Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Csaba Nyárádi, Paks, Hungary


Paks Nuclear Power Plant is the only commercial nuclear facility in Hungary, which has been operational since 1982.There are four VVER-440/215 (PWR) units was build with semicontainment (confinement) as border between the natural environment and nuclear equipments.

The main prerequisite of working is the leak rate of confinement has to be below limit told by national nuclear authority.

The leakage rate has been measured at the end of outage to change fuel. This process is fulfilled every year once on every unit - as usual.

The planned maximum pressure is 250 [kPa] absolute. The first leakage rate test was execute at 120 [kPa], 170 [kPa] and 250 [kPa] starting pressure, with two point method, at the time unit's first commissioning.

We have made the repeated leakage rate test at 120 [kPa]. The executing ILRT requires 12 hours.

Our company's management decided to shorten outage time. By this meaning we have been searching shorter ways to make ILRT.

We have been developing assessment of ILRT's results, equipment and measurement technology.

We have been executing more than a dozen time repeated ILRT with two different methods at same time cooperating with the Institute for Electrical Power Research Co., Division of Power Engineering.

The results and data was analyzed by us, and we have been initiating authorize the stabile pressure method as an authorized process by national nuclear authority to use appointing ILRT.

2 Contents

  1. Abstract
  2. Contents
  3. Construction and mean characteristics of the confinement.
  4. Limit and measuring method of ILRT.
  5. Principle of FASO.
  6. Execution of FASO.
  7. Outcome, conclusions.

3 Construction and mean characteristics of the confinement

The building contains all equipments generating nuclear energy into heat power connecting to first circuit, steam generators, coolant pumps, main closing valve, water cleaning systems, safety depressurisation systems, and other supplements we call confinement.

Fig 1: Confinement building with the Bubble Conde.

The confinement's wall is the third border between the nuclear radiation's source and environment.

Shape of confinement is extremely complicated, it has a lot of small compartments having corner joint welded on liner.

Thickness of concrete is 800÷1500 [mm]. Thickness carbon steel liner 6÷8 [mm], welded with hermetic joints.

Around steam generator box the carbon steel liner is on the outer side of concrete- include rector compartment -, elsewhere on the inner side.

Fig 2:

External dimensions of confinemet building:

lower stage:- 06,50 [m]
higher stage:+ 50,40 [m]
net volume:50 000 ÷ 52 000 [m3]

Containment characteristics during the normal operation:

Temperature:40÷50 [°C]
Pressure:- 2 [mbar]
Relative humidity:20÷30 [%]

Containment characteristics during the Designed Base Accident case:

Max temperature:127 [°C]
Max pressure:0,25 [MPa]
Max. relative humidity:100 [%]
Overpressure's time:12 [min]

There are about 400 hermetical penetrations for different technological pipes - steam, feedwater, cooling materials, sample, measuring purpose -, wiring to control and checking processes, cables for power supply through the liner, four compartment with airlock doors for personal traffic, 32 sealed equipment hatches, one reactor dome, and some different hermetic closures.

4 Limit and measuring method of ILRT

The leakage of confinement is strictly limited, which is one of the fundamental conditions of plant operation. Hungarian Nuclear Authority determines the maximal leakage rate and the error of measure.

The base of this limitation is the inhalation of radioactive methyl-iodine by a subject on the border of a ring has a radius3 [km] near power plant at the Designed Base Accident.

To assess the integrity of the confinement, the utility has to determine the leak rate of the confinement, even there has been a locally uncontrolled hole opened on the barrier of hermetic volume.

Outage for maintenance and refueling is a typical occurrence.

We had carried out a leakage test in full designed pressure - 250 [kPa] - at first start-up phase, and we had measured deformation of walls most loaded by pressure. We had measured the leakage of confinement at three level gauge:

120, 170, 250 [kPa]

We generated the leakage rate curve by pressure and leaka rate from database of first measure. We calculated coefficients for the extrapolation from low pressure to maximal gage by leakage curve. Repeated ILRT are achieved on pressure 120 [kPa].

We had managed a leakage test at 170 [kPa] on each unit in 1994÷1997.

We use two point method , while pressure drops. The first ILRT was term 24 hours at each pressure level, repeated ILRT requires measuring time 6 hours.

We can characterize the volume of hermetic compartment by

Relative Humidity(RH)[%]

We monitories the mean physical parameters of air closed in hermetic volume.

relative humidity10places

We generate this quantity as average of measured data counted average by volume proportion.

The two point method calculates with the mass of closed air at the first and last measuring point.

The instruments used for test:

temperaturePT-100 resistant thermometer
range: 0 ÷ 100 [°C]
accuracy : 0,1 [%] for full scale
relative humidity:Ahlborn FH9716-51B capacity method
range: 20 ÷ 100 [% RH]
accuracy: 0,1 [%] for full scale
pressure:RUSKA 6220-1 absolute pressure gauge meter
range: 0 ÷ 260 000 [Pascal]
accuracy 10-4 [%] for full scale
data compilerAhlborn THERM 5500-3. V4.5

5 Principle of FASO

Company management has decide shortening the outage time. It has assigned a duty to staff of ILRT.

We measured the mass of filling air while pressurized confinement due ILRT to determine net volume of hermetic compartment at each unit.

We decided developing this method based on experiences gained due measuring processes.

Fig 3:

FASO method suppose that,-if the pressure of compartment is stabile, while we charge pressure air to space - the quantity of feed air end leakage are equal. Scheme of principle you can see on picture 3.

We has have produce a measuring system according to assumption of accuracy and responsibility. Requires range to realise task is 20 ÷ 900[kg/hour].

6 Execution of FASO

We selected the measuring edge and measuring flange. It has have made three measuring pipeline overlapping range between them. You can see the arrange of pipelines and measuring part on picture 4. The measuring process assemblages three part.

Fig 4:

First of all we pressurize hermetic volume to 120 000 [kPa]. After a stabilization period starts the measuring process.

First step is a short measure with method loss of mass to determine leakage rate approximately. We can get possibility to count the quantity of feed air required.

The next action is to adjust the airflow, while overpressure keeps at 120 000 [Pa].

The last measure intervention is measuring airflow.

At the end we calculate leakage rate of actual process.

7 Outcome, conclusions

  • We can execute ILRT program using the FASO system during shorter time period as using two point method. The required time with FASO method was 1÷1,5 hour, with two point method was 6 hours.
  • The leakage rates are nearly same value, difference is smaller then limit of accuracy
  • The value of accuracy is lower then limit decided by authority
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