RELAP5/MOD3.2 Modeling Guidance for ADS-IDAC

Introduction

The current version of ADS-IDAC utilizes the RELAP5/MOD 3.2 computer code to provide a transient thermal-hydraulic simulation of nuclear power plant operation. RELAP5 can simulate a wide variety of accident initiators and provides the capability to model key safety systems, controls, and instruments.

The RELAP thermal-hydraulic model provides two key functions for ADS-IDAC:

1. Dynamic, realistic representation of plant parameters — all indicator values displayed on the ADS-IDAC control panel originate from the RELAP model.
2. Interactive plant control — the ADS-IDAC operator model manipulates the thermal-hydraulic model by changing component and system operating states.

Scope Limitation

Due to intrinsic limitations of RELAP5, it is not currently possible to model core damage states or severe accident scenarios. ADS-IDAC is therefore limited to analysis of scenarios up to the start of core damage. Adaptation to a more versatile code such as TRACE or MELCOR has been identified as a future research activity.

Information Perception

The operator cannot instantly use all information available on the control panel. Similar to an actual control room crew, information must first be perceived by the ADS-IDAC operator model before it can be used in decision-making.

ADS-IDAC supports four operator control modes for each controllable component:

1. Changing component operating mode (e.g., automatic vs. manual)
2. Setting a specific control value (e.g., throttling a control valve to 50% open)
3. Incrementing the control setting (e.g., opening a valve an additional 10%)
4. Setting a control value based on a perceived parameter (e.g., setting the steam dump target pressure equal to the perceived main steam header pressure)

Although many RELAP plant models exist for regulatory safety analyses, these input decks often require modification to exploit ADS-IDAC's full capabilities. Conservative analysis assumptions must be replaced with realistic best-estimate parameters, simplified boundary conditions must be replaced with component-level models, and manual control interfaces must be added.

Reference Document

For a comprehensive description of the RELAP5 input deck, see NUREG/CR-5535, RELAP5/MOD3 Code Manual, Appendix A, "Input Requirements."

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Pre-Modification Checklist

Before modifying an existing RELAP input deck, the following features should be evaluated for addition or correction:

• Replace all conservative analysis assumptions with realistic best-estimate parameters (trip setpoints, initial reactor power, timing delays for automatic safety actuations)
• Add significant non-safety control systems and safety features not previously included
• Replace simple thermal-hydraulic boundary conditions in safety system models with realistic component representations (redundant trains, key pump and valve controls, critical support system water supplies, electrical power)
• Add systems and components providing significant mitigative functions described in abnormal and emergency operating procedures
• Implement control interfaces for all interactive components (including manual control modes for auto-controlled components)
• Add modeling elements for initiating event conditions (coolant or steam leakage paths, equipment failures)

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Detailed Guidance

1. Identify Controllable Components and Plant Indicators

Identify all controllable components and plant indicators already included in the original RELAP input deck:

Major controllable components: valves, pumps, heaters, reactor fuel

Plant indicators typically present:

• Significant inventory/level indicators (reactor vessel, steam generators, pressurizer, safety injection accumulators)
• Significant mass flow rates (steam flow, feedwater flow, reactor coolant flow)
• Significant thermal-hydraulic parameters (RCS temperature and pressure, steam pressure)
• Control systems (pressure or inventory control systems)

A noding diagram for the plant model is extremely useful for locating key components. Because RELAP input decks are plain text files, key features can also be found by searching for specific keywords.

Key RELAP component type keywords:

Keyword	Component Type	Typical Use
TMDPVOL	Time Dependent Volume	Models a mass source or sink using a controlled pressure, temperature, and/or steam quality condition. Often used for relief valve exhaust volumes, leakage paths, or inexhaustible water sources.
TMDPJUN	Time Dependent Junction	Models a mass source or sink using a controlled mass flow rate boundary condition. Often associated with safety injection system connections, makeup/letdown systems, or inventory control systems.
ANNULUS	Annulus Hydraulic Component	Often used to model the reactor vessel or steam generator downcomer volume. Water levels for these components are measured in the annulus region.
PRIZER	Pressurizer	Models the pressurizer in a PWR. Spray valves and relief valves are connected to this volume. Pressurizer pressure and level are significant control panel indicators.
VALVE	Valve	Used to model flow limiting devices for parameter control or initiating accident conditions.
PUMP	Pump	Used for large pumps where coast-down flow following a pump trip is significant (reactor coolant pumps, feedwater pumps). Smaller pumps are usually modeled with time-dependent junctions.
ACCUM	Accumulator	Models a liquid/gas accumulator. Typical application: passive safety injection accumulators in PWRs.

VALVE subtypes:

Subtype	Description
TRPVLV (Trip Valve)	Two states only: fully open or fully closed. Repositions immediately when the associated logical or variable trip (card CCC0301) changes state. Typically used to model system leaks or ruptures.
MTRVLV (Motor Valve)	Two normal states (fully open/closed) but repositions at a finite rate specified in card CCC0301. Can be in an intermediate state during the transition. Typically used for large isolation valves (main steam stop valves, turbine trip valves, feedwater isolation valves).
SRVVLV (Servo Valve)	Can be positioned fully closed, fully open, or any intermediate position. Valve position set by normalized flow area in card CCC0301. Used for throttle valves, power operated relief valves, and control of mass flow rates. Typical examples: pressurizer PORV, steam generator atmospheric steam dump valves, feedwater regulating valves.

2. Identify Automatic Control Systems

Automatic control systems can be found in the 20500000 card series and are usually associated with FEEDCTL (feedwater control) or PROP-INT (proportional-integral controller) types.

At a minimum, automatic control systems should be provided for the following PWR control functions:

• Pressurizer pressure control (spray valves, power operated relief valve, heaters)
• Pressurizer level control (reactor coolant system makeup and letdown flow)
• Steam generator level control (feedwater regulating valve)
• Steam generator pressure control (atmospheric relief valves)
• Steam header pressure control (condenser steam dump valves)

Control System Identification Shortcut

Control systems can also be identified by back-tracing the control variable for the associated component. For example, the pressurizer pressure control logic control variable will be referenced in the servo valve description for the spray valve.

Control systems also provide model stability benefits: without control systems, small initialization mismatches (e.g., steam vs. feedwater flow rate) can cause significant plant deviations over time.

3. Convert Automatic Control Systems to Interactive-Variable-Driven Models

Modify automatically controlled components to permit manual control from the ADS-IDAC environment. Ideally, three control modes should be available for each automatic control system:

1. Fully automatic — the control system positions final actuating device(s) to maintain target parameter at the nominal setpoint
2. Fully manual — the operator positions the final actuating device(s)
3. Setpoint control — the control system positions final actuating device(s) to maintain the target parameter at a user-selected setpoint

Feedwater Regulating Valve (FWRV) Control System Example

The following procedure adapts a feedwater control system for ADS-IDAC compatibility:

a) Set up an interactive control variable (cards 800–999)

Interactive control variables are specified in cards 800–999. Card 810 sets up the interactive variable sgcfrv with an initial value of −1.0:

0000810    sgcfrv    -1.0    $ SG C feedwater reg valve

b) Add a variable trip to toggle automatic/manual mode

Variable trip 475 acts as an auto/manual toggle. When the trip is true (sgcfrv > −0.01), the control system is in automatic mode:

20604750    sgcfrv    1000000000 gt    null    0    -0.01    n

The threshold of −0.01 allows 0.0 to serve as the fully-closed manual control input. The n specifies the trip does not latch and can be reset during simulation.

c) Add manual/automatic control signal selection

Assuming control variable 725 was previously the automatic control output:

20572600 "sgcfwman" tripunit  1.0 0.0 1 3 0.0 1.0
20572601  475
20572700 "sgcfwaut" sum       1.0 1.0 1 3 0.0 1.0
20572701  1.0  -1.0  cntrlvar 726
20572800 "sgcfw_mc" mult      1.0 1.0 1 3 0.0 1.0
20572801  cntrlvar 726  sgcfrv  1000000000
20572900 "sgcfw_ac" mult      1.0 1.0 1 3 0.0 1.0
20572901  cntrlvar 727  cntrlvar 725
20573000 "sgcfw_tc" sum       1.0 1.0 1 3 0.0 1.0
20573001  0.0  1.0 cntrlvar 728  1.0  cntrlvar 729

• Cards 20572600/20572700: tripunit toggles between auto and manual — one is always 1.0, the other 0.0.
• Card 20572800: manual control signal — equals 0.0 in automatic or sgcfrv in manual.
• Card 20572900: automatic control signal — equals 0.0 in manual or the auto control signal (cntrlvar 725) in automatic.
• Card 20573000: sums both signals; since only one can be nonzero, this selects the active control signal. The final value is limited to 0.0–1.0 (0.0 = fully closed, 1.0 = fully open).

d) Revise the final actuating device to use the new control variable

Assuming the FWRV is hydraulic component 500:

5000301    730

Non-Destructive Deck Modification

In RELAP, if more than one version of the same card number appears in the input deck, the last version listed is used. Therefore, all ADS-IDAC additions can be placed at the end of the RELAP input file without deleting or modifying the original coding. This preserves the integrity of the original plant model.

4. Add Interactive Controls (IC_ Channels, Cards 800–999)

Based on review of plant procedures, operator behavior rules, and training materials, determine what plant control functions to include. Any component providing a substantial mitigative function following an abnormal event or accident should be controllable by ADS-IDAC.

Three control modes are available, each requiring corresponding interactive variables in cards 800–999 and cross-referencing in the RELAP_channels.txt input file:

Setpoint Control (Control_Panel_Controller in ControlPanel.txt):

0000801    sgcporvp    1040.0    $ SG atmospheric relief valve setpoint (psia)
0000802    stmdumpp    1020.0    $ Condenser steam dump setpoint (psia)

Component Operating State (Control_Panel_Controller in ControlPanel.txt):

0000803    scram      -1.0    $ Reactor trip (activated when > 0.0)
0000804    turbtrip   -1.0    $ Turbine trip (activated when > 0.0)

Initialized to a value less than zero to place trip functions in standby.

Fine Adjustment Control (Control_Panel_Fine_Adjust in ControlPanel.txt):

0000805    spray    -1.0    $ Pressurizer spray valve (auto when < 0.0, manual position when >= 0.0)
0000806    pzrporv  -1.0    $ Pressurizer PORV (auto when < 0.0, manual position when >= 0.0)

The initial value of −1.0 places the control system in automatic. When the value is ≥ 0.0, the system is in manual and the valve is positioned to the equivalent stem position (e.g., 0.5 = 50% open).

Default Values

The second word of each card 800–999 entry is the default value of the interactive variable. Set this to the normal full-power value (nominal setpoints, control systems in automatic, standby systems aligned for automatic start).

5. Improve Overall Plant-Level Control

RELAP models focused on accident analysis often omit or simplistically represent components needed for normal operation and a full range of operator interactions. Review the input deck and add:

• Turbine generator load control
• Non-safety related interlocks or protective features (e.g., turbine runback)
• Charging/letdown system operation
• Nuclear reactivity control systems (control rods, emergency boration)
• Condenser steam dump control

6. Activate the Point Kinetics Model

RELAP provides two methods to control reactor power:

1. Time-dependent power — a constant (conservative) power level pre-scram, followed by a tabulated decay curve. Does not model reactivity feedback.
2. Point kinetics model — calculates reactor power based on reactivity feedback. Recommended for ADS-IDAC.

To use point kinetics:

• Locate nuclear fuel heat structures (card series 1CCCGXNN)
• Heat source data cards 1CCCG701–1CCCG799 specify how power output is determined
• Provide core kinetics data in the 30000000 card series (temperature and density reactivity coefficients, core power shaping factors, scram reactivity worth, delayed neutron lifetimes)
• Link reactivity feedback control systems to the point kinetics model in cards 30000011–30000020

Add the following system functions for improved realism and stability:

• Control rod reactivity
• Emergency boron addition
• Safety injection boron addition

Each is modeled as a control system (205CCC000 card series) representing reactivity added by actuation, then linked to the point kinetics model.

Point Kinetics Stability

Activation of point kinetics can cause unexpected stability issues. If problems are encountered:

• Reduce the maximum allowable RELAP time step (cards 201–299)
• Ensure sufficient negative reactivity is added following a scram or safety injection actuation
• Increase scram rod worth or post-safety-injection boration to reduce potential for core restart events and associated model instability

Boron Concentration Modeling

Although RELAP is capable of modeling boron concentration, this option is not recommended within the ADS-IDAC environment. The effects of boron concentration changes and control rod motion are more effectively modeled with control variables.

7. Enhance Safety System Models (Figures 10 & 11)

Front-line safety systems in existing RELAP input decks are often modeled using only time-dependent junctions at each fluid injection point.

Figure 10 — Typical RELAP Mitigating System Model (simplified): The high pressure safety injection system for a three-loop PWR uses three separate Time Dependent Volume / Time Dependent Junction pairs, one per reactor coolant loop injection point. This approach decouples injection flow between loops and lacks control elements, alarm functions, and realistic pump head/flow characteristics.

Figure 11 — Modified ADS-IDAC Mitigating System Model: The improved model includes:

• Two separate subsystem trains (Time Dependent Volume A + Junction A, and Volume B + Junction B) feeding a Common Hydraulic Volume
• Individual loop servo control valves (Loop 1, 2, 3 Servo Control Valves) between the common header and each reactor coolant loop injection point

Benefits of the modified model:

• Independent control of multiple injection pump trains and individual loop flows
• More realistic pump head/flow characteristics matching the actual plant configuration
• Improved modeling of injection flow dependencies between reactor coolant loops (high injection to loop 1 reduces flow to loops 2 and 3 via the shared common header)

It is recommended that the analyst review system-level modeling for all front-line safety systems (high pressure safety injection, low pressure safety injection, auxiliary feedwater) and revise any overly simplified models.

8. Add Support System Dependencies

RELAP plant models generally do not include detailed support system models. Support system dependencies can be modeled with trip variables linked to key components:

• A loss of offsite electrical power can be modeled with an interactive and trip variable that toggles the availability of offsite power. Components normally powered from offsite power (reactor coolant pumps, turbine-generator auxiliaries, condenser circulating water) reference this trip in their individual trip logic.
• A similar approach applies to cooling water and lubricating oil support systems.

This approach simulates support system dependencies without requiring a detailed system model for each support function.

9. Create Derived or Unique Control Panel Indicators

ADS-IDAC includes capabilities for basic parameters (pressure, temperature, flow). Derived or unique parameters must be added directly into the RELAP input deck as control variables.

SI Units

Parameters obtained directly from the RELAP model are in SI units. Unit conversion control variables must be added for indicators displayed in British units.

Unique RELAP parameters available for control panel indicators:

RELAP Parameter	Description
ACVLIQ	Liquid volume in the referenced accumulator tank, standpipe, and surge line
VLVSTEM	Ratio of current valve stem position to fully open position (motor and servo valves)
HTCHF	Critical heat flux for the referenced heat structure
HTCHFR	Critical heat flux ratio for the referenced heat structure
HTMODE	Heat transfer mode number (heat transfer regime in effect) for the referenced heat structure
RKRECPER	Reciprocal reactor period (inverse seconds) from the point kinetics model
RKTPOW	Total reactor power from the point kinetics model

Example — valve position indicator for servo valve (component 350):

*ctlvar  name      type  factor  init  f c  min  max
20541500 "vlv_pos" mult  1.0     0.0   0 3  0.0  1.0
*ctlvar  variable name  parameter no.
20541501 vlvstem        350

This control variable can be added to the ADS-IDAC control panel by referencing control variable 415 in the ControlPanel.txt input file.

Example — average RCS temperature in degrees Kelvin (volumes 150, 250, 350) and Fahrenheit:

*ctlvar  name       type  factor  init   f c  constant  scale
20529000 "avgTempK" sum   0.333   550.0  1 0
*ctlvar  variable name  parameter no.
20529001  0.0   1.0  tempf  150010000
20529002             1.0  tempf  250010000
20529003             1.0  tempf  350010000

*ctlvar  name       type  factor  init   f c  constant  scale
20529100 "avgTempF" sum   1.0     530.0  1 0
*ctlvar  variable name  parameter no.
20529101  -459.7  1.8  cntrlvar  290

Control variable 290 computes the average in degrees K. Control variable 291 converts to degrees F using the factor −459.7 + 1.8 × T_K.

10. Expanded Trip Option

RELAP provides two types of trip devices — variable trips (Boolean state when a parameter exceeds a threshold) and logical trips (Boolean combinations of variable and logical trips).

Default trip limits:

• Variable trips: cards 401–599 → maximum 199 variable trips
• Logical trips: cards 601–799 → maximum 199 logical trips

This may be insufficient for complex ADS-IDAC reactor plant models. The expanded trip option removes this limitation.

Activating the expanded option:

Add the following card to the RELAP input deck:

20600000    expanded

Expanded trip ranges:

Trip Type	Card Range	Maximum Count
Variable trips	20600010–20610000	1000 (trips numbered 1–1000)
Logical trips	20610010–20620000	1000 (trips numbered 1001–2000)

Converting variable trip card numbers from default to expanded format:

If original trip cards use eight-digit format (e.g., trip 450 in card 00000450), replace the first four digits with 206 and append a trailing zero:

00000450    →    20604500

The trip number (450) remains the same in both formats. References to trip numbers elsewhere in the input deck do not need to be updated.

Converting logical trip card numbers:

Logical trip numbers must change: add 1000 to the default logical trip number (e.g., default trip 650 becomes trip 1650):

00000650    →    20616500

Logical Trip Renumbering

Every reference to the default logical trip number throughout the entire RELAP input deck must be updated to the new number. Use a word search to locate references, but carefully distinguish between logical trip numbers and control variable numbers — both may appear as three-digit integers. Also update complementary trip references (e.g., change all references to −650 to −1650).

All variable trip cards (401–599) must be revised to expanded card numbering format or a RELAP error will occur.

11. Time Step Control

RELAP end time limit and time step are controlled by input cards 201–299.

• Set the RELAP end time limit to a value greater than the ADS-IDAC sequence truncation time (specified in ZCtrlPar.txt) to prevent RELAP from prematurely terminating the simulation.
• Set the maximum time step to the highest value that permits stable model behavior.
• If the RELAP model runs slowly, increase the maximum time step or select a different time step control option (e.g., implicit or nearly implicit time step control).