1.3. ALMASI Gamma Spectrometry for Burnup Determination of Spent Fuel Assemblies at the Paks NPP

7/31/2019 1.3. ALMASI Gamma Spectrometry for Burnup Determination of Spent Fuel Assemblies at the Paks NPP

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Gamma spectrometry for burnupdetermination of spent fuel assemblies at

the NPP Paks

I. Almasi, Z. Hlavathy, C. T. Nguyen, P. Nagy, L. Lakosi, T. Parko and I. Pos

TECHNICAL MEETING

ONHOT CELL POST-IRRADIATION EXAMINATION AND POOL-SIDE

INSPECTION OF NUCLEAR FUEL

23-27 May 2011

Smolenice, Slovakia


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Goal

Develop a method for determining the burnup of theassemblies.

Validate the calculation of burnup from the reactor

parameters

Should be independent measurement method


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Why is the measurement important?

If the results of the measurement correspond withcalculated values, the safety margin could be

decreased in the calculation, thus the assemblies

could be used for higher power output

Could be cecked anomalies in the core

Safeguards aspect more precise values


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Method description

Production of Cs-137 is linearly proportional to theneutron fluence, i.e. the number of U-235 fissiled.

Cs-134 is produced in two steps: the fission product

Cs-133, then, by neutron capture Cs-134

direct Cs-134 production from fission is of lowerprobability by orders of magnitude.

The production of Cs-134 is proportional almost the

square of neutron flux.

If the activity of Cs-134 and Cs-137 is known, theburnup can be calculated as

BU= k(ACs-134/ACs-137)

K calculated from reactorphysic parameters


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Method description (cont.)

Why not used directly the activity of Cs-137?

CountrateCs-137(662 keV)=

ACs-137*Ig*f(AbsU)*f(Abscm)*f(Absw)*f(Abscol)*R-2*Deteff

well known or measurable

calculated good precision/can be only estimated.

The Cs-134 has two strong peaks 605 keV, Ig=97.6%,and 796 keV, Ig=85.5%.

The Cs-137 has one peak at 662 keV, Ig=85.1%

Peeks of Cs-134 determined the relative efficiency ofdetection the 662 keV gamma by intrinsic calibration

In this way the geometry dependent parameters will beeliminated


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Equipment


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Dependence of Cs ratio on the distance

y = 4E-05x + 0,5108

0,500

0,600

0,700

0,800

2700 2800 2900 3000 3100

Coordinate (mm)

Cs-134/Cs-137ratio

Only 1% changes at used measuring distance

Cs-134/Cs-137 activity ratio vs. distance


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Measurements

Four assembly groups /6 assemblies, as the core isdivided into 6 sectors same position in each sector

High burnup (44GWd/tU)

Low burnup (10GWd/tU)

39GWd/tU burnup 3,8% initial enrichment

26GWd/tU burnup 2,4% initial enrichment Two control assemblies

Two the neighbor of control assemblies

Altogether 28 assemblies were measured

Scanning of the assemblies of all the 6 sides 5-12 positions by side

measured ~1200 spectra 300-600s each

Took 5 weeks


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Measurements

Burnup profile

0,2

0,3

0,4

0,5

0,6

0,7

1800 2300 2800 3300 3800 4300

vertical coordinate (mm)

Cs-134/Cs-137

ratio

A typical burnup profile and its measured points


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Results

69260

0

0,25

0,5

0,75

11

2

3

4

5

6

69235

0

0,25

0,5

0,75

11

2

3

4

5

6

69249

0

0,25

0,5

0,75

11

2

3

4

5

6

69261

0

0,25

0,5

0,75

11

2

3

4

5

6

69248

0

0,25

0,5

0,75

11

2

3

4

5

6

69275

0

0,25

0,5

0,75

11

2

3

4

5

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Azimuthal profiles of 39 GWd/tU

group at the center of pins


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69249

0,5

0,6

0,7

0,8

0,9

1

1,1

1800 2300 2800 3300 3800 4300Vertical coordinate (mm)

Cs-1

34/Cs-1

37

-180

-120

-60

0

60

120

Burnup profile of a working fuel assembly


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67835

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

1800 2300 2800 3300 3800 4300

Vertical coordinate (mm)

Cs-1

34/Cs

-137

-180

-120

-60

0

60

120

Burnup profile of a control fuel assembly


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67753

0,1

0,3

0,5

0,7

1800 2300 2800 3300 3800 4300

Vertical coordinate (mm)

Cs-134/Cs-137

calc. (-180)

meas. (-180)

calc. (-120)

meas. (-120)

calc. (-60)

meas. (-60)

calc. ( 0)

meas. ( 0)

calc. ( 60)

meas. ( 60)

calc. ( 120)

meas. ( 120)

The Cs-134/Cs-137 ratio from measurements and calculation


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Measured vs. calculated isotop activity ratio Cs-134/Cs-137

y = 1,0149xR

2= 0,9955

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8

Calculated ratio

Measuredratio


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Summary

By this method the burnup of assemblies can bedetermined.

Uncertainty of the method is around 4%. To decreasethe uncertainty the measurement time should beincrease by decreasing the number of measured

positions and/or assemblies. For the best results 2-4 years old assemblies should

be measured.

The method should be checked at other ponds in NPP

Paks first. Its also planned to modify the detector system to

measure ponds without built in collimator, to be ableuse the method for other reactors too.


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Thank You for Your attention!

www.iki.kfki.hu

1.3. ALMASI Gamma Spectrometry for Burnup Determination of Spent Fuel Assemblies at the Paks NPP

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