Optimization of a Cold Neutron 

Source for a Proposed LEU 

Reactor at NIST

RYAN BONK

SURF STUDENT COLLOQUIUM

NIST CENTER FOR NEUTRON RESEARCH

GAITHERSBURG, MD
The NBSR

• The lifetime of the National Bureau 

of Standards Reactor (NBSR) will be 

coming to an end sometime in the 

middle of this century.

• NCNR hosts over 2,000 researchers 

annually

• 70% use cold neutrons in their 

experiments.

• Feasibility study for a Low-Enriched 

Uranium (LEU) reactor underway
Cold Neutrons

Type Energy Range Wavelength 

Range (Å)

Fast 1 MeV – 20 MeV .0003 - .0001

Thermal .025 eV – .625 eV 1.8 - .04

Cold < .005 eV > 4 

• Materials with structural spacings on the order of 100 Å have become more prominent in 

science and technology (Ex. Polymers).

• With their low energy and long wavelength, cold neutrons are better suited for probing 

these materials.

Note: 1 Å = 1 x 10−10 m
Producing Cold Neutrons

• To increase the production of cold neutrons 

from a reactor core, a cold neutron source 

(CNS) is used.

• Cryogenically cooled to around 20 K to shift the 

spectrum of neutrons to lower energies.

• Atoms with low Z (Example: hydrogen or 

deuterium) make ideal moderators.

•The new CNS design will be filled with liquid 

Deuterium (2021).

Unit 2 - Existing LH2 Cold Neutron Source
Neutron Moderation

• Neutrons collide with the atoms of the medium 

they reside in much like air molecules colliding in 

the room around us.

• Their kinetic energies depend only on the 

temperature of the surroundings according to the 

Maxwell-Boltzmann Distribution.

• D2O Moderator ≈ 300 KK

• LD2 Moderator ≈ 20 KK

Probability

Neumann, Dan. "Neutrons at NIST." NIST. Gaithersburg, MD. 15 Jan. 2014. Web. 19 July 2016.
Why Switch to Deuterium?

• Spectrum shifts to lower 

energies.

• Gains of up to 2 for the 

longest wavelengths.

• Up to a 50% loss at 15 meV

(2.5 Å).

• Conversion of NBSR to LEU 

will result in 10% reduction in 

thermal and cold neutron 

beams.

• New CNS/guides intended to 

make up for this reduction.

Relative Fluxes of LD-2 vs. LH-2 in CT Port

0

2

4

6

8

10

0 5 10 15 20

Energy (meV)

Relative Flux

Existing LH-2 Source

38-cm LD-2 Source
Deuterium Cold Neutron Source for 

Replacement Reactor

CNS

Thermal 

Beams

Heavy 

Water

Light 

Water

Neutron Beam 

Guide

Gaseous Deuterium 

Re-entrant Cavity

Liquid Deuterium 

Moderator

Plane View (Top) Plane View (Side)
Optimizing the Cold Source Location

• Monte Carlo N-Particle (MCNP) computer simulations were used previously to perform flux 

calculations on the new reactor design.

CNS CNS
MCNP Neutron Brightness Calculations

• The quantity used to measure the performance of the cold neutron source is the Brightness.

• Units: n/cm2-s-meV-ster or n/cm2-s-Å-ster

A surface source is 

generated around the CNS 

using a whole-core 

criticality calculation.

This surface source is 

used to perform surface 

current calculations on 

the CNS.

The DXTRAN feature is 

used to force “pseudo” 

particles to a tally plane at 

the neutron guide 

entrance.
Optimizing the Cold Source Geometry

• The effects of changing the size and shape of the re-entrant cavity were investigated using MCNP.

# Geometry (X x Y x Z) (cm)

1 Original (Trapezoid)

2 16 x 12 x 20 Box

3 Cylinder: d = 15, h = 20

4 38o Cone (vertex @ center)

5 8 x 12 x 8 Ellipsoid

6 No Re-entrant Cavity
Brightness Gain Compared to the 

Original Deuterium CNS Design

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5

GAIN

WAVELENGTH (Å)

NEUTRON BRIGHTNESS GAIN FOR VARYING ELLIPSOID CAVITY 

DEPTH (WIDTH = 8 CM, HEIGHT = 8 CM)

Depth = 12 cm Depth = 10 cm Depth = 8 cm

Depth = 6 cm Depth = 16 cm No Re-entrant Cavity

Unit 2
Brightness Gain Compared to the 

Original Deuterium CNS Design

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5

GAIN

WAVELENGTH (Å)

NEUTRON BRIGHTNESS GAIN FOR VARYING ELLIPSOID CAVITY 

WIDTH (DEPTH = 12 CM, HEIGHT = 8 CM)

Width = 8 cm Width = 4 cm Width = 12 cm

Width = 16 cm Width = 20 cm No Re-entrant Cavity
Brightness Gain Compared to the 

Original Deuterium CNS Design

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5

GAIN

WAVELENGTH (Å)

NEUTRON BRIGHTNESS GAIN FOR VARYING ELLIPSOID 

CAVITY HEIGHT (DEPTH = 12 CM, WIDTH = 8 CM)

Height = 4 cm Height = 8 cm Height = 12 cm

Height = 16 cm Height = 20 cm No Re-entrant Cavity
Nuclear Heat Load

• The Deuterium Cold Source is more massive than the existing Hydrogen cold source, meaning it 

will take on a greater nuclear heat load.

• MCNP used to calculate expected heat deposition rate in the LD2 and moderator chamber with 

an (8 x 12 x 8) cm ellipsoid re-entrant cavity .

• A new 7 kW cryogenic helium refrigerator is being installed to account for this threefold 

increase in heat load.

Deuterium (3806 g) Aluminum (2280 g)

Radiation 

Source

Rate 

(W/g)

Heat (W) Rate 

(W/g)

Heat (W)

Neutrons 0.0716 777 .0081 4

Beta Particles 4.32 679

Gamma Rays 1.53 1465 1.45 741

Subtotal 2242 1424

Total Cryogenic Heat Load = 3666 W
Conclusion

• An optimized LD2 cold source is needed to recover the 10% loss in neutron flux due to the 

switch to LEU fuel.

• The position and re-entrant cavity geometry were optimized to produce brightness gains 

between 1.3 - 1.5 for the desired range of 4 - 9 Å.

• (8 x 12 x 8) cm Ellipsoid re-entrant cavity consistently performs the best in terms of brightness 

in the 4 - 9 Å range.

Plane View (Top) Plane View (Side)
Future Research

• Cold Source re-entrant cavity has been optimized to within a few minor geometrical variations.

• The size and shape of the moderator chamber warrants further optimization research.

• This model must be adapted to the actual beam guide geometry.

Ellipsoid Sphere Annular 
Acknowledgements

• National Institute of Standards and Technology (NIST)

• NIST Center for Neutron Research (NCNR)

• Mentors: Robert Williams and Zeyun Wu

• NCNR SURF Directors: Julie Borchers and Joseph Dura

• National Science Foundation (NSF) – Center For High Resolution Neutron 

Scattering (CHRNS)