(7) Control of nuclear reactors and reactor self-regulating...

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Nuclear and various energy systems

(7) Control of nuclear reactors and

reactor self-regulating property

Department of Energy and Environmental SystemLaboratory of Nuclear Reactor Engineering

Go ChibaThis slide was originally prepared by Prof. Tsuji.

Critical state

If the number of neutrons (the number of fissionreactions) is practically constant over time, it is called the "critical state".

Temporal flow

Loss by leakage

Loss by Absorption

Period of one cycle

Light water reactor (LWR)：～10-5 secVery short

The period of one cycleTemporal flow

Loss by leakage

Loss by Absorption

Neutron multiplication factor,

： Critical

： Sub-critical

: Super-critical

Another, but consistent definition of

： Critical

： Sub-critical

: Super-critical

Reactivity,

： Critical

： Sub-critical

： Super-critical

Reactor power control

Power increase:

Critical ⇒ Super-critical ⇒ Critical

Power decrease:

Critical ⇒ Sub-critical ⇒ Critical

Reactor power control

ReactorPower

Reactivity

0 (= critical)

Released just after fission reactionsAveraged number： 2.4 (U-235)

Prompt neutrons

Fission products Fission

Prompt Neutrons

Two types of neutrons born from nuclear fissions

Prompt neutron

Fission product

Delayed neutron

Radioactive decayFission Delayed

neutron precursor

Released from radioactive decays of some unstable fission products with time delayProduction yield ( ) of delayed neutrons to total

number of fission neutrons is only 0.64% (U-235)


Delayed Neutrons

Period of one cycle: 2×10-5[sec] (LWR) Total number of cycles during 1 Sec. : 50,000 Neutron multiplication per cycle：

Neutron multiplication after 1 sec. :

After1 second

After10 seconds

1.0001 148 5.2×1021

1.00001 1.65 148

Uncontrollable !

Neutron multiplication

Power control only with prompt neutrons

After one cycle

neutrons

precursors

The relative abundance of the total number of generated neutrons are delayed neutrons (i.e., delayed neutron precursors).

neutrons and precursors

neutrons

If is less than unity, the number of neutrons might decrease for a while.


The number of neutrons in this “fission family” will become zero, but some precursors are generated. These precursors will generate neutrons in future and they produce new fission family.

……Order of seconds


Suppose a situation that delayed neutrons are needed to attain the criticality condition.

A period of “effective” cycle becomes longer(～0.08 sec)

Very important !

Roles of delayed neutrons in reactor dynamics

……

• Period of effective cycle：0.08 sec.• Total number of cycles during 1 sec.：12.5

After 1 second After 10 seconds1.001 1.01 1.131.003 1.04 1.45

PossibleControl of → Power controlThe existence of only 0.6% yield of delayed neutrons enable us to use nuclear power in a controlled way.

In the case where delayed neutrons are needed to attain the criticality condition

Power control with delayed neutrons

Neutron multiplication

It’s time to answer the question 1.

Delayed critical and prompt critical

Suppose that there are several delayed neutron precursors in a critical system.

Critical state


If one of them emits a delayed neutron by decay, can this neutron bring about immediate sustainable fission chain reaction?

Critical state

* This neutron might generate other precursors, but neutron emissions from these precursors require long time, so we ignore them here.


This neutron generates neutrons via one fission cycle, so after cycles, there are

neutrons in this system.

Critical state


This neutron generates 0.993 neutrons via one fission cycle, so after 50,000 cycles, there are

neutrons in this system.

Critical state

=0.007, =50,000


This neutron generates 1.0005 neutrons via one fission cycle, so after 50,000 cycles, there are neutrons in this system.

Super-critical state

=1.0075, =0.007, =50,000


Sub-critical state

“Delayed” critical state

“Prompt” critical state


Sub-critical state


“Prompt” critical stateUncontrollable

Controllable

Reactivity in dollar unit

Reactivity definition:

Its unit is “∆k/k”.

If reactivity is normalized by β, we have another unit of reactivity:

Its unit is “dollar”.


[$]

Sub-critical state


“Prompt” critical stateUncontrollable !

Controllable !

Reactor Power Change

Variations of fuel temperature, moderator temperature, etc

Atomic densities of moderator change.

Relative speed between a neutron and a target nucleus changes.

…

A nuclear reactor is designed so as to suppress reactor power change by various reactivity feedback effects.

Reactivity feedback

Reactor self-regulation

increasedecreasePower

Increase of nuclear fissions

Decrease of nuclear fissions

Increase of fuel/moderator temperatures

Increase of resonance neutron

absorption of uranium-238

Decrease of neutron slowing down property

due to decrease of water density

Examples of reactor self-regulation

Decrease of fuel/moderator temperature

Decrease of resonance neutron absorption of

uranium-238

Increase of neutron slowing down property due

to increase of water density

Examples of reactor self-regulation

Increase of nuclear fissions

Decrease of nuclear fissions

Transient with feedback

Reactivity(=0)

Fuel temperature

Moderator temperature

①Please consider time sequences of these quantities if positive reactivity (smaller than β) is accidentally inserted.

β

Reactor Power

Reactivity(=0)

β

Fuel temperature


Reactor Power


Reactivity(=0)

β

Fuel temperature


Reactor Power


②Please consider time sequences of fuel/moderator temperatures and reactivity if positive reactivity (larger than β) is accidentally inserted.

Reactivity(=0)

②Please consider time sequences of fuel/moderator temperatures and reactivity if positive reactivity (larger than β) is accidentally inserted.

β

Fuel temperature


Reactor Power



Reactivity(=0)

β

Reactor power and fuel temperature rapidly increase.

Fuel temperature


Reactor Power


Reactivity(=0)

β

Due to negative feedback effect of fuel temperature, reactivity begins to decrease and becomes below β.

Fuel temperature


Reactor Power


Reactivity(=0)

β

There are NOT so many precursors in the system, the number of neutrons, i.e., the reactor power, rapidly decreases.

Fuel temperature


Reactor Power


Reactivity(=0)

β

Fuel temperature


Reactor Power


Since the reactor power decreases, the fuel temperature begins to decrease.

Reactivity(=0)

β

Fuel temperature


Reactor Power


Moderator temperature begins to increase with time delay, and it gives also negative feedback effect.

Reactivity(=0)

β

Fuel temperature


Reactor Power

If the initial reactivity is large, the peak fuel temperature is also large.


If the peak fuel temperature reaches the melting point, fuel failure occurs.

“Cold” and “hot” state of nuclear reactor

Control rod

“Hot” full power state “Cold” state (room temperature)

Let us consider a reactor which is critical at hot full power state; if this reactor becomes cold state, is this reactor critical, sub-critical, or super-critical?

Reactivity change during reactor operation

Reactivity decreases with the operation.

Excess reactivity

Positive reactivity at room temperature when reactivity control elements such as control rods are removed from a reactor core.

Reactivity in a operation state

= 0 (critical)= Excess reactivity

– Power defect– Reactivity by control elements

Excess reactivity

Positive reactivity at room temperature when reactivity control elements such as control rods are removed from a reactor core.

Hot, CR insertion

Cold, no CR insertion

Hot, no CR insertion

ρ=0

ExcessReactivity

“Power defect”

“Reactivity by control elements”

Excess reactivity

Operation of Power Plant：

Long term operation without refueling (12 - 24 months)

Addition of a proper amount of excess reactivity at refueling

BWR：～0.25Δk/k

PWR：～0.29Δk/k≫β

Maximum reactivity manipulated in normal power operation

～0.0065

Long-term reactivity controlRefueling Refueling Refueling

Variation of excess reactivity (large →small）

Excess reactivity control along with fuel depletion

Control by soluble poison（PWR)Neutron absorber (boron) is dissolved in coolant.

Control by burnable poison（BWR)Neutron absorber ( boron, Gadolinium ) is contained in

fuels（absorption effect decreases with its depletion).

12～24 months

Example of application of burnup poison (BP)

Operation modes of nuclear power plant

Base Load OperationConstant power operation with a rated powerBasic operation method used at nuclear power

plants in Japan Low fuel cost For keeping thermal stresses of fuel and

cladding as low as possible

Daily Load Following Operation

Load Following Operation


Base Load Operation

Daily Load Following OperationFor meeting power demand variations during a day This is required when a fraction of electric power

generated with nuclear power plants to the total grid demand is large.

Load Following Operation


Base Load Operation

Daily Load Following Operation

Load Following OperationTo temporal power demand variations

Turbine–follow-reactor rule Reactor-follow-turbine rule

Please prepare to answer the question 4.

Methods of reactivity control (1)

Control of neutron absorption• Insertion or withdrawal of absorber :

movable control rod• Addition of poison materials : burnable

poison and soluble poison

Methods of reactivity control (2)Control of neutron generation

・ Insertion/withdrawal of fissionable material

FCA, Fast Critical Assembly at JAEA/Tokai

Methods of reactivity control (3)Control of neutron leakage

• Adjustment of reflector width of insertion/withdrawal of reflector

4S,Super-SafeSmall and Simple reactor of Toshiba

Methods of reactivity control (4)Control of neutron slowing down

• Adjustment of an amount of moderator(spectrum shift control)

(Spectral shift operation in BWR)

At beginning of cycle: Smaller amount of moderator is used and conversion from uranium-238 to plutonium-239 is promoted.

At end of cycle: Larger amount of moderator is used and fissions by plutonium-239 is utilized.

Reactivity Initiated Accident（RIA)Instantaneous addition of reactivity above 1$ (β)

Control rod ejection/drop accident (LWR)

Rapid increase of reactor powerAdiabatic rapid increase of fuel temperature

Failure of nuclear fuels Steam explosionDestruction of reactor core

Prompt critical:criticality is attained only with prompt neutrons

If the fuel temperature becomes larger than its melting point,

RIA (1) SL-1 accident

1961 US navy’s nuclear reactor for trainingCause

A control rod was withdrawn accidentally by hand of operators during maintenance.

RIA (1) SL-1 accident

Damages

Power excursion with a reactor period of 3.5～4 [msec]

Death of 3 operators

Melting of 16 nuclear fuel assemblies located in the core center

Destruction of a nuclear reactor core due to steam explosion

1961 US navy’s nuclear reactor for training

1986 Former Soviet Nuclear power plantGraphite-Moderated and Water-Cooled Nuclear Reactor

Reactor room（Chernobyl-1） Schematic view of nuclear reactor system

reactor building

steam drumwater

steam

water

water

pump

turbine

generator

pressure tube

fuel assembly

graphite blocks

condenser

RIA (2) Chernobyl accident


Cause- The reactor was under unstable state where positive feedback was expected.

- Control rod had a defect that it gives positive reactivity when it is inserted to the core (graphite was attached at the bottom of the control rod).


Damages- Power excursion (power reached 100 times of the rated value.)

- Hydrogen/stream explosions and graphite fire occurred.

- Death of 31 persons due to acute radiation.

Study on RIA

Some knowledge have been obtained from RIA accident study for power reactors.

Power excursion can be suppressed by reactor self-regulating property.

Destructive power would not be produced unless melted nuclear fuels are scattered into water region of the reactor core.

Safety evaluation can be possible through RIA experiments using few fuel rods (NOT full mock-up).

Power excursion experiments using pulse nuclear reactors

NSRR（Trigger-type experimental nuclear reactor）in Japan（JAEA）

ＲＩＡ evaluation guideline for light water nuclear power plants has been established

by Japan Nuclear Safety Commission

(USA, Japan, France, Former Soviet Union）

Study on RIA

Nuclear safety research reactor (NSRR)

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Nuclear reactor specially designed for studying nuclear fuel behavior during RIA

Built at 1975 in JAEA

Features• Power excursion at RIA under

realistic core cooling conditions

• Observation of behavior of nuclear fuel under realistic RIA conditions

• Experiment for spent fuels

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Reactor Type TRIGER-ACPRSwimming pool type annulus coreSteady power & Pulsed power

Thermal Power Steady power operation : 300kWPulse power operation : 23,000MW

Reactor core Active height ～38cmEquivalent diameter ～63cmModerator ZrH and H2O Reflector Graphite and H2O

Fuel rods Type Enriched Uranium–ZrHShape RodDimension ～3.75cmΦ、～65cmLEnrichment 20%The number of fuel assemblies 157Fuel clad SUS304

Control rods Safety rods 2Control rods 6Transient rods 3

Reactor Vessel Swimming pool type Wide 3.6m Depth 4.5m Water Depth H8m

Main design parameters of NSSR

Cut view of NSRR

Specification of NSRR

Core configuration of NSRR

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実験孔

regulating rod

安全棒

transient rodカプセル

試験燃料体

Core Configuration of NSRRouter capsule

inner capsule

test fuel

Test Capsule

test fuelcapsule fuel elements

safety rodexperimental

hole

Power excursion experiment at NSRR

Rapid and large negative reactivity feedback occurs responding to fuel temperature increase.

Realization of pulsed power operation simulating power excursion accident

Nuclear Fuel: Uranium hydride-zirconium alloy

Power excursion experiment at NSRR

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Experiment with addition of reactivity 4.67＄

An Example of Pulse Power Operation of NSSR

Pow

er (M

W)

Cum

ulat

ive

Pow

er(M

W・se

c)

Time (sec)

Cumulative powerPower

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Rapid power increase caused by addition of larger reactivity

Overheating of nuclear fuels

Melting or rupture of fuel clad due to the increase of inner pressure

Destruction of fuel clad due to rapid heat expansion （Pellet and Clad Mechanical Interaction, PCMI）

Subsequent release of radioactive materials (FPs)

Generation of pressure wave caused by melted fuels scattering into water (steam explosion)

pellet

clad

cut view of fuel

Ejection of control rod

destruction by PCMI

rapid increase of reactor power and temperature

Behavior of nuclear fuels during RIA

Reactivity addition : 2.85$

• RIA was simulated at the most severe condition （low power steady state）.

• Fuel temperature reaches the maximum temperature adiabatically within 0.1 Sec.

• Heat energy produced in the adiabatic condition determines subsequent behavior of nuclear fuels.

Reactor power Fuel center temperature

Fuel surface temperature

Rea

ctor

Pow

er (M

W)

Tem

pera

ture

(℃)

Elapsed Time form Control Rod Drop (sec)

Reactivity addition*

*This reactivity is equivalent with the one which is added when a control rod is dropped in BWRs with 8ｘ8 fuel assembly type, rated power of 1100MWe and initial power of 10-6 /the rated power.

Numerical simulation for RIA

• Oxidization of fuel clad

• Fuel failure

Melting of fuel pellets in very large heat release

Increase of fuel clad temperature

反応度投入による被覆管表面温度の測定結果

DNB

Fuel clad temperature after reactivity additionElapsed time from reactivity addition (sec)

Fuel

cla

d su

rfac

e te

mpe

ratu

re (℃

)

TC break

heat value

(cal/g・UO2)

Behavior of nuclear fuel clad

• Scattering of melted fuel into coolant water

• Steam explosion

Safety regulation standard of nuclear fuel against RIA

Hea

t （ca

l/gU

O2) Allowable

Design Limit of Nuclear Fuels

Pressure Difference between outer/inner of Fuel Rod (Kg/cm2)

One standard was established by Japan nuclear safety commission in 1984.

Two standards are simultaneously adopted.

Recommendation was issued by the special committee under the Japan nuclear safety commission in 1998.

Burn-up (MWd/kg)

Hea

t（ca

l/g・fu

el)

Two standards are simultaneously adopted.

PCMI thresholdHea

t（kJ

/g・fu

el) Black: Failed

White: Not-failed

Safety regulation standard of nuclear fuel against RIA

Please see the Youtube video (8:38) :Nuclear Reactor Meltdown: "SL-1 Accident Briefing

Film Report" 1961 Atomic Energy Commission

You can find this by searching “SL-1” and “accident” by Google.

A video “10,000th cycle of the Annular Core Research Reactor” is also recommended.

Your homework

Please submit a short report to the post of N-317. You can describe/discuss ANYTHING related to this video, my lecture, this lecture series and others. Japanese language can be used.

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Materials used in this lecture can be downloaded from the Web site of the nuclear reactor engineering laboratory.

(7) Control of nuclear reactors and reactor self-regulating...

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