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Friday, June 20, 2014

How does Nuclear Plant construction done?

Many might be wondering how does Nuclear Plant reactors (Domes) are constructed. 

Reactor Building

The 88 meters tall structure of the main Reactor building has been done with a novel raft design for the reactor structure. The reactor building raft has a foundation at 8.85 meters below the ground level. The thickness of the raft foundation is 4.6 meters at the end and 1.6 meters in the middle. The containment base slab is at 1.1 meters above ground level and 5.35 meters above the foundation which is of 1800mm thick and the total concrete quantity of this slab is 6000 Cum. The containment base slab supports the core of the nuclear reactor placed in the reactor cavity at the center of the containment structure.

Inner Containment Structure

  • · Inner containment wall starts from +5.35 M above ground level and goes up to +43.9 meters as cylindrical part with a dia of 44 meters.
  • · The hemispherical dome starts from +43.9 meters with radius of 22 meters. 
  • · The top of Inner Containment Dome is +67.10 meters.
  • · The inner containment wall is 1200mm thick and outer containment wall is 600 mm. The annular space between inner containment and outer containment is 2200 mm

A special feature undertaken in India for the first time is a completely steel lined dome of the inner containment wall and dome for the reactor building. The dome was constructed in three major parts.
  • · Part I – Liner segment from elevation + 43.9M to elevation +49.5M, was fabricated and erected in 15 segments, similar to cylindrical liner.
  • · Part II – Liner segment from elevation +49.5M to elevation +57.1M was fabricated in 15 segments and pre-assembled at ground.
  • · Part III – Liner segment from elevation +57.1M to elevation +65.7M was fabricated in 15 segment and pre-assembled in the ground, inside the dome Part-II assembly area.
  • · Part IV – Small part of the apex from elevation +65.7 to elevation +65.91 was fabricated and erected separately.
Erection of the Dome:
  • · The erection of the dome Part-II & III were carried out using 650MT Liebherr Crane.
  • · A circular evener beam of diameter 16.9M weighing 15MT was specifically designed, analyzed by finite element method, verified, and fabricated.
  • · There were 15 slinging points from the dome parts to the evener beam and 4 slinging points from the evener beam to the crane hook.
  • · Before erection of dome Part-II, dome Part-III was lifted and matched with Part-II on the ground level itself, and connecting plate welded to the part-II portion.
  • · The crown portion from EL +65.7 to EL +65.91 was fabricated and erected separately.
  • · The fabrication and erection of the dome is carried out with in strict tolerance limits.
Another special feature of the construction is the Unbounded Prestressing system adopted for a Reactor Containment Structure for the first time in the world. The prestressing system use tendons consisting of 55 strands of 15.7mm diameter of high strength steel wires. There are 68 numbers of horizontal tendons, circular in plan and 60 numbers of inverted U-shaped vertical tendons which are anchored at both ends of the stressing gallery which is a circular corridor located on the ground level of the Reactor building. 219.1mm steel pipes are used as the tendon ducts for the inverted U shaped vertical tendons and 200mm diameter dross Bach ducts are used for the horizontal tendons. Generally use of a prestressing system reduces the thickness of the inner containment wall while offering additional strength. In the case of the unbounded system the advantages are

High efficiency of pre-stressing is possible due to very low friction co-efficient (0.05). This may help in:
  • · Possibility of using tendons of longer length
  • · Possibility of using less no. of tendons 
  • · Possibility of higher spacing of tendons
  • · More uniform stress in the tendons and in concrete due to low variation of force along the strands.
Construction team has been instrumental in modifying & refining the techniques and procedures for strand threading and grouting to achieve the required perfection by study and analysis of problems during a full scale mock-up of a horizontal circular tendon, which has been accepted by AERB, India.

Unique features of project:

The Main Reactor Buildings are the heart of each unit designed to produce 1000 MW power each – by far the largest Reactors ever built in India. These are not only the largest but the first to have the following construction features

1. The inner and outer containment structures have complete hemispherical dome. The outer containment dome is further protected by a PHRS (Passive Heat Removal System) dome.

2. The inner containment floor, wall and dome are completely lined with steel liner on the inner face.

3. The inner containment also has 45 steel brackets; over which 15 steel beams of box section ultimately supports 30 rails. This rail of 21m radius is provided for supporting a 350t capacity Polar Crane.

4. The inner containment structure is pre-stressed by 55C15 type tendons, each tendon constituting 55 HDPE sheathed strands of 15.7mm nominal diameter. This is the first time in the World that an un-bonded pre-stressing system has been adopted for a reactor containment

Overview of the VVER-1000 Reactor operations:

The reactor vessel for VVER-1000 plant is designed to contain the vessel internals and fuel assemblies of the core. The reactor along with control rod drive units has overall height of 19 meters and diameter of 4.5 meters. It is a vertical cylindrical container made of 190mm thick high purity low alloy steel ring forging, welded together and cladded inside with stainless steel.

The reactor is placed in a concrete pit inside the containment. The reactor coolant system (RCS) transfers the heat generated in the reactor core to the steam generators where steam is produced to drive the turbine-generator. The borated demineralised water coolant of RCS also acts as a neutron moderator and reflector and as a solvent for the neutron absorber. The RCS pressure boundary provides a barrier against the release of radioactivity generated in the reactor and is designed to ensure a high degree of integrity throughout the life of the plant.

The turbine is designed to operate at 3000rpm on saturated steam in conjunction with VVER-1000 reactor having thermal output of 3000MW(th). The rated output at the generator terminal will be 1000MW based on cooling water temperature of 32oC and steam dryness factor of 0.995. The electricity generated will be supplied to the southern grid.

Challenges encountered in project execution

Nuclear Power Plant construction involved many new features and challenges which were not anticipated earlier.

Fabrication and erection of liners for Inner Containment

  • · Detailed planning was required to fabricate the liners for the 44m diameter walls.
  • · The cylindrical part of liners to cover a height of 38.5m, have been made in 8 tiers with 15 segments. 
  • · The maximum height of one tier is about 6.5m.
  • · Each segment is made out of 6mm thick steel sheet stiffened by structural sections on the back side.
  • · The liners were fabricated to stringent dimensional tolerance of - 0 / +10mm radius and all welded joints were subjected to 100% leak tightness test by Vacuum Box and 20% Ultrasonic examination.
  • · In order to take care of correct fabrications, jigs were fabricated, inspected and cleared for each      type of panel for mass production.
  • · A massive fabrication shop of 15m x 57m was set up with 15mt EOT crane. 
  • · Annexed shop for Grit Blasting and Painting was set up.
  • · To facilitate transfer of the large fabricated panels to grit blasting & painting shop, in-house tailor-made transfer trolleys on rails were erected.
Erection of Dome Liners

The containment dome liner is of hemispherical shape of 22m radius, made out of 6mm thick steel plates with circumferential and radial stiffeners on the outer side.

· Part I of the dome liner of 5.60m height was assembled of 15 segments in-situ.

· Part II of height 7.6m & Part III of height 8.8m were assembled at ground level, lifted and then placed in the position.

Lifting of Part II Dome Liner

This part of truncated hemispherical shape, weighing 90 tones, was lifted and placed in position over the existing Part I dome of RB-1 on 17th July 2006. 650 tones capacity crane was used for erection. The special features of this erection work are as follows:

  • · This is the first time in India such a huge dome was fabricated and erected. There was no direct experience available in the country.
  • · Considering the low stiffness and open shape, it has become essential to ensure that the dome does not distort during lifting. It has also become essential to balance the structure and restrict the possible sway of the structure during lifting.
  • · Considering the Safety of the crane against wind load acting on the dome, the erection had to be carried out in relatively calm weather. The acceptable wind velocity was 35 Km/Hr maximum. But during this time of the year, there was wide fluctuation of wind speeds in this coastal area and quite often it goes up to 60 Km/Hr.
  • · However, several steps were taken to take care of the above problems, at the planning and execution stage. 
  • · A no-load mock-up of the 650 tones crane was carried out in the initial stage to locate the starting position of the crane and the dome and the vertical clearance required for taking and placing to position. The sequence of lifting, marching and swinging was meticulously worked out well in advance to avoid any unforeseen problems.
  • · An evener beam of 16.9m diameter was designed and fabricated by CONSTRUCTION for controlling distortion and sway of the dome. Safety of the evener beam was checked with advanced analytical tools using finite element technique.
  • · The evener beam is connected to the main hook of the crane with 4 slings of 80mm size.
  • · In turn the dome is connected to the evener beam with 15 slings of 38mm size. These slings have been specifically made for this job and load tested, before its use, and certified by a third party agency.
  • · This evener beam as well as the sling hooking points was checked for integrity by carrying out non-destructive tests after load testing.
  • · Two of the four tower cranes at RB-1 were lowered to below 53m level to ensure adequate clearance from the bottom of the dome as well as from the mast and boom of the crane during the swinging and lowering operation.
  • · The time of erection was decided after studying the hourly wind speed variation data available from the Project Meteorological laboratory. The afternoon period from 2PM to 7PM was found to be most favorable.
However, continuous monitoring of wind speed was done during the erection.

Time Consumed

Actual lifting was started at 4PM and the placing in position including arresting of support points, was completed by 6PM - in just two hours. Just after 16 days from the date of lifting, i.e. on 2nd August 06, the complete fit-up and welding of dome Part II to the dome Part I was completed.

Lifting of Part –III Dome Liner
  • · This part of hemispherical shape, weighing 180 tones (including 25 tones weight of the sprinkler system fixed on the inner face of the liner) was lifted and placed in position over dome Part II 
  • · The methodology followed was similar to erection of dome Part II, except the following features.
  • · This part was load tested after fixing of the sprinkler system, and the integrity of the evener beam and the slinging points were again checked by non-destructive tests (including UT, RT and DPT)
  • · This time the available meteorological data indicated that the wind speed would be low in the morning, between 6AM to 9AM.
  • · The completion of welding of Part II liner and checking of its dimensions in-situ had to be ensured before lifting Part III.
  • · Protocols related to satisfactory installation of the Polar Crane within the containment had to be completed by NPCIL before allowing this erection.

Time Consumed

Total duration was 1hr 20min.

Passive heat removal system structure (PHRS)

This was indicated as a simple structure with a tertiary dome starting from a cantilevered slab at 52.87m and going up to 81.00m & PHRS enclosure. However when detailed drawings were given the work involved complex works of rib walls, slabs and load requirement for the slab which involved fabrication and erection of about 650MT of special staging, to support the slab and retain the same till some design considerations were met.

The main features of this structure were:

1. Structure starts at 36.00m El cantilevered from Outer-containment wall

2. Cantilever portion: 7.70m

3. Thickness of slab is: 600mm

4. Circular wall from the edge of the cantilever starts at 36.60m and up to 52.27m.

5. Partial 600mm slab connected from outer-containment wall to outer wall of PHRS at 43.20m.

6. Rib walls starts at 43.20m above the slab and connected to outer wall of PHRS.

7. Load on the staging for the below portion of outer wall is to be considered PHRS wall up to +52.27m and slab load at two elevations.

Polar Crane

Polar Crane Main Beam assembly over supporting structures
The 350ton capacity polar crane runs on a circular rail-beam assembly on brackets fixed on the Inner-containment wall. The fabrication of brackets and beams and erection of the same posed real challenges. The concerned item says “fabrication & erection of brackets and beams for crane assembly“. However when detailed drawings were issued the complexity involved and amount of details required to be attended, as well as the complex sequence turned this job into “Precision engineering work”, totally different from what was anticipated.

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