ML20212L175

Safety Evaluation Supporting Util Methodology in Standby Gas Treatment Suction Analysis & Interim Operation for Fuel Cycle 11 Only

ML20212L175
Person / Time
Site:	Cooper
Issue date:	03/06/1987
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20212L171	List: ML20212L167 ML20212L175
References
NUDOCS 8703100282
Download: ML20212L175 (4)

Text

.

[pu cs o UNITED STATES

{. sq 'T,j NUCLE AR REGULATORY COMMISSION

.p , W4SHINGTON, D. C. 20555 k ....

j' SAFETY EVALUATION BY THE 0FFICE OF NUCLEAR REACTOP PEGULATION RELATING TO INTERIP OPERATION FOR FUEL CYCLE 11 COOPER NUCLEAR STATION NEBRASKA PUBLIC POWER DISTRICT DOCKET NO.52-298 INTRODUCTION In October 1986 Nebraska Public Power District identified 58 out of 88 supports on the suction side of the Standby Gas Treatment (SBGT) System that were structurally inadequate for seismic class I loads at the Cooper Nuclear Station.

Subsequently, the licensee also found structural deficiencies on supports in the discharge side of the SBGT system and the Control Poom Ventilation (CRV)

System. The licensee has since completed support modifications of the SBGT Suction system. The SBGT Discharge and the CRV Systems will be modified during the next refuelino outage beginning in February 1988.

In a letter dated December 5, 1986, the staff permitted interim operation of the Cooper Station for the current fuel cycle (Number 11) based on a pre-liminary review of the licensee's submittals of November 25 and December 5, 1986, that provided analyses for the effected systems against interim oper-ability criteria. This Safety Evaluation is a more detailed review of the aforementioned and other additional submittals that the staff has requested.

Although the SBGT Discharoe and the CPV Systems have not been modified at this time, the licensee has inspected both Systems to assess their interim oper-ability and is currently analyzing both systems to prepare them for modif-ication in 1988. The focus of the staff review was on: 1) the acceptability of the licensee's methodology and criteria with respect to the Updated Safety Analysis Report (USAR) comitment; and 2) the acceptable criteria for the interim operation of the SBGT Discharge and the CRV Systems.

EVALUATION i The staff reviewed 1) three sets of stress calculations of the SBGT Suction i

side for modified ductwork and supports, 2) the interim operability of the SBGT Discharge System and CRV Systems; and 3) the discussion of the seismic margin from a viewpoint of probability risk assessment.

The first set of the three calculations, Report 86140/2/F (Ref 4), provides the stress analysis for the ductwork at elevation 976'-0". The ductwork at this elevation consists of regular 12" schedule 105 piping which is within the generally accepted diameter to thickness ratio for the standard piping computer code. The ADLPIPE code was used to perform the deadweight and seismic analyses.

l The forces and moments from the deadweight and seismic analyses were combined to evaluate the supports, and the stresses were combined to determine if these

' 8703100202 070306 PDR ADOCK 05000298 P PDR

t were within the ASME cede allowables. After the support modificatinnt were completed, as-built recenciliation_ analyses were performed to determine whether the self-weicht of the modified supports affected the stresses of the ductwork..

The staff reviewed both anclyses, concentrating er 1) modeling of pipe sections compared to the ductwork isometrics, 2) modeling of concentrated weights'such .

as motors and valves, 3) seismic acceleration input, 4) pipe material proper-ties, 5) forces and moments at pipe nodal points, 6) load combinations for pire support design, and 7) stress intensificttion factors.

The second Report. 86140/2/F, set B (Ref. 5), generated the support loads for the sheetmetal fabricated ductwork sizes 36", 24", and 12" sec.tions which connect the torus and drywell penetrations to the aforementioned 12" pipe section at elevation 976'-0". The third Report. 86140/2/F (Ref. 3), calculated stresses due to deadweight, internal pressure, and seismic loads on sheetmetal ductwork. The stresses were evaluated using equivalent static beam theory.

. Because of the laroe diameter-to-thickness ratio of the sheetmetal fabriceted duct (12" to 36") and the inherent limitation of-beam theory which does not consider cross sectionel deformation, thin shell finite element models were also developed to calculate the stresses as a comparison. The staff reviewed the static beam analysis, stress calculations and the finite element analysis.

Based on the review of these three reports, the staff finds that the licensee's stress analysis methodology and stress levels in.the SBGT suction ductwork are acceptable.

At the time of the November 1986 submittals, the licensee had not performed duct or support calculations for the SBGT Discharge and CRV Systems. The licensee, however, had inspected the duct and supports to determine their structural integrity for interim operability and to compare them with that of the SBGT Suction System. The primary requirement for the operability are that

1) a minimum factor of safety of 2.0 must be maintained; 2) the slenderress ratio of the support steel must satisfy AISC limits; and 3) the span between vertical supports be acceptable. For the ducts, the licensee observed that the support span (about 8 feet apart) among the three systems are similar, j The SBGT Suction System analysis showed that its duct is not overstressed; i therefore, the ducts of the SBGT Discharge and CRV Systems will most likely I

not be overstressed. The stresses in the ductwork under normal conditions is due to deadweight. Under upset conditions the stresses are due to deadweight and earthquake; the pressure and thermal stresses are insignificant. Even if i the ducts are defomed in an DBE, their complete collapse or rupture would be unlikely because of the flexibility and non-linear behavior of the sheetmetal.

All three systems have similar support design; trapeze type for the vertical supports and angle beams for the lateral supports. For the SBGT Discharge t Supports, the licensee observed that 1) the Discharge Supports are of a short trapeze type with an average height less than 4 feet, hence, their slenderness l

j ratios will be acceptable; 2) the Discharge duct contributes less deadweicht i to the supports due to the smaller diameter; 3) the Discharge System has no concentrated weights such as motors or valves; and 4) a section of the duct is

! embedded in the concrete which provides additiorel three directional restraints for the system.

The CRV ductwork is larger and heavier; therefore, to assure the structural integrity of the supports, the licensee analyzed three typical supports (Ref. ,

2) using the guidelines of I&E Bulletin 7c-02. The support calculations showed r

t that the stress levels are below the norraal allowables and a minicum factor cf safety cf 2.0 is most likely meintained for the Phillips Red Head (shell type) anchor bolts in teth systems. The trapere supports in the CRV System are also short; hence, their slenderness ratios are also within the AISC limits.

' Although a safety factor of 5.01s reouired for shell type anchors, the staff has pennitted interim creration of the plants of several licensees by the use of a factor of 2.0 in response to IAE bulletin 79-02. Therefore, the lictosee's interim criteria for the aforementioned anchor bolts are acceptable for the current operating cycle.

The licensee used the results of a probabilistic risk assessment argument tu show that both systems have enough seismic margin to be operable in a DBE. The measure of seismic margin is represented by the high confidence, low probab-ility of failure (HCLPF) capacity (Ref. 7). This is an esticate of seismic capacity, expressed by the fragility curves, that corresponds to a 95% con-fidence level of not exceeding a 57 chance of failure. Based on Reference 7 the HVAC systems have HCLPF capacity of at least 0.39 peek ground acceleratier (PGA). The licensee reasoned that since the ductwork at the Cooper Station is qualified for 0.29 PGA there is sufficiert margin in the seismic capacity in the SBGT and CRV systems.

CONCLUSIONS The staff concludes that the litersee's methodology in the SBGT suction analysis is based on generally acceptable engineering practice and the acceptance criteria satisfy the commitment in the Ccoper Station's USAR. The licensee has shown that for fuel cycle 11 the ductwork and supports of the existing SBGT Discharge System and CRV System satisfy the interim operability criteria and; therefore, there is reasonable assurance that system functions would not be lost should a desion basis earthquake occur. This Safety Evaluation finds interim operation is acceptable for fuel cycle 11 only. The remaining Systems should be completely modified prior to the startup of fuel cycle 12.

Principal Contributor: John Tsao Dated: March 6, 1987

N G

\' REFERENCES.

1. Letter, Villiam 0. Long from J. M. Pilant of Nebraska Public Power-District, November 2S, 1985 s

,'e E. .Cygna Erergy Services, " Preliminary Structural Evalbation of Main Contro.

Rocr. HVAC supports for Cooper Nuclear Station," 66140/5/F, Set B, Revisier 0,

3. Cygna Energy Services. " Stress Analysis", 86140/4/F set A, Revision D.

4 Cyana Energy Services, "Strucutral Evaluation of the SBGT System" 86140/2/F, set A, Revision'0.

5. Cygna Energy Services, " Support Evaluation," 86140/2/F, Set B, Revision 0.

6. Cygna Energy Services, " Seismic Evaluation Criteria and Pethodoloay,"

86140/DC-1, Revision 2.

7. Budnitz, R.J., et al., " An Approach to the Ouantification of Seismic Margins in Nuclear Plants," NURFC/CR-4334, Nuclear Regulatory Commission, August 1985. ,

I C

t 1

- - , , , - - - -- - - -,- - , ---- ,,,,,.,e - ~- , - - - - - - - - .