ML12065A170

B 3.1 Reactivity Control Systems, B 3.1.1 Shutdown Margin (SDM)

ML12065A170
Person / Time
Site:	Callaway
Issue date:	03/01/2012
From:	NRC Region 4
To:
References
FOIA/PA-2012-0110
Download: ML12065A170 (6)

Text

SDM B 3.1 RE-AC IiViTY CUONTROL SYSTEMS B 3.1.1 SHUTDOWN MARGIN (SDM)

BA3r-S BACKGROUND Acmrdling tn CGrOC 9R (s{

1)1 thi rewctivity control ystems must be redundant and capable of holding the reactor core subcriticai when shut down under cold conditions. Maintenance of the SDM ensures that postulated reactivity events will not damage the fuel.

SDM requirements provide sufficient reactivity margin to ensure that acceptable fuel design limits will not be exceeded for normal shutdown and anticipated operational occurrences (AOOs). As such, the SDM defines the degree of subcriticality that would be obtained immediately following the insertion of all shutdown and control rods, assuming that the single rod cluster assembly of highest reactivity worth is fully withdrawn.

The system design requires that two independent reactivity control systems be provided, and that one of these systems be capable of maintaining the core subcritical under cold conditions. These requirements are provided by the use of movable control assemblies and soluble boric acid in the Reactor Coolant System (R,,,.

The Rod Control System can compensate for the reactivity effects of the fuel and water temperature changes accompanying power level changes over the range from full load to no load. In addition, the Rod Control System, together with the boration system, provides the SDM during power operation and is capable of making the core subcritical rapidly enough to prevent exceeding acceptable fuel damage limits, assuming that the rod of highest reactivity worth remains fully withdrawn. The Chemical and Volume Control System can control the soluble boron concentration to compensate for fuel depletion during operation and all xenon burnout reactivity changes and can maintain the reactorsubcritical under cold p ndoins..........................

During power operation, SDM control is ensured by operating with the shutdown banks fully withdrawn and the control banks within the limits of LCO 3.1.6, "Control Bank Insertion Limits." 'When the unit is in the shutdown and refueling modes, the SDM requirements are met by means of adjustments to the RCS boron concentration.

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DMM B 3.1.1 APPLIC L

nimum recuired 3DM is assumed as an in.ti. cndtion in.afe,.v SAFETY analyses. The safety analysis establishes an SDM that ensures specified ANALYSES acceptable fuel design limits are not exceeded for normal operation and A0Os, with the assumption of the highest worth rod stuck out on scram.

For MODE 5, the primary safety analysis that relies on the SDM limits is U4 if VJI J!

U1IULIUi!! anal-r-is The acceptance criteria for the SDM requirements are that specified acceptable fuel design limits are not exceeded. This is done by ensuring that:

a.

The reactor can be made subcritical from all operating conditions, transients, and Design Basis Events;

b.

The reactivity transients associated with postulated accident conditions are controllable within acceptable limits (departure from nucleate boiling ratio (DNBR), fuel centerline temperature limits for AOOs, and _< 200 cal/gm average fuel pellet enthalpy at the hot spot in irradiated fuel for the rod ejection accident); and

c.

The reactor will be maintained sufficiently subcritical to preclude inadvertent criticality in ihe shutdown condition.

The most limiting accidents for the SDM requirements are the main steam line break (MSLB) and inadvertent boron dilution accidents, as described in the FSAR (Refs. 2 and 3). In addition to the limiting MSLB transient, the SDM requirement is also used in the analyses of the following events:

a.

Inadvertent boron dilution;

b.

An uncontrolled rod withdrawal from subcritical or low power condition (automatic rod withdrawal is no longer available);

Startupn of an in..c.ive reactor coolant pump (RCP); and

d.

Rod ejection.

The increased steam flow resulting from a pipe break in the main steam system causes an increased *-nergy removal frobIn the affected steam generator (SG), and consequently the RCS. This results in a reduction of the reactor coolant temperature. The resultant coolant shrinkage causes a reduction in pressure. In the presence of a negatve moderator temperature coefficient, this cooldown causes an increase in core (continued),

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SDM BASES A---N LA-L_-

SAFETY, ANALYSES (continued) reactivit,. As RCS temperature decreases, the severity of an MSLB decreases until the MODE 5 value is reached. The most limiting MSLB, with respect to Potential fuel damage before a reactor trip occurs, is a guillotine break of a main steam line inside containment initiated at the

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decrease ov" terinate when the affected SG boils dry, thus terminating RCS heat removal and cooldown. Following the MSLB, a post trip return to power may occur; however, no fuel damage occurs as a result of the post trip return to power, and THERMAL POWER does not violate the Safety Limit (SL) requirement of SL 2.1.1.

In the boron dilution analysis, the required SDM defines the reactivity difference between an initial subcritical boron concentration and the corresponding critical boron concentration. These values, in conjunction with the configuration of the RCS and the assumed dilution flow rate, directly affect the results of the analysis. This event is most limiting at the beginning of core life, when critical boron concentrations are highest. The SDM must be adequate to allow sufficient time for the BDMS to detect a flux multiplication greater than its setpoint and initiate valve swapover to prevent inadvertent criticality.

Depending on the system initial conditions and reactivity insertion rate, the uncontrolled rod withdrawal -transient is terminated by either a high power level trip or a high pressurizer pressure trip. In all cases, power level, RCS pressure, linear heat rate, and the DNBR do not exceed allowable limits.

The startup of an inactive RCP is administratively precluded in MODES !

and 2. In MODE 3, the startup of an inactive RCP can not result in a "cold water"' criticality, even if the maximum difference in temperature

-exists between the SG and the core. The maximum positive reactivity -,

addition that can occur due to an inadvertent RCP start, is less than half the minimum required SDM. Startup of an idle RCP cannot, therefore, produce a return to power from the hot standby condition.

The ejection of a control rod rapidly adds reactivity to the reactor core, causing both the core power level and heat flux to increase with corresponding increases in reactor coolant temperatures and pressure.

The ejection of a rod also produces a time dependent redistribution of core nower. Depending on initial power e this accident iC t.m..nated by the power range neutron flux - high or low reactor trip setpoint in the FSAR analyses.

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SDM i 3.!.11 BASES (continued)

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t uo 1CFR5.36(c)(2)(i). Even thouhn it is noi directly observed from the control room, SDM is considered an initial ConditiOn process variable because it is per-iodically monitored t Osra, that the unit is operating within the bounds of accident analysis assumptions.

LCO SDM is a core design condition that can be ensured during operation through control rod positioning (control and shutdown banks) and through the soluble boron concentration.

The MSLB (Ref. 2) and the boron dilution (Ref. 3) accidents are the most limiting analyses that establish the SDM value of the LCO. For MSLB -..

accidents, if the LCO is violated, there is a potential to exceed the DNBR

-tdoc 10 CFR 100, "Reactor Site Criteria," limits (Ref. 4).

For the boron dilution accident, if the LCO is violated, the minimum required time assumed for operator action to terminate dilution may no longer be sufficient. The required SDM limits are specified in the COLR.

APPLICABILITY In MODE 2 with kf_ < 1.0 and in MODES 3. 4. and 5 the SDM requirements are applicable to provide sufficient negative reactivity to meet the assumptions of the safety analyses discussed above. In MODE 6, the shutdown reactivity requirements are given in LCO 3.9.1, "Boron Concentration." In MODES I and 2, SDM is ensured by complying with LCO 3.1.5, "Shutdown Bank Insertion Limits," and LCO 3.1.6, "Control Bank Insertion Limits."

The Applicability is modified by a Note stating that the transition from MODE 6 to MODE 5 is not permitted while LCO 3.1.1 is not met. This Note specifies an exception to LCO 3.0.4 and prohibits the transition when SDM limits are not met. This Note assures that the initial.

assumptions of a postulated boron dilution event in MODE 5 are met.

ACTIONS A.1 If the SDM requirements are not met, boration must be initiated promptly.

A Completion Time of 15 minutes is adequate for an operator to correctly align and start the required systems and components. It is assumed that boration will be continued until the SDM requirements are met.

(continued)

CALLAWAY PLANT B 3-.1.- 4 Revision 0

S -. M P. ^* i. -1 BASES A. i jcontin~ued')

ite diatlermination of the required combination of boration flow rate and boron concentration, there is no unique requirement, that must be satisfied. Since it is imperative to raise the oron oncentration of the RCS as soon as possible, the borated water source should be a highly concentrated solution, such as that normally found in the boric acid storage tanks, or the refueling water storage tank. The operator should borate with the best source available for the plant conditions.

SURVEILLANCE REQUIREMENTS SR 3.1.1.1 In MODES 1 and 2, SDM is verified by observing that the requirements of LCO 3.1.5 and LCO 3:1.6 are met. in the event that a rod is known to be untrippable, however, SDM verification must account for the worth of the untrippable rod as well as another rod of maximum worth.

In MODES 2 (with kf < 1.0), 3, 4, and 5, the SDM is verified by performing a reactivity balance calculation, considering the listed reactivity effects:

a.

RCS boron concentration (may include allowances for boron-10 depletion);

b.

Control and shutdown rod position;

c.

RCS average temperature;

d.

Fuel burnup based on gross thermal energy generation;

e.

Xenon concentration;

f.

Samarium concentration; and

g.

Isothermal temperature coefficient (iTC).

Using the ITC accounts for Doppler reactivity in this calculation because the reactor is subcritical, and the fuel temperature will be changing at the same rate as the RCS.

(continued)

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SDM BASES fREQUiRENIENTS SR( 3.i.1.i (continued)

In the event that a rod is known to be untrippabie and not fully inserted, SDM verification must account for the worth of the untrippable rod as well as another rod of maximum worth.

The Frequency of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> is based on the generally slow change in required boron concentration and the low probability of an accident occurring without the required SDM. This allows time for the-operator to collect the required data, which includes performing a boron concentration analysis, and complete the calculation.

REFERENCES

1.

10. CFR 50, Appendix A, GDC 26.

2.

FSAR, Chapter 15, Section 15.1.5.

3.

FSAR, Chapter 15, Section 15.4.6.

4.

10 CFR 100.

CALLAWAY PLANT B 3.1.1-6 Re vis'ýio-.n 01