ML20086H988

Rev 1 to Drywell Temp Response to Small Steam Break, Realistic Environ Qualification Envelope

ML20086H988
Person / Time
Site:	Limerick
Issue date:	11/18/1983
From:	David Jones, Dang Nguyen GENERAL ELECTRIC CO.
To:
Shared Package
ML20086H987	List: ML20086H979 ML20086H988
References
RTR-NUREG-0588, RTR-NUREG-588 NUDOCS 8401230028
Download: ML20086H988 (12)

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. DRYWELL TEMPERATURE RESPONSE TO A SMALL STEAM BREAK

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A REALISTIC ENVIRONMENTAL QUALIFICATION ENVELOPE LIMERICK GENERATING STATION, UNITS 1 AND 2

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.V. Nguyen Containment and Radiological Engineering

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.D. Jones Containment ad Radiological Engineering l

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GENERAL ELECTRIC COMPANY San Jose, California November 18, 1983 l

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1. INTRODUCTION The bounding drywell temperature envelope for environmental qualification

( is specified by NUREG-0583. " Environmental Qualification of Safety-Related Electrical Equipment" Rev. 1, dated July, 1981. This snvelope is specified as 340*F for 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. This report provides a realistic assessment of the drywell temperature response to a small steam break, sontrolled depressurization transient. TWo sensitivity studies requested by Philadelphia Electric Co. (PECO) to evaluate 1) uncertainty in the drywell structural steel volume in the drywell and 2) an alternate scenario with NSIV isolation are also discussed.

2. ANALYSIS ,

The drywell temperature response is controlled by several key parameters:

1) break and SRV flow from the RPV, 2) heat load from hot pipes and equipment in the drywell and 3) heat transfer to structures in the drywell and the wetwell. Each of these contributing parameters are discussed below.

The small break transient scenario is described in Table 1. The RPV  ;

response to this scenario was conservatively estimated with a l thermodynamic analysis based on the assumption of a controlled depressurization of 100*F/hr. The resulting RFV pressure vs. time is shown in Figure 1. The reactor pressure is assumed to decrease to the turbine stop valve closure pressure in -2 minutes, and then remain constant until the operator initiates the controlled.depressurization.

Reactor pressure during the controlled depressurization was determined by assuming saturated conditions with RPV. The resulting break flow for a

[ 0.01 ft steam break, shown in Figure 2, was calculated as saturated steam critical flow at reactor pressure, using Noody's Homogeneous Equilibrium Model (EEM) (Reference 1).

The model accounted for structural heat sinks in the drywell and wetwell, by modelling convective heat transfer with the Uchida heat transfer coefficients, as defined in Reference 2. No credit was taken for drywell sprays.

The drywell hest load was based on the drywell fan coolers heat load specification of Reference 4. This specification included test load from:

- Recirculation pump assumed has tripped and does not contribute af ter 2 minutes.

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- Recirculation piping heat load assumed to be constant during the

transient.

The CRD scram load is assumed to act for 30 minutes.

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The main steam line heat loads are reduced as a function of time based on decreasing KI.

The heat loads from the pipes and equipment are described in Table 2.

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Tho dryw211 t:cparctaro rosp2aso ces calc 31stod olth a stcadard

.. containment analysis which models the drywell and wetwell and the mass and energy flows associated with these volumes. These models are

( consistent with the containment models described in Reference 3.

I Also, as described in Table 1, the only f ailure considered was the small break since any other failure (such as RHR heat exchanger) would tend to result in more severe conditions to the point where rapid depressurization may be necessary. That would be non-conservative with respect to maniano duration of high drywell temperature. The transient described in Table 1 results in only break flow entering the drywell, since the main condenser is ava!!able to accomodate the reactor depressurization steam flow (no SRV flow to suppression pool). ,

2.1 Sensitivity Studies TVo additional analyses were performed to evaluate 1) the effect of nacertainty in the determination of the volume of structural steel in the drywell and 2) an alternate scenario with NSIV isolation. The alternate steel volume analysis assumed a factor of ten increase in the steel volume, with a compatible decrease in the drywell free volume. The base case volumes and the revised voinmes are shown in Table 3. The NSIV isolation case assumed that the MSIV's remain isolated, the main condenser is not available, and thus SRV's are used to depressurize the RFY.

3. RESULTS

- The base caso drywell temperature response is shown in Figure 3. The f

peak temperature is 258*F. As'can be seen, the peak drywell temperature is reached at ~2 hours into the transient and turns over Thus, withinthis the hour.

SBA At twelve hours the temperature has decreased to 210*F.

analysis results in lower peak temperature and shorter duration for the drywell equipment qualification envelope, compared to NUREG-0588. The drywell and wetwell pressure histories for tais transient are shown in Figure 4.

3 .1 Sensitivity Studies The effect of increased drywell structural steel volume (decreased airspace volume) was to increase the peak drywell temperatne by ~3*F.

The increased temperature results from the smaller airspace volume, demonstrating that an 8% decrease in airspace volnae dominates a 100%

increase in heat sink volnae.' This study shows that the drywell temperature results are relatively insensitive to uncertainties in the structural steel voinne.

The effect of MSIV isolation is to necessitate SRV discharge to the suppression pool. The previons analysis showed a maximum drywell temperature of 255'F. It's concluded that the effect of MSIV isolation is insignificant.

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4. CONCLUSION

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An assessment of extended high temperature in the drywell has shown that

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the peak temperature is 2588F and that this temperature occurs for a period of ~3 hours af ter a small steam break. This temperature envelope is much less restrictive than the NUREG-0588 specified envelope of 340ep ,

for 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

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• . . TABLE 1 COMMENT 1N: (KIN) EVENT

0. Small steam break at 0.01 11: break size power occurs.

0.5 2 psig in DW, scram, Operator switche s containment isolation, Noasswitch to loss of DW fan coolers Shutdown

10.5 Operator initiatas con- Operator controls

- trolled RPV depressuri- reactor water level.

sation at 100*F/hr: RER Nain condenser avail (2 loops) in pool cooling. Able.

Shutdown cooling inter- Ecactor pressure 140.

lock (75 psig) reached, held at 75 psis operator initiates switch- until sautdown over from pool cooling to cooling initiated.

(2 loops). .

Pool cooling not availabis.

156. Switch over to sautdown Switchover assumed cooling completed, drywell to take 16 minutes.

' peak temperature turns over. ,

720. DW temperature is 210*F. Transient ends when temperature drops below 212*F within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

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s TABLE 2 DRYWELL BEAT LOAD VS. TIME

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0 0.031 0.285 1.128 0.375 120 0 0.285 1.128 0.364 1100 0 0.2 85 1.128 0.278 1800 0 0.285 0 0.265 3600 0 0.285 0 0.220 4800 0. 0.285 0 0.190 8400 0 0.285 0 0.099 9360 0 0.285 0 0.009 13320 0 0.285 0 0.009 1.0E6 0 0.285 0 0.009

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TABLE 3

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CokPONENT VOLUMES FOR SBA Uncertainty '

base Case Evaluation.

Drywell 248,950 it: 230,284 Airspace 2,074 20,740 Drywall Steel f

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, FIGURE 4 LIMERICK SMALL BREAK ACCIDENT. PRESSURE VS. .TI}E I

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• f ., e REFERENCES

1. F.J. Moooy, " Maximum Discharge Rate of Liquid / Vapor Mixtures from ,

(- Vessels", NEDO-21052, September, 1975.

2. USNRC Standard Review Plan, Section 6.2.1.5, " Minimum Containment Pressure Analysis for Emergency Core Cooling System Performance Capability Studles", NUREG-0800.

3. " Pressure Suppression Containment Analytical Model", NEDO 10320.

4. Design Specification for Drywell Cooling System, Document No. 22A2715AA, Rev. 1.

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