a. molnár, a. keresztúri, e. temesvári , l. korpás * kfki atomic energy research institute

KFKI Atomic Energy Research InstituteKFKI Atomic Energy Research Institute

Statistical evaluation of the on line core monitoring Statistical evaluation of the on line core monitoring effectiveness for limiting the consequences of the fuel effectiveness for limiting the consequences of the fuel

assembly misloading eventassembly misloading event

A. A. Molnár, Molnár, A. Keresztúri, E. A. Keresztúri, E. TemesváriTemesvári, L. , L. KorpásKorpás**


*NPP *NPP PaksPaks

HungaryHungary

AER Symposium, Yalta, Crimea, Ukraine, September, 200AER Symposium, Yalta, Crimea, Ukraine, September, 20077

2


OutlineOutline

The safety related function of theThe safety related function of the online core monitoring online core monitoring in the defense in depth concept in the defense in depth concept Indication methodsIndication methodsUncertainties of the measurements of the temperature Uncertainties of the measurements of the temperature risesrisesDescription of the applied Monte Carlo methodDescription of the applied Monte Carlo methodThe results: confidenceThe results: confidence level of the indicationlevel of the indicationConclusionsConclusions

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The role of onlineThe role of online corecore monitoring monitoring according to according to „„DEFENSE IN DEPTH IN DEFENSE IN DEPTH IN NUCLEAR SAFETY, INSAG-10, A report by the International Nuclear Safety NUCLEAR SAFETY, INSAG-10, A report by the International Nuclear Safety Advisory Group, INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, Advisory Group, INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 19961996””

Level1 Prevention of abnormal operationand failures

Conservative design and highquality in construction andoperation

Level2 Control of abnormal operation and detection of failures

Control, limiting and protectionsystems and other surveillance features

Level3 Control of accidents within the design basis

Engineered safety features and accident procedures

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The functions of the on line monitoringThe functions of the on line monitoring

1. Indication of the abnormal events like1. Indication of the abnormal events like- fuel assembly misloading- fuel assembly misloading; ; onlyonly thethe onlineonline monitoringmonitoring cancan detect,detect,- inadvertent misalignment of the Control Assemblies,- inadvertent misalignment of the Control Assemblies,-- partial or full blockage of one or more coolant channels (for example partial or full blockage of one or more coolant channels (for example due to crud deposition).due to crud deposition).

2. Utilizing the measurements for refining the online calculated 2. Utilizing the measurements for refining the online calculated “frame parameters” of the local power limitations, like maximum “frame parameters” of the local power limitations, like maximum linear heat rate, pin power, sub-channel outlet temperature.linear heat rate, pin power, sub-channel outlet temperature.

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The objectives, the questions to be answeredThe objectives, the questions to be answered

How to indicate the abnormal event of misloadingHow to indicate the abnormal event of misloading.. WWhat combination hat combination of the online measured (and calculated) quantities can be applied as of the online measured (and calculated) quantities can be applied as „frame parameters” for this purpose.„frame parameters” for this purpose.

How to evaluate the uncertainties of the measured data, determining How to evaluate the uncertainties of the measured data, determining the above “frame parameter” uncertainties.the above “frame parameter” uncertainties.

How to built the above uncertainties into the appropriate Operational How to built the above uncertainties into the appropriate Operational Indication Level („OIL”) of the abnormal event - in order to indicate it Indication Level („OIL”) of the abnormal event - in order to indicate it with high confidence level but to avoid the erroneous indication in with high confidence level but to avoid the erroneous indication in normal operation at high probability, too.normal operation at high probability, too.

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The objectives, the questions to be answeredThe objectives, the questions to be answered (Cont.) (Cont.)

What are those satisfactory configurations of the measurements (number and What are those satisfactory configurations of the measurements (number and position of the detectors) at which the above, to some extent contradictory position of the detectors) at which the above, to some extent contradictory requirements are met, or with other words, the given safety related function of requirements are met, or with other words, the given safety related function of the online monitoring is fulfilled.the online monitoring is fulfilled.

In the given special case, all the above questions are to be answered as a In the given special case, all the above questions are to be answered as a function of the reactor power, because the abnormal event must be indicated function of the reactor power, because the abnormal event must be indicated in due time, after the reloading during the uploading phase at lower power in due time, after the reloading during the uploading phase at lower power rate, when the consequences of the local power perturbations are still milder.rate, when the consequences of the local power perturbations are still milder.

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Symbolic axes of limits of different sorts and for the consequences

Best estimate safety limits (SL) to avoid fuel failure due to DNB (LEVEL3)

Safety margin

Indication levels

TIL: “True” indication level (LEVEL2),

enveloping frame parameter for the DBA analyses

Normal operation level

OIL: Operational early indication level (LEVEL2)

M1: Margin for

uncertainties

M2: Margin for

uncertainties

„True” characteristics

of itnitial and end states

of the DBA anelyses

(examples)

„True” local power limit (LEVEL1),

enveloping frame parameter for the DBA analyses

Local power limitations at normal operation

Margin for

uncertainties

Operational local power limit (LEVEL1)

Reactor states not covered by the DBA analyses and to be indicated with high confidence level in due time

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Indication methodsIndication methods

)(

),()(max)(

iT

jiTiTjiAF

Evaluation of the measured asymmetry factors for the Evaluation of the measured asymmetry factors for the ii-th -th radial position radial position ofof a symmetry sector a symmetry sector

wherewhere

- - tthe he jj index index stands for the equivalent positions of the stands for the equivalent positions of the different symmetry sectorsdifferent symmetry sectors

- is the average temperature rise of the equivalent - is the average temperature rise of the equivalent positions of the positions of the ii-th position inside the sector-th position inside the sector

Comparison of the calculated (not refined!) and measured Comparison of the calculated (not refined!) and measured temperature rise values temperature rise values

T i

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Evaluation of the temperature rise measurement Evaluation of the temperature rise measurement uncertaintiesuncertainties

Symmetry scattering:Symmetry scattering:

wherewhere

- the - the jj index stands for the equivalent positions of the index stands for the equivalent positions of the different symmetry sectorsdifferent symmetry sectors

- is the average temperature rise in the - is the average temperature rise in the ii-th -th position of the symmetry sector at the position of the symmetry sector at the kk-th reactor state-th reactor state

- - N(i)N(i) is the number of available measurements in the is the number of available measurements in the symmetric positionssymmetric positions

)(

),,(

),(

)(

1

iN

kjiT

kiT

iN

j

1)(

),,(),(),(

2)(

12

iN

kjiTkiTkiT

iN

j

,T i k

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Evaluation of the temperature rise measurement Evaluation of the temperature rise measurement uncertaintiesuncertainties

Power Symmetry scattering Relative symmetry scattering

100 % 0.53 oC 1.6 %

55 % 0.33 oC 1.8 %

30 % 0.25 oC 2.3 %

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Reading the basic input parameters: TIL, OIL, selection parameter for the

abnormal states, number of Monte Carlo runs, …

Selection of the precalculated “true” normal and abnormal temperature rise distribution according to the reactor

power

Sampling the calculated and measured normal and abnormal temperature rise distributions

Sampling the detector availability

Cycle according to the selected abnormal states

Cycle according to the Monte Carlo runs

Algorithm for indication of the abnormal state,

Counting of the indications

The Monte Carlo The Monte Carlo algorithm for algorithm for evaluation of P(M1) evaluation of P(M1) and P(M2)and P(M2)

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The investigated variantsThe investigated variants

Reactor powerReactor power

100 %, the relative error of the temperature rise is 1.6 %100 %, the relative error of the temperature rise is 1.6 %55 %, the relative error of the temperature rise is 1.8 %55 %, the relative error of the temperature rise is 1.8 %30 %, the relative error of the temperature rise is 2.3 %30 %, the relative error of the temperature rise is 2.3 %

Available temperature measurements above the assemblies Available temperature measurements above the assemblies

All the 210 measurements are available.All the 210 measurements are available.75 % of the measurements is available and at least in the second 75 % of the measurements is available and at least in the second neighbor of each radial position a measurement is existing.neighbor of each radial position a measurement is existing.

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The investigated variants (Cont.)The investigated variants (Cont.)

Abnormal (and normal) state(s)Abnormal (and normal) state(s)

The less recognizable but not covered by the DBA analyses abnormal The less recognizable but not covered by the DBA analyses abnormal state. According to the investigations this is an exchange of two state. According to the investigations this is an exchange of two neighbor assemblies (No. 221 and 222 for Unit 1 Cycle 15) leading to an neighbor assemblies (No. 221 and 222 for Unit 1 Cycle 15) leading to an asymmetry factor of 7.5 %asymmetry factor of 7.5 %Statistical selection of the exchanges with equivalent probabilityStatistical selection of the exchanges with equivalent probabilityNormal operation state (must not be indicated)Normal operation state (must not be indicated)

Indication method:Indication method:

Measured asymmetry factorMeasured asymmetry factorComparison of the calculated and measured temperature distributionsComparison of the calculated and measured temperature distributions

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70

75

80

85

90

95

100

105

4.5 4.7 4.9 5.1 5.3 5.5 5.7 5.9

Operational Indication Level (OIL) of asymmetry factor (%)

P(M

1) a

nd

P(M

2) (

%)

P(M1): Probability of indication of theabnormal state; 75% of the det.


P(M2): Probability of not indication of thesymmetric state; 75% of the det.


Probabilities of indicating the less recognizable abnormal state and Probabilities of indicating the less recognizable abnormal state and that for the normal operation, 100 % power, indication based on the that for the normal operation, 100 % power, indication based on the asymmetry factor asymmetry factor

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70

75

80

85

90

95

100

105

5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7

Operational Indication Level (OIL) of asymmetry factor (%)

P(M

1) a

nd

P(M

2) (

%)





Probabilities of indicating the less recognizable abnormal Probabilities of indicating the less recognizable abnormal state and that for the normal operation, 55 % power, state and that for the normal operation, 55 % power, indication based on the asymmetry factorindication based on the asymmetry factor

16


70

75

80

85

90

95

100

105

6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8

Operational Indication level (OIL) of asymmetry factor (%)

P(M

1) a

nd

P(M

2) (

%)





Probabilities of indicating the less recognizable abnormal Probabilities of indicating the less recognizable abnormal state and that for the normal operation, 30 % power, state and that for the normal operation, 30 % power, indication based on the asymmetry factor indication based on the asymmetry factor

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90

92

94

96

98

100

102

6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8


P(M

1) a

nd

P(M

2) (

%)

P(M1): Probability of indication of theabnormal state; 75% of the det.P(M1): Probability of indication of theabnormal state; 100% of the det.P(M2): Probability of not indication of thesymmetric state; 75% of the det.P(M2): Probability of not indication of thesymmetric state; 100% of the det.

Probabilities of indicating abnormalProbabilities of indicating abnormal,, states selected states selected randomlyrandomly,, and that for the normal operation, 30 % power, and that for the normal operation, 30 % power, indication based on the asymmetry factorindication based on the asymmetry factor

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80

85

90

95

100

105

9 9.5 10 10.5 11 11.5


P(M

1)

an

d P

(M2

) (%

)





Probabilities of indicating the less recognizable Probabilities of indicating the less recognizable abnormal state and that for the normal operation, 100 % abnormal state and that for the normal operation, 100 % power, indication based on the comparison of power, indication based on the comparison of calculation with measurements calculation with measurements

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SummarySummary

A Monte Carlo method was developed for the statistical evaluation A Monte Carlo method was developed for the statistical evaluation of the on line core monitoring effectiveness. The method was applied of the on line core monitoring effectiveness. The method was applied for the case of the fuel assembly misloading indication. for the case of the fuel assembly misloading indication.

The standard deviation of measurements, necessary for the Monte The standard deviation of measurements, necessary for the Monte Carlo sampling procedure, was obtained from the on line monitoring Carlo sampling procedure, was obtained from the on line monitoring system measurements, performed at symmetric states. The great system measurements, performed at symmetric states. The great advantage of the outlined procedure is that both the uncertainty advantage of the outlined procedure is that both the uncertainty estimation and the abnormal state indication are based on the same estimation and the abnormal state indication are based on the same type of measurements. type of measurements.

The investigations proved the satisfactory effectiveness of the The investigations proved the satisfactory effectiveness of the online core monitoring down to 55 % power even in case when only online core monitoring down to 55 % power even in case when only the 75 % of the temperature measurements are available and if the the 75 % of the temperature measurements are available and if the indication is based on the measured asymmetry factor.indication is based on the measured asymmetry factor.

a. molnár, a. keresztúri, e. temesvári , l. korpás * kfki atomic energy research institute

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