ML20105C706

Proposed TS Bases 3/4.1.3 Re Control Rods

ML20105C706
Person / Time
Site:	LaSalle
Issue date:	09/16/1992
From:	COMMONWEALTH EDISON CO.
To:
Shared Package
ML20105C704	List: ML20105C701 ML20105C706 ML20105C713 ML20105C720
References
NUDOCS 9209220365
Download: ML20105C706 (4)

Text

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% $ (u ,e k m C REACTIVITY CONTROL SYSTEMS &( IS BASES .

3/4.1.3 CONTROL RODS The specification of this section ensure that (1) the minimuru SHUTDOWN MARGIN is maintained, (2) the control rod insertion times are consistent with those used in the accident analysis, and (3) the potential effects of the rod drop accident are limited. The ACTION statements permit variations from the basic requirements but at the same time impose more restrictive criteria for continued operation. A limitation on inoperable rods is set such that the resultant effect on total rod worth and screm shape will be kept to a minimum.

The requirements for the various scram time measurements ensure that any indication of systematic problems with rod drives will be investigated on a timely basis.

Damage within the control rod drive mechanism could be a generic problem, therefore with a control rod immovable becauce of excessive friction or mechanical interference, operation of the reactor is limited to a time period which is reasonable to determine the cause of the inoperability and at the san.e time prevent operation with a large numbcr of inoperable control rods.

Control rods that are inoperable for other reasons are permitted to be taken out of service provided that those in the nonfully-inserted position are consistent with the SHUTDOWN MARGIN requirements.

The number of control rods permitted to be inoperable could be more than the eight allowed by the specification, but the occurrence of eight inoperable rods could be indicative of a ger.eric problem and the reactor must be shutdown for investigation and resolution of the problem, The control rod system is designed to bring-the reactor subtr itical at a rate fast enough to prevent the MCPR from becoming less than the fuel cladding safety limit during the limiting power transient analyzed in Section 15.0 of the FSAR. This analysis shows that the negati N reactivity rates resulting from the scram with the average response of all the drives as given in the specifications, provide the required protection and MCPR remains greater than the fuel cladding safety limit. The occurrence of scram times longer then those specified should be viewed as an indica * .n of a systemic problem with the rod drives and therefore the surveillance interval is reduced in crder to prevent operation of the reactor for long p~ iods o) time with a potentially serious problem.

The scram discharge volume is required to be 6DERA8LE so that it vill be available when needed to accept discharge water irce the control rods daring a reactor scrata and will isolate the reactor coolant system from the envir onment when required.

Control rods with inoperable accumulators are declared i..perable and Specification 3.1.3.1 then applies. This prevents a pattern of inoperable accumulators that would result in less reactivity insertion on a scram than has been analyzed even though control rods with inoperable accumulators may still be inserted with normal drive water pressure. Operability of the accumu-lator ensures that there is a means available to insert the control rods even under the most unfavorable depressurization of the reactors.

LA SALLE - UNIT 1 B 3/4 3-2 Amerdment No. IS 92092203eS 920916 PDR ADOCK 05000373 p P11R

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. ' REACTIVITY CONTROL SYSTEMS.

BASES ._

CONTROL RODS (Continued)

In addition, the automatic CRD charging water header low pressure scram (see Table 2.2.1-1) initiates well before any accumulator loses its full capa- ,

bility to insert the control rod. With this added automatic scram feature, j the surveillance of each individual accumulator check valve is no longer  !

necessary to demonstrate adequate stored energy is available for normal scram l action.

Control rod coupling integrity is required to ensure compliance with the I analysis of the rod drop accident in the FSAR. The overtravel position feature

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provides the only positive means of determining that a rod is properly coupled and therefore this check must be performed prior to achieving criticality after completing CORE ALTERATIONS that could have affected the control rod drive coupling integrity. The subsequent check is performed as a backup to the _ initial demonstration.

In order to ensure that the control rod patterns can be followed and there-fore that other parameters are within their limits, the controi rod position' indication system must be OPERABLE.

IM' The control <fod housing support restricts the outward movement of a control rodtoCiesi[than[)VTnchesintheeventcfahousingfailure..Theamountofrod

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reactivity which could be added by this small amount of rod withdrawal is less than a normal withdrawal increment and will not contribute to any damage _to the primary coolant system. The support is not required when there is no pressur' to act as a driving force to rapidly eject a drive housing.

The required surveillance intervals are adequate to determine that the rods are OPERABLE and not so frequent as to cause excessive wear on the system components.

3/4.1.4 CONTROL ROD PROGRAM CONTROLS Control rod withdrawal and insertion sequences are established to assure that the maximum insequence individual control rod or control _ rod segments which are withdrawn at any time during the fuel cycle could not be worth enough to result in a peak fuel enthalpy greater than 280 cal /gm in the event of a control rod drop accidint. The specified sequences are characterized by homogeneous, scattered patterns of control rod withdrawal. When THERMAL POWER is greater than 20% of RATED THERMAL POWER, there is no possible rod worth which, if- dropped at the design rate of the velocity limiter, could result in a peak enthalpy of

~280 cal /gm. Thus requiring the RSCS and RWM to be OPERABLE when THERMAL POWER p is less than or equal to 20% of RATED THERMAL POWER provides adequate control.

l l The RSCS and RWM provide automatic supervision to assure that out-of-sequence rods will not be withdrawn or inserted.

The analysis of the rod drop accident is presented in Section 15.4.9 of the FSAR and the techniques of the analysis are presented in a topical report, Reference 1, and two supplements, References 2 and 3.

LA SALLE - UNIT 1 8 3/4 1-3 Amendment No. 33

y, ., ,, h m OA J N. (1 c. UW REACTIVITY CONTROL SYSTEMS BASES 3/4.1.3 CONTROL RODS The specifications of this sectinn ensure that (1) the minimum SHUTOOWN MARGIN is maintained, (2) the control rod insertion times are consistent with those used in the accident analysis. and (3) the potential effects of the rod drop accident are limited. The ACTION statements permit variations from the basic requirements but at the same time impose more restrictive criteria for

. continued operation. A limitation on inoperable rods is set such that the resultant effect on total rod worth and scram shape will be kept to a minimum.

The reo"irements for the various scram time measurements ensure that any indica-tion 01 systematic problems with rod drives will be investigated on a timely basis.

Damage within the control rod drive mechanism could be a generic problem, therefore with a control rod iminovable beccuse of excessive friction or mechanical interference, ope,'ation of the reactor is limited to a time period which is reasonable to determine the cause of the inoperability and at the same time prever.t operation with a large number of inoperable control rods.

Control rods that are inoperable for other reasons are permitted to be taken out of service provided that those in the nonfully inserted position are consistent with the SHUlDOWN MARG:N requirements.

The number of control rods permitted to be inoperable could be more than the eight allowed by the specification, but the occurrence of eight inoperable rods could be indicative of a generic problem and the reactor must be shut down for investigation and resolution of the problem.

The control rod system is designed to bring the reactor subcritical at a rate fast enough to prevent the MCPR from becoming less than the fuel cladding ufety limit during the limiting power transient analyzed in Section 15.0 of the FSAR.

This analysis shows that the negative reactivity rates resulting from the scram with the average response of all the drives as given in the specifications, provide the required protection and MCPR remains greater than the fuel cladding safety limit. The occurrence of scram times longer then those specified should be viewed as an indication of a systemic problem with the rod drives and therefore the surveillance interval is reduced in order to prevent operation of the reactor for long periods of time with a potentially serious problem.

The scram discharge volume is required to be OPERABLE so that it will be available when needed to accept discharge water from the control rods during a reactor scram and will isolate the reactor coolant system from the environment when required.

Control rods with inoperable accumulators are declared inoperable and Specification 3.1.3.1 then applies. This prevents a pattern of inoperable accumulators that would result in less reactivity insertion on a scram than has been analyzed even though control rods with inoperable accumulators may still be inserted with normal drive water pressure. Operability of the accumu-3 lator ensures that there is a means available to insert the control rods even under the most unfavorable depressurization of the reactors. i I

LA SALLE - UNIT 2 8 3/4 1-2

REACTIVITY CONTROL SYSTEMS BASES CONTROL RODS (Continued)

. In additicn, the automatic CRD charging water header low pressure scram (see Table 2.2.1 ') irdtiates well before any accumulator loses its full capa-bility to insert ti. control rod. With the added automatic scram featu.e, the surveillance of each individual accumulator check valve is no longe necessary to demonstrate adequate stored energy is available for normal scram action.

Control rod coupling integrity is required to ensure compliance with the analysis of the rod drop accident in the FSAR. The overtravel position feature provides the only positive means of determining that a rod is properly coupled and therefore this check must be performed prior to achieving criticality after cr'oleting CORE ALTERATIONS that could have af fected the control rod drive coupling integrity. The subsequent check is performed as a backup to the initial demonstration.

In order to ensure that the control rod patterns can be followed and there-fore that other parameters are within their limits, the control rod position indicationsystemmusybeOPERABLE.

The_t.ontr_olyrod housing support restricts the outward movement of a control rod to less than F inches in the event of a housing failure.

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The amount of rod reactivity which could be added by this small amount of rod w:thdrawal is less than a normal withdrawal increment and will not contribute to any damage to the primary coolant system. The support is not required when there ig no pressure to act as a driving force to rapidly eject a drive housing.

The required surveillance intervals are adequate to determine that the rods are OPERABLE and not so frequent as to cause excessive wear on the system components.

3/4.1.4 CONTROL ROD PROGRAM CONTROLS Control rod withdrawal and insertion sequences are established to assure ,

that the maximum insequence individual control rod or control rod segments which are withdrawn at any time during the fuel cycle could not be worth enough to result in a peak fuel enthalpy greater than 280 cal /gm in the event of a control rod drop accident. The specified sequences are characterized by homogeneous, scattered patterns of control rod withdrawal. When THERMAL POWER is greater than 20% of RATED THERMAL POWER, there is no possible rod worth which, if dropped at the design rate of the velocity limiter, could result in a peak enthalpy of 280 cal /gm. Thus requiring the RSCS and RWM to be OPERABLE when THERMAL POWER is less than or equal to 20% of RATED THERMAL POWER provides adequate control.

The RSCS and RWM provide automatic supervision to assure that out-of-sequence rods will not be withdrawn or inserted.

The analysis of the rod drop accident is presented in Section 15.4.9 of the FSAR and the techniques of the analysis are presented in a topical report, Reference 1, and two supplements, References 2 and 3.

The RBM is designed to automatically prevent fuel damage in the event of erroneous rod withdrawal from locations of high power density during high power operation. Two channels are providea. Tripping one of the channels will block erroneous rod withdrawal soon enough to prevent fuel damage. This system backs up the written sequence used by the operator for withdrawal of control rods.

LA SALLE - UNIT 2 B 3/4 1-3 Amendment No. 6

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