Safety consideration in HTR-PM

Fu LI

INET, Tsinghua University, Chinalifu@tsinghua.edu.cn

VIC, Vienna, AustriaJuly 10-12,2012

IAEA Technical Meeting on Re-evaluation of Maximum Operating Temperatures and Accident Conditions for High

Temperature Reactor Fuel and Structural Materials

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Outline

1 Safety philosophy 2 Limiting conditions 3 Current implementation 4 Uncertainty analysis 5 Other topics

1 Safety philosophy

Based on current licensing framework Basically for LWR Ensure the integrity of primary circuit, or

flooding of the core With help of engineered safety features

Safety functions: Control of Power—reactivity Decay heat removal Retention of FP

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1 Safety philosophy

Basic philosophy Defense in depth - DiD

Multiple barriers First layer: TRISO --most important in HTGR Second layer: primary circuit Third layer: confinement/containment

Most important for LWR Forth layer,…: ESF, EOP,…

Large time span for accident treatment and slow development of accident is another layer

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1 Safety philosophy

Basic philosophy ALARA

As low as reasonable achievable

Safety class systems plus non-safety class systems Such as helium purification system, sub-

atmosphere ventilation system,… To reduce the radioactive release to environment

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1 Safety philosophy

Experiences Adoption of mature technology

Feedback from HTR-10 Based on steam cycle Test of systems/components before

installation Research on advanced technology

Gas turbine Hydrogen production Advanced fuel …

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2 Limiting conditions

Limiting conditions for each operation conditions Normal operation (NO) Anticipated operation occurrence (AOO) Design basis accident (DBA) Beyond design basis accident (BDBA)

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2 Limiting conditions

Types of limiting conditions Fuel temperature Component temperature

For core internal For primary circuit boundary, supporting for

RPV For structure material

CR, blower, SG tube For component in cavity

Cavity cooling system Cavity concrete

For auxiliary system 8

2 Limiting conditions

Temperature limit for fuel Normal operation: 1200C Accident: 1620 C

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2 Limiting conditions

Temperature Limit for structure

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Components Operation temp(C) Accident temp（24h）(C)

RPV 350 425

Supporting for RPV 350 425

Hot gas duct 800 1000

Hot gas duct vessel 350 600

Core barrel 375 500

Cavity concrete 70 100

2 Limiting conditions

Comments: Fuel temperature:

Safety related FP release

Large margin Long time span, small percentage of fuel

Structure temperature: Investment related?

For LWR, safety related, for the decay heat removal and fuel cooling

Based on good TRISO fuel performance Very low failure rate

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3 Current implementation

Most of safety features in HTR-PM are similar to HTR-MODUL The system arrangement, the parameters

are similar HTR-MODUL was reviewed by TUV

Most of the components are similar to those in HTR-10

Safety features of modular HTR is well recognized

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3 Current implementation

Most of the components will be tested in advance, in full scale Control rod, small absorber ball system,

fuel handling system, blower, steam generator,

More experiments are undergoing Fuel irradiation Pebble flow, pebble bed heat conduction

coefficient, graphite dust, hot gas mixture, natural circulation of cavity cooling system 13

3 Current implementation

The preliminary safety analysis report (PSAR) of HTR-PM was reviewed by NNSA, and was accepted in Sept 2009 Structure and scope: SRP Classification of operation conditions Criteria for each operation conditions Main results

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3 Current implementation

SRP: Proposed by NRC 20 chapters

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3 Current implementation

Operation conditions: NO, AOO(10-2), DBA(10-4 ,10-6), BDBA Criteria:

Operation of the unit, damage to the unit, radioactive release to the environment

NO, AOO, DBA: systems of non-safety class will not be assumed to be available

Borrowed from LWR DBDA was analyzed

Through PSA by some special topic reports

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3 Current implementation

Interested accidents: DLOFC

Most challenge for fuel temperature For capability of cavity cooling system For temperature limit for core barrel and RPV

PLOFC For capability of cavity cooling system For temperature limit for core barrel and RPV

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3 Current implementation

Interested accidents: Water ingress

For set point of primary circuit safety valve For FP release

FP deposition in SG Additional FP release from failed coated particles

Requirement for the sub-atmosphere ventilation system No requirement for safety class

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3 Current implementation

Interested accidents: ATWS (BDBA)

Time for re-criticality Requirement for the small absorber ball system No safety impact

Continuous operation of blower (BDBA) Fail to trip For the temperature of primary circuit

boundary For diversity actuation system, to trip blower

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3 Current implementation

Interested accidents: Failure of cavity cooling system (BDBA)

No effect on max fuel temperature Challenge for RPV temperature

Manual depressurization of primary circuit For concrete temperature

With/without concrete cooling system Additional supporting cooling system

Additional water supply from fire fighting system (accident management)

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3 Current implementation

Interested accidents: Air ingress accident (BDBA)

Configuration of break in primary circuit No big challenge for fuel temperature Experiment data for particle failure under air

condition (German data and Japan data) Fuel performance under normal condition and

oxidation condition Difference data from difference countries

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3 Current implementation

Interested accidents: Rod ejection accident (BDBA)

Not included in PSAR How fast of the control rod ejection Fuel temperature is not a problem Criteria for max power of kernel

250mW/Particle? More for short period

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3 Current implementation

Interested accidents: Failure of hot gas duct vessel (BDBA)

Not included in PSAR DiD

Vessel concept(basic safety) Pipe concept with LBB Movement restrain Air ingress Accident management

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3 Current implementation

Other topics Vented lower pressure containment

(VLPC) Burst release system Sub-atmosphere ventilation system Filtering system

Based on function requirement Physics protection for reactor ALARA

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3 Current implementation

Other topics Cavity cooling system

Safety class 3 redundant trains

Natural circulation Residual heat removal system? Just investment protection related?

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3 Current implementation

Other topics Non-safety sub-atmosphere ventilation

system With filtering system: for aerosol, for Iodine For normal pressure

Ventilation for small break Burst release for large break

Coincide with delayed release phenomenon of coated particles, lower failure rate of coated particles, permeation and non-condensation of helium

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3 Current implementation

Other topics Acceptance criteria

NO: AOO:

0.25mSv/reactor year DBA:

5mSv/accident 10mSv/accident

BDBA: 50mSv, 10-6/reactor year Through PSA

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3 Current implementation

Other topics For fuel temperature

Margin for normal operation AVR experience Difference for modern modular design

Requirement for cooling after reactor trip Main heat transfer system, helium purification

system, cavity cooling system Margin for accident

1620C Just few percent of fuel

Uncertainty analysis?28

3 Current implementation

Other topics For fuel temperature

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200

300

400

500

600

700

800

900

1000

0 200

400 600

800 1000

1200

温度 (℃)

轴向

坐标

(cm)

冷却剂

温度

燃料表

面温

度

燃料中

心温

度

颗粒中

心温

度

3 Current implementation

Other topics Reactor inlet /outlet temperature

IAEA TECDOC 1674 HTR-PM: 250/750C ANTARES: 400/850C GT-MHR: 490/850C GTHTR300: 587/850C HTR-MODUL: 250/700C PBMR: 500/900C

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3 Current implementation

Other topics For normal/accident fuel temperature

IAEA TECDOC 1674 HTR-PM: 930/1620C ANTARES: GT-MHR: 1320C /1600C GTHTR300/300C: 1400/1600C HTR-MODUL: 837 /1620C PBMR-400: 1080/1570C

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3 Current implementation

Other topics For normal/accident RPV temperature

IAEA TECDOC 1674 HTR-PM: SA508, 250/350C ANTARES: Mod9Cr, GT-MHR: SA508/SA533, 440C / GTHTR300/300C: SA533,140C helium/ ? HTR-MODUL: PBMR:

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3 Current implementation

Other topics Structure temperature for control rod

Under DLOFC accident INCOLOY 800 Investment protection related?

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4 Uncertainty analysis

To evaluate the max temperature of fuel and structure material under normal and accident condition Conservative assumption + conservative

analysis Best estimation + uncertainty analysis

More reasonable

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4 Uncertainty analysis

Uncertainty on Fuel temperature

For normal operation, for accident Large margin for normal accident, Less margin for accident (DLFOC accident)

Real challenge?

Structure accident For normal operation, for accident

Challenge condition: DLOFC with failure of cavity cooling system

Not real safety related?

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4 Uncertainty analysis

Uncertainty sources Nuclear cross section Other material property, correlations

Thermal conduction coefficient, pebble bed Manufacturing uncertainty Model error, numerical error

Few group, diffusion, FD, semi-equilibrium Operation history deviated from design

conditions Pebble bed movement

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4 Uncertainty analysis

Pebble bed movement Random in microscopic Random in macroscopic Error in model

Such as model in VSOP

Effect from multiple pass mode Will reduce uncertainty?

Need quantitive evaluation

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4 Uncertainty analysis

Experience from melt-wire experience in AVR Bypass Pebble flow model

2-zone fuelling Capability of code

2D- 3D Many kinds of fuel Complicated operation history

Not exact samples for modern design38

5 Other topics

Future options for V/HTR Super critical steam cycle Process heat application

Co-generation Gas turbine

Silver release

Requirements Increase of inlet temperature Increase of outlet temperature

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5 Other topics

Challenges Higher fuel temperature

For normal operation, for accident ZrC coating? UO2*?

Structure material temperature For RPV

Active cooling, 9Cr-1Mo For Component: IHX

Inconel 617? ODS?

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Conclusion remarks

HTR-PM PSAR is finished already Based on current LWR licensing

framework Maximum temperature on fuel and

structure material for normal and accident condition is very important for HTR Max fuel temperature is more important Most challenging condition is DLOFC,

plus failure of cavity cooling accident41

Comments

Assessment of AVR experiment is very important Melt-wire experiment Features of AVR, or features for all

pebble bed HTR, or features for all HTGR?

Operation on 950/1100C outlet temperature proven the robustness of pebble bed HTR?

Positive of negative lessons are learned?42

Comments

FP release, instead of fuel temperature, is the real concern For normal operation, monitoring of FP

inside primary circuit is a good indicator In helium, in graphite dust

For accident condition, qualification of fuel performance (irradiation & annealing experiment) is important

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Comments

Based on good performance of modern TRISO fuel, temperature on structure material is mainly investment protection related, not safety related. This is the fundamental difference

compared with LWR, where keeping of coolant is most important (to prevent the core melt)

This may be the key factor for future development of HTGR 44

Comments

One of important way to re-evaluate the max temperature of fuel and structure material is uncertainty analysis Uncertainty aroused from pebble flow is

an interested topic Multiple pass model will reduce the

uncertainty?

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Comments

According to basic safety function (power control, decay heat removal, FP retention), the performance of FP retention in TRISO particle is the key issue, it will relief the requirement on max temperature of fuel and structure material Based on the Defense-in-Depth and

ALARA principles, control of fuel temperature and structure temperature are also very important 46

Comments

In order to evaluate the max temperature of fuel and structure material, another methodology is risk informed analysis, probability safety analysis (PSA) Which risk? What level of probability?

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