Nuclear Design and Analysis of the 

NIST Replacement Reactor

Patrick Park

pjp2136@columbia.edu

SURF Colloquium 2021
What this talk will cover:

Introduce Replacement Reactor project

Discuss series of standard reactor physics calculations that characterize reactor performance

Talk Overview

Calculation Significance

Control blade calibration How does reactor power change as control blades are withdrawn?

Control blade heat expansion How does the heat from reactor power affect control blade physical properties?

Moderator dump In an emergency, how much heavy water must be dumped for the reactor to shutdown?

Reactivity coefficients How does heat affect reactor power over time?

Goal of talk:

Not for everyone to understand all the technical details, but to learn why and how we calculate certain reactor 

parameters to determine performance and safety.
• Research reactors produce neutrons for unique experiments, e.g.

• Measure element compositions

• Examine stress fractures at atomic levels

• Map behaviors of very strong magnets

• Our current source of neutrons (NBSR) has been active since 1967--

over 50 years!

• A new neutron source (NNS) allows

• Easier maintenance

• More operating time

• More instruments

• = More experiments!

Need for a New Neutron Source

National Bureau of Standards Reactor 

(NBSR)
Proposal New Neutron Source (NNS) Site

Beam Tubes

NNS

Piping

Instrument

Area

Instrument

Area

Proposal NNS
Proposal NNS

Primary System and Core
Proposal NNS

• Updated cold neutron sources

• More beam tubes

• More room for new 

instruments

Cold Neutron Reactor Core

Source (CNS)

Beam Tubes

Light Water Tank
Basic Core Characteristics

• 9 Fuel Assemblies (U-10Mo)

• Produce neutrons through fission

• Curved plates for structural robustness

• 6 Control Blades (hafnium)

• Control rate of fission by absorbing 

neutrons when blade is inserted in core

Reactor Core

Control Blade

Fuel Assembly

30 

cm

30 cm

70 cm tall
NNS

• 20 MW

• H2O coolant (Lower cost)

• Open pool (Easier maintenance)

• LEU fuel (New requirement)

• 9 fuel assemblies

• 50 day cycle (More op time)

• 2 instrument halls

NBSR and NNS Comparison

NBSR

• 20 MW

• D2O coolant

• Closed vessel

• HEU fuel

• 30 fuel assemblies

• 38.5 day cycle

• 1 instrument hall
NEUTRON INTERACTIONS

U-235 fission produces fission products, energy, and 2 neutrons.

Biff the Trainee says...

FISSION

Fission is a process in which a nucleus splits into two smaller nuclei. Nuclear reactors use 

induced fission, where neutrons bombard a heavy radioisotope, like U-235, producing an 

unstable, compound nucleus that then fissions. 

About 2.4 neutrons and 200 MeV are produced per fission event.

Fission is a probabilistic event, where there exists a non-unitary probability of inducing fission.

Spontaneous fission events also occur, where a heavy radioisotope fissions without an incident 

particle but in negligible amounts.
NEUTRON INTERACTIONS

Fission requires thermal neutrons but produces fast neutrons.

Biff the Trainee says...

NEUTRON ENERGY SPECTRUM AT 20 C

CLASSIFICATION ENERGY (eV)

cold 0.0-0.025

thermal 0.025

epithermal 0.025-0.4

intermediate 0.4-1 MeV

fast 1-20 MeV Fissioning 

uranium releases 

fast neutrons

Uranium needs 

thermal neutrons 

to fission To sustain fission, 

these fast neutrons 

need to be slowed 

down to thermal 

energies.

The kinetic energy of a particle is 

proportional to the temperature of its 

surroundings. Thermal neutrons are called 

“thermal” because they are in equilibrium 

with the thermal motion of the surroundings. 

So, these energies are relative to surrounding 

temperature.
What is a Control Blade?

Six control blades (70 cm tall, 7 cm wide, 0.7 cm thick) made of halfnium plates control reactor power. 

When they are fully inserted, the reactor is shutdown. When fully withdrawn– full power.

Control Blade Calibration
Control Blade Calibration

What is a Control Blade Calibration?

Essentially, measure how much reactor power 

changes from blade movement

Observations

• Control blades in the middle of the core are 

“worth” more because they affect a larger 

amount of neutron flux
get elevation view

Reflector Dump

• Reflector (NNS: heavy water) bounces neutrons back into the core, reducing neutron leakage

• Should be non-fissionable and have high scattering/low absorption cross sections

• Evenly-distributed flux allows a higher average power and more predictable temps and burnup

Higher avg power with reflector to contain neutrons

Reflector tank of pure heavy water
Reflector Dump

Observations

• Reactor is most reactive at startup (SU)

• At least 35% of the heavy water reflector must 

be dumped for the reactor to be subcritical

Reflector with 35% 

dumped by noble gas, 

elevation view
Reactivity Coefficients

Types of Reactivity Coefficients How is reactor power affected when...

Moderator temperature coefficient When fission heats up light water moderator

Void coefficient If air gets mixed in with light water moderator

Fuel temperature coefficient When fuel elements heat up

Mixing coefficient If the heavy water reflector and light water moderator accidentally mix
MODERATOR

Our water moderator slows down fast fission neutrons to 

thermal neutrons, which allows us to sustain fission. 

Biff the Trainee says...

Fission neutrons are produced with energies of about 2 MeV.

Thermalization is the process of reducing the energy of a neutron to the thermal region 

(~0.025 eV at 20 C).

A moderator is used to thermalize neutrons.

A good moderator reduces neutron speeds in as few collisions as possible without absorbing 

them. This requires a large scattering and small absorption cross sections with large energy loss 

per collision. In most fission reactors, a good reflector is a good moderator.

The energy absorbed by a moderator (ξ) is measured in units of lethargy (u), where: 

ξ = - ln (Ei

/Ef)

Our moderator reduces 2 MeV fast neutrons to 0.025 eV thermal neutrons by absorbing 

99.999999% of the initial energy. By the formula, this is a lethargy of 18.4 u.
Moderator Temperature Coefficient

Mechanism

• Water heats up due to thermal energy from 

fission

• Temperature goes up, density goes down

• Lower density = less effective water molecules 

in given volume

• Less water molecules = less neutrons causing 

fission

Observations

• Different cycle states are differently affected by 

changes in moderator temperature (startup vs. 

end-of-cycle)

• Moderator temperature coefficient is constant 

over all temperatures
Void Coefficient

Observations

• Calculates reactor power as a function of 

constant temperature but decreasing density

• Might be an issue if water is boiled, equipment 

added to pool, or new beam lines are added (all 

reducing effective water density)

• 0% void = pure water, 100% void = pure air

Observations

• Different cycle states are differently affected by 

changes in void (startup vs. end-of-cycle)

• Not expected to be a problem for the NNS
What we’ve learned:

Summary

Calculation Conclusion

control blade calibration Helps us predict how much reactor power changes as we move control blades

control blade heat expansion How does the heat from reactor power affect control blade physical properties?

moderator dump In an emergency, about 35% of our heavy water needs to be dumped for a shutdown

moderator temperature 

coefficient

As light water heats up during operation, it becomes less effective at facilitating fission

void coefficient Any bubbles or air pockets in the core significantly decreases reactor power

Overall takeaways:

• Plans for a new reactor are underway (on House of Rep. floor for funding soon)

• New reactor will have more instruments, operate longer, be cheaper to maintain long-term

• Series of nuclear calculations help predict reactor performance and safety