Current Affairs- FAQ on Kudankulam issue- Part 5

5.8) VVER Reactor Design and Engineering

      a)   VVER is a pressurized light water cooled and moderated reactor with four  independent cooling loops. The reactor has horizontal steam generators in each loop that gives high water storage capacity. It uses hexagonal fuel assemblies which have low enriched fuel in oxide matrix, housed in sealed Zirconium‐ Niobium alloy tubes.

      b)   KKNPP VVER 1000 adopts the basic Russian design by model marked V320 with Enhanced Safety Features to make it in line with IAEA GEN III reactors. Further, certain additional safety features were incorporated like Passive Heat Removal System taking it to GEN III+ category. Russian Federation has marked KKNPP reactor as V412.

c) Salient Normal Operating Parameters of KKNPP Reactors:

·        Electrical Power 1000 MWe

·        Thermal Power 3000 MWt

·        No. of FAs 163

·        Coolant inlet temp 291C

·        Coolant outlet temp 321C

·        Coolant Pressure 15.7 MPa

·        No. of Loops 4

·        No. of Control Rods 103

·        Pressure Maintenance by Pressurizer

d) Enhanced Safety Features: Key Safety Features incorporated in KKNPP as required India:

·        Quick Boron Injection System

·        Passive Heat Removal System

·        Second Stage Hydro Accumulators

·        Passive Hydrogen Re‐combiners

·        Annulus passive filtering system (passive system)

·        Core Catcher

·        Emergency Control Room

     

The above systems have been developed based on extensive R & D and simulated testing by Russian design institutes. Functional performance of these systems are established during commissioning stage. These systems are described in subsequent sections.

e) VVER 1000

Plant model	Site (units)	Status	No. of Units
V‐320	Balakovo NPP (1‐4), Zaporozhe NPP (1‐6), Rovno NPP (3,4), Khmelnitsky NPP (1,2), South Ukraine NPP (3), Rostov NPP (1,2), Temelin NPP (1,2), Kalinin NPP (3), Kozloduy NPP(5,6)	Operating	22
V‐412	Kudankulam NPP (1,2)	Under Construction	2
V‐428	Tianwan NPP (1,2)	Operating	2

* In addition to the above:    

·        VVER ‐1000 reactors are under construction in Russian Federation.

·        Recently VVERS are planned in Vietnam, Turkey and Bangladesh.

f) IAEA Safety Review Of VVER1000 ( V-320 )

This review was done by international Experts in 1994 and recommendations have been incorporated in the V‐320 and are part of KKNPP ‐ V412 also.

g) Safety Functions for a NPP

     

      The following safety functions shall be performed in all operational states, i.e. during normal operation, during and following design basis events conditions and specified       beyond design basis events (BDBEs):

     

·        Control of the Reactivity (control of fission chain reaction)

·        Heat removal from the core and

·        Confinement of radioactivity

     

h) Safety during Normal Operation:

     

      During Normal Operation (NO) & Operational Transients (such as Turbine trip, pump trips etc), the reactor is controlled by the controllers within certain operational limits and conditions. The control is achieved by following parameters :

     

·        Control of Reactivity:

     

      i) CPSAR (Control and Protection System Absorber Rods)

      ii) CVCS (Chemical Volume Control System)

     

·        Heat Removal from Core:

     

      i) Primary Coolant Circuit (four independent loops)

      ii) Steam Generator (one in each loop)

      iii) Turbine & Condenser

·        Confinement of Radioactivity by following multiple barriers:

      i) Fuel Matrix and sealed Fuel Clad

      ii) Reactor Coolant System with Chemistry control

      ii) Containment and Containment filtration Systems

·                    Plant operation shall be carried as per Technical Specifications for operation approved by AERB which ensures that the plant is operated within safe parameters.

i) Systems Catering to Design Basis Events (DBE):

      Though a detailed design analysis indicates that the rector will operate within the design       parameters, safety systems have been provided to ensure safety during postulated  events, known as Design Basis Events (DBEs).

      DBE postulations have been made as per AERB guidelines which follow international    practices. An example of DBE is break of main coolant pipe resulting in loss of coolant accident, known as LOCA.

      During DBEs, reactor is shutdown by the control rods.

      The Reactor core cooling will be maintained by the following safety systems, which are four train independent systems:

·        High Pressure Emergency Injection System: Starts injecting borated water to the reactor core when primary pressure falls below 7.9 MPa

·        First Stage Hydro Accumulators ( Passive system): Starts injecting borated water to the reactor core when primary pressure falls below 5.9 MPa

·        Long term decay heat removal System: Starts injecting borated water to the reactor        core when primary pressure falls below 1.9 MPa

·        Emergency Safety Boron Injection System: Injects borated water to the pressuriser to depressurize the reactor during steam generator tube leak, so as to minimize the leakage   of primary coolant.

j) Backup Systems for Control Rods (4 Trains):

      Control rods are passive systems which are designed to drop under gravity. They are    tested extensively in the test set ups and during commissioning. During reactor operation           and annual shutdown, the performance of the rods is monitored. However, even under the postulated failure of control rods (event known as Anticipated Transient Without Scram or ATWS), reactor is designed to shutdown  using following additional safety systems:

·        Emergency Boron Injection System: Injection of boric acid solution to the reactor at high pressure ‐ 16 MPa

·        Quick Boron Injection System (Passive System): Injection of concentrated boric acid            solution to the reactor.

     

k) Systems for catering to Beyond Design Basis Events BDBE (Enhanced Safety Features)

      In line with the current international practices, certain beyond design basis events have              been postulated. To ensure the safety under these conditions, following systems have        been provided. These enhanced safety features are additional systems in KKNPP.

·        Passive Heat Removal System (PHRS):

      Decay heat removal from the core following complete loss of power supply, known as     station black out (SBO).

·        Additional Core Passive flooding system ( passive II stage accumulator):

      Supplies borated water to the reactor core during a multiple failure such as simultaneous       occurrence of LOCA and SBO.

·        System for retaining and cooling of molten core ( Core Catcher):

      Retention and long term cooling of molten core under a postulated severe accident condition.

l) Reactor Containment

      Nuclear steam supply systems are housed in a Reactor Containment, to contain any       release of radioactivity. It also provides protection against external hazards.

·        Salient Features of Containment structure

      i. Double containment structure: Pre‐stressed inner Containment (IC) with leak tight inner           steel liner & Reinforced concrete Secondary Containment.

      ii. Air locks with double doors;

·     Design pressure is 0.4 MPa (g) based on estimated pressure due to loss coolant accident

·        Design temperature is 120ºC

·        Containment has been tested up to a test pressure of 0.46MPa

·        Permissible containment leakage rate is 0.3% volume/day. Leakage rate  observed during containment leak rate test conducted during pre‐commissioning was 0.18 % volume/day.As part of in‐service inspection, containment leak test is carried out periodically.

·        Secondary Containment Designed to withstand

      i. Aircraft Crash (such as Cessna and lear jet aircraft)

      ii. Air Shock wave

· Sub‐atmospheric pressure maintained during normal operation and under accident conditions so as to minimize ground level releases

m) Containment Systems:

      Following systems are provided to maintain the integrity of the containment and its

      functional capability under abnormal conditions:

     

·        Containment Spray System: Condenses steam due to any leakage from the primary or                    secondary system, thus limiting pressure rise in the containment.

·        Annulus passive filtering system (passive system): The annular space between the primary and secondary containments is always maintained at a negative pressure which prevents any ground level releases. During an SBO condition, this negative            pressure is maintained by the natural draught created due to the PHRS operation.

·        Passive Hydrogen Re‐combiners: Hydrogen, if generated during accident conditions, is recombined in Passive Hydrogen Re‐combiners to convert it to water. This prevents any hydrogen ignition within the containment. They are located at various locations within the containment.

n) Supplementary Control Room

      Supplementary control room (SCR) is provided in the shielded control building, to enable essential safety functions and monitoring of all the important parameters in case of main         control room (MCR) becoming inaccessible.

o) Training and Qualification

·        Training ‐ Three Phase Programme

      i.    Operators are graduate engineers with adequate experience

      ii.   Phase – A Orientation course and Examination

      iii. Phase – B Theory & Simulator Training in RF

      iv.  Phase – C Participation in commissioning activities and Simulator Training –

            in India

·        Qualification of O & M

      i.    Licensing of O&M personnel by AERB and their periodic

      ii.   Requalification including managerial cadre.

      iii.  Details of Qualification Methodology Finalized

p) In Service Inspection

     

·     Monitoring of healthiness of equipments and components is conducted as per ISI program.

·        ISI data is compared with baseline data collected during Pre‐Service Inspection

·        Typical systems monitored are

      i.    Reactor coolant pressure boundary.

      ii.   Systems essential for safe reactor shut down and/or safe cooling of nuclear fuel.

      iii.  Containment Systems

      iv.  Other systems and components whose functioning is essential for systems mentioned above.

q) Material Surveillance

·        Material surveillance coupons are installed inside the reactor to assess state of RPV material typically due to neutron irradiation & temperature effects.

·   These set of coupons are withdrawn at specified interval of reactor operation and subjected to destructive testing to assess change in mechanical properties of RPV material.

·     This method provides sufficient lead time for actions, if required.

5.9) VVER Performance Information

            Given in Annexure‐1