Safety Issues for High Temperature Gas Reactors

Andrew C. KadakProfessor of the Practice

Major Questions That NeedGood Technical Answers

• Fuel Performance– Normal operational performance– Transient performance

• Ejected Rod (maximum energy insertion capability)• Reactivity insertions (seismic, water)

– Accident Performance– Weak fuel issues– Mechanistic source term for high burn-up fuel– Fuel fabrication quality assurance

• Risk Dominant Accident Sequences– Establish risk informed design to identify risk

dominant accident sequences to be analyzed.– Use either IAEA1 or NRC2 risk informed

approach to establish safety requirements of plant.

– Use of safety goal as a design guide– Application of risk informed “Defense in

Depth”– Scope of risk analysis may be easier due to

inherent robustness of basic design.1. “Development of Technology Neutral Safety Requirements for Innovative Reactors”, IAEA

TECDOC Draft Dec. 20042. “Regulatory Structure for New Plant Licensing, Part 1: Technology Neutral Framework, Dec.

2004, Draft, US NRC.

Consequences

FInitiating Event/Y

Lev. 1Lev. 2

Lev. 3

Lev. 4

Lev. 5

10-2

10-6

10-7

Failure of Level 1Initiating Event

Minimal emergencyactions beyonddefined distancefrom the plant

No off-site actionsbeyond defineddistance from theplant

Off-site Actions

NOTE 2: Dosesare derived fromIAEA-SS No 115

5 mSv/a(For 1 year periodfollowing the accident)

5 mSv/a(For 1 year periodfollowing the accident)

5 mSv/a1 mSv/a (average 5 y)

1 mSv/a(10 µ Sv/a–target)

Doses to thepublic

NOTE 1: Dosesfor NO, AOO, ACare derived fromIAEA-SS No 115

500 mSv (limit)(This value derivedfrom Finnishregulation)

50mSv/a (Could beexceeded for rearrecovery events)

50 mSv/a20 mSv/a (average 5 y)(5 mSv/a target)

50 mSv/aALARA(5 mSv/a target)

Doses toOperators

Severe plantconditions* (SSPC)

Accident conditions(AC)

AnticipatedOperationalOccurrences (AOO)

NormalOperations(NO)

Plant conditions

Consequences

AOO

AC

SPC

Challenges

DESIGN BASIS

* Severe challenge to the Fission Products Confinement Function

Risk Informed Safety Profile

LEVELS OF DEFENCE IN DEPTH (From INSAG-10)

Control, limiting and protection systemsLevel 2 Control of deviations from normal operation and detection of failures and other surveillance features

ObjectiveLevels ofdefence

Essential means

Level 1 Prevention of deviations from normal operation and failures

Conservative design and high quality inconstruction and operation

Level 3 Control of accident conditionswithin the design basis

Engineered safety features and accidentprocedures

Level 4 Control of severe plant conditions Complementary measures and accidentmanagement

Level 5 Mitigation of radiological consequencesof significant releases of radioactivematerials

Off-site emergency response

Acceptablefailures of the

Level of Defence(events/year)

< 10-2

10-2- 10-6

10-6- 10-7

< 10-7

• Expected Significant Accident Sequences– Air Ingress– Water Ingress (reactivity insertion)– Seismic Events (reactivity insertion)– Loss of Load– Rod Ejection (more significant in block reactors)– Failure of reactor cavity cooling system– Recuperator By-pass events (overcooling)– Graphite dust, plate-out, lift off– Impact of Terrorism– Identification of “cliff edge” effects

Knowledge Required• Improved understanding of core behavior • Improved understanding of heat transfer in core and vessel

- pebble and block - bypass flows• Materials behavior at high temperature in helium (plus

contaminants) including radiation effects and chemical attack on graphite

• Blow down loads and timing of accident event sequences.• Behavior of fuel, fission product release behavior in

reactor building and structures under accident conditions.• Development and validation of computer codes used in the

analysis• Validation of passive performance of safety systems -

natural circulation - heat conduction and convection.

Issues

• Fuel Temperature limits (1600 C ?)• Regulatory Credit for Basic Design

Strengths • Need new risk informed licensing process

to allow credit for innovative systems.

Containment • Based on design and accident analysis of source

term and sequences - a containment of radioactive materials strategy is developed to assure that safety goals are met.– Full pressure containment– Confinement - low pressure - not pressure tight– Dynamic containment/confinement (time

dependent)– Performance is quite different than water

reactors.

Classification of Safety “Systems”

• Ideally safety system classification should be done on importance to safety function in a risk informed manner.

• Some “systems” are not components but parameters in analysis for passive performance (ex. emissivity of reactor vessel).

Expectations• Water Ingress - generally understood and can be

limited by amount of water ingress - some German experience at AVR

• Seismic - reactivity simulations can assess reactivity impact.

• Rod ejection - more significant for block reactors but fuel energy limits like for LWRs can be established for rod worths.

• Testing on heat transfer and flow can be verified by South African tests and Chinese pebble bed reactor including reactor cavity cooling systems.

• Fuel behavior data to be provided by past German and focused South African and US testing programs

Challenges• Verification of high temperature material

behavior (fuel, graphite, metals, carbides)• Validation of analysis tools• Air ingress

– Most visible concern among the public– Most significant in terms of potential offsite

consequences– Can not be eliminated by “design”

Air Ingress Status• Most “eliminate” connecting “vessel”

failure as too low a probability event (10-8).• Break sizes limited to largest connecting

“pipe”.• Two breaks (top and bottom) considered

unlikely but are analyzed (chimney effect)• Graphite corrosion behavior not well

modeled in existing codes.• CFD analysis and confirmatory experiments

needed.

Air Ingress Tests

• Japanese series on prismatic configuration– Diffusion– Natural Circulation– Corrosion (multi-component)

• German NACOK tests - pebble bed– Natural circulation– Corrosion

• MIT CFD (Fluent Methodology Development)

Experimental Apparatus - Japanese

C4

270

Nitrogen

Helium

Valves

C3

C1

C2

H4

H3

H2

H1

Figure 16: Apparatus for Isothermal andNon-Isothermal experiments

Figure 17: Structured mesh

Isothermal Experiment

0.00

0.20

0.40

0.60

0.80

0 50 100 150 200 250 300Time (min)

Mol

e fra

ctio

n

H-1 & C-1(Calculation)H-2 & C2 (Calculation)H-3 & C3 (Calculation)H-4 & C4 (Calculation)H-1 & C-1(Experiment)H-2 & C2 (Experiment)H-3 & C3 (Experiment)H-4 & C4 (Experiment)

Figure 18: Mole fraction of N2 for the isothermal experiment

Thermal Experiment

Figure 19: The contour of the temperature bound4ary condition

Pure Helium in top pipe,

pure Nitrogen in the

bottom tank

N2 Mole fractions are

monitored in 8 points• Hot leg heated• Diffusion Coefficients as a

function of temperature

Thermal Experiment

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Time (min)

Mol

e fra

ctio

n of

N2

H-1(FLUENT)C-1(FLUENT)H-1(Experiment)C-1(Experiment)

Figure 20: Comparison of mole fraction of N2 at Positions H-1 and C-1

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200

Time(min)

Mol

e Fr

actio

n

H2(Experiment)C2(Experiment)H-2(FLUENT)C-2(FLUENT)

Figure 21: Comparison of mole fraction of N2 at Positions H-2 and C-2

Thermal Experiment (Cont.)

0

0.2

0.4

0.6

0.8

1

0 50 100 150 200 250

Time(min)

Mol

e Fr

actio

n of

N2

H4(Exp)C4(Exp)

H-4(Calc)C-4(Calc)

Figure 22: Comparison of mole fraction of N2 at Positions H-1 and C-1

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0 2 4 6

Time (Second)

Velo

city

(m/s

econ

d)

Figure 23: The vibration after the opening of the valves.

Multi-Component Experiment

2

1

3

4

Heated Graphite

Air

Helium

Graphite InsertedMultiple gases: O2, CO, CO2, N2, He, H2OMole fraction at 3 points are measuredMuch higher calculation requirementsDiffusion Coefficients

Figure 34: Apparatus for multi-Component experiment of JAERI

Multi-Component Experiment(Cont.)

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0 20 40 60 80 100 120 140

Time(min)

Mol

e Fr

actio

n

O2(Experiment)O2(Calculation)CO(Experiment)CO(Calculation)CO2(Experiment)CO2(Calculation)

Figure 36: Mole Fraction at Point-1 (80% Diffusion Coff.)

Multi-Component Experiment(Cont.)

Figure 37: Mole Fraction at Point-3

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0 20 40 60 80 100 120 140Time(min)

Mol

e Fr

actio

n

O2(Experiment)O2(Calculation)CO(Experiment)CO(Calculation)

CO2(Experiment)CO2(Calculation)

Multi-Component Experiment(Cont.)

Figure 38: Mole Fraction at Point-4

0.00

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100 120 140Time (min)

Mol

e Fr

actio

n

O2(Experiment)O2(Calculation)CO(Experiment)CO(Calculation)CO2(Experiment)CO2(Calculation)

NACOK Natural Convection Experiments

Figure 39: NACOK Experiment

Boundary Conditions

Figure 41: Temperature Profile for one experiment

The Mass Flow Rates

Figure 42: Mass Flow Rates for the NACOK Experiment

0.0E+00

1.0E-03

2.0E-03

3.0E-03

4.0E-03

100 300 500 700 900 1100

Temperature of the Pebble Bed (C)

Mas

s Fl

ow R

ate

(kg/

s

5.0E-03

)

T_R=200 DC(Exp.)T_R=400 DC(Exp.)T_R=600 DC(Exp.)T_R=800 DC(Exp.)T_R=200 DC(FLUENT)T_R=400 DC(FLUENT)T_R=600 DC(FLUENT)T_R=800 DC(FLUENT)

Future NACOK Tests

• Blind Benchmark using MIT methodology to reproduce recent tests.

• Update models• Expectation to have a validated model to be

used with system codes such as RELAP and INL Melcor.

Air Ingress Mitigation

• Air ingress mitigation strategies need to be developed– Realistic understanding of failures and repairs– Must be integrated with “containment” strategy

to limit air ingress– Short and long term solution needed

Overall Safety Performance Demonstration and Validation

• China’s HTR-10 provides an excellent test bed for validation of fundamentals of reactor performance and safety.

• Japan’s HTTR provides a similar platform for block reactors.

• Germany’s NACOK facility vital for understanding of air ingress events for both types.

• PBMR’s Helium Test Facility, Heat Transfer Test Facility, Fuel Irradiation Tests, PCU Test Model.

• Needed - open sharing of important technical details to allow for validation and common understanding.

Chinese HTR-10 Safety Demonstration

• Loss of flow test– Shut off circulator– Restrict Control Rods from Shutting down

reactor– Isolate Steam Generator - no direct core heat

removal only but vessel conduction to reactor cavity

Video of Similar Test

Loss of Cooling Test

Power

Loss of Cooling Test

Power

Summary• Safety advantages of High Temperature

Reactors are a significant advantage.• Air ingress most challenging to address• Fuel performance needs to be demonstrated in

operational, transient and accident conditions.• Validation of analysis codes is important• Materials issues may limit maximum operating

temperatures and lifetimes of some components.

• International cooperation is essential on key safety issues.