spectra were collected either on the same day or
on consecutive days.
Statistical and systematic uncertainties are provided with the measurements.
5. Description of Results and Analysis:
-----------------------------------
Gross neutron counting measurements acquired with the LANL SNAP are listed in Table 22.
Measurements were performed with the SNAP polyethylene cover both on and off. Table 23
lists the SNAP neutron count rate and uncertainty versus the thickness of the
polyethylene reflector. The measured count rates are plotted in Figure 47.
Neutron multiplicity counting measurements acquired with the LANL NPOD are listed in
Table 24. The measured count rate and its uncertainty are given in Table 25 versus
the thickness of the polyethylene reflector, and the count rate is plotted in Figure 48.
Figure 49 shows the entire neutron multiplicity distribution versus coincidence gate width,
and Figure 50 shows the measured neutron multiplicity distribution for a coincidence gate
width of 1024 μs. Figure 51 and Figure 52 respectively show the measured Feynman-Y and Rossi-α versus reflector thickness.
Note that both the gross neutron counting measurements and the neutron multiplicity
measurements demonstrate competition
• Increasing neutron multiplication with increasing reflector thickness
• Decreasing neutron leakage with increasing reflector thickness
As a result, most of the neutron metrics are maximized for the 1.5-inch thick reflector,
where the product of multiplication and leakage probability are near their maximum.
Gamma spectrometry measurements acquired with the HPGe detector are listed in Table 26.
Recall that measurements were performed both with and without the NPOD and SNAP present.
For each reflected configuration of the plutonium source, gamma spectra were collected for
10, 20, and 60 minutes.
6. Special Features:
----------------
None
7. Author/Organizer
----------------
Experiment and analysis:
John Mattingly
Sandia National Laboratories
1515 Eubank Boulevard Northeast
Mail Stop 0782
Albuquerque, New Mexico 87123 USA
8. Availability:
------------
Unrestricted
9. References:
----------
SAND2009-5804-R2.pdf John Mattingly "Polyethylene-Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements"
Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550 SAND2009-5804 Revision 2 (December 2009)
10. Data and Format:
---------------
DETAILED FILE DESCRIPTIONS
--------------------------
Filename Size[bytes] Content
---------------- ----------- -------------
| 1 | berp_poly_2009-a.htm | 31,898 | This information file |
| 2 | berp_poly_2009-e.htm | 37,935 | Description of experiment |
| 3 | berp_ball_construction.pdf | 1,047,389 | berp_ball_construction.pdf: Internal memorandum on assembly of Pu-239 ball |
| 4 | berp_ball_drawing.pdf | 8,386 | berp_ball_drawing.pdf: Schematic of berp-ball (high quality) |
| 5 | HPGe Measurements.xlsx | 20,778 | HPGe Measurements.xlsx: Excel Spreadsheet listing HPGe detector measurement file names |
| 6 | instrument_configuration.pdf | 74,591 | instrument_configuration.pdf: Measurement locations (high quality) |
| 7 | neutron_counter_stand.pdf | 158,813 | neutron_counter_stand.pdf: schematic of neutron detector stand (high quality) |
| 8 | NPOD Measurements.xlsx | 21,695 | NPOD Measurements.xlsx: Excel Spreadsheet listing NPOD detector measurements file names |
| 9 | room_layout.png | 170,660 | room_layout.png: Drawing detailing source and detector placement |
| 10 | SAND2009-5804-R2.pdf | 7,871,598 | SAND2009-5804-R2.pdf: Sandia report of Polyethylene-Reflected Plutonium Metal Sphere: Subcritical Neutron and Gamma Measurements |
| 11 | SNAP Measurements.xlsx | 55,311 | SNAP Measurements.xlsx: Excel Spreadsheet listing SNAP detector measurements |
| 12 | steel_cart.pdf | 60,912 | steel_cart.pdf: Schematic steel cart used in experiment (high quality) |
| 13 | Synopsis.pdf | 78,664 | Synopsis.pdf: Brief description of this experiment |
| 14 | ba133_certificate.pdf | 1,027,580 | ba133_certificate.pdf: activity certificate for Ba-133 calibration source |
| 15 | calibration_sources.pdf | 21,291 | calibration_sources.pdf: List of calibration source |
| 16 | calibration_sources.xlsx | 1,244,548 | calibration_sources.xlsx: list of calibration sources |
| 17 | cf252_certificate.pdf | 1,244,548 | cf252_certificate.pdf: certificate for Cf-252 calibration source |
| 18 | cf252_details.xlsx | 15,522 | cf252_details.xlsx: detailed calculations of Cf-252 neutron source strength |
| 19 | co60_certificate.pdf | 1,005,626 | co60_certificate.pdf: activity certificate for Co-60 calibration source |
| 20 | cs137_certificate.pdf | 980,351 | cs137_certificate.pdf: activity certificate for Cs-137 calibration source |
| 21 | nist_sources.pdf | 115,340 | nist_sources.pdf: document describing NIST source construction |
| 22 | u232_info.pdf | 31,494 | u232_info.pdf: information on NIST U-232 calibration source |
