Quantitative Phenomena Identification and Ranking Table
(QPIRT) for Reactor Safety Analysis
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Citation
Yurko, Joseph, and Jacopo Buongiorno. "Quantitative
Phenomena Identification and Ranking Table (QPIRT) for
Reactor Safety Analysis." Transactions of the American Nuclear
Society, Vol. 104, Hollywood, Florida, June 26-30, 2011.
As Published
http://epubs.ans.org/?p=trans:104&pg=5
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American Nuclear Society
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Author's final manuscript
Accessed
Wed May 25 22:08:47 EDT 2016
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http://hdl.handle.net/1721.1/87057
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Quantitative Phenomena Identification and Ranking Table (QPIRT) for Reactor Safety Analysis
Joseph Yurko* and Jacopo Buongiorno
MIT 77 Massachusetts Avenue Cambridge, MA 02139
*jyurko@mit.edu, (617)715-2621
INTRODUCTION
Next generation reactor safety analysis codes are
intended to make use of advanced numerical methods and
higher fidelity models with built-in sensitivity analysis
(SA) and uncertainty quantification (UQ) [1]. However,
due to the complex nature of uncertainty propagation in
thermal-hydraulic codes, it is crucial to first narrow the
focus to only the most important processes contributing to
a particular figure of merit (FOM). Traditionally,
Phenomena Identification and Ranking Tables (PIRTs)
based on expert opinion have been used to guide selection
of the “most important processes”. For example, during
the blowdown phase of a large break loss of coolant
accident (LB-LOCA), the fuel rod stored energy is
usually judged to be very important (ranked a 9 out 9 in
the PIRT), and during the reflood phase in the core,
quenching heat transfer is judged to be very important
(also ranked a 9) [2]. However these expert opinion
PIRTs are qualitative and subjective in nature, and fail to
identify exactly what parameters contribute to the FOM
the most.
The goal of the current work is to develop
aQuantitative PIRT (QPIRT), which, using the output of a
system code (e.g. RELAP5), allows tracking of the
dominant processes through both time and space and
results in an automatically generated ranking of said
processes. The QPIRT can be used to downselect the
processes to be investigated through a formal SA and UQ.
QPIRT FORMULATION
A “Top-Down” approach is used to assess the
contribution of different processes on the FOM. To
illustrate the methodology, the peak clad temperature
(PCT) during a LB-LOCA is chosen as the FOM here;
however, the formulation is applicable to any other
transients and/or FOM. The methodology comprises the
following steps:
i
Run a code (e.g. RELAP5) to simulate the LBLOCA
ii Identify the location (node) and time of the
maximum PCT
iii For that node and time step, re-cast the clad
energy equation as PCT=…
iv Using the code output in that re-cast energy
equation, identify the processes that affect the
PCT value the most at that location and time
step. For example, you may find that the PCT is
most affected by the heat transfer to the coolant
vapor phase.
v Re-cast the energy equation for the vapor phase
as Tv=…
vi Using the code output in that re-cast energy
equation, identify the processes that affect T v the
most at that location and time. For example, you
may find that Tv is most affected by the
convective inflow of vapor energy in that cell. If
so, next shift the attention to the cell from which
that inflow is coming from, and determine the
dominant processes determining the convective
outflow of vapor energy from that cell.
vii Repeat steps v and vi for all important processes
identified in step iv.
viii Repeat procedure for all processes identified in
step vi and so on.
The result is a ‘tree’ of dominant processes
through space and time eventually leading to some
initial and/or boundary conditions, where the analysis
stops. All intermediate processes are tracked and
assigned a quantitative weight based on the
importance of their contribution in the respective
energy equations. In the end, it is possible to state
that the max PCT at location x and time step y is
most affected by, say, the temperature of the vapor
entering the core at time step z or the decay heat
generation at location x from time step y to time step
z. A complete description of the new QPIRT is
reported in [3].
RESULTS
As an example, results are given for the
blowdown PCT, which occurs at 7 seconds after the break
for the reference PWR used for the analysis. Table I
provides the QPIRT results for the most important
processes with their location (node number in the core)
and their corresponding time. The term “fuel-clad
coupling” represents heat transfer between the fuel pellet
and the clad, i.e. conduction within the pellet and clad +
conduction and radiation through the gap. The heat
generation at 5 seconds corresponds to the end of the
steady-state operation. Therefore, the combination of the
heat generation rate and fuel-clad coupling is equivalent
to the “fuel rod stored energy” process of the traditional
expert PIRT. Work is underway on the next step of the
QPIRT process, which is a “Bottom-Up” determination of
the key parameters that govern the processes identified by
the “Top-Down” approach..
TABLE I. QPIRT for Blowdown at 7 seconds
Weighted
Process
LocationContribution
Cell Number
0.2925
Fuel-Clad
10
Coupling:
cell 10, 7sec
0.0846
Heat
10
Generation:
cell 10, 5sec
0.0644
Fuel-Clad
10
Coupling:
Cell 10, 6sec
0.0569
Heat
10
Generation:
cell 10, 6sec
0.054
Fuel-Clad
10
Coupling:
cell 10, 5sec
0.0292
Vapor-Clad
10
Coupling:
cell 10, 7sec
REFERENCES
Time [s]
7
5
6
6
5
7
1. NOURGALIEV, R., DINH, N. and YOUNGBLOOD,
R., Development, Selection, Implementation, and Testing
of Architectural Features and Solution Techniques for
Next Generation of System Simulation Codes to Support
the Safety Case of the LWR Life Extension, Idaho National
Laboratory, September 30, 2010.
2. BOYACK et al., “Quantifying Reactor Safety Margins
Part 2: Characterization of Important Contributors to
Uncertainty,” Nucl Eng Des., 119, 17 (1990).
3. BUONGIORNO, J., and YURKO, J., Quantitative
Phenomena Identification and Ranking Table for
Multiphase Heat Transfer, MIT, Final Report,
Release No. 00043, September 30, 2010.