• Maintain primary containment integrity
• Protect equipment located within the primary containment.

• Combustible gas concentration (H₂): 2%
• Torus Temp: 90^o F
• Torus water level: below -4" or above +3"
• Drywell Temp: 135^o F
• Primary containment pressure: 2 psig (NOTE: scram setpoint is 1.84 psig. However, EOP entry is still 2 psig as of ILT '09

Preventing fuel clad rupture by (in order of preference):

• Core submergence
• 2/3 core height with Core Spray Injection
• Steam cooling with injection of makeup water to RPV

• DW or torus hydrogen above 2%
• Torus Temp above 90 degrees F
• Torus level below -4.0 inches or above +3.0 inches
• DW temp above 135 degrees F
• DW pressure above 2 psig

Maximum Subcritical Banked Withdrawal Position:

(for Monticello: position 04 or less.)

• Tech Spec 3.1.1 Shutdown Margin (one rod full out with all others at 00)
• Rod pattern analyses performed by nuclear engineers.

• ADS actuation can imposed a severe thermal transient on the RPV and may complicate efforts to control RPV level
• If only steam-driven systems are available for injection, ADS actuation may directly lead to loss of adequate core cooling and subsequent core damage
• The conditions assumed in the design of the ADS actuation logic (e.g., no operator action for 10 minutes after event initiation) may not exist when the actions specified in RPV Control are being carried out
• The operator can draw on much more information than is available to the ADS logic and can better judge...

• Pressure capability of primary containment
• The maximum primary containment pressure at which vent valves sized to reject all decay heat from the primary containment can be opened and closed
• The maximum primary containment pressure at which ADS valves can be opened and will remain open

• Metal-water reactions
• Radiolysis of water
• Release of hydrogen dissolved in reactor coolant (HWC)

• Reduced capacity for condensing steam
• Loss of ECCS pump NPSH
• Pressurization of containment

High water levels result in higher stresses on primary containment structural components during a LOCA or SRV actuation.

Low water levels may result in reduced capacity for condensing steam, loss of ECCS pump NPSH, and vortex formation at ECCS pump suction inlets.

2

1.84 psig

The highest torus temperature which provides adequate net positive suction head for pumps taking suction from Torus.

NOTE: only applies to RCIC and HPCI when suction aligned to Torus.

110 degrees F.

• The scram reduces the rate of energy production and thus the heat input to the torus
• If torus temperature continues to rise, a blowdown will be required. The reactor should be scrammed before the blowdown is performed
• Monticello Tech Specs require a reactor scram (Place the MODE switch in SHUTDOWN) if torus temperature reaches 110 degrees F

• C.5-1100 coordinates control of RPV water level and pressure following a scram
• It also branches to C.5-2007 (Failure to Scram) if all rods are not inserted by the scram
• An override in the Pressure branch of C.5-1100 allows rapid depressurization through the main turbine bypass valves in anticipation of a possible blowdown

The highest torus temperature at which a blowdown will not result in exceeding either:

• Torus design temperature, or
• The drywell pressure limit

while the rate of energy transfer from the RPV to the primary containment is greater than the containment vent capacity.

A blowdown must be performed to preclude:

• Loss of primary containment integrity
• Loss of the pressure suppression function of primary containment

The lower of:

• The maximum pressure suppression primary containment water level, and
• The highest torus water level at which opening an SRV will not result in exceeding the code of allowable stresses in the SRV tail pipe, tail pipe supports, quencher, or quencher supports.

4.2 feet.

If torus level exceeds this elevation, the vacuum breakers may not function as designed to relieve noncondensibles into the drywell and equalize drywell and torus pressures.
Therefore, drywell spray is permitted only if torus water level is below 4.2 feet and the sprays must be secured if torus level may exceed the limiting elevation.

-3.7 feet

If not maintained above this level, HPCI must be secured to prevent direct pressurization of the torus. THIS ACTION IS TAKEN IRRESPECTIVE OF WHETHER THE CORE WILL REMAIN ADEQUATELY COOLED, SINCE FAILURE TO DO SO COULD LEAD TO A COMPLETE AND UNCONTROLLED LOSS OF PRIMARY CONTAINMENT INTEGRITY. This could result in a loss of core cooling and release of substantial amounts of radioactivity to the environment."

Design bases assumes use of DW spray at 10 minutes after the break to limit DW temperatures.

NOTE: small and intermediate steam line breaks will result in DW temperatures above 281 degrees F.

The highest temperature at which initiation of drywell sprays will not result in an evaporative cooling pressure drop to below the high drywell pressure scram setpoint.

The goal is avoiding the Drywell EXTERNAL limit of 2 psig

De-inertion of the primary containment or loss of primary containment integrity.

Loss of primary containment integrity would be due to exceeding the EXTERNAL pressure limit for primary containment. At temperatures above the drywell spray limit, the containment pressure drop may be too rapid to control/prevent.

• The core has remained covered. Dry or superheated steam is needed to exceed the DW spray limit. If the core has remained covered, steam leaving the RPV will remain saturated
• Drywell pressure is above 14 psig. The DW spray limit is independent of DW temperature above this pressure;
• The Mollier Diagram indicates isentropic expansion will result in a steam temperature within the DW spray limit.

4.2 feet.

If torus level exceeds this elevation, the vacuum breakers may not function as designed to relieve noncondensibles into the drywell and equalize drywell and torus pressures. Therefore, drywell spray is permitted only if torus water level is below 4.2 feet

Verify recirc pumps tripped and place DW cooling FAN CONTROL switches in OFF.

This is because recirc pumps and DW fans are not qualified for continuous operation in a spray environment.

NOT RECOMMENDED

Yes. May be delayed until DW temperature approaches the max temperature at which ADS is qualified (335 degrees F) or drywell structural integrity is challenged by an adverse trend in drywell airspace temperature.

Before DRYWELL PRESSURE reaches 12 psig: initiate torus spray.

Secure before torus pressure drops to 0 psig.

Also, secure DW sprays before DW pressure reaches 0 psig.

• The rate of change of DW temperature rise
• The margin to 281 degrees F
• The likelihood of reversing the DW temperature trend before 335 degrees F (ADS qual temp) or challenging the 281 degrees F DW structural limit and,
• The nature of the prescribed actions, i.e. drywell sprays.

• Reducing containment pressure below the scram setpoint will clear scram logic and maximize the margin to containment limits
• If containment has failed or if primary containment venting is anticipated, it may be advisable to continue spray operation at low pressures to scrub the containment atmosphere
• Reducing primary containment pressure will also reduce NPSH available for pumps drawing suction from the supression pool. If there is no need for continued spray operation, sprays may be terminated at higher pressures to avoid exceeding the NPSH limits.

Concerning pump operation beyond NPSH and Vortex Limits, what factors should be considered?

• Availability of other systems
• Current trend of plant parameters
• Anticipated time that such operation will be required
• Degree to which the limit will be exceeded
• Sensitivity of the system to operation beyond the limit
• Consequences of NOT operating a system beyond the limit

Considerations of whether or not drywell temperature can be restored and maintained below 281 degrees F.

• Energy being added to the drywell cannot be significantly reduced
• Energy being removed from the drywell cannot be significantly increased
• Drywell temperature is rising and will reach 335 degrees F (max ADS qual temp) within a short time or the structural integrity of primary containment will be challenged.

Purpose of performing blowdown

2 psig

Drywell and Torus sprays

Spray operation reduces primary containment pressure by condensing any steam present and absorbing heat from containment atmosphere.

Drywell temperature is below the DW Spray Limit, and

Torus water level is below 4.2 feet

NOTE: the drywell spray limit is technically defined as the highest temperature at which initiation of drywell sprays will not result in an evaporative cooling pressure drop to below the high drywell pressure scram setpoint.

BEFORE DW pressure reaches 12 psig, what must be done?

At what level does torus spray become ineffective?

14.1 feet

This is the elevation of the torus spray nozzles. Torus spray will not be effective with level beyond 14.1 feet.

To reduce and maintain pressure below Figure D, DW Pressure Limit. Venting is NOT intended to provide complete relief of primary containment pressure.

The preferred vent path is through the torus so the containment atmosphere is scrubbed by the suppression pool. Futhermore, if radiation levels inside primary containment are significant, venting through SBGT is preferred.

Preferred path for venting primary containment

Which is more important: venting primary containment or not exceeding normal offsite release limits?