DRIVEN SUBCRITICAL FISSION SYSTEMS USING A CYLINDRICAL INERTIAL ELECTROSTATIC CONFINEMENT (IEC) NEUTRON SOURCE

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DRIVEN SUBCRITICAL FISSION SYSTEMS USING A
CYLINDRICAL INERTIAL ELECTROSTATIC CONFINEMENT
(IEC) NEUTRON SOURCE
Miley G. H., Thomas R., Takeyama Y., Wu L.,
Percel I., Momota H., Hora H2., Li X. Z3. and P.
J. Shrestha4 1University of Illinois, Urbana,
IL, USA 2University of New South Wales, Sydney,
NSW, Australia 3Tsinghua University, Beijing,
China 4NPL Associates, Inc., Champaign, IL, USA
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Introduction IEC driven subcritial systems

• The IEC is already a commercial fusion neutron
  source at low levels!!
• Replaced Cf-252 in neutron activation analysis
  at
• Ore mines in Germany
• Coal mines in USA
• In these cases ease of licensing, long lived
  target (plasma), on-off capability, simplicity
  of construction (low cost), compactness, low
  maintenance requirement, flexibility in neutron
  spectrum (2.54 or 14 MeV), ease of control gave
  the IEC the edge. The features can carry over
  to a driver for a hybrid.
• Possibility of small size/power opens door to
  several near term applications university
  training and research facilities.

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Flexible geometry offers new types of drive
configurations ---
Fig. 1 Spherical IEC Device
Fig. 2 Cylindrical IEC Device
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Cylindrical IECs

• Cylindrical IECs offer many advantages for the
  present sub-critical reactor system .
• The prototype cylindrical IEC version , C-device,
  is a particularly attractive.
• Deuterium (or D-T) beams in a hollow cathode
  configuration give fusion along the extended
  colliding beam volume in the center of the device
  a line-type neutron source.

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Figure 3 Diagram of the C-device with calculated
ion trajectories and equi-potential surfaces
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IEC Modular Cylindrical Design Advantage

• Accelerator approaches to date have used an
  accelerator spallation-target system.
• The large size and cost of the accelerator remain
  an issue. Also, the in-core target system poses
  significant design and engineering complications.
  
• The IEC fits in fuel element openings of the
  sub-critical core assembly. This provides a
  distributed source of neutrons
• Replaces both the accelerator system and
  spallation-target by by multiple modular sources
  assembly.
• Provides flexibility in core design and in flux
  profile control.
• Small IEC units can be produced at a lower cost
  than the accelerator

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Vertical Cross-Section Showing C-device Modules
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Top Cross-Section View Shows C-Device Modules in
Channel Locations
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IEC Status

• Considerable research on the IEC concept has
  already been carried out on a laboratory scale
  1012 n/s neutron driver for low power
  sub-assemblies . (student labs or research
  assemblies)
• A key remaining issue for power reactors or
  actinide burners is concerns the ability to scale
  up to the high neutron rates required using the
  small volume units that are envisioned. Requires
  an intermediate prototype at 1014 n/s,
  followed by a full demo unit at 1018 n/s.
  Modular design allows testing on single small
  units speeding this development up.
• Engineering issues include materials development
  (common to other drivers), blanket (here core)
  design, rad hardening of the high-voltage
  components.

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Flowchart of MCP Code used to scale up from
presnt low level devices
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Neutron Production Fits Reasonably Well With
Earlier Data
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Neutron source strength predictions

• Present experiments give 1010 DD n/s (1012 DT
  n/s at 90 kV and 20 mA.
• Extrapolation to 1014 n/s (prototype research
  reactor goal) at 100 kV requires 0.3 A or 30 kW
  input.
• With improved potential profile control, might be
  reduced to lt10 kW.
• Research focusing on power reduction.
• Further extrapolation to higher power systems
  also promising.

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Near Term Use in Low Power Research Reactors or
Teaching Labs

• Present experimental IEC devices are close to
  neutron yields required of this application.
• Calculations for a representative graphite
  moderated subassembly next.

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Graphite Modulated Sub-critical System

• Figure presents the power obtained per unit
  source as a function of the multiplication factor
  k8.
• assumed to be a cylindrical homogeneous reactor,
  fueled by uranium dioxide.
• The fuel enrichment is adjusted to give the
  desired value of k8.
• the fraction of core volume occupied by the fuel
  fixed at 5.
• the graphite-moderated system can deliver 1 kW of
  power with a source of 1012 neutrons/sec at Keff
  0.99
• Specifications summarized in Table .

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Figure 4 Power level per unit source (P/S) as a
function as a function of k? for two different
moderators
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Table 1 Parameters for a 1 kW graphite-
moderated sub-critical system.
Fuel UO2 (0.5 U-235)
Moderator material Graphite
Moderator volume fraction 95
Multiplication factor 0.97
Radius (cm) Height (cm) 3050
Source strength (neutrons/s) 1x1012
Power (kW) 1.2
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Summary

• A 1-kW IEC driven graphite moderated research
  reactor appears attractive from a cost and safety
  point of view.
• Also, existing experimental IEC devices very
  close to the target of 1012 n/s.
• This is consistent with the source levels in
  Garching II research reactors (Cf-252 neutron
  source with 4x109 neutrons/sec).

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Future Driven Reactor Designs vs Accelerator
Target designs

• Designed to ensure safety against criticality and
  loss-of-cooling accidents as is done in the
  conventional accelerator-target designs.
• But important differences exist in the method
  used .
• Accelerator designs use a passive beam shut-off
  device based a combination of thermocouple
  readings and a melt-rupture disk in the side-wall
  of the beam guide tube.
• The IEC uses a temperature sensitive fuse in the
  in-core electrical circuit to shut down the
  high-voltage.
• A melt rupture disk on the IEC wall is added as
  backup to spoil the IEC vacuum.
• A very rough conceptual design for a 1000 MWe
  plant has been developed. The reactor core
  employs distributed IEC units as in the low power
  subcritical applications. Very important here for
  flux profile control.

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Design RD Comments

• Cylindrical IEC units occupy multiple fuel
  channels. a distribute d neutron source and
  modular source design.
• 15 units stacked in each channel in preliminary
  design for 1000Mwe unit.
• Selected to optimize neutron profiles in both the
  radial and vertical directions
• This distributed source design is to be
  contrasted to a central core spallation target
  (STET)
• Waste heat from IEC is deposited on the large
  area hollow electrodes and removed by coolant
  flow around the fuel channels.
• A keff lt 0.97 requires 1018 n/s per IEC ( vs
  present experimental values of 1012 D-T n/s)
  MCR calculations indicate feasible.
• Since IEC scaling involves velocity space
  increasing the yield does not require a
  significant increase in unit size.
• Instead, higher beam currents and improved ion
  recirculation are key other special crucial
  issues include high-voltage stand-offs that are
  radiation hardened. Material development, etc.
  are common issues to other drivers.
• These issues can be studied starting from a
  simple small unit, allowing a rapid development
  cycle..

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Conclusions

• An alternative to the standard driven reactor
  accelerator-spallation target design is proposed
  which employs IEC neutron sources which can be in
  a central location or distributed across a number
  of fuel channels. Such a modular design has
  distinct advantages in reduced driver costs, plus
  added flexibility in optimizing neutron flux
  profiles in the core. The basic physics for the
  IEC has been demonstrated in small-scale
  laboratory experiments, but a scale-up in source
  strength is required for ultimate power reactors.
  
• However, the IEC source strength is already near
  the level required for low power research
  reactors or for student sub-critical laboratory
  devices. This application would be advantageous
  since the safety advantages of these reactors
  should enable a next generation of research
  reactors to be constructed quickly, meeting the
  educational and research needs facing us as there
  is a rebirth of interest in nuclear power.