Thermal-Hydraulics Feasibility for an

Ultra-Compact Nuclear Reactor Core

Andrew Seamone

2019 SURF Symposium

6 August 2019

Reactor Operations and Engineering
 Operation started in 1967

 Split core

NBSR

2
 Wavelengths

 Energies

 Selectivity

 Magnetism

 Neutrality

 Capture

Why Neutron Research?

3

10 m
NBSR

D2O 

reflector 

H2O

pool

Concept Reactor

Compact 

core

20 MW

“Loose” core

Closed vessel

HEU (U3O8/Al) fuel

30 fuel assemblies

20 MW

Compact core

Open pool

LEU (U-10Mo) fuel

9 fuel assemblies

Replacement Reactor
 Ultra-compact

◦ High Neutron Flux

 Greater accessibility

◦ Maintenance

Replacement Reactor Features

5

Zircaloy

Aluminum

Light Water

Hafnium

Heavy Water

26 cm
Reactor: FRM-II NIST concept OPAL

Crosssectional plan 

view of 

reactor core

Power (MW) 20 MW 20 MW 20 MW

Volume 28 L 41 L 69 L

Peak thermal 

neutron flux 

in reflector

8×1014 

cm-2s-1

5.6×1014

cm-2s-1

4×1014

cm-2s-1

Motivation for Compact Reactor 

Core

6
 Compact core geometries

 Structural robustness

◦ Larger forces acting on fuel elements

 Cooling

◦ High heat flux per fuel element

Replacement Reactor Challenges

7
Structural Stability

8

 Miller velocity

 2/3 of Millers velocity for 

maximum actual flow

 Miller velocity design limit

◦ High flow velocity can cause 

fuel plates to deform

E – Youngs Modulus ρ – Coolant Density

a – Plate Thickness b – Wetted Width

h – Channel Width ν – Poisson’s Ratio Plate deflection with increasing velocity

21 Fuel Plate Cross-Section
 High coolant flow velocity

 First step for optimized design

Structural Stability

9

23 plates

19 plates

21 plates
 Curving the fuel plates offers more structural 

stability

Structural Stability

10
 Failure of the

heated surface may

occur once the CHF

is exceeded

 Design for free

convection phase

Critical Heat Flux (CHF)

11

Fuel Plate Excess Temperature (K)

Heat Flux (W/m

2)
 Low flow and high flow 

design case

 Average heat flux

=1,160 (kW/m2)

Critical Heat Flux (CHF)

12

Low flow

9.9 m/s

High flow 

16.3 m/s

Max Heat Flux

CHFR < 2

CHFR > 2

 Max heat flux

=2,900 (kW/m2)

 CHF Ratio > 2
 Multiphysics solver ◦ Computational fluid dynamics ◦ Heat transfer ◦ Nonisothermal flow  Evaluate ◦ Pressure drop ◦ Temperature increase ◦ Change in flow velocity

COMSOL

13
 2D single element

 Temperature model

COMSOL Modeling

14

Velocity (m/s) Temperature (C)
 3D single fuel element

COMSOL Modeling

15

Velocity (m/s) Pressure (Pa)
 2D Core slice

◦ Flat fuel plates

◦ Bypass flow (7.1%)

◦ Thermodynamics across core

COMSOL Modeling

16

Velocity (m/s) Pressure (Pa)

1 2 3
 COMSOL capabilities can extend to full reactor core

 Structural advantage with curved plates

 Critical heat flux and critical velocity are designed

 Greater neutron flux can be achieved

Conclusion

17

Future Direction

 Run a full core simulation

 Virtual reactor

 Fluid structure interaction
 Danyal Turkoglu

 Daniel Mattes

 Julie Borchers

 Joe Dura

 Reactor Operations and Engineering

Acknowledgements

18
 Mantecón, Javier González. Evaluation of mechanical stability 

of nuclear fuel plates under axial flow conditions. Diss. 

Universidade de São Paulo, 2019.

 Miller, D.R. CRITICAL FLOW VELOCITIES FOR COLLAPSE OF 

REACTOR PARALLEL-PLATE FUEL ASSEMBLIES. United States: 

N. p., 1958.

 Groeneveld, D.C. & Shan, Jianqiang & Vasić, A.Z. & Leung, 

Laurence & Durmayaz, A & Yang, Jun & Cheng, S.C. & Tanase, 

A. (2007). The 2006 CHF look-up table. Nuclear Engineering 

and Design. 2007.02.014. 

Reference

19
 Radius of Curvature of 25 cm increases the 

designed strength

 Factor of 1.7x higher

velocity

 Alpha = 0.1392

 b/a = 58

 Vrf = 1.7

Structural Stability

20
Limited COMSOL Availability

21
Pathway to a new source

 First began looking into a replacement reactor in 2013 

 Several concepts have been investigated in an effort to 

optimize a reactor design for cold neutron science

 A succession plan that minimizes time between 

operation of NBSR and the replacement reactor is ideal

22

NBSR operation

?

Concepts  Specifications

Design, Construction, Testing

NBSR-2 operation { Replacement 

Reactor 

Project

?

?
LEU Fuel Assembly Design

23

NBSR Concept Reactor

Foil thickness 0.0216 cm 0.0250 cm

Foil width 6.134 cm 6.5 cm

Foil height 27.94 cm 70 cm

Foils per FA 34 (17×2) 21

U-235 mass per FA 383 g 726 g

Fresh FAs per cycle 4 3

Cycle length 38.5 d 50 d

Cd wires as 

burnable absorbers

Square profile

(8.05 cm × 8.05 cm) 

allows rotations 

during refueling
Core design

24

Plan view (xy)

Cd wires as 

burnable 

absorbers

Hf shims (×6)

Zircaloy 

follower 

rods (×6)

Zircaloy 

chimney 
Fuel management scheme

25

 3 fresh fuel assemblies per cycle for a 50 d cycle
Power distribution

26

Stripe PPFs at startup Axial power profiles

 Hot spot power peaking factor: 2.13

◦  Maximum power density: 9.3 kW/cm3 × 2.13 = 19.8 kW/cm3

◦  Maximum heat flux: 116 kW/cm3 × 2.13 = 247 W/cm2 

 Heat flux exceeds NUREG-1313 limit for U3Si2 fuel
Fission density distribution

27

Plate #

Fission density versus plate number for 

discharged FA with highest burnup

Fission 

density 

(×1021

f/cm3)

 Potential for high fission 

densities: 6×1021 fissions/cm3
Peak power density and fission density

28

NIST replacement reactor
Cold neutron source performance

29

 High unperturbed thermal neutron flux in the 

reflector: 5.6×1014 cm-2 s-1

◦ More than a factor of 2 greater than NBSR

 Opportunity to optimize cold source designs and 

locations for neutron science

◦ Large gains (>2) 

in cold source brightness 

over NBSR are possible para-LH2 CNS

LD2 CNS

NBSR LD2 CNS

para-LH2 CNS LD2 CNS Cold 

flux