New Approaches to Prevention and

Mitigation of Severe Accidents in the Light

of Fukushima-1 NPP Events

Joint-Stock Company of Open Type

Atomenergoproekt

Main solutions to ensure nuclear, radiation and environmental safety

of nuclear power facilities are based on national regulations

supported by IAEA, ICRP recommendations, as well as design,

construction and operation experience of civil nuclear power

facilities built as per Russian designs.

Russian National regulations are being updated as per the State

regulatory procedure with consideration of plant operation events

both in Russia and internationally, e.g. Three Mile Island, Chernobyl

and Fukushima accidents.

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Russian Approach to Identification of Ways of Modern NPP Design

Upgrading with Consideration of Fukushima Events

Технологические особенности ВВЭР

The core composed of hexagonal cassettes;

Horizontal steam generators;

Reactor pressure vessel (RPV) of forged core barrels without

longitudinal joints;

Transportability of the main equipment by railroad;

No penetrations in the reactor bottom;

Location of fuel pond inside the containment;

Reactor pressure vessel (RPV) of carbon alloyed steel;

SG tubes of carbon steel with a relatively thick wall

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VVER Technological Features

Advantages of VVER

Specific features of VVER reactor plants are high level of inherent

protection implemented in the design bases of systems and equipment in

all VVER designs :

Large primary coolant inventory (reactor coolant circuit (RCC) &

pressurizer (PRZ) with respect to fuel mass and core heat capacity;

Large water inventory in horizontal steam generators via secondary

circuit;

Actuation of control rods to scram the reactor by gravitation forces;

Inherent restriction of core energy release by negative reactivity

coefficients;

Using of passive components, isolation, restricting and discharge

devices;

Using inertia coastdown of reactor coolant pump (RCP) special

flywheel masses to ensure the required decrease of flowrate through

the core during blackout.

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Extreme External Impacts

1. Examples of external impacts:

• earthquake, flooding;

• storm, tornado;

• airplane crash.

2. Extreme impact with intensity above the design-basis

causing multiple equipment damage and failures

Extreme impacts may cause beyond-

the-design-basis initiating events

including a BLACKOUT

Initiating Events and Failures at Fukushima NPP

Seismic impact with a magnitude above 8 points on MSK scale;

Loss of normal and emergency (Diesel-generators) power supply

(black-out);

Tsunami and resultant failure of ultimate heat sink (seawater);

Hydrogen generation due to steam-zirconium reaction, hydrogen

release into the reactor building and its damage due to hydrogen

explosion;

Reactor building foundation damage and activity release into the

environment

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Systems and components are to be designed

for seismic impacts:

Safe Shutdown Earthquake (SSE) – 0.25g – maximum horizontal

ground acceleration (8 points on MSK-64 scale);

Operation Basis Earthquake (OBE) – 0.12g – maximum horizontal

ground acceleration (7 points on MSK-64 scale);

VVER TOI withstand an earthquake with 40% margin by maximum

horizontal ground acceleration during Safe Shutdown Earthquake

(SSE) (this impact is considered as a beyond-the-design-basis impact).

In order to build NPP at sites with higher seismicity, it is possible to

design a plant for Safe Shutdown Earthquake (SSE) level – 0.41g (9

points on MSK-64 scale) without considerable modification of layout

solutions.

VVER-TOI Seismic Stability

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VVER TOI Safety Analysis for Fukushima Accident Conditions

1. Seismic impact

According to the Russian Federation (ОСР-97) seismic zoning map, the

anticipated VVER-TOI sites has the maximum earthquake magnitude of

7 points once in 10,000 years.

Therefore, in VVER-ТОI design the safe shutdown earthquake (SSE)

level is assumed as 8 points on MSK-64 scale. This value is used for

design of all safety systems, as well as equipment, valves and pipeline

of normal operation systems, important to safety, involved in safety

function performance.

Also, VVER TOI plants are to be proven for seismic impact beyond safe

shutdown earthquake (SSE) by 40%, with realistic (non-conservative)

approaches (EPRI NP6041 recommendations). During this impact no

activity release is allowed. Possibility of further commercial using of

NPP may be lost.

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VVER TOI Safety Analysis for Fukushima Accident Conditions

2. Blackout (without primary circuit accident)

During station blackout, the reactor core residual heat is removed by the

passive heat removal system (PHRS) for a long time. After 15-20 days,

operator interference is required to fully open the passive heat removal

system (PHRS) controller in order to use hydro accumulator-2 (HA-2)

inventory

Heat from the spent fuel in fuel pond is removed by water boiling. Fuel pond

(FP) water inventory is sufficient for 10 days in case of regular refueling.

Subsequent fuel pond (FP) makeup for 10 days during regular unloading is

possible from 2nd stage hydro accumulators (HA) located on the maintenance

platform inside the containment

Rate of containment pressure rise due to fuel pond boiling is such that

approximately after 10–15 days the pressure will reach the design

containment pressure. Further measures are required to limit the pressure

rise.

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3. Ultimate heat sink failure

Reactor core residual heat is removed by passive heat removal system (PHRS) with

atmospheric air-cooled heat exchangers. Thus, system operation does not depend on other

heat sinks, e.g. service water, seawater, cooling pond water, etc.

The heat exchangers are located at a height of around 40m and protected by civil structures.

Thus, their failure due to flooding or other natural or man-induced impacts (hurricanes,

tornadoes, air shock waves resulting from on-site or nearby explosions, airplane crash, etc.)

is ruled out.

VVER TOI Safety Analysis for Fukushima Accident Conditions

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4. Hydrogen release inside the containment and its damage by hydrogen explosion

Reactor building in VVER TOI design consists of two containment shells: 1) primary

pre-stressed concrete containment designed for 0.4 МPа (gage) internal pressure with

1.5 reliability factor and inner sealed steel liner; 2) secondary reinforced concrete

containment designed to withstand man-induced and natural impacts

Passive hydrogen recombiners are arranged inside the primary containment. They

prevent hydrogen concentration rise to hazardous limits in all accident modes including

beyond-the-design basis conditions

Thus, both hydrogen explosion and reactor building

damage are excluded. Therefore, possibility of a

beyond-the-regulation release of activity products

into the environment is ruled out. Additional

protection against activity release into the

environment is ensured by annulus underpressure

with the help of active and passive (annulus

passive filtration) systems

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VVER TOI Safety Analysis for Fukushima Accident Conditions

5. Reactor building raft damage and activity release into the environment

VVER TOI design includes a core catcher on the containment bottom for molten

core isolation and cooling in case of hypothetic accident, which may cause reactor

core damage

Core catcher keeps the

containment integrity and thus

excludes activity release into the

environment even in case of

hypothetical severe accidents

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VVER TOI Safety Analysis for Fukushima Accident Conditions

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Main Safety Features

Passive safety features in VVER TOI design

Reactor

Pressurizer

RCP

Steam generator

Passive heat removal system

from the steam generator

Annulus

System of 1st-stage hydro

accumulators

System of 2nd-stage hydro

accumulators

Passive annulus filtration

system

Inner containment

Outer containment

Primary

circuit Corium catcher Active emergency core

cooling system (ECCS)

Reacto

r

Pressurize

r

RC

P

Steam

generator

Passive heat removal

system from the steam

generator

Annulus

System of 1st-stage hydro

accumulators

System of 2nd-stage hydro

accumulators

Passive annulus filtration

system

Inner containment

Outer

containment

Primary

circuit Corium catcher Active emergency core

cooling system (ECCS)

VVER-TOI Safety Assessment Under More Severe Than Fukushima

Conditions

Blackout with primary (Large Break, Loss-Of-Coolant Accident) LB LOCA

Under this accident conditions, reactor core

residual heat is removed by combined

operation of passive heat removal system

(PHRS) and 2nd stage hydro accumulators.

Self-sufficiency (no core damage) of VVER

TOI in this mode depends on water

inventory in 2nd stage hydro accumulators.

Water inventory in the fuel pond ensures

self-sufficiency in case of any leak rate for

at least 72 hours. Fuel pond (FP) is

connected with hydroaccumulator-2 (HA-2)

pipeline.

After hydroaccumulator-2 (HA-2) and fuel

pond (FP) water inventory is over, and if

normal or emergency power supply is not

recovered, reactor and fuel pond makeup

may be needed with the help of pumps fed

from mobile air-cooled diesel-generator

(water as diesel generator (DG) coolant

may not be available).

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Power Plant Robustness to Blackout

Reactor plant robustness to BLACKOUT depends on availability

of passive safety systems to perform all main safety functions.

Analysis of blackout conditions including coincidence with

primary loss-of-coolant-accident (LOCA) inside the containment

has demonstrated: if passive safety systems perform their

functions the reactor plant is maintained in a safe state for 24 h.

This time can be extended up to 72 h.

Reactor heat is removed to the ultimate heat sink by Passive

Heat Removal System (PHRS).

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Conclusions and Further Steps in VVER TOI Safety Analysis

VVER-TOI design includes full complex of design features, which

1) ensures NPP safety;

2) exclude a beyond-the-regulation activity release into the environment in

case of external (natural and man-induced) impacts combined with

internal initiating events and additional failures.

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To enhance plant stability to low probable hypothetical events and self-sufficiency during beyond-the-design-basis (BDBA) accidents it is proposed to ensure:

Spent fuel pond heat removal and prevention of long-term containment

pressure rise;

Long-term reactor makeup when primary circuit is leaktight and during

primary loss-of-coolant accidents (LOCA);

Safety and other plant parameters monitoring.

The proposed measures include installation of additional equipment such as mobile diesel-generators with heat removal only to atmosphere.

Conclusions and Further Steps in VVER TOI Safety Analysis

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Design Improvements Based on Fukushima Lessons

Fukushima plants demonstrated their design survivability under

combined external impacts; however, the design did not envisage

safety functions recovery.

Fukushima lessons:

Combine active and passive safety systems

Ensure safety functions performance at different accident

stages (redundant power supply sources with guaranteed

connection, long-term stable functioning of passive safety

systems);

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Ensure safety systems self-sufficiency and diversity,

protection against dependent failures under extreme

conditions, common-cause;

Ensure NPP accessibility for emergency services during

accidents and disasters;

Ensure the possibility of replenishment of media and energy

sources in case of destruction and blockage (by air inclusive);

Develop within the INSAG group recommendations to

enhance operational stability and safety of water-cooled

reactors, based on proposals made at this conference.

Design Improvements Based on Fukushima Lessons

VVER Design Developments in JSC Atomenergoproekt

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NVNPP Unit 5

Small series

U- 87 (V-320) AES-92

•4 trains of active safety systems;

• passive safety systems for all critical safety

functions (CSF);

• double-shell containment with controlled

annulus;

• emergency heat removal via secondary circuit is

not limited in time both in active and passive

mode;

• long-term (no less than 24 h) ability to prevent

damage to fuel above the limits established for

design-basis accidents;

• a certificate in compliance with the EUR

requirements has been documented

• 3 trains of active safety systems;

• single-shell containment;

• emergency heat removal via

secondary circuit is limited in time

by water inventory in chemically

demineralized water tanks;

• core damage after 2-3 hours in

case of active safety systems

failure

Kudankulam NPP Belene NPP

AES-2006

• 2 trains of active internally

redundant safety systems;

• passive safety systems for all

critical safety functions (CSF);

• double-shell containment with

controlled annulus;

• emergency heat removal via

secondary circuit is not limited in

time both in active and passive

mode;

• long-term (no less than 24 h)

ability to prevent damage to fuel

above the limits established for

design-basis accidents under SBO

conditions and without operator

intervention;

• performing, jointly with the EUR

club, an analysis of the project for

compliance with the EUR

requirements

AES VVER-ТОI

•2 trains of active safety systems;

• passive safety systems for all

critical safety functions (CSF);

• double-shell containment with

controlled annulus;

• emergency heat removal via

secondary circuit is not limited in

time both in active and passive

mode;

• the power unit has enhanced

resistance to extreme external

impacts;

• long-term (no less than 72 h)

ability to prevent damage to fuel

above the limits established for

design-basis accidents under SBO

conditions and without operator

intervention;

• performing, jointly with the EUR

club, an analysis of the project for

compliance with the EUR

requirements

Novovoronezh-2 NPP

Government

Order No 1026 of

28.12.92 and

within the

Environmentally

Cleary Energy

National

Program

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VVER-TOI NPP

Thank You for Your Attention!

Address: Bldg.1, 7, Bakuninskaya str.,

Moscow, 105005,

E-mail: info@aep.ru

www.aep.ru

Joint-Stock Company of Open Type

Atomenergoproekt