Fuel Safety Criteria - Nucleus...Limits and margin Design criteria AOO fuel safety criteria LOCA fuel safety criteria RIA fuel safety criteria 2013-12-13 2 Page 3 International Atomic

2013-12-13

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Fuel Safety Criteria

International Atomic Energy AgencyPage 2

Outline

� Limits and margin

� Design criteria

� AOO fuel safety criteria

� LOCA fuel safety criteria

� RIA fuel safety criteria

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Limits and Margins

� Layers are intended to

protect the ultimate

limit

� Operator “owns”

operating and design

margin

� Regulator “owns”

analytical margin and

safety limits


Safety vs. Operational Criteria

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Applicable Safety Standards

Compliance with Safety Standards intended to maintain

plant operation within boundaries of safety analysis


Design Criteria

“Design criteria are primarily intended to increase fuel

reliability and performance”

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CRUD

� “Chalk River Unidentified Deposit” or CRUD• Primarily Nickel and Iron that concentrate and combine on top of

hotspots on the fuel

� No quantitative limits exist, but CRUD is known to be

a contributor to fuel leakage• River Bend NPP in the US during Cycle 8 and 11


Cladding Dimensional Changes

� Fast neutrons cause cladding to elongate during

prolonged operation

� Design must accommodate this phenomena or rod

bowing and failure will occur

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Peaking Factors

• Control radial loading and burnup

• Design control rod patterns and fuel performance to control axial offset


Stress, strain and fatigue

� Typically apply the “1% strain” criterion• Tangential strains due to long term operation

� Swelling, fission gas buildup, etc.

• Short term strain caused by PCMI and PCI events

� Operational limits are intended to limit local power increases so generally not very restrictive

� Fatigue limits established by vendor analyses• Fatigue cycles typically much lower than design values

• Algorithms typically based on vendor proprietary information

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Oxidation and hydriding

� Limits established to improve fuel performance and

ensure compliance with cladding strength and

ductility requirements

� Zircaloy-4 mainly replaced with low Tin Niobium

based Zirconium alloys• Superior resistance to hydriding and oxidation

� PWRs can add hydrazine to scavenge oxygen from

coolant• Not proven to enhance performance

� Some countries have established oxide layer thickness

limits• Typically on the order of 100 microns


Anticipated Operational

Occurrence Fuel Safety Criteria

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Maintain Cladding Integrity

� Needed to ensure integrity of first fission product

barrier

� No such thing as a “cladding integrity instrument”

� Need to develop a surrogate value• Must be able to linked to measurable quantities

• Have sufficient margin to account for uncertainties

� Criteria developed to avoid boiling transitions• Implementation differs by fuel type, vendor and country


What is Boiling Transition

� Also known as “boiling crisis”

� Refers to the “transition” portion on the boiling curve

between nucleate and film boiling• Associated with a dramatic drop in heat transfer coefficient

• Rapid increase in cladding surface temperature

� Significant increase in heat removal needed to return

to nucleate boiling

� Values generally derived from AOO Analyses

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Boiling Crisis and limits (CHF, DNBR)

� the detachment of bubbles, formation of “vapour columns” and “patches”, tending to oppose to liquid flow to the heater;

� thermal crisis starts developing, CHF, beyond which heat transfer rapidly deteriorates, “departure from nucleate boiling” (DNB)

� the phenomenon occurs as a hydro dynamical instability

Boiling crisis in pool boiling

IB – CHF �� from PARTIAL to

FULLY developed nucleate boiling

CHF – B �� sudden increase in

heater temperature that may lead to

its “burn-out”

MFB – A �� decrease of the heater

temperature caused by its sudden

rewetting (“quenching”)


Implementation of CHF/DNB Limits

� Derived from measurable values:• Core inlet and outlet temperatures

• Core flow

• Power (both ex-core and in-core)

� Complexity of physics captured in empirical correlations derived from prototypical testing• Hundreds of correlations

• Vendor proprietary - GEXL, ANFB, WRB

• Open source - Tong (W-3), Levitan

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Criticality / Shutdown Margin

� Directly derived from one of the three critical safety

functions

� Implemented as a reactivity margin to maintain

assuming most reactive rod stuck out of the core• Accounts for SL-1 experience

• Technical specification values typically 0.3 – 0.5 ΔK/K

� Calculated at Hot Zero Power conditions

� Limiting value derived from Main Steam Line Break

Analyses


Typical MSLB Transient

Core Average Density Total Reactivity

Source: “PRESSURISED WATER REACTOR MAIN STEAM LINE BREAK (MSLB) BENCHMARK,

Volume III, NEA/NSC/DOC(2002)12

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Linear Heat Generation Rate

� Quantified as Power per axial dimension (W/cm or kW/ft)

� Usually related to maximum or “hotspot” value

� Typically quantified from DBA analysis• Ensures that plant operates

within boundaries of safety analyses

� Not directly measurable• Inferred from ex-core or in-

core instruments


Peak Centerline Temperature

� Designed to prevent fuel rod melting

� Typically derived from normal operations and AOO

conditions

� Intended to protect cladding from failure because

molten UO2 has 13 % additional volume

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Pellet Cladding Mechanical Interaction

� Refers to stress and strain on cladding from pellet

expansion

� Two main causes:• Long term operation leading to pellet growth

• Transient power increases

� Burnup limits control pellet growth

� Power ramp limits control transient effects• Derived from vendor proprietary correlations


Reactivity Coefficients

� Introduced to simplify analysis of feedback mechanisms• Fuel temperature

• Moderator temperature

• Steam volume (or void fraction)

• System pressure

• Boron concentration

� Requirements for either moderator temperature or total reactivity coefficient to be negative

� Fuel temperature coefficient generally insensitive to burnup• Enrichment and burnup effects offseting

� BWR void coefficient very large and negative

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Shutdown Margin

� Ensures the sufficient negative reactivity always

available from control rods• PWR control rods achieve hot shutdown

• BWR control rods needed for cold shutdown

� Technical specification limits typically 0.3 – 0.5 % Δk/k• 1 % Δk/k design limits are typical

� Verified at least once per cycle

� Core designs such as the use of MOX or high burnup

fuel negatively effects SDM• Offset the introduction of:

� More control rods (or higher worth)

� The use of enriched boron


Internal Gas Pressure

� Typically limiting parameter for fuel mechanical

performance

� Increased rod internal pressure leads to:• Increased stress on cladding

• Reduced heat transfer from pellet

� Fission gas release primarily dependent upon fuel

microstructure and temperature

� Two limits have been deemed acceptable• Limit rod internal pressure to below system pressure; or

• Below cladding lift-off limit (pellet to clad gap)

� Typically predicted with vendor proprietary methods

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LOCA Fuel Safety Criteria


Typical Response to a LOCA

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Ballooning and Rupture

� Rod internal pressure limited to system pressure by

design

� During LOCA, system pressure is lost leading to large

differential pressure across cladding

� Subsequent heatup weakens cladding and it could

lead to ballooning and rupture


Effect of Ballooning and Rupture

� Considering original

objective to maintain

ductility, two concepts

emerge:

� Modelling must

demonstrate that

balloon size does not

inhibit cooling; and

� Predictions should

account for enhanced

oxidation

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Oxidation

� Zr oxidation rate in steam environment is strongly dependent on temperature• Kinetics governed by an Arrhenius relation of the form: k = a exp (-Q/RT)

� Breakaway oxidation also needs to be considered• Related to H uptake during LOCA

• Can be addressed at design stage, but also need to be considered in analyses


Effect of Oxidation

� Consideration of

oxidation leads to the

following criteria:

� Limits placed on

maximum temperature

� Limits on maximum

allowed oxidation

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Burnup Effects on Oxidation Threshold

� H uptake increases with burnup• Increased H concentration lowers embrittlement threshold


Burnup Effect on Oxygen Pickup

� Pellet-to-clad gap closes at low burnup

� Operation for extended periods causes diffusion welding between pellet and cladding inside diameter• Fully developed by 50 – 60 GWd/MTU

� Bonding layer is primarily ZrO2

• Source of Oxygen during LOCA

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Burnup Effects from Fuel Relocation

� UO2 is a brittle ceramic

� Burnup causes factures

in the pellet during

operation

� Fuel can relocate to

balloons during a LOCA

• Can cause increased heat

which will accelerate the

embrittlement process


LOCA Safety Criteria

� Most countries have adopted the approach developed

in the US during the 1973 AEC ECCS hearings• 1200 ◦◦◦◦C

• 17 % local oxidation

• 1 % total oxidation

• Maintain coolable geometry

• Ability to provide long term cooling

� Criteria are intended to maintain some degree of

ductility in the cladding

� Original criteria developed largely from fresh fuel data

� Recent research has been directed to address the

effect of burnup

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RIA Fuel Safety Criteria


Sample RIA Results with Fresh Fuel

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Results of Testing with Fresh Fuel


Consideration of RIA in Design

• Design minimizes maximum

possible enthalpy deposition

• Design should accommodate

possible expansion of UO2

• Sufficient gap sizes and

tolerances to allow for

expansion without failure

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Factors to Consider with High Burnup

� Fission gas bubbles in the pellet

� “Edge effects”• Buildup of Pu on the rim of the pellet

• Increased fuel surface temperatures

� Pellet to cladding gap is closed

� Increase cladding H content


High Burnup Fuel Fission Gas Distribution

� Fission gas expands

when heated

• Increase pellet fracturing

� Pellet fracturing

increased at surface

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Edge Effects

� Fissile material

concentrated at pellet

edge

� Can exacerbate irregular

pellet expansion


Thermal Expansion

� Different physics govern

high burnup thermal

expansion

• Most models not updated

� Maximum measured

strain of 2.9 percent

• May lead to fuel failures

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High Burnup Fuel H Content

� Low burnup cladding

retains sufficient

ductility

� Postulated that high

burnup failures primarily

PCMI induced

• More data needed

� Maximum predicted

enthalpy change is

approximately 60 cal/gm

• Some failures evident


RIA Criteria

� Must eliminate possibility of steam explosion and

meet DNB limits

� Original regulatory limit of 280 cal/gm• Original testing conducted with fresh or slightly irradiated fuel

• Burnup effects not considered

� Recent testing suggests new criteria needed to

account for high burnup effects• Typical interim criteria of less than 170 cal/gm

� More testing is planned• NEA CABRI water loop tests planned

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Summary

� Regulatory authority must be maintain technical

capability to challenge operators and designers safety

case

� Regulatory authority must stay current with ongoing

R&D• Regulations may need to be updated to address new knowledge

� Above all, the regulatory authority must be able to

“ask good questions”

Documents

Fuel Safety Criteria - Nucleus...Limits and margin Design criteria AOO fuel safety criteria LOCA fuel safety criteria RIA fuel safety criteria 2013-12-13 2 Page 3 International Atomic