ML20207H312

Requests Exemption from Installation of Alternate Rod Injection Sys.Capability of Bypassing Full Steam Flow Prevents Occurrence of High Reactor Pressure Conditions & Actuation of Safety Relief Valves.Fee Paid

ML20207H312
Person / Time
Site:	Big Rock Point File:Consumers Energy icon.png
Issue date:	12/29/1986
From:	Frisch R CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8701070504
Download: ML20207H312 (4)

Text

d Consumers Power ro wsmaus AIE8MEAM5 PRSERE55 General offices: 1946 West Parnalt Road, Jackson, MI 49201 . (517) 788-0550 December 29, 1986 Director, Nuclear Reactor Regulation US Nuc1 car Regulatory Commission Washington, DC 20555 DOCKET 50-155 - LICENSE DPR BIG ROCK POINT PLANT -

REQUEST FOR EXEMPTION FROM 10CFR50.62(C)(3), ALTERNATE ROD INJECTION-In July, 1984 the NRC published requirements (10CFR50.62) for the reduction of risk from anticipated transients without scram (ATWS). This rule required licensees to submit implementation schedules for meeting the requirements of the rule. Consumers Power Company letter dated October 14, 1985 provided the required information. With respect to the Alternate Rod Injection require-ments of 10CFR50.62(c)(3), Consumers Power Company committed to perform risk based evaluations to determine the efficacy of Alternate Rod Injection at Big Rock Point. Our letter dated October 1, 1986 provided the risk evaluation which is currently undergoing NRC review.

The October 1, 1986 risk evaluation concludes that the benefits of an Alter-nate Rod Injection system are minimal at Big Rock Point and that other design features provide an equal or more substantial impact on reducing the risk of operation than does a diverse scram system. For the reasons presented in detail in the October 1, 1986 submittal Consumers Power Company concludes in accordance with the requirements of 10CFR50.12(a)(2)(ii) that the diverse scram system is not required at Big Rock Point to achieve the underlying purpose of the rule. This letter requests exemption to 10CFR50.62(c)(3) for the Big Rock Point Plant and provides a brief summary of the purpose of the rule and the October 1, 1986 submittal. The October 1, 1986 risk evaluation provides the basis for our exemption.

In the statements of consideration for 10CFR50.62, the Commission states that the purpose of the alternate rod injection portion of the rule is directed at accident prevention and diversity in the reactor trip system. The Commission further suggests, on the other hand, that some of the older operating plants may be granted exemption if they demonstrate that the risk from ATWS is sufficiently low. Factors to consider important in this demonstration could j- be power level, unique design features that could prevent or mitigate the consequences of ATWS, remaining plant lifetime, or remote siting.

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Director, Nuclear Reactor Regulation 2 Big Rock Point Plant Request for Exemption - Alternate Rod Injection December 29, 1986 The analysis submitted to the NRC on October 1, 1986 takes into consideration all of these factors in determining the benefits of alternate rod injection and concludes that the risk associated with ATWS is sufficiently low so as to not warrant backfitting of the Big Rock Point Plant with the suggested diverse scram system. Aside from features such as its low power level (240MWT) and limited remaining lifetime (14 years), several plant design features were identified that are not found in the design of the generic plant used by the NRC Staff in its regulatory analysis. Detailed discussion of these design features and their importance to the consequences of an ATWS are presented in the October 1,1986 submittal. A brief summary of several of the more impor-tant features follows.

Fast Response of the Liquid Poison System The standby liquid control system at Pig Rock Point once actuated depends on a siphoning action to draw the poison solution into the lower plenum of the vessel where it mixes with the reactor coolant before entering the core. There is no reliance on charging pumps to inject the poison solution. Reactor shutdown can be attained within minutes as opposed to time frames on the order of a half hour at larger BWRs. The importance of this feature is that even during rapidly evolving transients, there is time for the operator to actuate poison and attain reactor shutdown before reactor core cooling is threatened.

Large Dry Containment The volume of the Big Rock Point containment is similar to large PWR containments. There is no supprescion pool and there is no potential for unstable conditions at the discharge of the relief valves to cause unacceptable containment loading, as is postulated to occur at large BWRs. Furthermore, containment failure does not lead to a degradation of core cooling capabilities. This is the single biggest contributor to the risk of an ATWS at BWRs as described in the NRC Staff regulatory analysis.

Full Load Rejection Capability The turbine bypass line is sized to accept steam flow at a rate equivalent to full power operation. For accident sequences in which the main condenser and bypass valve operate (more than 70% of all sequences),

reactor high pressure conditions are prevented and safety valves do not get a demand. This is unlike BWRs with small bypass capability in which steam is discharged to the suppression pool for almost any transient except those from low reactor powers. The significance of this feature is that for most transients, even though a reactor trip may fail to occur, it is possible for steam to be be bypassed to the main condenser and returned by way of the feedwater system similar to conditions associated with normal power operation.

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Director, Nuclear Reactor Regulation 3 Big Rock Point Plant Request for Exemption - Alternate Rod Injection December 29, 1986 In general, these features tend to mitigate the consequences of an ATWS at Big Rock Point. The last feature, full load rejection, is also to some degree preventative in that if it performs as designed it results in an essentially indefinite amount of time for the operator to shut down the reactor either by using conventional rod insertion methods or by poison injection.

While these unique design features of the Big Rock Point Plant minimize the principle sources of risk that exists at bigger BWRs, the October 1, 1986 submittal also pointed out ATWS risks at Big Rock Point not present in newer designs. As an example, it was identified that above given power levels, operation of the bypass valve results in a level swell in the hotwell and a trip of the feed pumps on low suction as this additional inventory is rejected to the condensate storage tank. The resulting loss of feedwater injection can aggravate an ATWS by significantly reducing the time to core uncovery, which in turn reduces the time for the operator to respond to the event.

Because of this unique response of Big Rock Point to an ATWS event, before taking credit for full load rejection capabilities as a mitigative or preventive feature for ATWS, Consumers Power Company commits to improving the reliability of feedwater during bypass valve opening events. This activity is already in progress and is identified as Issue BN-16, Secondary System Instabilities, of the Big Rock Point Integrated Plan.

In the event that it is shown not to be possible to eliminate these feedwater instabilities, then the October 1, 1986 report suggests that there is some benefit to a modified version of alternate rod injection.

This modified diverse system would be desirable because of the short time frame in which the operator has to actuate the poison system during loss of feedwater without scram. The modification as described in the October 1 submittal endorses the redundant scram solenoid portion of the alternate rod injection system which would be actuated by an electrically independent circuit attached to the manual scram button.

In summary, 1) because of the capability of bypassing full steam flow preventing the occurrence of high reactor pressure conditions and the actuation of safety relief valves during the largest majority of ATWS sequences, 2) because of the fast acting nature of the liquid poison system requiring only minutes to bring the reactor subcritical, and

3) because of the large dry containment not subject to relief valve discharge instabilities, it is concluded that installation of an alternate rod injection system in accordance with 10CFR50.62(c)(2) is not required in order to meet the underlying purpose of the rule. In accordance with the statements of considerations governing exemptions to the ATWS rule, an exemption to this rule for the Big Rock Point Plant is warranted not only because of its power level and remaining lifetime, but because of these unique mitigating design features which help to minimize the consequences of ATWS at Big Rock Point.

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Director, Nuclear Reactor Regulation 4 Big Rock Point Plant Request for Exemption - Alternate Rod Injection December 29, 1986 Pursuant to the requirements of 10CFR170.12(c) a check in the amount of

$150.00 is remitted with this application.

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Ral R risch Senior Licensing Analyst CC Administrator, Region III, USNRC NRC Resident Inspector - Big , Rock Point Plant OC1286-0170-NLO4 m