ML20133G354
| ML20133G354 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 05/28/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20133G357 | List: |
| References | |
| NUDOCS 8508080646 | |
| Download: ML20133G354 (12) | |
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REVISED SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SOPPORTING AWENDMENT NO.120 TO LICENSE NO. DPR.49 IOWA ELECTRIC LIGHT AND POWEP COMPANY CENTRAL IOWA POWER COOPERATIVE CORN BELT POWER COOPERATIVE DUANE ARNOLD ENERGY CENTER DOCKET NO. 50-331
1.0 INTRODUCTION
By letter dated August 17, 1984, the Iowa Electric Light and Power Comoany (the licensee) requested chan Technical Specifications (TS)ges to the Duane Arnold Energy Center (DAEC) to implement Extended Load Line Limits derived from its analysis. Subsequently, by letters dated January 11 and March 15, 1985, the licensee proposed imorovements to the Average Power Range Monitor (APRM)andRodBlockMonitor(RBM). The APRM RBM and T5 (ARTS) improvements are intended to increase the plant operating efficiency, update the compliance with the themal margins requirements, improve the accuracy and response of the pertinent instrumentation, and to improve the man / machine interface.
The licensee has provided Extended Load Line Limit Analysis (ELLLA) as a basis for nomal reactor operation in the region of power / flow map above 100 percent power and 100 percent flow limits. The operation in the extended region is achieved by changing the slope of the flow bias algorithms and revising the APRM red block line. The ARTS improvements involve (1) elimination of APRM trip setdown requirements, (2) changes from flow to power referenced setpoints for the RBM, (3) power and flow dependent limits on Maximum Average Planar Linear Heat Generation Rate (MAPLHGR) and Minimum Critical Power Ratio (MCPR), (4) reconfiguration of Local Power Range Monitor (LPRM1, (5) changes in normalization procedure and new trip logic in the RBM providing definition of a limiting rod pattern for RBM bypass decisions, and (6) an altered rod withdrawal error at power analysis.
2.0 EVALUATION Extended Load Line Limit Analysis The extended load line limit operation permits higher powers for low flow conditions by changing the slope of the APRM rod block line. The effect is to allow operation at 100 percent power for 87 percent flow or' greater and to increase the permitted power at 40 percent recirculation flow by about 7 percent to 73.2 percent. We have reviewed the impact of the reactor operation in the extended power / flow region, on the evaluation of transients and core stability.
8500080646 850002 DR ADOCK 050
The transient and accident analyses described in the evaluation of the APTS program have all assumed operation with the extended load line limit. The changes in core behavior caused by the extended operation range have been
. accounted for in the revised analyses discussed under ARTS improvement program.
The operation in the extended power / flow region was previously approved for the Hatch, Dresden, and Monticello plants (see Monticello Amendnent No. 29).
Additionally, the compliance of General Electric Company's (GEi boiling water reactors with thermal-hydraulic stability criteria has been generically confimed by the staff for operation in the extended power / flow region. We, therefore, conclude that the thermal-hydraulic stability of DAEC Cycle B in the extended power / flow region is assured. The operation of DAEC in the proposed extended power / flow region is, therefore, acceptable.
APRM System Imorovements Each APRM channel consists of a number of LPRMs which are chosen in such a way that the channel output is proportional to core power.
The APRM signals are compared to a fixed scram trip (at 120% full power) and to a flow biased rod withdrawal block trip.
1 Current DAEC Technical Specifications require that the flow biased APRM setpoints be lowered (set down) if the core maximum fraction of limiting power density (CMFLPD) exceeds the fraction of rated power (FRP). This may be accomplished by increasing the APRM channel gain. If CMFLPD exceeds FRP and the core power is raised to its full value the operating limit value for MAPLHGR or MCPR could be exceeded and the assumptions used in the plant transient analyses violated.
In the proposed APRM system the setdown requirement would be removed.
It would be replaced by power and flow dependent MAPLHGR and MCPR limits.
Analyses have been perfomed to obtain the multipliers to be applied to the full power values of MAPLHGR and MCPR in order to prevent violation of safety criteria during transients and accidents.
The.LOCA and limiting transients were reanalyzed without the APRM system setdown requirements.
Previous analyses of the LOCA at less than rated flow have been performed under the assumptions that the extended load line was in effect and that the APRP setdown was present.
These analyses showed that below 70 percent flow a five percent reduction in MAPLHGR limits was required. However, the reanalysis performed for Cycle 8 (approved by the staff in Amendment No.
115) showed increased margins to LOCA limits at both full and reduced flow.
4 The analyses showed that the reouired reduction for low flows in the absence of the APRM setdown is bounded by that required for reasons other than LOCA.
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In order to restore safety margins which might be reduced when the APRM setdown is removed, the limiting transients were reanalyzed assuming the absence of this feature. The analyses assumed operation within the proposed extended power / flow domain with flows up to 100 percent of rated flow. Analyses of the transient events were made as a function of initial power and flow and the results used to determine multipliers to be applied to full power-full flow values of MCPR and MAPLHGR. The power dependence was most sensitive at full flow and the feedwater controller failure was the transient showing the largest sensitivity.
This event was then used to construct a curve of MCPR multiplier, K
and MAPLHGR multiplier, MAPFAC c8r,ves were drawn in order to boun$, future cycles.as a function of core power. Conse Flow dependence of MCPR and MAPLHGR was determined from analyses of flow runout events in which the core flow is ramped rapidly upward to the maximum value permitted by the setting of the recirculation pump scoop. The flow multipliers, K maximum flow an,d a family $f curves is drawn.and MAPFAC, are thus a function of The multipliers are chosen so that a flow runout to the maximum flow will not result in a violation of MCPR l
or LHGR safety limits. The MAPFAC, curves are combined with the results of LOCA analysis described above and the combined family of curves is used in i
the Technical Specifications. For inclusion in the Technical Specifications, 4
the Kf curve family is transposed to a MCPR family by assuming a value of f
l 1.2 for the full flow MCPR. This is the lowest value that may be used for DAEC.
The above discussion applies to the power range from 30 to 100 percent of full power. Below 30 percent of full power the turbine stop and control valve scrams are bypassed and the analyses do'not apply. Below 25 percent of full power no MCPR and MAPLHGR limits are defined.
In the interval between 25 and 30 percent of full power flow dependent effects are taken into account by having two power dependent curves - one for flows greater than 50 percent of rated and one for lower flows. Analyses are then performed to obtain limiting MCPR and MAPLHGR values in these domains.
Approved methods were used to perform the analyses described above except for those used for the loss of feedwater heater event. For that event the trend analysis was performed with the BWR simulator code.
However, this event is not limiting and safety analyses for the event are done by approved methods described in GESTAR II.
We find this acceptable.
We conclude that deletion of the APRM setdown requirement is acceptable when it is replaced by the power and flow dependent operating limits described above.
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Rod Block Monitor System improvements The Rod Block Monitor (RBM) System is used to prevent violation of fuel themal-hydraulic limits in the event of inadvertent continuous withdrawal of a control rod. When a rod is selected for withdrawal the surrounding LPRM strings are selected. Their response to the withdrawal is monitored and a withdrawal block is initiated by the RBM if that response exceeds certain limits. These limits are selected so that no violation of fuel limits occurs. The RBM has two independent channels either of which will 1
initiate a rod block if tripped.
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The proposed Rod Block Monitor improvements include:
1.
Reordering of the assignment of LPRM detectors to the two RBM channels in order to increase instrument sensitivity and provide more uniformity of response between the two channels.
2.
Changing the baseline noma 11 ration of the RBM from an APRM channel to a fixed signal in order to reduce the number of unnecessary rod blocks.
4 3.
Replacing the flow-biased trip setpoints with fixed power-dependent trip setpoints.
4.
Elimination of the resettable trips in order to make operation simpler.
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in addition the electronics hardware has been updated to increase the
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reliability of operation.
The change in LPRM assignments is described in the licensee's request and a comparison of the RBM channel responses to those of the current design is made. The revised design shows sistlar responses for the two channels each of which has a response similar to that of the most responsive channel in the current design.
A block diagram of the revised RBM system is presented and a discussion of the electronics change given in the licensee's application. We conclude i
that sufficient information is given in the report to pemit the conclusion that the proposed revisions to the R8M system design are acceptable. The electronics changes are discussed in the next section.
i The revisions of the RBM system necessitate the reevaluation of t_he Rod Withdrawal Error Event. The present deteministic, bounding, cycle.-
specific analysis is replaced with a statistical analysis valid for application to all DAEC cores using GE fuel up to type P8x8R inclusive. A data base calculated from actual plant operation states was created which j
covers the spectrum of plant sizes and power densities. The data base construction began with the selection of operating states at near full power which had low MCPRs and/or high MAPLHGRs in bundles near deeply i
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i l inserted control rods.
the MCPR values to approximately 1.20.The rod configurations were then adjusted l
Thirty-nine such configurations were chosen.
In order to investigate power and flow dependence, the rod configuration in 26 of the above cases was held constant, the flow was reduced to 40 percent of rated and xenon allowed to equilibrate.
for the 26 cases, the flow was held constant at 40 percent and the rodFinally, I
pattern altered to yield 40 percent power with no xenon. For each of the 91 cases described above 100 rod withdrawal error (RWE) analyses were performed assuming a random distribution of starting points for the error rod (and thus initial MCPR values MCPR ) and random failures of the LPRMs which r
provide inputs to the Rod 81oct Monitor.
All cases which did not result in
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a rod block were rejected from the. data base unless the rod started from the fully inserted position.
A 15 percent random failure rate was assigned to each LPRM.
Experience has shown this value to be high.
The Rod Block Monitor response was generated as a function of error rod position for each RWE.
in the analyses.
The currently used and approved methods were employed Rod Block Monitor setting.The results were tabulated as error rod position vs assu of normalized MCPR change ( g g results were then transformed into values MCPR ) and the mean and standard deviation of the distribution for each set of 1 RWE analyses were detemined for each RBM setting. These data were then combined to obtain a mean and standard deviation for the entire data base at each power / flow state for each RBM channel at each assumed R8M setting.
A plot of the required initial MCPR value (MCPR,) as a function of Rod Blocf Monitor trip setting was constructed.
The required value of MCPR is that which assures that 95 percent of the rod withdrawal errors which a,re initiated from it do not violate the MCPR safety limit (1.07) with a 95 percent confidence level.
The final step is the selection of suitable setpoints for the Rod Block Monitor.
i-At any power level the required operating limit MCPR for this e greater than that required for other transients as described in Section 3.2 above. A value of 1.20 at full power / full flow is assumed.
i the three trip settings of the present system, the power range from 30In keeping with percent to full power is divided into three intervals with a constant setpoint in each interval. For DAEC the intervals are 30-65, 65-85, and 85-100 percent of full power. The analytic setpoint for the intervals are respectively, 118,112 and 108 percent of the reference signal.
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The effect of the absence of LPRM strings for certain rods near t'IIe periphery of the core has been analyzed and it was shown that the setpoints described above are adequate to mitigate the consequences of the Rod Withdrawal Error in the periphery of the core.
A downscale trip at about 94 percent of the reference signal also inhibits rod withdrawal.
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. The anaYy'tes described above assumed unfiltered LPRM signal inputs to the RBM.
time constant of up to 0.55 seconds.However, provision is made in the instrumen Use of such a filter would necessitate the reduction of the setpoints given above by an amount which depends on the time constant chosen.
adjustments and values are given in NEDC-30813-P. Analyses were perfomed to filtering is chosen, the maximum time constant is recommended.If anything other than no a delay occurs between the time when the input signal reaches the setpointIn addition, and the imposition of the rod block.
this delay and no greater value may be pemitted.A value of 2.0 seconds was assumed for into the Technical Specifications.
This value is incorporated In order to confirm the use of a 15 percent failure probability in the up to 30 percent was assumed. statistical analysis a sensitivity study was performed value had a negligible effect on the results. Increasing the failure rate to the higher The Rod Block Monitor is currently req (30uired to be operable when c greater than some low power setpoint percent of full power). Additional i
defined to be a pattern which causes the core to be at the o i
if the complete withdrawal of any single rod in the core would vi limits.
Analyses have been perfomed - using the data base described above -
to obtain operating limit MCPR values above which no rod withdrawal error could lead to violation of the limits.
levels greater than 90 percent full power and one for levels from 25Two values ar percent full power.
If the plant is o to 90 is on a " limiting control rod pattern"perating at or below these limits it It may be bypassed when operating above these limits.and the RBM is required to be opera
_ Electrical Instrumentation and Controls The RBM system is designed to automatically detect and block control rod withdrawal that could violate Technical Specification safety limits during a single control rod withdrawal error (RWE) transient.
core is operated in compliance with plant Technical Specifications beforeIt is assum the RWE event.
rod block (i.e., prevent control rod withdrawal).There are two RBM channe The RBM channels are powered from the Reactor Protection System (RPS) buses (RBM channel A is powered from RPS bus A, and RBM channel B is powered from RPS bus B).
l Although the RBM system is not safety related, separation i channel to be bypassed if necessary.
via a single three position bypass switch such that only one RBM channelRBM can be bypassed at a time.
is placed in the center Both RBM channels are operable when the switch of a RBM channel bypass a(nomal) position. Both local and remote indication re provided via indicator lights.
stated that to the maximum extent possible, the new RBM system design meetsThe lic e
e w
. The only exceptic.ns are the sharing of LPRM signals from the
'C' levelthe same separation and isolation requirements as t detectors by both RBM channels.
Since the new RBM system is fail safe for failed LPRM input signals and the RBM channel is declared inoperable if too few detectors are available, we find that it would not pose a safety problem and is, therefore, acceptable.
located on the reactor operator's console, local meters, trip unitsThe RBM the on-line computer) will remain unchanged, although in some cases,the and signals used for these functions have been modified.
assigned (and unbypassed) local power range monitor chann to reach its asymptotic value and then automatically ampl The average same as a fixed reference signal.
sequence) is reinitiated each time a new rod isThis process (referred to as the RBM null rod motion is blocked during the null sequence. selected for movement. Control Each RBM channel then compares the calibrated (nulled) signal to an automatically selected preset rod block alarm / trip level (one of three power biased upscale trip levels is selected dependent upon the current reactor power level).
is selected based on the magnitude of a reference APRM.
The trip level flux level increases to the upscale trip setpoint, further control rodIf the local neutron withdrawal is blocked, thus limiting the change (increase) level) biased trips. flow biased (recirculation flow) trip feature with power (neut This modification will be implemented by changes to the pC card electronics (averaging cards, null sequence cards, RBM setpoint ~
cards, and quad trip cards).
It should be noted that an adjustable time delay (t seconds)hasbeenaddedtodelaythecalibrated(nubeIto50secondst0.5 d) average local neutron flux signal to the RBM trip logic. The purpose of this delay is to allow minimum rod movements despite abnonnally high signal noise not remove by filtering.
This delay is typically set at a value of 1 to 2 seconds.
oe design of the control rod drive system is for a normal speed of 3 inches per The second 0.6 inches per second.
the delay is short enough to limit rod movement well below that which c cause a thermal limits violation.
the minimum value, it is considered a bypass of the associated RBM chan since the analyses did not consider time delays in excess of the minimum value.
the ARTS modificationGeneral Electric Co. report submitted by the licensee in support o states that time delay t the minimum value as a, means of bypassing the RM.shall only be set above that manual adjustment of the t The staff's position is channel in lieu of using the exNting RBM channel bypass switch (whichsetp provides automatic indication of the bypass condition) is not acceptable and should not be permitted.
switch will be used to effect a RBM channel bypass.The licensee has stated t
. The trips include too few LPRM inputs, downscale rod withdrawal block (RRM signal abnormally. low). upscale rod withdrawal block. instrument inoperative, mode switch in other than operate, a module removed, number of unbypassed inputs too few and failure to null to the reference source signal. The licensee states that the response time of the trip logic and drift of the setpoints equals or is less than that of the logic being replaced. The staff finds it acceptable.
All rod blocks are alarmed. The upscale rod block alarm can only be reset by activating a reset switch or selecting another rod for movement. Locally mounted color coded lights are provided to indicate the type of rod block (upscale - amber. instrument inoperative - white and downscale - white).
The RBM system is required to be operable whenever a limiting rod pattern exists. A limiting rod pattern exists when any control rod in the core would result in violation of the safety limit MCPR if it were fully withdrawn.
During operation with a limiting rod pattern, both RBM channels should be operable.
If onl of the operable (y one RBM channel is operable, an instrument functional test bypassed) channel must be performed prior to withdrawal of any control rods. If the inoperable channel is not restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, then all control rod withdrawal shall be blocked.
If both RBM channels are inoperable, then all control rod withdrawal shall be blocked until operability of at least one channel is restored.
It should be noted that the operators are responsible for detennining whether a limiting rod pattern exists (and there-fore, for detenr.ining RBM system operability requirements) prior to control rod withdrawal in accordance with plant operating procedures. The staff has -
found this to be acceptable. The APRM and RBM instrument surveillance requirements (i.e.
instrument functional tests and calibrations) have not changed as a result of implementation of the ARTS improvement program.
The staff finds the proposed Technical Specification requirements for PBM system operability ~and the associated limiting conditions for operation to be acceptable.
Based on our review of the electrical, instrumentation, and control aspects of the Ouane Arnold Energy Center ARTS improvement program, we conclude that implementation of this design complies with the requirements of Section 7.7 (Control Systems) of the Standard Review Plan (NUREG-0800), and therefore, is acceptable.
The separation provided between redundant RBM channels and the isolation provided between the RBM system and safety related circuits have not been compromised as a result of the ARTS modification.
Technical Specification Changes Implementation of the hardware changes and revised analyses described above requires changes in the DAEC Technical Specifications. These changes are discussed below:
6.
. APRM Technical Specification Changes The requirement for the setdown of the trip setpoint is deleted from the specification and the setdown factor (Fraction of Rated Power divided by Core Maximum Fraction of Limiting Power Density) is removed from the equation for the trip setpoint.
block line and of the APRM/STPM flow biased scram are altered to pe operation within the domain defined by the extended load line limit analysis.
Rod Block Monitor Technical Specifications The RBM biased trip equation is replaced by power dependent setpoint definitions and incorporate RBM filter and time delay setpoints.
Current operability of the limiting control rod pattern. requirements are replaced by the new ones Thermal-Hydraulic Operating Limit Specifications The following changes are required in the Power Distribution Limit Specification:
1.
A curve of MCPR multiplier K, as a function of power must be added.
p 2.
The K, family of curves must be replaced with curves of MCPR function of flow.
f as a 3.
The MCPR Technical Specification must be altered to Sefine the manner in which the two curves are combined with the full power, full flow value of the operating limit MCPR to obtain the power / flow dependent limit.
4 Power and flow dependent multiplier factors (MAPFAC be added and the MAPLHGR Technical Specification muft be altered toand MAPFAC,)
define the manner in which the two curves are combined with the full MAPLHGR limits. power / full flow MAPLHGR curves to obtain the power and flow depen 5.
The bases for the various Technical Specifications must be modified to account for the altered Technical Specifications.
We have confirmed that the proposed DAEC Technical Specifications meet the requirements given above and are acceptable.
Impact of Other Licensing Action on Technical Specifications
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other Licensing Actions are also proposed.In addition to the introduction Two of these - use of Single i
's
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. Loop Operation and the introduction of Lead Test Assemblies (LTAs) in ihe core have an impact on the ARTS Technical Specifications.
Effect of Presence of LTAs As indicated above, the MAPLHGR reduction factor as a function of flow required by the LOCA analysis is bounded by that required by other transients.
P8x8R fuel in the core.This conclusion was based on the presance of " standard" 8x8R or However, the LOCA analysis for the LTA-311 fuel results in a MATLHeR reduction factor that is not bounded by the other transients.
Accordingly, a separate curve of MAPLHGR Flow Factor as a the LTA-311 assembly. function of core flow has been added to Technical Spec rated) has a stepwise reduction in the factor to 0.95 at 70 percent flow This is acceptable.
factors for MAPLHGR as described below.In addition the LTA-311 bundle l
Single Loop Operation DAEC makes several revisions to the Technical Specifica These include:
1.
LOOP OPERATION (SLO). Expanding the definition of REACTOR POWER O 2.
Increasing the safety limit value for MCPR from 1.07 to 1.10 in order to account for increased uncertainties in the measurement of core parameters.
3.
SLO in order to maintain operating margins. Increasing the allow 4.
Reducing the MAPLHGR limits to accommodate the reduced flow of SLO.
The particular changes in the Technical Specifications are described below.
- Defintion 8 " REACTOR POWER OPERATION" This definition has been expanded to include a definition of SINGLE LOOP l
OPERATION (SLO).
loop operation in the Technical Specifications and is acceptable.Such S
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- Specification 1.1.A i
This specification has been amended to include a value of 1.10 as the safety limit MCPR.
This is conservative with respeit to the approved value (see Amendment No.119 for evaluation of core stability and single loop operation) i and is acceptable.
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- Specification 2.1.A Neutron Flux Trips and Table 3.1-1 and 3.2.C The APRM High Flux Scram Trip has been reduced by 3.5 percent full power in order to protect the core from violation of the 1.10 MCPR value.
This reduction is consistent with the increase in safety limit and is acceptable.
- Specification 3.12.A MAPLHGR This specification has been changed to include a separate MAPLGHR Flow Factor Curve for SLO.
confusion to operators caused by having too much informat figure. This is acceptable.
The SLO reduction factor for 8x8 fuel has been increased from the value of 0.7 to 0.87 The reduction factor for LTA-311 fuel is 0.74 (see separate evaluation of stability and single loop operation).
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- Specification 3.12.C MCPR MCPR value obtained for the power and flow conditions in is consistent with the change in the safety limit MCPR and is acceptable.
This Based on the above review, we conclude that the proposed Extended Load Line Limits and' ARTS Improvement Program are acceptable for use in the Duane Arno Energy Center.
This conclusion is based on the following:
1.
The analysis methods used for the safety analyses presented in the report are those which have been previously used and approved for reload safety analyses.
7.
The revised operating limits and procedures do not result in reductions to safety margins relative to current values.
In general, margins are increased.
3.
The revised operating procedures are simpler to follow which' tends to increase operating safety.
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3.0 ENVIRONMENTAL CONSIDERATION
S This amendment involves a change in the installation or use of a facility component located within the restr.icted area as defined in 10 CFR Part 20.
The staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents i
that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission
'has previously issued a proposed finding that this amendment involves no significant hazards consideration and there has been no public comment on such finding.
Accordingly, this amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of this amendment.
4.0 CONCLUSION
We have concluded, based on the considerations discussed above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such activities will be conducted in compliance with the Commission's regulations, and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
W. Brooks and N. Trehan Dated:
May 28, 1985
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