PWR Containment Modeling¶

Source

Rev 1 Modeling Guidance, §11.6 — PORV & SRV BREAK Boundary Condition Modeling; Appendix A.11 — Pressurizer Bases (BREAK BC discussion); Appendix A — Generically Applicable Bases (BREAK DXIN/VOLIN); BWR Containment Guidance (CONTAN component reference).

Gap in Rev 1 Guidance

The Rev 1 PWR Modeling Guidance does not contain PWR-specific CONTAN component modeling guidance. The Appendix A.11 bases state:

"Existing NRC staff experience with PWR plant models does not incorporate CONTAN components, so no guidance is currently provided but the option is made available. NRC staff plans to develop modeling guidance on use of CONTAN components with PWRs in the near future, drawing upon more extensive experience using CONTAN components in BWR plant models."

The generically applicable bases (Appendix A) further note:

"This modeling guidance currently focuses on conventional PWR applications. Development of CONTAN modeling guidance for incorporation in this modeling guidance is planned for the future."

For CONTAN component principles (COMPARTMENT, passive junctions, fan coolers, drywell/wetwell modeling), analysts should consult the BWR Containment page, which provides comprehensive CONTAN guidance developed from BWR plant model experience.

Overview¶

For most PWR analyses in TRACE, the containment is not modeled explicitly with CONTAN components. Instead, the containment environment is represented through BREAK components that serve as pressure boundary conditions at discharge points (PORVs, SRVs, break locations). This approach is simpler and, for many transient types, adequate — particularly when PORV/SRV flow remains choked and the downstream backpressure is unimportant.

The Rev 1 guidance provides detailed instructions for setting up these BREAK boundary conditions as part of the pressurizer modeling (§11.6). This page consolidates that guidance and discusses when more detailed containment modeling may be needed.

Containment Backpressure Approach¶

When Simple BREAK BCs Are Sufficient¶

For most short-duration transients, the PORV/SRV flow will be choked and the downstream pressure is unimportant. In these cases, a constant-pressure BREAK at a sufficiently low value to ensure choked flow is adequate.

Basis — Appendix A.11

"In most transients (especially those of short duration), the pressure specified by the BREAK components is unimportant as long as it is specified at a sufficiently low value because the flow through the valves will be choked."

When Detailed Containment Modeling May Be Needed¶

More detailed backpressure treatment is needed when:

Subsonic PORV/SRV flow — The containment pressure rises to a point where the pressure differential results in subsonic fluid velocities (e.g., long-term cooling calculations)
Stuck-open PORV/SRV at lower pressures — If a PORV or SRV is assumed to open or be stuck open at pressures where choked flow cannot be assured
LOCA analyses requiring containment response — Where the containment pressure-temperature history feeds back into break flow or ECCS performance

Basis — Appendix A.11

"If the flow through the PORVs/SRVs is expected to become subsonic, then the BREAK pressure becomes an important variable in determining the mass release. Such a situation may exist, for example, if a PORV or SRV is assumed to open or be stuck open at lower pressures."

Available Options¶

The Rev 1 guidance identifies three approaches, in increasing order of complexity:

Approach	IBTY	Description	When to Use
Constant pressure	0	No tables — fixed backpressure	Short transients with choked PORV/SRV flow
Time-dependent pressure	1	Input pressure table	Subsonic flow possible; containment P available from other analysis
Coupled CONTAN	7	BREAK coupled to CONTAN component	Self-consistent containment response needed

Basis — Appendix A.11

"In the meanwhile, analysts may also supply a constant minimum containment pressure (to maximize release where this type of conservatism is desired) or a time-dependent curve based on other analyses. The latter option may also be preferred to use of CONTAN components because this approach can be used to account for the effect of containment geometry as well as containment sprays, fan cooler units, and other equipment in the containment."

PORV/SRV BREAK Boundary Condition Setup¶

This section provides the complete Rev 1 guidance for the BREAK components that serve as containment-side pressure boundary conditions for the PORV and SRV valve discharge paths.

Break Type (IBTY)¶

The analyst is responsible for determining the most appropriate break type:

IBTY = 0 (No tables) — Models a constant pressure boundary. Used for transients where PORV/SRV flow is expected to remain choked. The backpressure is unimportant and can be set to a sufficiently low pressure to ensure choked flow conditions.
IBTY = 1 (Input Pressure Table) — Used to specify containment pressure as a function of time. Appropriate for transients where containment pressure may increase to a point at which the pressure differential results in subsonic fluid velocities (e.g., long-term cooling calculations).
IBTY = 7 (CONTAN Component) — Couples the BREAK to a CONTAN component, with all BREAK fluid properties set by the connecting CONTAN component. Note that guidance is not currently provided for modeling CONTAN components in PWR applications.

Temperature Table Option (ISAT)¶

The liquid and gas temperatures should be set to saturation temperature:

ISAT = 3 — Set liquid and gas to T_sat

Fluid State Option (IOFF)¶

When the BREAK table controlling trip is off after being on, the last interpolated constant should be held:

IOFF = 0 — Last Interpolated State Held Constant

Length (DXIN)¶

The BREAK length should be set to a very small value so that the junction thermodynamic properties will be weighted toward those of the upstream connected volume:

DXIN = 10⁻⁶ m

Basis — Appendix A (Generically Applicable)

"In the special case of modeling breaks, however, the containment volume is very large relative to the break. Consequently, DXIN is set to a very small value and VOLIN is set to a very large value. This ensures that the BREAK properties will essentially be eliminated from the internal TRACE weighting of cell quantities on each side of the break plane."

Volume (VOLIN)¶

The BREAK volume should be set to a very large value so that the junction thermodynamic properties will be weighted toward those of the upstream connected volume:

VOLIN = 10⁶ m³

Initial Gas Volume Fraction (ALPN)¶

Assume all vapor at transient initialization:

ALPN = 1.0

Initial Mixture Temperature (TIN)¶

Set equal to the containment temperature at transient initialization:

TIN = initial containment temperature

Initial Pressure (PIN)¶

Set equal to the containment pressure at transient initialization. With IBTY = 0, this will be held constant throughout the transient:

PIN = initial containment pressure

Initial Non-condensable Partial Pressure (PAIN)¶

Set equal to the containment pressure at transient initialization:

PAIN = initial containment pressure

Adjacent Pressure Flag (ADJPRESS)¶

The adjacent pressure flag gives the option to always set the break pressure equal to the upstream volume pressure. This option should not be used:

ADJPRESS = False

Max Pressure Change Rate (RBMX)¶

Set to a very large number:

RBMX = 10²⁰ Pa/s

Break Table (PTB) — IBTY = 1 Only¶

For pressure specified as a function of time, the PTB array contains data pairs defining pressure at specified times. TRACE will interpolate between data points as needed.

The table should cover the entire duration of the transient with no extrapolation
Ensure input format is consistent with intent (pressures evaluated according to problem time vs. time since trip)
For IBTY = 0 or 7, this table is not input

Rate Factor Table¶

Rate factor tables are not recommended for use.

Break Signal (IBSV)¶

For IBTY = 0 or 7: no break signal is used — IBSV = 0
For IBTY = 1: set to the signal variable used to evaluate pressure from the break table — IBSV = signal variable corresponding to problem time

Break Trip (IBTR)¶

For IBTY = 0 or 7: no break trip is used — IBTR = 0
For IBTY = 1: set to the trip that indicates when TRACE should start evaluating pressure from the break table

Compartment (COMPID) — IBTY = 7 Only¶

Indicates which CONTAN component this BREAK is coupled to:

|COMPID| = ID number for the CONTAN component that will supply fluid properties at the BREAK during the transient

Compartment Region — IBTY = 7 Only¶

PORVs/SRVs typically discharge below the surface of the liquid in the containment to allow for cooling/condensation of steam:

Compartment Region = Liquid Space (represented in TRACE input by a negative COMPID number)

Vapor Equilibrium Fraction (BDSPRAY) — IBTY = 7 Only¶

This input is applicable to subcooled liquid being injected into the vapor region of a CONTAN component, and is not applicable to typical PORV/SRV discharges:

BDSPRAY = 0.0

Fluid Equilibrium Fraction (BDCOND) — IBTY = 7 Only¶

This input is applicable to superheated steam/air being injected into the liquid region of a CONTAN component. Set to an appropriate value for the analysis of interest:

BDCOND = assumed mass fraction of steam which reaches thermal equilibrium with liquid in the containment

Pressure Scale Factor (PSCL)¶

Set equal to 1.0 to prevent this option from affecting the pressure table inputs:

PSCL = 1.0

BREAK BC Summary Table¶

Parameter	Value	Notes
IBTY	0, 1, or 7	Analyst determines most appropriate type
ISAT	3	Liquid and gas set to T_sat
IOFF	0	Last interpolated state held constant
DXIN	10⁻⁶ m	Very small — weight toward upstream
VOLIN	10⁶ m³	Very large — weight toward upstream
ALPN	1.0	All vapor initially
TIN	Containment temperature	At transient initialization
PIN	Containment pressure	At transient initialization
PAIN	Containment pressure	At transient initialization
ADJPRESS	False	Do not use adjacent pressure option
RBMX	10²⁰ Pa/s	Very large — no rate limiting
PSCL	1.0	No scaling applied

ECCS BREAK/FILL Containment Coupling¶

The ECCS injection and drawdown systems (FILL components) are not generally associated with a CONTAN component for PWR applications. The analyst should verify that this assumption is valid, since some long-term transients may involve liquid addition systems (such as LPCI/RHR) taking suction from the containment.

Couple to CONTAN = False (represented in TRACE input by the sign of IFTY)

Known Issue: CONTAN-PIPE BREAK Connection¶

An NRC staff assessment (Krotiuk) identified a potential issue with BREAK component connections between CONTAN and PIPE components. The study found:

When break flow is added directly to the CONTAN component, mass and energy balances are consistent
When break flow is added to a PIPE component connected to CONTAN via a BREAK, the enthalpy flow shows significant oscillations and reduced values, suggesting incorrect energy flow calculation
The resulting pressures and temperatures in the CONTAN component were lower than expected for the PIPE-to-CONTAN flow direction

Recommendation

If using CONTAN components coupled to the primary system via BREAK connections, ensure that high-energy break flow is added directly to the CONTAN component rather than to a PIPE component connected to CONTAN through a BREAK. Review all mass and energy flow calculations at BREAK connections between PIPE and CONTAN components for consistency.

PWR Containment Types — Reference Information¶

The following descriptions of PWR containment types are provided for analyst reference. These are not part of the Rev 1 TRACE modeling guidance but provide context for understanding the containment environment being represented by the BREAK boundary conditions.

Large Dry Containment¶

The most common PWR containment design:

Steel-lined reinforced concrete structure
Free volume: 50,000–80,000 m³
Design pressure: 0.4–0.6 MPa
Design temperature: 130–150°C
Passive heat sinks (walls, floors, equipment)
Active containment spray systems
Containment air coolers (fan cooler units)

Ice Condenser Containment¶

Used in some Westinghouse designs:

Smaller free volume: 25,000–35,000 m³
Lower design pressure: 0.1–0.15 MPa
Ice baskets provide passive pressure suppression
Compartmentalized design (lower, ice condenser, upper)
Divider deck separates ice condenser from upper compartment

Subatmospheric Containment¶

Maintained below atmospheric pressure during normal operation:

Similar structural design to large dry
Continuous vacuum pumping maintains sub-atmospheric conditions
Reduced initial pressure provides additional margin
Lower potential for outward leakage

Cross-References¶

Pressurizer — §11.3–11.5 covers PORV/SRV VALVE components that connect to these BREAK BCs
BWR Containment — Comprehensive CONTAN component modeling guidance (drywell, wetwell, passive junctions, vacuum breakers, fan coolers)
Primary Loop — LOCA break modeling at primary system locations

This page consolidates containment-related guidance from Rev 1 §11.6 (pressurizer BREAK boundary conditions) and Appendix A.11/A (associated bases). The Rev 1 guide does not provide PWR-specific CONTAN component modeling guidance; that development is identified as planned future work. BWR CONTAN experience is documented in the BWR Containment guidance. The CONTAN-PIPE BREAK assessment is from a separate NRC staff study (Krotiuk).