Overview of Framatome’s Simulation-assisted Works to

1 Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in SFP – Braun – IAEA – 2-5 September 2019

Restricted Framatome – C1 – AL: N – ECCN: N © Framatome - All rights reserved

Overview of Framatome’s Simulation-assisted Works to Implement SFP related post-Fukushima Measures BRAUN Matthias

2-5 September 2019, Vienna, Austria

Restricted Framatome – C1



1. Incidents and Accidents in the Spent Fuel Pool (SFP) usually considered in EOM / SAMG

2. MELCOR SFP Model Development based on the Behavior of the Fukushima Daiichi SFPs

3. Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR)

4. Mitigation of Long-term Boiling SFP

Content









► International recognition of need for an emergency operation manual (EOM) or severe accident management guidelines (SAMG)

► After Fukushima, many utilities / regulators expanded scope of EOM / SAMG from dealing with the reactor core during/after power operation to ● The Spent Fuel Pool (SFP)

● Outage states with closed RPV

● Outage states with open RPV

► For this expansion of scope to the SFP, one of the first question to answer was: How to classify / rate different accident situations in the SFP?

Incidents and Accidents in SFP usually considered in EOM / SAMG



► Long-term boiling of a SFP with water makeup ● Occurrence in case of long-term SBO (Fukushima)

● No safety risk with appropriate plant qualification

● Likely not isolated incident (additional emergency measures necessary for re-establishing power supply, for ensuring core cooling, supplying neighboring units, …)




► Boiling and dry-out of the SFP (no water makeup) ● Long grace periods of days to weeks

until FA may get exposed

● Extremely unlikely accident scenario without crediting any mitigating emergency measures

● Scenario not considered as an initiating event for FA damage accident

● (Bounding) reference scenario for evaluating accessibility of service floor with respect to

▪ Temperature ▪ Humidity




► High-lying water leakages from the SFP ● Caused e.g. by break of SFP slot gate

● This event is considered a design base accident

▪ If leakage can be over-fed or ▪ If SFP drains into a closed pool (storage pool,

reactor well) so that the SFP level can be restored by filling these pools

● If SFP drops non-recoverably by few meters, transition into DEC

● Induces failure of SFP closed loop cooling systems as water extraction ports at top of pool

● For substantial drop of SFP liquid level > ~6 m, service floor becomes inaccessible (lack of shielding)

● With SFP LB-LOCA and if radiation prevents a repair of the damage the consequence is either

▪ Long-term SFP boiling or ▪ Long-term SFP overfeeding




► Low-lying water leakages from the SFP (in active zone) ● Small SFP liner leakages which can be overfed are DBC

● Large leakages (which can not be overfed) are

▪ rather unlikely due to solid SFP walls, but ▪ hard to mitigate

● Some approaches to mitigate FA heat-up by e.g. spray systems (question if these systems survive initiating events)

● At best avoided by protecting SFP from events like air plane crash etc.




► Not considered: Re-criticality within the SFP ● Drop of a FA from handling machine on top of

storage rack is DBC – must not lead to re-criticality

● Re-criticality in SFP only conceivable after massive mechanical damage to storage rack e.g. by load drop (and low / no boron concentration like in a BWR)

● Risk of dry-storage cask drop can be reduced by administrative measures to not lift it above FA

● Fukushima showed that not even a collapse of the concrete building roof structure caused corresponding damages to storage racks




► Loss of core or SFP cooling during outage with open RPV (connected or disconnected to the SFP / fuel transport pool) ● Fast escalation into nuclear accident possible

● Beneficial that during critical outage phase additional safety regulations are in effect

● Short grace periods encumber execution of possible emergency actions

► Mechanical or thermal damage of one or few fuel elements (e.g. load drop, handling error, failure of fuel handling equipment, ..) ● Already happened numerous times, e.g. NRU Reactor, Bohunice A1, Paks, …

● Only limited environmental impact

▪ Low number of FA affected ▪ Activity retention capabilities of the reactor building remains intact










Model Development based on the Fukushima Daiichi SFPs Observation



► Peak temperature of SFP depends on

● Heat loss into walls: ~ 0.5 MW after start of accident, less at a later stage due to low heat conductance of concrete

● Subcooled evaporation of SFP water

▪ Limited by convection of humid air away from the water surface

▪ Promoted by large surface, low decay heat, and good ventilation of service floor

▪ Was dominant mechanism in Fukushima

● Boiling – vaporization of SFP water

▪ High decay heat / core unload

▪ Service floor atmosphere approaches 100% humidity

► Why of importance: Evaporation limits accessibility to service floor (temperature / humidity) long before SFP boiling

Model Development based on the Fukushima Daiichi SFPs Peak Temperature of the SFP

SFP 1F4, 6/29/2011

Absence of boiling

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► A lumped parameter model of the SFP must…. ● … allow the formation of a humid air surface layer Separation of service floor & pool in two control volumes

● … allow a convective heat transport of humid air into service floor connect service floor & pool by 2 flow paths

● … must not overestimate the convection flow throttle convective flow, as it is usually overestimated

Model Development based on the Fukushima Daiichi SFPs Lumped Parameter Modelling

Service floor volume

Pool volume

Throttle flow by major loss and

flow area



► Available experiments / correlations ● Boelter et al.,

● Shah,

● ….

Model Development based on the Fukushima Daiichi SFPs Test of Simulated Evaporation









Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR)

► Assumption: ● Leakage in the SFP in or below the active zone of the FA

● Leakage area is too large to overfeed the leakage flow

● Sudden drop of the SFP level into or below the active zone of the FA

● Lack of shielding of direct radiation from used FA encumbers repairs

► Consideration of this scenario ● Unlikely to occur (solid reinforced concrete walls)

● Hard to mitigate (How likely would e.g. a SFP spray system survive an initiating event destroying massive SFP concrete walls)

Introduction



Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR) Cooling Mechanisms for Fuel Assemblies



Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR) Representation of the Fuel Assemblies in the MELCOR Code



Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR) Drop of the SFP Liquid Level into the FA Active Zone

► Partially exposed FA ● Evaporation cooling of lower FA region

● Steam cooling of upper region

● Water blockage prevents convection through FA

► Simulation results ● SFP liquid level must substantially

drop into active zone to cause thermal damages to FA

● The longer the cool-down time of FA, the higher the exposure can be

● Reactor service floor inaccessible long before due to lack of radiation shielding 0

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Analytics



Limits of FA Cooling after Low-lying SFP Leakage (e.g. for BWR) Complete Drainage of SFP

► Natural convection of air through FA ● FA act as small stacks with natural draught

● Cool air enters the bottom of the canister

● Hot air rises to the service floor

► Simulation results ● Air-coolability up to about 1 kW per FA

● Corresponds to a FA cool-down time of 1-2 years

► Conclusion: Relatively narrow range of SFP levels which actually can cause thermal damage to a large number of FA





3. Limits of FA Cooling after Low-lying SFP Leakage




Mitigation of Long-term Boiling SFP Fukushima Experience

► Fukushima showed that after a massive external event, a long-term SBO can occur failure of the SFP cooling

► Detrimental factors for SFP feeding in Fukushima ● Lack of instrumentation to control SFP level

● Encumbered accessibility to plant site by

▪ Damaged infrastructure ▪ Radiation from the core damage events

► ‘Beneficial’ factors in Fukushima ● Accessibility to SFP due to roof failure

● Ventilation of SFP limited pool temperature well below boiling point

► Nowadays many plants have to have means to mitigate a long-term boiling SFP with high reliability

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Mitigation of Long-term Boiling SFP Tasks to realize a reliable mitigation of a long-term boiling SFP

a) Access of plant personnel to the SFP itself or an interconnected system to establish the water makeup ● Access possibly limited by temperature / humidity / radiation (not crediting operating HVAC)

● The lack of SFP cooling likely is not an isolated event (core damage in same or nearby reactor unit….)

b) Establish ways for water injection into SFP

● Dedicated / pre-planned injection path (back-fitted dry pipe, or port at existing interconnected system)

● Equipment to cope with total loss of electric power (usage of fire engines / mobile diesel driven pumps, …)

● Pre-planning of water reservoir usage

▪ Unborated water to be used when pool boils, as boron remains in pool prevention of boron crystallization in the long term

▪ Depending on storage rack design, evaluation of criticality possibly necessary ▪ Possible water resources: Condensate storage (BWR), emergency feedwater tanks (PWR),

fire / drinking water systems, surface fresh water ▪ Due to low power density in spent fuel, water impurities do not endanger heat removal (but may affect corrosion)




c) Surveillance of the current pool water level ● For PWR especially important to not overfeed SFP with demineralized water, to avoid deborating of SFP

● Qualified for the expected environmental conditions (physical as well as administrative like unavailable MCR)

● For international customers Framatome developed accident / severe-accident resistant SFP measurement systems

▪ Contact-less radar wave level measurement(1) ▪ Fixed pressure transducer measurement(1)

▪ Magnetic severe-accident level measurement device(2) (200°C, 11 bar-a, 20 MGy radiation, 5 g acceleration)

▪ All systems retro-fit-able without welding to the pool liner

(1) http://us.areva.com/home/liblocal/docs/Solutions/Product %20Sales/SFP-Instrumentation_By_AREVA_Presentation_V0.pdf

(2) http://www.framatome.com/EN/customer-671/ accident-level-measurement-device-severe-accident-instrumentation.html

http://us.areva.com/home/liblocal/docs/Solutions/Product Sales/SFP-Instrumentation_By_AREVA_Presentation_V0.pdf

http://us.areva.com/home/liblocal/docs/Solutions/Product Sales/SFP-Instrumentation_By_AREVA_Presentation_V0.pdf

http://www.framatome.com/EN/customer-671/accident-level-measurement-device-severe-accident-instrumentation.html

http://www.framatome.com/EN/customer-671/accident-level-measurement-device-severe-accident-instrumentation.html




d) Preparation of a suitable steam release path into the environment ● Some plant designs have SFP inside containment

(VVER, German PWR1300, MARK III BWR)

● Evaluation of necessary steam release flow rate and reachable containment pressure (with e.g. lumped parameter code)

● Thereby a suitable conservative nodalization for building response calculations is necessary (beside e.g. for a German PWR)

▪ Service floor & pool are one control volume ▪ Not a best-estimate approach as discussed previously in the talk ▪ Overestimates subcooled evaporation Conservative concerning containment pressure buildup

● Based on the simulation results, design of a vent path from the containment into the environment

▪ Usage of existing HVAC systems, or ▪ Usage of existing filtered containment venting systems, or ▪ Installment of new vent path




e) Confirmation that the pool liner and load-bearing concrete structures can endure the increased water temperature

● SFP often only qualified for sub-cooled water e.g. 80°C in Germany

● In Fukushima the SFP liner and the concrete structure endured ~90°C without damage in Unit 4, however unclear in how far this observation can be generalized to other plants

● Especially for plants with SFP inside containment, SFP temperatures >100°C expectable due to containment pressure buildup

● Solution:

▪ Material stress and strain can be determined numerically by e.g. finite element methods

▪ Framatome experience is that a post-qualification of the liners to higher temperatures is usually possible



► Historically, most incident / accidents in a SFP are caused by FA handling errors / equipment failure ● Low environmental consequences

● Prevention of occurrences at best by well-trained and reliable service operators

► Catastrophic thermal damage to FA only realistic in case of a low-lying leakage of the SFP ● Highly unlikely occurrence ( air coolablity)

● Limited possibilities to mitigate such an occurrence

● At best avoidance of such a scenario in new builds by external protection of SFP (e.g. APC shell)

► Mitigation of long-term boiling SFP ● Possible by suitable emergency preparations including

▪ Personnel accessibility to relevant equipment (radiation, humidity, temperature) ▪ Emergency equipment (pumps, generators, injection lines) ▪ Confirmation of robustness of pool liner / concrete to (pressure-dependent) boiling temperatures ▪ Steam release path into environment (if necessary) ▪ Robust SFP level measurement

● At best: Exclusion of this event via passive cooling systems ( Talk by Mr. Fuchs)

Summary



Thank you for your attention!

To discover more about how we can help, visit us at www.framatome.com/solutions-portfolio

http://www.framatome.com/solutions-portfolio



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Overview of Framatome’s Simulation-assisted Works to