1. End of Mission Report
Nuclear System Instability Modeling Implementation Review of the
Laguna Verde Nuclear Power Station Classroom Analysis Simulator
ADMINISTRATIVE INFOMATION
Project No.: MEX/9/045 Task 04
Project Title: A Classroom Analysis Simulator for the Laguna Verde Nuclear
Power Plant
Mission/Task Title: Nuclear System Instability Modeling (Implementation Review)
Prepared by: Dr. Robert M. Edwards, Agency Personnel No: T538154
rmenuc@engr.psu.edu
Dates of Assignment: September 27-October 3, 1998
Duty Station: IIE, UREN, Cuernavaca, Morelos, MEXICO
Contact: Dr. Carlos Chavez-Mercado, Instituto de Investigaciones Electricas,
UREN, Av. Reforma No: 113, Col. Palmira, 62490 Temixco,
Morelos, MEXICO. cchavez@iie.org.mx
TERMS OF REFERENCE (A description of the specific objectives of the assignment and
the duties performed by the expert as they relate to the objectives ...)
The objective of the assignment was to review the classroom BWR simulation modeling,
concentrating on neutronics aspects, especially treatment of BWR instabilities. Robert M.
Edwards is Associate Professor of Nuclear Engineering at the Pennsylvania State University,
USA. He teaches senior and graduate level courses in nuclear reactor control, chemical
process control, nuclear reactor physics experiments, power plant modeling and control,
and simulator design. His research has included application of advanced control techniques
to nuclear reactors and power plants, development of real-time plant-wide simulation, and
stability modeling and monitoring of Boiling Water Reactor (BWR) systems.
STATUS OF PROJECT IMPLEMENTATION (An assessment at the time of arrival of
the situation regarding staffing, training, equipment and facilities as applicable within the
context of the assignment)
Carlos Chavez-Mercado Control and Supervision Processes Department
Jaime Morales Nuclear Energy Department
Hector Ocampo Nuclear Energy Department
Pedro Mendoza Nuclear Energy Department
Conzepcion Hernadez Simulation Department

2. Simulation Department
The Classroom Analysis Simulator (CAS) [1,2] is described as a multi-computer, multiple
code-based system with a sophisticated direct manipulation advanced graphical interface. It
includes virtual representation of control panels and is being designed to fulfill training and
on/off line analysis requirements under normal, abnormal and severe accident operational
modes.
At present, two codes are independently implemented for simulation. First, the MAAP code
[3] is used to model a loss of coolant event where the reactor is scrammed;i.e., neutronic
power is rapidly decreased. The reactor primary system is also isolated from the rest of the
plant, which thus does not require to be modeled. Second, the VASIJA code [4,5] is used to
model normal operating transients in the operating power production range; i.e., exclusive of
startup and shutdown operations and accident conditions. It is during the normal operation
of the plant where BWR instability events occur. Two types of instability events are of
concern and have been observed in operating reactors. In one case referred to as an
in-phase instability event, the total reactor power and coolant flow rate time response
develops to a nonlinear limit cycle of potentially large amplitude. This type of event
occurred at Laguna Verde Nuclear Power Station in 1995 [6]. In a second case, referred to
as an out-of-phase instability event, local regions of the reactor undergo the development of
nonlinear local power and local flow limit cycle where localized fuel damage is of concern,
but the total reactor power and coolant rate are not indicative of a potential problem. Much
work has been performed throughout the world to fully understand the phenomena and
properly avoid and mitigate the potential consequences of potential BWR instability. The
principal technique to avoid and respond to an instability event is through operating
procedures and operator training, thus motivating an essential application of the CAS to
enhance training to avoid core damage and its associated potential economic, safety, and
public relations consequences.
The VASIJA code was developed in Mexico. It models the neutronics of the reactor with
the point-kinetics equations that provide the total power of the reactor. The reactor core
thermal-hydraulics is represented with a single channel with 13 or 25 axial nodes where 2
mass equations, for liquid and vapor, momentum, and energy equation are solved. A loop
form of the momentum equation is employed. The axial power profile is an input to the
simulation. Upper and lower plenums of the reactor vessel and recirculation flow loops are
also modeled. Reactivity feedbacks to the point kinetics equations are provided through
polynomial curvefits to the core-average void fraction, fuel temperature, and coolant
temperature. Although information displays have been developed for the balance of the
plant, including steam turbines and feedwater systems, a dynamic model of the balance of the
plant has not been implemented in the CAS. The feedwater flow and steam boundary
conditions of the reactor primary system are inputs to the CAS VASIJA model.
In principle, the VASIJA code may have the capability to replicate the in-phase instability
event such as has occurred at Luguna Verde. However, the current implementation on the
CAS does not demonstrate that capability.

3. Discussions indicated that the IAEA has arranged to soon provide a five-year license for the
RELAP R5-3DI code [7] and the computers capable of real-time execution. This system of
codes is expected to provide a detailed 3-dimensinonal neutron kinetic and thermal hydraulic
simulation of a BWR core during both normal and severe accident conditions. Some work
has already been initiated to develop the necessary network communications to interface an
earlier version of RELAP/SCDAP and MELCOR [8] simulation system to the CAS display
and human-machine interface system. The system should be capable of both in-phase and
out-of-phase instability event simulation.
The organization and staffing level to support continued development of the CAS has been
reduced since the end of 1997. At the end of 1997, the expected development of the CAS
for utilization at the Laguna Verde plant was indefinitely deferred, presumably until there is a
hard requirement that plant operators be trained in severe accident scenarios. The principal
CAS manager, Carlos Chavez-Mercado, was reassigned from the Nuclear Energy
Department to the Control and Supervision Processes Department. All of the CAS equipment
was also moved from the Nuclear Energy Department to the new department except for the
ceiling mounted NEC 9PG Plus Multisync Projector. The moved equipment appears to be
in good working order in its new environment. (See the Document 1D Attachment on
Technical Co-operation Implementation Division – Project Equipment Status.)
The assignment of staff to support the project has been reduced from 2.8 full-time equivalent
to 1.15 full-time-equivalent personnel. Carlos Chavez-Mercado has been allocated 15% of
his time to manage the project with the remaining support to come from the Nuclear Energy
Department. It was explained that continued development of the CAS is desired in case it
should be needed at the Laguna Verde plant at some point in the future and for other
simulation opportunities. However, the loss of the anticipated financial support from CFE
for the Laguna Verde Plant implementation has apparently made continued IIE assignment
of personnel at the previous level difficult to reconcile at the present time.
WORK PROGRAMME: (A description of activities, emphasizing matters relating to
project implementation...)
 Discussions of organization and staffing at the IIE to support the CAS were
conducted as a way of introduction to the project.
 Pedro Mendosa presented his work on stability monitoring. The MOPIN (Pilot
Monitor for Nuclear Instability [9]) system utilizes an Adaptive Digital Filter and
Time Series Analysis to rapidly estimate the decay ratio of typical signals
encountered in BWR stability events. The system was developed using
sinusoidal inputs with superimposed noise and also verified using actual data
recorded from the Laguna Verde instability event.
 Demonstration of the CAS graphical presentation and human-machine interface
for the BB0-09 Emergency Systems was performed. The graphical interface
provides an accurate replication of the Emergency Systems Control panel at the
Laguna Verde Power Plant and provides a mechanism for a student trainee to

4. interact with the simulation models using realistic graphical replication of the
Laguna Verde hardwired panel.
 A simulation scenario for a loss of coolant accident was demonstrated using the
MAAP code interfaced to the BB-09 replica simulator
 A simulation scenario was demonstrated for transient operation using the
VASIJA code interfaced to a simplified graphical control panel and process
schematic diagram with real-time data display. User control of the simulation
included changing the valve positions in the recirculation lines, recirculation
pump hi-lo speed setting and trip status.
 The documentation of the modeling of the VASIJA code was studied
 Discussions with personnel from the IIE Simulation department were conducted
to obtain an understanding the modeling capability, status, and plans of the
Laguna Verde replica simulator currently utilized for operator training.
 Closing discussions to summarize the mission were conducted.
CONCLUSIONS: (An assessment of the results and impact of the expert’s assignment,
relevant conclusions, including an evaluation of the degree of success in solving the
problems encountered ...)
The conclusions and recommendations that follow are the Expert’s and are not to be
considered a commitment on the part of the International Atomic Energy Agency
Broaden Customer Base for the CAS: The impression obtained from discussions is that
the loss of the potential implementation of a CAS as part of Laguna Verde training operations
has resulted in a deemphasis and reduced internal support for aggressive development of the
CAS. It was suggested that the developers seek ways to develop a broader utilization of the
CAS so as not to be so severely dependent on a single customer. Based on the expert’s
experience in academia, it was suggested that the CAS and its development could afford a
wealth of opportunities in university education and research. In addition to nuclear reactor
modeling and simulation, the CAS involves general computer science, human machine
interaction, and electrical and mechanical engineering disciplines. Increased interaction
within the national and international university communities can also further the general
public understanding of nuclear power as an environmentally clean and safe energy source
and thus derive public support for government agencies to allocate resources to projects such
as CAS.
Expand and Validate Simplified Modeling of BWR Stability Phenomena: Although there is
an already planned expansion of the CAS capabilities with the RELAP/SCDAP codes, it is
suggested that some work be performed to implement some simplified simulation modeling
that accurately represents observed and possible instability behavior at Laguna Verde. The
possible advantages of this approach are 1) the computer capabilities, development, and
maintenance requirements would be less, 2) the explanation of the phenomena may be more
easily adaptable for initial instability event training purposes, and 3) a simpler level system
would be easier to use for preliminary studies of stability monitoring methods and
man-machine interface research and design. The RELAP/SCDAP system, once fully

5. developed and validated, could be used for advanced testing and training, including severe
accident conditions.
One approach to address simplified modeling for stability events may be to further study
some modifications of the VASIJA model, which is already operational. Other simplified
modeling capabilities will become available when Miguel Cecenas-Falcon completes his
Ph.D. degree in Nuclear Engineering at Penn State and returns to Mexico in 1999. He has
already developed some simplified modeling, which includes the capability of out-of-phase
stability events.
All simulation models must be capable of replicating the Laguna Verde stability event as a
minimum requirement.
Expand Development and Testing of MOPIN: It appears that the MOPIN stability
monitoring methodology may be suitable for continued development as a stability monitor,
especially if it could gain consideration for utilization at an actual plant. To help gain this
consideration, MOPIN and an appropriate human-machine interface could be developed and
implemented on a validated CAS instability event simulation. The possibility of producing
a CAS simulation of the currently stability monitoring techniques at Laguna Verde should be
investigated. If implementation of the Laguna Verde stability monitoring techniques on the
CAS were feasible, the CAS would be an ideal environment to evaluate and benchmark
performance of alternative stability monitoring techniques, human-machine-interface, and
human performance.
Add Balance of Plant Simulation to CAS: Simulation of Balance of Plant (BOP) (steam
turbine and controls, electrical generator and controls, condenser, feedwater heaters and
controls, and feedwater pumps and controls) has not been added to the CAS. The BOP may
interact with the primary system to help initiate, reinforce, or mitigate the consequences of an
instability event or accident. The BOP behavior may distract or confuse operators and must
be included for comprehensive training purposes. One approach to add the Laguna Verde
BOP simulation capabilities is to obtain and integrate the model that is currently used in the
Laguna Verde replica simulator. Another approach suggested is to investigate the Modular
Modeling System (MMS) and Simulator Development Tools (SDT) which are marketed by
Framatome Technologies (FT), Lyncburg, VA, USA[10]. The FT simulation products have
been in use at Penn State University for over 12 years and have been used in numerous
university research projects incorporating real-time simulation of complete fossil and nuclear
power plants. The FT simulation products have also been used for teaching courses in
power plant modeling, simulation and control. Currently, the FT simulator products are
being used in a multi-year simulator design course at Penn State. The simulator design
course is converting the SEL computer system of the General Public Utilities (GPU) Three
Mile Island (TMI) basic principles simulator to a Windows-NT system. THE GPU TMI
simulator was donated to Penn State in July 1998. A possible advantage of adopting the FT
MMS-SDT for the CAS BOP simulation is that it can probably be more readily adapted to
other types of power plants than using the dedicated modeling deployed on the Laguna Verde
replica simulator. A possible disadvantage is that it may initially cost more than adapting
the Laguna Verde replica simulator BOP modeling. At Penn State, the cost of the FT
simulation products has been minimal due to special pricing available to universities.

6. Continue Development and Implementation of RELAP/SCDAP and MELCOR: Since the
IAEA has already planned to obtain a five-year license for RELAP R5-3DI and associated
computer systems to operate in real-time for integration with CAS, this effort should
continue. There is not presently any apparent technical reason why these systems cannot be
successfully integrated to provide severe accident training capabilities at Laguna Verde as
well as excellent facility to be adapted to other nuclear power system training at other
facilities around the world.
Training Recommendations: Currently planned training with the RELAP/SCDAP software
should be continued. Additional training is recommended to provide expanded
understanding of Balance of Plant (BOP) Simulation. BOP simulation training could be
provided at Penn State using the Framatome Technologies simulation products.
SUMMARY:
Of the recommendations, the continuation of development and implementation of
RELAP/SCDAP and MELCOR for the CAS would require the most effort. A two-year
project proposal extension, beginning in 1999, was submitted by the IIE to the IAEA in
February 1998 and is currently pending approval. The proposed effort by IIE provides for a
100% commitment by Carlos Chavez-Mercado and four additional IIE researchers. This
level of effort is necessary and is also far greater than the 1.15 full-time equivalent personnel
that are currently allocated to the project. In particular, a full-time commitment is required
by Carlos Chavez-Mercado because of his extensive experience, developed over the last 10
years, to interface a wide variety of simulation codes to the CAS environment. Other
supporting expert personnel from the nuclear energy department are required to setup input
data and validate performance of simulation codes to accurately represent Laguna Verde, but
these experts cannot easily interface the codes to the CAS environment without extensive
additional training.
REFERENCES:
1. Chavez-Mercado, Carlos, Ä Classroom Analysis Simulator for the Laguna Verde
Nuclear Power Plant,¨ The 1996 American Nuclear Society International Topical
Meeting on Nuclear Plant Instrumenation, Control, and Human Machine Interface
Technologies, NPIC&HMIT´96, University Park, PA, May 6-9, 1996.
2. Chavez-Mercado, Carlos, Än Advanced Graphical Human Machine Interface For A
Classroom Analysis Simulator of Nuclear Processes,¨Proceedings of the IERE
Workshop Human Factors in Nuclear Power Plants, TEPCO R&D Center, Tokyo Japan,
1996
3. Valdes-Oaura, S., Ïncorporaciou del Cadigo MAAP en el Prototipo del Simulador de
Aula,¨ Reporte Interno IIE/13/10565/I002/F/N, Instituto de Investigaciones Electricas,
Mexico, 1997

7. 4. Juan Carlos Ramos-Pablos, ¨Simulacion en Tiempo Real de la Termohidraulica de un
Reactor de Aqua Hirviente,¨ A Masters of Science Thesis in Nuclear Engineering,
Instituto Politecnico Nacional, Departamento de Ingenieria Nuclear, Diciembre de 1991.
5. Ocampo-Mansilla, Hector, ¨Adaptacion al Simulador de Aula Prototipo de un Modelo
Para Simular la Termohidraulica de la VASIJA del Reactor Nuclear de Laguna
Verde,¨Reporte Interno IIE/13/10565/I003/F/N, Instituto de Investigaciones Electricas,
Mexico, 1997
6. Gonzales-Mercado, V.M., R. Amador Garcia, R. Castillo, J.L. Hernandez, Änalisis del
Evento de Osciñaciones de Potencia en la CN Laguna Verde,¨CNSNS-TR-13, rev. 0,
Preliminary Report, April 1995.
7. RELAP5-3D Code Executive and Short Description, Idaho National Engineering &
Environmental Laboratory, GWJ/01/06/98.
8. Workshop on Severe Accident Codes MELCOR and SCDAP/RELAP Application,
Cuernavaca, Morelos, MEXICO, September 1998.
9. Mendoza E., P.R., ¨MOPIN: Monitor Piloto de Estabilidad Nuclear, Reporte Interno
IIE/13/11054/I002/F/N. Instituto de Investigaciones Electricas, Mexico, 1998
10. MMS