AT ARMENIAN NPP Eduard YEGHOYAN Head of Laboratory, Armatom - - PowerPoint PPT Presentation

IAEA Technical Meeting on the Phenomenology, Simulation and Modelling

• f Accidents in Spent Fuel Pools

Eduard YEGHOYAN

Head of Laboratory, “Armatom” Institute

Artashes HOVHANNISYAN

I Cat. Engineer, “Armatom” Institute

2-5 September 2019, Vienna, Austria

APPROACHES TO THE MANAGEMENT OF SEVERE ACCIDENTS IN SFP AT ARMENIAN NPP

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The nuclear energy sector of Armenia includes one nuclear power plant - Armenian NPP, which is located in the western part of Ararat valley, 30 km west of the capital Yerevan.

Introduction

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Armenian NPPconsists of two units with Soviet design WWER-440- 270 model reactor that is a version of the WWER-440-230 serial model with special seismic considerations in the design.

➢

Unit 1 started its commercial operation in 1976 and the Unit 2 in 1980; Both units were shut down shortly after the earthquake of December 7th, 1988;

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Following the completion of repair and safety upgrading activities Unit 2, after 6.5 years of shutdown, restarted operation in 1995 and it has been operational since then. Unit 1 remains in long-term shutdown.

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The Spent Fuel Pools (SFPs) of both units are currently in

• peration - the fuel discharged from the operating unit 2 reactor

core is put first in the SFP of that unit, then, after several years of storage, is transferred to the SFP of the unit 1.

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Plant layout

SFPs at Armenian NPP are of “at-reactor” type. The SFPs are located close to the reactor, but outside of the containment hermetic

• boundary. The pools are constructed of reinforced concrete with a

two-layer steel liner – stainless steel and carbon steel with 4mm of thickness each one and 4mm gap between them. The so called “Central hall” of the reactor building, to which the SFPs are connected, is a relatively big area – width 39m, length 126m, height 28.3 m (the Central hall is common for two units of the plant; it is an airtight room in the upper part of the reactor building, not designed for overpressure). When the fuel pool is not in refueling mode, it is covered by panels (panels don’t ensure tightness of the pool and the pool is considered to be connected to the Central hall).

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Plant layout

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FIG 1. Simplified layout of the Reactor building SFP1 SFP2

Plant layout

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FIG 2. Section of the Reactor building

SFP

Refueling Pool

SFP ACCIDENT STUDY SPENT FUEL POOL AND COOLING SYSTEM DESIGN

The SFP has rectangular form, and has 2 separate parts – main pool and container compartment. The racks in the SFP can have 2 levels. The lower level is used for permanent storage of the fuel assemblies, and the upper level of racks is used temporarily - installed and used in short term in case of full off-load of reactor core. The number of fuel assemblies’ cells in the SFP lower racks is 372 and in the upper level -

• 351. The number of fuel assemblies in reactor core – 347. The pools

have also several cells for installing special containers in the SFP for the storage of damaged or leaking FAs. Depending on operation mode, different coolant levels are maintained in the pool. Heat removal from SFPs is ensured by forced circulation

• f the coolant through the heat exchangers of the dedicated cooling
• system. The lowest level penetration of the pool is the connection of

circulation pumps suction line (pool outlet line).

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SFP ACCIDENT STUDY SPENT FUEL POOL AND COOLING SYSTEM DESIGN

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FIG 3. SFP heat removal system simplified principal scheme

SFFP-1 SFPP-2 SFPHE-1 SFPHE-2

Cooling water to heat exchangers

Passage connecting with refuelling pool

+5.47 +5.67

• 1.8

+1.4 +11.8 +6.0 +4.5

• 1. 8

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• 1.48

+ 715 1. +5 44 . +2 24 .

Lower level racks

Spent Fuel Assemblies

+1.03 Upper level racks

Container compartment

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SFP ACCIDENT STUDY ACCIDENT SCENARIOS

The main scope of studies of accident scenarios in SFPs is performed within the development of plant EOPs for SFPs and corresponding analytical justification documentation. Main two categories of accident scenarios considered in the studies are:

1)

“Loss of SFP coolant”

2)

“Loss of cooling of SFP”. “Loss of SFP coolant” scenario can take place in case of break of cooling system tube. For the case of coolant inlet pipe break, in order the siphon effect is “broken” there is special small size line connected to the containment atmosphere to insure air inlet to the cooling system in case the highest point is under vacuum. The elevation of tube connection to the pool prevents uncovering of fuel when only lower level racks contain spent fuel assemblies.

SFP ACCIDENT STUDY ACCIDENT SCENARIOS

The scenario with most serious consequences - both level of racks full of spent fuel assemblies (reactor core off-load mode) and break

• f cooling system tube (pump suction line).

In such a scenario coolant level in SFP can decrease very fast till reaching the elevation of cooling system suction tube (see figure 4 ). Thus, the higher part of fuel assemblies in the upper level racks can be uncovered very soon after beginning of the transient . According to the calculations’ results, in the beginning phase of the transient, decrease of the coolant level in SFP is very fast. The fuel top level is uncovered in less than 5 min. From this moment increase

• f fuel temperature starts at the uncovered part and takes place

significantly fast.

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FIG 4. Change of coolant level in the SFP in the scenario of cooling system tube break (without SFP make-up)

SFP ACCIDENT STUDY ACCIDENT SCENARIOS

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Elevation, m

Coolant level Fuel top level Outlet tube level

Time, hours

SFP ACCIDENT STUDY ACCIDENT SCENARIOS

Quick loss of coolant ceases when the coolant level reaches the elevation of the tube connected to the pool. Heat removal from the pool is lost and the coolant temperature increases continuously. Starting from this period, during about 50 minutes the coolant level is unchanged – the loss of coolant (mainly through evaporation) is compensated by its thermal expansion. About 1 hour after beginning of the transient coolant temperature reaches the boiling temperature and decrease in coolant level starts again due to more intensive evaporation process (with no more effect

• f thermal expansion of the coolant).

In parallel to the warming up of the coolant in the lower part of the pool (below upper level racks), the rate of steam generated in the upper level assemblies increases, thus, improving the heat removal by steam from the uncovered part of the fuel, and starting from about 45-46 min. of the transient the fuel temperature starts to decrease (see figure 5).

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FIG 5. Change of fuel maximum temperature in the scenario of cooling system tube break (without SFP make-up)

SFP ACCIDENT STUDY ACCIDENT SCENARIOS

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Time, hours 0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Fuel hottest point temperature, C

• 1200

1000 800 600 400 200

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SFP ACCIDENT STUDY ACCIDENT SCENARIOS

Continuous decrease in coolant level results in less steam generation and bigger part of fuel uncovered, i.e. bigger decay power to be removed by steam. After 25 minutes of fuel temperature decrease, it starts to increase again. Overheating of the fuel cladding till 1200 oC takes place about 4.23 hours after starting of the transient. Another accident scenario with the same initiating event and early SFP make-up was studied. Due to significant loss of coolant through the break the coolant level is almost the same as in the first scenario. Some kind of “feed-and-bleed” takes place in the pool, and the mean temperature of the coolant does not practically increase. However, heat removal conditions for upper level rack assemblies are deteriorated – low temperature of coolant feeding the upper level assemblies results in low rate of steam generation and, thus, in continuous increase in fuel temperature in the upper uncovered part

• f fuel. In this 2nd scenario the conditions of fuel damage are reached

significantly earlier (less than 1.5 hours after transient beginning).

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SFP ACCIDENT STUDY ACCIDENT SCENARIOS

In the scenario with loss of cooling of the SFP, even in case of reactor core full off-load, the fuel top part will be uncovered in a time longer than 34 hours (coolant boiling starts about 2 hours and 40 minutes after the beginning of the transient). For the scenarios with only lower level racks containing spent fuel assemblies the cooling system line break will not create quick change

• f heat removal conditions – at least 2.5 m layer coolant inventory will

be available in the beginning phase of the transient. Severe accidents in SFPs of ANPP are not studied yet. Draft versions

• f SFP SAMGs were developed based on known general phenome-

nology of the accident progression in SFPs. Currently the model is under development (MELCOR 1.8.6 version is used) to support analytical justification of the strategies considered in the SAMGs.

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

Armenian NPP SAMG development program was divided in 2 phases. In phase 1 set of SAMG documentation was developed for conditions

• f closed reactor and closed (tightened) containment. In the 2nd phase

(launched in 2018) draft versions of SAM guidelines were developed based on known phenomenology of the accident progression in SFPs and the approaches to severe accident management used by Westinghouse Owners Group. The main way to stop the progress of fuel damage in the SFP is providing coolant inventory in the pool to ensure heat removal from fuel – through evaporation in the beginning, then, if the inventory is enough, through circulation of the coolant via cooling system. The attempts to ensure SFP make-up from different sources and through different ways are considered in EOPs that are used before transition to SAMGs. If transition from EOPs to SAMGs was done, this means the attempts were not successful.

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

After the onset of fuel damage in the SFP there are at least 2 pheno- mena that will influence the priorities of tasks to be completed within the severe accident management: 1) release of radioactive materials from SFP to the Central hall, 2) generation of hydrogen in the SFP and its accumulation in the Central hall. The priorities of strategies comparing to EOPs should be changed making main priority limitation of radioactive releases, both current and potential. The way of radioactivity release to the environment is the flow of radioactive materials from SFP atmosphere to the Central hall, and then from there directly to the environment or to the adjacent premises in the reactor building (Central hall is not a hermetic area). If no strategy is implemented, continuous flow of Central hall atmosphere to the environment will take place in result of generation

• f steam and incondensable gases in the SFP.

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

In the current configuration of the plant, the only way for limitation of releases is the “direction” of gas flow through the ventilation system filters. Although Central hall is not hermetic, it has capability to localize to some extent the radioactive materials and limit the releases. Thus, the protection of Central hall boundary is an important task for SAM. Due to the design of the Central hall, burn of hydrogen even in slow deflagration mode could result in damage of this area boundary and in

• pening of ways for unobstructed propagation of radioactive materials

to the environment. Thus, formation of flammable gas mixture in Central hall should be excluded during the progression of the severe accident.

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

The major steps considered in draft SAMGs to be implemented within the management of severe accident in SFP are as follows: ✓ Blocking/closing ways of radioactive materials propagation from Central hall to the environment; ✓ Organizing filtered venting of the Central hall with the purpose of limiting the unfiltered releases of the gases from the Central hall as well as limiting accumulation of hydrogen in the Central hall and adjacent areas; ✓ Identifying/recovering ways for SFP make-up; ✓ Identifying and evaluating anticipated impacts of pool make-up at the current conditions and with available means of make-up; ✓ Control of the strategy of pool make-up; ✓ Transfer of SFP heat removal to its cooling system.

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FIG 6. Transitions from EOPs to SAMGs

SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

EOP

• n power

( )

NO

Shutdown EOP shutdown reactor ( ) SFP EOP SFP-1,2 ( )

SAG-1 : 0 “ ” Severe accident management on SFP-2 SAG- : 11 “ ” Severe accident management on SFP-1 SAG- : Severe accident management

• n open reactor

9 “ ” DFC-2

SAMG package

DFC

Closed reactor

YES

Closed containment

YES NO

SAG-12: “ ”

• n shutdown reactor with open containment

Severe accident management

Severe accident symptoms Severe accident symptoms Severe accident symptoms

Management of severe accident in case of radioactive materials releases through the Reactor Building Central Hall Management of severe accident in case of radioactive materials releases from the containment

Management of severe accident

• n reactor in case of

tightened containment

Transitions from EOP to SAMG

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

EOPs documentation includes 3 standard sets of procedures:

• EOPs on power;
• Shutdown EOPs;
• SFP EOPs.

Fragmentation of SAM guidelines was made based on anticipated pathway of radioactivity releases: 1) releases from containment, 2) releases through reactor building Central hall. In the first case the source of radioactivity is the reactor, and in the second case the source of radioactivity can be the SFP of unit 1, SFP

• f unit 2 as well as the reactor (open reactor or closed reactor but
• pen containment - configuration of the plant possible at the end of
• utage during short time period).

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SFP ACCIDENT STUDY MANAGEMENT OF SEVERE ACCIDENT IN SFP

In ANPP SAMGs plant diagnostics during severe accident will be ensured using 2 different diagnostics flow-charts – DFC and DFC-2. DFC is developed for the configuration of the plant with closed reactor and closed containment. It controls such parameters, as dose rate on the plant site, hydrogen concentration in containment, pressure and temperature in containment, steam generators coolant levels, primary pressure, core exit temperature, reactor vessel failure symptoms. DFC-2 is referenced in case of severe accident in SFP or in open reactor or in closed reactor with open containment - in cases when the radioactivity release will mainly take place through Central hall. For each of these mentioned cases there is a special guideline which will be used to mitigate the severe accident (transitioned from DFC-2). These guidelines can be used in parallel, if needed, depending on number of facilities containing damaged nuclear fuel.

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SFP ACCIDENT STUDY MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG

Recently activities for severe accident study for spent fuel pools (SFP) and open reactor configuration started. The MELCOR model (using MELCOR 1.8.6 version) is under development. The SAMG developers defined a set of tasks for analytical team. Some of these tasks are given below:

• The model must include also the ventilation systems (including the

vent stack) to ensure the modeling of filtered venting from of the plant Central Hall. It must model also the natural draft of the vent stack - flow of gazes without operation of the ventilators.

• The model must be detailed enough to ensure the consideration of

the possible air ingress into the assemblies from lower head.

• The model must consider (at the extent possible) the heat exchange
• f steam-gas mixture with constructions and walls of the Central

Hall (to consider the steam condensation rates and its influence on the flammability of the gas mixture and the scope of releases).

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SFP ACCIDENT STUDY MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG

• The model should allow to assess the efficiency of the heat removal

by air in case the lower heads of assemblies are uncovered - to assess how it can mitigate the heating up of fuel or if it can prevent the overheating of the fuel, what temperatures of cladding are anticipated in case of cooling by air.

• The calculations must include scenarios without any operator

actions to assess the phenomenology of the severe accident and different phases of its progression as well as the scenarios with implementation of strategies considered in the guidelines.

• The calculations must reveal the conditions (decreased level of

coolant in the pool) when the failure of fuel cladding is anticipated as well as the tendency of change of the temperature in the SFP (thermocouples measurement values) during the heating up of the fuel, the runaway oxidation of fuel cladding and later phases of severe accident. Analytical part has to define the scale of tempera- ture measurements to be sufficient for the severe accident phase.

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SFP ACCIDENT STUDY MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG

• The calculations must reveal the quantity (tendency of generation)
• f the hydrogen that can be generated in the pool in case of

different accident scenarios as well as the tendency of hydrogen concentration increase in Central Hall. It must reveal if flammable mixtures can be formed in the Central Hall and the minimal anticipated timeframes for its formation.

• The calculations must reveal the influence of radioactive aerosols

natural deposition in Central Hall .

• The calculations must reveal the efficiency of Central Hall venting –

mitigation of releases and prevention of flammable mixture formation (particularly for the case without availability of ventilators and use of vent stack natural draft).

• The calculations must include the reflooding of uncovered fuel.
• Calculations must justify (or disprove) the fact that the make-up of

the pool (containing uncovered overheated fuel) with bigger flow rates will result in better results.

Thanks for your attention!

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