# GRS Method for Uncertainty and Sensitivity Evaluation of Code Results and Applications

Authors: Horst Glaeser
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

During the recent years,  an increasing interest in computational reactor safety analysis is to replace the conservative evaluation model calculations by best estimate calculations supplemented by uncertainty analysis of the code results. The evaluation of the margin to acceptance criteria, for example, the maximum fuel rod clad temperature, should be based on the upper limit of the calculated uncertainty range. Uncertainty analysis is needed if useful conclusions are to be obtained from “best estimate” thermal-hydraulic code calculations, otherwise single values of unknown accuracy would be presented for comparison with regulatory acceptance limits. Methods have been developed and presented to quantify the uncertainty of computer code results. The basic techniques proposed by GRS are presented together with applications to a large break loss of coolant accident on a reference reactor as well as on an experiment simulating containment behaviour.


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## Body

## 1. Introduction

Best estimate computer codes are used to calculate
postulated loss of coolant accidents and transient in a realistic way and not
in a conservative way. There is an increasing interest in computational reactor
safety analysis to replace the conservative evaluation model calculations by
best estimate calculations supplemented by a quantitative uncertainty analysis.
The USA Code of Federal Regulation (CFR) 10 CFR 50.46 [1], for example, allows either to use a best estimate code
plus identification and quantification of uncertainties, or the conservative
option using conservative computer code models listed in Appendix K of the CFR,
Title 10, Part 50.Code predictions are uncertain due to several sources of
uncertainty, like code models as well as uncertainties of plant and fuel
parameters. These uncertainties, for example, come from scatter of measured
values, approximations of modelling, variation and imprecise knowledge of initial
and boundary conditions. Computer code models are developed based on
experiments which can simulate the complex behaviour of a reactor plant under
accident conditions in a simplified way only. Most of the experiments are
performed in small scale compared to plant size. Uncertainty due to imprecise
knowledge of parameter values in calculations is quantified by ranges and probability
distributions. These distributions should be taken into account for input
parameters instead of one discrete value only.Stochastic
variability due to possible component failures of the reactor plant is not
considered in an uncertainty analysis. The single failure criterion is still taken
into account in a deterministic way. This is a superior principle of safety
analysis and requirements of redundance. The probability of system failures is
part of probabilistic safety analyses, not of demonstrating the effectiveness
of emergency core cooling systems.The aim of the uncertainty analysis is at first to
identify and quantify all potentially important uncertain parameters. Their
propagation through computer code calculations provides probability distributions
and ranges for the code results. The evaluation of the margin to acceptance
criteria, for example, the maximum fuel rod clad temperature, should be based
on the upper limit of this distribution for the calculated temperatures, see
Figure1. Uncertainty
analysis is needed if useful conclusions with regard to prediction capability,
such as maximum cladding temperature, are to be obtained from “best estimate”
thermal-hydraulic code calculations, otherwise single values of unknown
accuracy would be presented for comparison with limits for acceptance.Figure 1
Margin illustration.Section2 describes
the GRS method, Section 3 presents examples of application of the GRS method,
and Section 4 provides conclusions.


## 2. Description of the GRS Method

Among others, GRS method [2] has been developed for the determination of
uncertainties. The state of knowledge about all
uncertain parameters is described by ranges and probability distributions, Figure 2. In order to get
information about the uncertainty of computer code results, a number of code
runs have to be performed. For each of these calculation runs, all identified
uncertain parameters are varied simultaneously. Uncertain parameters are uncertain
input values, models, initial and boundary conditions, numerical values like
convergence criteria and maximum time step size, and so forth. Model
uncertainties are expressed by adding on or multiplying correlations by
corrective terms, or by a set of alternative model formulations. Uncertainties
in noding, to describe the important phenomena, are to be taken into account in
the code validation process. However, alternative noding schemes can be
included in the uncertainty analysis. Code validation results are a fundamental
basis to quantify parameter uncertainties.Figure 2
Consideration of input parameter value ranges instead of discrete values in the GRS method.The selection of parameter values according to their specified
probability distributions, their combination, and the evaluation of the
calculation results requires a method. Following a proposal by GRS, the central
part of the method is a set of statistical techniques. The advantage of using
these techniques is that the number of code calculations needed is independent
of the number of uncertain parameters. In each code calculation, all uncertain
parameters are varied simultaneously. In order to quantify the effect of these
variations on the result, statistical tools are used. Because the number of
calculations is independent of the number of uncertain parameters, no a priori
ranking of input parameters is necessary to reduce their number in order to cut
computation cost. The ranking is a result of the analysis as described later.The number of code calculations depends on the requested probability
content and confidence level of the statistical tolerance limits used in the
uncertainty statements of the results. The required minimum number n of these
calculation runs is given by Wilks’ formula [3, 4], for example, for one-sided
tolerance limits: 1−an≥b,  where b×100 is the confidence level
(%) that the maximum code result will not be exceeded with the probability a×100 (%) (percentile) of the corresponding output distribution, which is to be
compared to the acceptance criterion. The confidence level is specified to
account for the possible influence of the sampling error due to the fact that
the statements are obtained from a random sample of limited size. For two-sided
statistical tolerance intervals, the formula is: 1−an−n(1−a)an−1≥b.
The minimum number of calculations can be found in Table 1.Table 1
Minimum number of calculationsn for one-sided and two-sided statistical tolerance limits.
One-sided statistical tolerance limitsTwo-sided statistical tolerance limitsb/a0.900.950.990.900.950.990.90224523038773880.95295929946934730.99449045964130662The probabilistic treatment of parameter uncertainties allows
quantifying their state of knowledge. This means, in addition to the
uncertainty range, the knowledge is expressed by probability density functions
or probability distributions. This interpretation of probability is used for a
parameter with a fixed but unknown or inaccurately known value. The classical
interpretation of probability as the limit of a relative frequency, expressing
the uncertainty due to stochastic variability, is not applicable here.The probability distribution can express that some values in the
uncertainty range are more likely to be the appropriate parameter value than
others. In the case that no preferences can be justified, uniform distribution
will be specified, that is, each value between minimum and maximum is equally
likely to be the appropriate parameter value. As the consequence of this
specification of probability distributions of input parameters, the computer
code results also show a probability distribution, from which uncertainty
limits or intervals are derived.A total number ofn code runs are performed varying simultaneously the
values of all uncertain input parameters, according to their distribution. The
n values of the considered output parameters are ordered: Y(1)<Y(2)⋯<Y(n−1)<Y(n). Therefore, the name-order statistics is used for Wilks’
formula. On the basis of this ranking, the 95th percentile value with a
confidence level of 95% is obtained by selecting Y(n) with n=59 for the one-sided
tolerance limit, for example. A 5th percentile value with a confidence level of
95% is obtained by selecting Y(1) with n=59. A (95%/95%) two-sided tolerance
limit is obtained by selecting Y(1) and Y(n) with n=93.Another important feature of the method is that one can evaluate
sensitivity measures of the importance of parameter uncertainties for the
uncertainties of the results. These measures give a ranking of input
parameters. This information provides guidance as to where to improve the state
of knowledge in order to reduce the output uncertainties most effectively, or
where to improve the modelling of the computer
code. Sensitivity measures like standardised rank regression coefficients,
rank correlation coefficients, and correlation ratios permit a ranking of
uncertainties in model formulations, input data, and so forth, with respect to
their relative contribution to code output uncertainty. The difference to other
known uncertainty methods, for example, [5], is that the ranking is a result of
the analysis and not of prior estimates and judgements. This prior setup of a phenomena identification and ranking
table (PIRT) by extensive expert staff-hours in [5] is known to be very costly.
Uncertainty statements and sensitivity measures are available simultaneously
for all single-valued (e.g., peak clad temperature) as well as continuous
valued (time dependent) output quantities of interest. The method relies only
on actual code calculations without using approximations like fitted response
surfaces. Similar methods based on the GRS method, and an alternative
uncertainty method is presented in [6].The different steps of the uncertainty analysis according
to the GRS method are supported by the software system for uncertainty and sensitivity analyses (SUSA) developed by GRS [7].
They provide a choice of statistical tools to be applied during the uncertainty
and sensitivity analysis.


## 3. Applications

The GRS method for uncertainty and sensitivity evaluation
of code results can be used for different codes to investigate the combined
influence of all potentially important uncertainties on the calculation results.
Several applications have been performed in GRS to investigate loss of coolant from
the primary and secondary coolant systems of pressurised water reactors, as
well as related experiments. For these
analyses, we used the thermal-hydraulic computer code ATHLET. Another
uncertainty and sensitivity analysis was performed calculating an experiment simulating
containment behaviour using the computer code COCOSYS.


### 3.1. Thermal-Hydraulic Applications Using the ATHLET Computer Code

Several uncertainty and sensitivity analyses
were performed by GRS using the thermal-hydraulic computer code ATHLET simulating
breaks of the primary and secondary side cooling systems of pressurised water
reactors. These are(i)
separate effects experiment OMEGA heater rod bundle Test 9,(ii)
integral experiment LSTF-CL-18, 5% cold leg break, accumulator
injection into cold legs,(iii)
PWR 5% cold leg break, accumulator injection into hot legs
(Siemens/ KWU reactor),(iv)
integral experiment LOFT L2-5,2×100% cold leg break, accumulator
injection into cold legs,(v)
PWR2×100% cold leg break, combined ECC injection into cold
and hot legs,(vi)
PWR 10% steam line break,(vii)
PSB-VVER 11% upper plenum break experiment, UP-11-08 (OECD
PSB-VVER Test1).
One out of these applications is described in the following section.


### 3.2. Application to a German PWR Reference Reactor,2×100% Cold Leg Break

A double ended cold leg offset shear break design basis accident of a
German PWR of 1300 MW electric power is investigated. The fuel rod peak linear
heat generation rate is 530 W/cm. Loss of off-site power at turbine trip is
assumed. ECC injection is into cold and hot legs. The accumulator system is
specified to initiate coolant injection into the primary system below a
pressure of 2.6 MPa. High- and low-pressure ECC injection is available.A single failure is assumed in the broken
loop check valve for ECC injection from accumulator, high- and low- pressure
system, and one hot leg accumulator is unavailable due to preventive
maintenance. These assumptions are considered to be the worst unavailability,
agreed between applicants and assessors.The uncertainty analysis considered 56 uncertain input parameters. These
consist of 37 model parameters, 4 parameters to select different model
correlations for heat transfer and friction, 2 for bypass flow cross sections
in the reactor vessel, 1 for temperature of accumulator water, 1 for core
power, 1 for decay heat, 1 for radial power distribution in the core, 1 for hot
channel factor, 5 for gap width (5 burn-up classes), 1 for fuel thermal
conductivity, and 2 for convergence criteria. The model parameters comprise
critical flow, heat transfer, evaporation, condensation, wall and interfacial
shear, form loss, main coolant pump head, and torque.A total number of 100 calculations were performed using the code ATHLET
Mod 1.2, cycle D [8].


### 3.3. Maximum Clad Temperature

Figure3 shows at any point of time, at least 95%
of the combined influence of all considered uncertainties on the calculated
clad temperatures is below the presented uncertainty limit (one-sided tolerance limit), at a
confidence level of at least 95%. For each instant of time, the desired
tolerance limits were selected from the 100 calculated code results. A
“conservative” calculation result is shown for comparison, applying the best
estimate code ATHLET with default values of the models and conservative values
for the initial and boundary conditions reactor power, decay heat, gap width of
fuel rods between fuel and clad, fuel pellet thermal conductivity, and
temperature of accumulator water.  All these conservative values were also
included in the distributions of the input parameters for the uncertainty
analysis. The maximum clad temperature of the conservative calculation does not
bound the 95%/95% one-sided tolerance limits of the uncertainty analysis over
the whole transient time, for example, after 75 seconds. The regulatory
acceptance criterion for peak clad temperature is 1200°C.Figure 3
Calculated one-sided95%/95% uncertainty limit and best estimate reference calculation compared with a “conservative” calculation of rod clad temperature for a reference reactor during a postulated double ended offset shear cold leg break.The “conservative” calculation is representative for the use of best
estimate computer codes plus conservative initial and boundary conditions. Such
an evaluation is possible in the licensing procedure of several countries, but
not in the USA
.
The uncertainty of code models is not taken into account. The selection of
conservative initial and boundary conditions will bound these model
uncertainties. That is obviously not the case for the whole transient in the
present example. An uncertainty analysis quantifies uncertain initial and
boundary conditions as well as model uncertainties. The peak clad temperatures,
however, are bounded due to cumulating conservative values of the highly
sensitive parameters gap width and pellet thermal conductivity. It is obvious
that the results are dependent on the extent of conservatism implemented in the
conservative calculations. Therefore, the US Code of Federal Regulation [1]
requires that “uncertainties in the analysis method and inputs must be
identified and assessed so that the uncertainty in the calculated results can
be estimated” when a best-estimate computer code is used for the analysis.According to the US
Code of Federal Regulations, Title 10, Section 50.46, the conservative method
requires conservative models to be applied in conformity with the required and
acceptable features listed in Appendix K, “ECCS Evaluation Models” of the
Federal Regulations [1]. This is the main reason why, in the USA
, an additional margin to
licensing criteria is available by changing from conservative evaluation to best
estimate calculations plus uncertainty analysis.The confidence level 95% denominates that the 95th percentile is
overestimated conservatively by 95% probability providing a (95%, 95%)
statement. This conservatism is the reason why some experts claim that a
coverage of a (95%, 95%) statement by a conservative calculation is not needed.
GRS requires coverage unless other suitable methods for comparison and
quantification of “conservatisms” are presented. This could be achieved by an
additional statistical test proving that the conservative calculation bounds
the 95th percentile.


#### 3.3.1. Sensitivity Measures

Sensitivity
measures indicate the influence of the uncertainty in input parameters on calculation
results. For example, the Spearman rank correlation coefficient is used as
sensitivity measure. The length of the bars indicates the sensitivity of the
respective input parameter uncertainty on the first peak clad temperature which
occurs during the blowdown phase; see Figure4. The sensitivity measure gives the variation of the result in terms of
standard deviations when the input uncertainty varies by one standard deviation
(if the input uncertainties are independent). Positive sign means that input parameter
value and result tend to move in the same direction, that is, an increase of uncertain
input parameter value tends to increase the clad temperature and vice versa.
For negative sign, the input parameter value and the result tend to move in opposite
direction, that is, an increase of the parameter value tends to decrease the
clad temperature and vice versa.Figure 4
Sensitivity measures of the blowdown PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.The most important
parameter uncertainties, out of 56 identified potentially important parameters,
with respect to the blowdown peak clad temperature uncertainty are(i)
fuel rod gap width for low burn up (positive sign),(ii)
fuel heat conductivity (negative sign),(iii)
minimum film boiling temperature (negative sign),(iv)
model for critical heat flux (negative
sign: Biasi correlation causes lower clad temperatures due to a later change
from nucleate to transition boiling compared to the Hench-Levy correlation),(v)
reactor initial power (positive sign),(vi)
2-phase multiplier in horizontal pipe (negative sign: higher resistance
of water transport to break location⇒higher water content in core due to
lower break flow ⇒lower clad temperature).
The most important parameters for the peak clad
temperature uncertainty during reflood are, according to Figure 5,Figure 5
Sensitivity measures of the reflood PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.(i)
fuel heat conductivity (negative sign),(ii)
fuel rod gap
width for low burn up (positive sign),(iii)
model for
1-phase convection to steam (positive sign, i.e., Mc Eligot correlation tends
to cause higher clad temperatures than Dittus-Boelter II),(iv)
number of
droplets (negative sign: number of droplets higher⇒higher condensation
⇒lower PCT),(v)
steam-droplet
cooling (negative sign: higher cooling tends to result in lower PCT).


#### 3.4. Application to the Experiment HDR T31.5 Simulating Containment Behaviour

The
experiment T31.5 on the HDR containment facility simulates a large break of a
main coolant pipe, investigating steam and gas release into the containment
according to the low pressure scenario of the German risk study. A short term phase was performed with
emphasis on pressure buildup in the containment and the temperature evolution.The hydrogen distribution was measured during a long term phase over 20 hours,
when steam and a helium-hydrogen
mixture
were injected.A total
number of 200 calculations were performed using the code COCOSYS V0.2 [9]. At
least 95% of the combined influence of all considered uncertainties on the
calculated pressure at a confidence level of at least 95% at any point of time is
shown in Figure 6. A total of 79
uncertain parameters were included, consisting of model parameters, of the
experimental facility, initial and boundary conditions.Figure 6
95%/95% uncertainty interval, reference calculation and experimental values for pressure in the upper part of the containment versus time.Sensitivity
measures about the influence of the
uncertainty in input parameters on the pressure in the upper part of
the HDR containment versus time are presented in Figure7. We see decreasing and increasing high importance
versus time on the maximum pressure. Decreasing influence with time is due to decreasing energy transport with
decreasing convection forFigure 7
Sensitivity measures for pressure in the upper part of the containment versus time.(i)
free convection,
parameter 72, negative sign,(ii)
forced
convection, parameter 73, negative sign,(iii)
condensation
at wall, parameter 74, negative sign.
Increasing with time are the following parameters because of decreasing
convection:(i)
thickness
of liner, parameter 79, negative sign,(ii)
surface
of liner, parameter 77, negative sign,(iii)
heat capacity
of concrete structures, parameter 69, negative sign.


## 3.1. Thermal-Hydraulic Applications Using the ATHLET Computer Code

Several uncertainty and sensitivity analyses
were performed by GRS using the thermal-hydraulic computer code ATHLET simulating
breaks of the primary and secondary side cooling systems of pressurised water
reactors. These are(i)
separate effects experiment OMEGA heater rod bundle Test 9,(ii)
integral experiment LSTF-CL-18, 5% cold leg break, accumulator
injection into cold legs,(iii)
PWR 5% cold leg break, accumulator injection into hot legs
(Siemens/ KWU reactor),(iv)
integral experiment LOFT L2-5,2×100% cold leg break, accumulator
injection into cold legs,(v)
PWR2×100% cold leg break, combined ECC injection into cold
and hot legs,(vi)
PWR 10% steam line break,(vii)
PSB-VVER 11% upper plenum break experiment, UP-11-08 (OECD
PSB-VVER Test1).
One out of these applications is described in the following section.


## 3.2. Application to a German PWR Reference Reactor,2×100% Cold Leg Break

A double ended cold leg offset shear break design basis accident of a
German PWR of 1300 MW electric power is investigated. The fuel rod peak linear
heat generation rate is 530 W/cm. Loss of off-site power at turbine trip is
assumed. ECC injection is into cold and hot legs. The accumulator system is
specified to initiate coolant injection into the primary system below a
pressure of 2.6 MPa. High- and low-pressure ECC injection is available.A single failure is assumed in the broken
loop check valve for ECC injection from accumulator, high- and low- pressure
system, and one hot leg accumulator is unavailable due to preventive
maintenance. These assumptions are considered to be the worst unavailability,
agreed between applicants and assessors.The uncertainty analysis considered 56 uncertain input parameters. These
consist of 37 model parameters, 4 parameters to select different model
correlations for heat transfer and friction, 2 for bypass flow cross sections
in the reactor vessel, 1 for temperature of accumulator water, 1 for core
power, 1 for decay heat, 1 for radial power distribution in the core, 1 for hot
channel factor, 5 for gap width (5 burn-up classes), 1 for fuel thermal
conductivity, and 2 for convergence criteria. The model parameters comprise
critical flow, heat transfer, evaporation, condensation, wall and interfacial
shear, form loss, main coolant pump head, and torque.A total number of 100 calculations were performed using the code ATHLET
Mod 1.2, cycle D [8].


## 3.3. Maximum Clad Temperature

Figure3 shows at any point of time, at least 95%
of the combined influence of all considered uncertainties on the calculated
clad temperatures is below the presented uncertainty limit (one-sided tolerance limit), at a
confidence level of at least 95%. For each instant of time, the desired
tolerance limits were selected from the 100 calculated code results. A
“conservative” calculation result is shown for comparison, applying the best
estimate code ATHLET with default values of the models and conservative values
for the initial and boundary conditions reactor power, decay heat, gap width of
fuel rods between fuel and clad, fuel pellet thermal conductivity, and
temperature of accumulator water.  All these conservative values were also
included in the distributions of the input parameters for the uncertainty
analysis. The maximum clad temperature of the conservative calculation does not
bound the 95%/95% one-sided tolerance limits of the uncertainty analysis over
the whole transient time, for example, after 75 seconds. The regulatory
acceptance criterion for peak clad temperature is 1200°C.Figure 3
Calculated one-sided95%/95% uncertainty limit and best estimate reference calculation compared with a “conservative” calculation of rod clad temperature for a reference reactor during a postulated double ended offset shear cold leg break.The “conservative” calculation is representative for the use of best
estimate computer codes plus conservative initial and boundary conditions. Such
an evaluation is possible in the licensing procedure of several countries, but
not in the USA
.
The uncertainty of code models is not taken into account. The selection of
conservative initial and boundary conditions will bound these model
uncertainties. That is obviously not the case for the whole transient in the
present example. An uncertainty analysis quantifies uncertain initial and
boundary conditions as well as model uncertainties. The peak clad temperatures,
however, are bounded due to cumulating conservative values of the highly
sensitive parameters gap width and pellet thermal conductivity. It is obvious
that the results are dependent on the extent of conservatism implemented in the
conservative calculations. Therefore, the US Code of Federal Regulation [1]
requires that “uncertainties in the analysis method and inputs must be
identified and assessed so that the uncertainty in the calculated results can
be estimated” when a best-estimate computer code is used for the analysis.According to the US
Code of Federal Regulations, Title 10, Section 50.46, the conservative method
requires conservative models to be applied in conformity with the required and
acceptable features listed in Appendix K, “ECCS Evaluation Models” of the
Federal Regulations [1]. This is the main reason why, in the USA
, an additional margin to
licensing criteria is available by changing from conservative evaluation to best
estimate calculations plus uncertainty analysis.The confidence level 95% denominates that the 95th percentile is
overestimated conservatively by 95% probability providing a (95%, 95%)
statement. This conservatism is the reason why some experts claim that a
coverage of a (95%, 95%) statement by a conservative calculation is not needed.
GRS requires coverage unless other suitable methods for comparison and
quantification of “conservatisms” are presented. This could be achieved by an
additional statistical test proving that the conservative calculation bounds
the 95th percentile.


### 3.3.1. Sensitivity Measures

Sensitivity
measures indicate the influence of the uncertainty in input parameters on calculation
results. For example, the Spearman rank correlation coefficient is used as
sensitivity measure. The length of the bars indicates the sensitivity of the
respective input parameter uncertainty on the first peak clad temperature which
occurs during the blowdown phase; see Figure4. The sensitivity measure gives the variation of the result in terms of
standard deviations when the input uncertainty varies by one standard deviation
(if the input uncertainties are independent). Positive sign means that input parameter
value and result tend to move in the same direction, that is, an increase of uncertain
input parameter value tends to increase the clad temperature and vice versa.
For negative sign, the input parameter value and the result tend to move in opposite
direction, that is, an increase of the parameter value tends to decrease the
clad temperature and vice versa.Figure 4
Sensitivity measures of the blowdown PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.The most important
parameter uncertainties, out of 56 identified potentially important parameters,
with respect to the blowdown peak clad temperature uncertainty are(i)
fuel rod gap width for low burn up (positive sign),(ii)
fuel heat conductivity (negative sign),(iii)
minimum film boiling temperature (negative sign),(iv)
model for critical heat flux (negative
sign: Biasi correlation causes lower clad temperatures due to a later change
from nucleate to transition boiling compared to the Hench-Levy correlation),(v)
reactor initial power (positive sign),(vi)
2-phase multiplier in horizontal pipe (negative sign: higher resistance
of water transport to break location⇒higher water content in core due to
lower break flow ⇒lower clad temperature).
The most important parameters for the peak clad
temperature uncertainty during reflood are, according to Figure 5,Figure 5
Sensitivity measures of the reflood PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.(i)
fuel heat conductivity (negative sign),(ii)
fuel rod gap
width for low burn up (positive sign),(iii)
model for
1-phase convection to steam (positive sign, i.e., Mc Eligot correlation tends
to cause higher clad temperatures than Dittus-Boelter II),(iv)
number of
droplets (negative sign: number of droplets higher⇒higher condensation
⇒lower PCT),(v)
steam-droplet
cooling (negative sign: higher cooling tends to result in lower PCT).


### 3.4. Application to the Experiment HDR T31.5 Simulating Containment Behaviour

The
experiment T31.5 on the HDR containment facility simulates a large break of a
main coolant pipe, investigating steam and gas release into the containment
according to the low pressure scenario of the German risk study. A short term phase was performed with
emphasis on pressure buildup in the containment and the temperature evolution.The hydrogen distribution was measured during a long term phase over 20 hours,
when steam and a helium-hydrogen
mixture
were injected.A total
number of 200 calculations were performed using the code COCOSYS V0.2 [9]. At
least 95% of the combined influence of all considered uncertainties on the
calculated pressure at a confidence level of at least 95% at any point of time is
shown in Figure 6. A total of 79
uncertain parameters were included, consisting of model parameters, of the
experimental facility, initial and boundary conditions.Figure 6
95%/95% uncertainty interval, reference calculation and experimental values for pressure in the upper part of the containment versus time.Sensitivity
measures about the influence of the
uncertainty in input parameters on the pressure in the upper part of
the HDR containment versus time are presented in Figure7. We see decreasing and increasing high importance
versus time on the maximum pressure. Decreasing influence with time is due to decreasing energy transport with
decreasing convection forFigure 7
Sensitivity measures for pressure in the upper part of the containment versus time.(i)
free convection,
parameter 72, negative sign,(ii)
forced
convection, parameter 73, negative sign,(iii)
condensation
at wall, parameter 74, negative sign.
Increasing with time are the following parameters because of decreasing
convection:(i)
thickness
of liner, parameter 79, negative sign,(ii)
surface
of liner, parameter 77, negative sign,(iii)
heat capacity
of concrete structures, parameter 69, negative sign.


## 3.3.1. Sensitivity Measures

Sensitivity
measures indicate the influence of the uncertainty in input parameters on calculation
results. For example, the Spearman rank correlation coefficient is used as
sensitivity measure. The length of the bars indicates the sensitivity of the
respective input parameter uncertainty on the first peak clad temperature which
occurs during the blowdown phase; see Figure4. The sensitivity measure gives the variation of the result in terms of
standard deviations when the input uncertainty varies by one standard deviation
(if the input uncertainties are independent). Positive sign means that input parameter
value and result tend to move in the same direction, that is, an increase of uncertain
input parameter value tends to increase the clad temperature and vice versa.
For negative sign, the input parameter value and the result tend to move in opposite
direction, that is, an increase of the parameter value tends to decrease the
clad temperature and vice versa.Figure 4
Sensitivity measures of the blowdown PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.The most important
parameter uncertainties, out of 56 identified potentially important parameters,
with respect to the blowdown peak clad temperature uncertainty are(i)
fuel rod gap width for low burn up (positive sign),(ii)
fuel heat conductivity (negative sign),(iii)
minimum film boiling temperature (negative sign),(iv)
model for critical heat flux (negative
sign: Biasi correlation causes lower clad temperatures due to a later change
from nucleate to transition boiling compared to the Hench-Levy correlation),(v)
reactor initial power (positive sign),(vi)
2-phase multiplier in horizontal pipe (negative sign: higher resistance
of water transport to break location⇒higher water content in core due to
lower break flow ⇒lower clad temperature).
The most important parameters for the peak clad
temperature uncertainty during reflood are, according to Figure 5,Figure 5
Sensitivity measures of the reflood PCT with respect to the selected 56 uncertain input parameters (rank correlation coefficient) for the reference reactor large break.(i)
fuel heat conductivity (negative sign),(ii)
fuel rod gap
width for low burn up (positive sign),(iii)
model for
1-phase convection to steam (positive sign, i.e., Mc Eligot correlation tends
to cause higher clad temperatures than Dittus-Boelter II),(iv)
number of
droplets (negative sign: number of droplets higher⇒higher condensation
⇒lower PCT),(v)
steam-droplet
cooling (negative sign: higher cooling tends to result in lower PCT).


## 3.4. Application to the Experiment HDR T31.5 Simulating Containment Behaviour

The
experiment T31.5 on the HDR containment facility simulates a large break of a
main coolant pipe, investigating steam and gas release into the containment
according to the low pressure scenario of the German risk study. A short term phase was performed with
emphasis on pressure buildup in the containment and the temperature evolution.The hydrogen distribution was measured during a long term phase over 20 hours,
when steam and a helium-hydrogen
mixture
were injected.A total
number of 200 calculations were performed using the code COCOSYS V0.2 [9]. At
least 95% of the combined influence of all considered uncertainties on the
calculated pressure at a confidence level of at least 95% at any point of time is
shown in Figure 6. A total of 79
uncertain parameters were included, consisting of model parameters, of the
experimental facility, initial and boundary conditions.Figure 6
95%/95% uncertainty interval, reference calculation and experimental values for pressure in the upper part of the containment versus time.Sensitivity
measures about the influence of the
uncertainty in input parameters on the pressure in the upper part of
the HDR containment versus time are presented in Figure7. We see decreasing and increasing high importance
versus time on the maximum pressure. Decreasing influence with time is due to decreasing energy transport with
decreasing convection forFigure 7
Sensitivity measures for pressure in the upper part of the containment versus time.(i)
free convection,
parameter 72, negative sign,(ii)
forced
convection, parameter 73, negative sign,(iii)
condensation
at wall, parameter 74, negative sign.
Increasing with time are the following parameters because of decreasing
convection:(i)
thickness
of liner, parameter 79, negative sign,(ii)
surface
of liner, parameter 77, negative sign,(iii)
heat capacity
of concrete structures, parameter 69, negative sign.


## 4. Conclusions

Two applications of the uncertainty method proposed by
GRS are presented. A significant advantage of this methodology is that no a
priori reduction in the number of uncertain input parameters by expert
judgement or screening calculations is necessary to limit the calculation
effort. All potentially important parameters may be included in the uncertainty
analysis. The method accounts for the combined influence of all identified
input uncertainties on the results. This would be difficult or even impossible
to achieve by a priori expert judgement of loss of coolant accidents or
transients.The number of calculations needed is independent of
the number of uncertain parameters accounted for in the analysis. It does,
however, depend on the requested tolerance limits, that is, the requested probability
coverage (percentile) of the combined effect of the quantified uncertainties,
and on the requested confidence level of the code results. The tolerance limits
can be used for quantitative statements about margins to acceptance criteria.Another important feature of the method is that it
provides sensitivity measures of the influence of the identified input
parameter uncertainties on the results. The measures permit an uncertainty importance
ranking. This information provides guidance as to where to improve the state of
knowledge in order to reduce the output uncertainties most effectively, or
where to improve the modelling of the computer code. Different to other known
uncertainty methods, the ranking is a result of the analysis and its inputs and
not of an a priori expert judgement. Uncertainty statements and sensitivity
measures are available simultaneously for all single-valued (e.g., peak
cladding temperature) as well as continuous valued (time dependent) output
quantities of interest. The method relies only on actual code calculations
without the use of approximations like fitted response surfaces. The method
proposed by GRS has been used in different applications by various international
institutions including licensing.A challenge in performing uncertainty analyses is the
specification of ranges and probability distributions of input parameters.
Investigations are underway to transform data measured in experiments and post
test calculations into thermal-hydraulic model parameters with uncertainties.
Care must be taken to select suitable experimental and analytical information
to specify uncertainty distributions. This is a general experience gained in applying
different uncertainty methods.



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*Source: 798901-2008-03-26.xml*