ML20154K857
| ML20154K857 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 08/24/1988 |
| From: | Parece M BABCOCK & WILCOX CO. |
| To: | |
| Shared Package | |
| ML20154K854 | List: |
| References | |
| 51-1172953, 51-1172953-00, NUDOCS 8809260030 | |
| Download: ML20154K857 (11) | |
Text
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i 51-1172953 00 Page 2 IVAluATICII 0F 000KE SLS AT RAIICND SECO 4
g by N. V. Parece for Sacramente Municipal Utillty District Prepared by Babcock & Wilcox Nuclear Power Division Lynchburg, Virginia L
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Introduction The Sacramento Municipal Utility District asked Babcock & Wilecx to assess the consequences of an instantsnoous break of both main steaa lines at the Rancho-Seco Nuclear Generating Station.
The evaluation was to include a coincident loss of non vital systems and equipment upon initiation of the postulated event.
The District is specifically interestad in the extent of core damage that would result from this event and the associated off site radiation dose.
In answer to this request, B&W identified a recent computer analysisl of a postulated main steam line break (MSLI) event at the Rancho Seco plant with double steam generator blowdown.
The computer analysis is very similar to the l
event postulated by the District.
The analysis predictions of systes and core response can be shown to envelope the response to the event postulated by the District, even with a coincident loss of non vital systems.
This document determines the appitcability of the recent SLB analysis results to the double MSLI event postulated by the District.
Also, the loss of non-vital systees is shown to lessen the severity of the postulated double MSLB event such that the system and core responses to the postulated event are f
bounded by the analysis.
Consequently, since no fuel damage was predicted to i
result from the analyzed MSLB and since the analysis bounds the postulated i
scenario, this evaluation concludes that ne fuel damage will result from the f
coincident rupture of both main steaa lines.
!!. Postulated SLB Event Description The event is initiated by the instantaneous rupture of both main steaa lines outside of the containment building.
In addition, coincident loss of reactor coolant pumps and/or main feedwater pumps (MFPs) could occur because both the off site AC power supply and the MFPs are considered non vital.
The large increase in steam flows from the steam generators (SGs) will cause a rapid depressurization of both steam generators resulting in act iation of the Emerge: ;y Feedwater Initiation and Control (EFIC) system.
EFIC will isolate main feedwater by closing the main feedwater isolation v. !ves (MFlys), main feedwater ccatrci valves (MFCVs) and the main feedwater start up control valves (M"SUCVs).
EFIC also initiates auxiliary feedwater (AFW) to both SGs by 8ABC0CK & WILCOX a McDermott Company L
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51 1172953 00 Page 4 starting the motor driven and turbine driven AFW pumps with suction from the condensate storagt tank (CST).
However, AFW flow will not begin until each SG liquid level falls below the low level control setpoint and flow from the turbine driven AFW pump will not be available due to the loss of steam supply.
The rapid SG depressurization and the large secondary steam flows result in an increase in the primary-to secondary heat transfer which rapidly cools the primary system. This cooling causes a contraction of the primary system liquid which results in a reactor trip on low primary system pressure.
Following reactor trip, the steam generators continue to blowdown and cool the primary system resulting in a Safety Feature Actuation System (SFAS) trip on low primary system pressure. This trip actuates high pressure injection (HPI) flow which delivers borated water to the reactor coolant system from the borated water storage tank (SWST).
Once the SGs are dry and depressurized, the primary system will continue to cool and depressurize due to the addition of AFW to both SGs via the motor driven AFW pump.
Since the SGs are at equal pressure, EFIC will not automatically isolate AFW to the SGs and the primary system cooldown will continue until the operator manually throttles AFW to match core decay heat.
Prior to operator action to throttle AFW, the primary system pressure may decrease below 600 psig resulting in flow from the core flood tanks.
The concern for this event is that the cooldown of the primary system will cause a positive reactivity insertion (and of life moderator temperature coefficient is negative) which may cause the core to return to critical with the control rods fully inserted.
This uncontrolled power excursion could result in fuel damage which would increase the off site radiation dose in the presence of a small primary +to secondary leak.
111.
Existing Analysis D ucription l for the District 84W recently performed a computer analysis of a SLB event which is very similar to the event postulated in Section II.
The analysis considered a double ended MSLB on one SG with a f ailure of one turbine stop valve (TSV) to close on tha unaffected S3 following turbint trip.
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failure allows the unaffected SG to blowdown with the affected SG (although at
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a slower rate).
The analysis, to be consistent with the NRC's increasing emphasis on safety grade systems and de emphasis of operator action to mitigate events, took no credit for integrated control system or operator action (prior to 10 minutes) to mitigate the event. Also, MFW isolation AFW initiation, HP! initiation and reactor trip were all performed by safety grade systems (vital systems).
Other conservative assumptfons made in the analysis include:
a, conservative initial SG mass inventory of 55825 lb/SG, b.
main feedwater pumps (non vital) ran out to maximize cooling, discharge coefficient, C, of 1.0 used on steam line break paths to d
c.
maximize steam flow, d.
most negative moderator and doppler reactivity coeffie ents (end-9f.
Itfe) to maximize reactivity insortion, e.
no credit taken for transioni xenon reactivity effects post reactor
- trip, highest worth control rod is stuck out of the core following reactor f.
- trip, g.
no credit taken for primary systes metal heat addition to the primary
- coolant, h.
off site power (nonvital) is available throughout the event to provide forced circulation in the primary system and to run MFW condensate pumps so that primary system cooling is maximized.
A detailed description of the SLB analysis and plots of the system response are given in reference 1.
The initiating event in the analysis was the double ended rupture of the main steam line on the A steam generator.
The rapid increase in steam flow caused the secondary pressure to decrease resulting in an EFIC low SG pressure actuation 0.7 sec after rupture (see Table 1).
Following a 5 sec delay, the NFCVs and MFSUCVs began to close.
EFIC started both AFW pumps but no flow was provided to the SGs because SG liquid levels had not decreased to the low level setpoint.
Meanwhile, due to the reduction in SG pressure, MFW flow increased l
- s both MFPs approached run out conditions.
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% :c. =s u.r su m 51 1172953 00 Page 6 The increase in steam flow and decrease in secondary pressura resulted in a large in<:rease in primary to secondary heat transfer.
This caused the primary systers tc contract until the plant tripped on low primary system pressure 1.4 seconds after rupture.
The turbine tripped shortly thereafter, which closed one TSV on the side with the unaffected 54 and reduced its depressurization rate.
However, the continued blowdown of the SGs caused the primary side to continue its contraction and low RC pressure SFAS actuated, thus initiating HPl flow following a 25 second delay.
The system continued to cooldown as the SGs depressurized.
The cooldown caused a positive reactivity addition to the core and the core power level began to increase due to subcritical multipitcation. As the SGs dried out, their energy removal was less than the core thermal power level and the system began to reheat. Consequently, the core shutdown margin reached a minimum value at 35.5 seconds and began to increase due to the negative moderator temperature, coefficient.
EFIC initiated AFW to the unaffected SG at 35 seconds when the collapsed liquid level in that SG reached the low limit.
AFW remained isolated from the affected SG until 56 seconds because the SG differential pressure was greater than 150 pst'.
When AFW was restored to the affected SG, only the motor driven AFW pump provided ' flow because the unaffected SG pressure was insufficient to drive the turbine driven AFW pump.
After AFW was restored to both SGs, they could remove only 7 8% full power from the systes. Consequently, the primary system did not begin to cool again until 110 seconds when the core thermal power decreased below that value.
- However, once the primary system began to cool again, it depressurized below 600 psig and flow initiated from the CFTs.
The CFT flow ensured long tern shutdown of the core by adding borated water to the primary system.
Operator action was assumed at 10 minutes to throttle AFW flow and tenninate the cooldown.
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The reactor remained subcritical during the analysis of the event.
In addition, the maximum return to power was 35.2% and should result in no pins in rWB. Consequently, the off site radiation dose consequences are bounded by the FSAR values and are well within 10CFR100 limits.
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51 1172953 00 Page 7 IV. Applicability of Analysis to Double MSL5 The cumputer analysis of the MSLB with TSV failure presented in Section 111 is very stallar to the event postulated by the District (Section !!).
The main differences are:
a.
the B SG will blowdown more quickly in the postulated event than it did in the analysis, b.
AFW will be initiated earlier than in the analysis and flow to both SGs due to the faster blowdown of the B SG, c.
MfPs (non. vital) could trip coincident with the MSLB in the postulated event, whereas, the MFPs ran out to maximum flow in the analysis, d.
offsite power could be lost coincident with the MSLB in the postulated event, whereas, offsite power was available in the analysis to maximize primary system cooling.
A more rapid SG blowdown will give a faster rate of primary systes cooldown than shown in the analysis. However, the primary system average temperature at the time of SG dry out (which determinis the core minimua subcritical margin).
is a
function of the total mass available for cooling in the SGs.
Consequently, the most important impact of the faster SG blowdown is that AFW will be initiated earlier, on low level, in the postulated event than in the analysis.
Also, since the SG differential pressure will be zero, flow will begin to both SGs.
Based on the blowdown time of the affected SG in the analysis, the additional AFW added to the SGs during the postulated event, prior to the time of minimum subcritical margin, would be approximately 1500 lb as compared to the snalysis.
This additional mass is relatively insignificant with respect to the total mass blown through both SGs (~160000 lb) and will have minimal impact on the prediction of minimum subcritical margin.
In addition, the conservative initial SG mass inventories and absence of primary metal heat in the analysis more than compensate for small differences in AFW flow and SG depressurization rates which would be attained in the postulated double MSLB.
Therefore, the analysis bounds a double MSLB with offsite power and MFPs available.
When the impact of a loss of non-vital systems is evaluated, the analysis of MSLB with TSV failure easily bounds that of the postulated double MSLB.
The vital systems and components required to operate during any MSLB are shown in BABC0CK & WILCOX a McDermott Coapany
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These are consistent 'with the assumptions of the analysis except for the availability of MFPs and the availability of offsite power.
Should a loss of MFPs occur coincident with a double main stas line rupture, the MFW flow to the SGs will rapidly decrease to zero (approximately 5-7 seconds).
The existing MSLB analysis assumed tt.f MFPs provided maximum flow until the MFSUCYs and MFCVs were closed by Eric.
Consequently, MFP trip would result in 27000 - 31000 lb less feedwater addition to the SGs than was assumed in the analysis.
This would significantly reduce the primary system cicling.
Therefore, the existing computer analysis of MSLB with TSV failure bounds a double MSLB with coincident loss of MFPs.
Similarly, a loss of-offsite power results in trip of the condensate pumps which provide suction to the MFPs.
This causes the MFPs to trip, rapidly terminating MFW addition to the SGs.
In addition, the reactor coolant pusgs trip and begin to coast down.
The reduction in primary system mass flow rate reduces the primary-to-secondary heat trr.nsfer coefficient which, in turn, retards the prizary system cooldown rate.
As a resuit, a double MSLB with coincident loss of offsite power will cool the primary system less than the existing MSL8 analysis which assumed offsite power was available.
Therefore, the existing computer analysis of MSL5 with TSV failure bounds a double MSLB with coincident loss-of offsite power.
As a direct result of this evaluation, it is concluded that the primary system l
and core responses predicted by the computer analysis of MSLB with TSV f atlura bound the response of the Rancho Seco plant to a coincident rupture of both main steam lines.
This conclusion remains valid for a loss of non vital systems and components coincident with the double MSl8.
V.
Summary and conclusions The Sacramento Municipal Utility District asked Babcock & Wilcox to assess the consequences of an instantaneous break of both wain steam lines at the Rancho-Seco Nuclear Generating Station including the effect of a coincident loss of non vital systems and equipment.
B&W identified a recent computer analysts! of a postulated MSLB event at the Rancho Seco plant with double steam generator blowdown. The computer analysis is very similar to the event postulated by the BA8C0CK & WILCOX a McDermett Company
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5101172953 00 Page 9 Olstrict and was shown to bound the ccnsequences of the postulated event, even with coincident loss of non vital components.
The computer analysis of the MSLI with TSV failure showed that the reactor remained subcritical throughout ',he event with r.o fuel pins predicted to depart from nucleate boiling.
Consequently, since no fuel damage was predicted to result from the analyzed MSLB and since the analysis bounds the postulated scenario, this evaluation concludes that no fuel damage will result from the coincident rupture of both main steam lines.
VI. Reference 86 1171141 01, M. V. Parece, 'SMU0 SLB W/TSV Failure,' B&W Doc. No.
1.
February 18, 1988.
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Sequence of Events for MSLB With Turbine Stop Valve Failure to close on the Unaffected SG Event Tina After tunturc h Double ended SLB occurs on A SG.
C.
EFIC actuation on low SG pressure.
0.68 Low RC pressure reactor trip.
1.36 Control rods begin to drop.
1.88 Turbine trip (TSV fails to close).
2.38 i
Control rods fully inserteJ.
4.28 Low RC pressure SFAS.
5.36 MFSUCVs and MFCVs close.
5.7 21.3 HP! begins.
30.4 AFW flow to the unaffected SG begins due to 35.0 1
low SG 1evel.
Maximum return to power of 35.2%
35.5 j
EFIC delta P falls below 150 psid. AFW 54.0 flow to affected S4 begins.
Core flooding tank flow begins.
164.0 Operator throttles AFW to terminate 600.0 cooldown (assumed).
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Table 2.
Vital Systems and Components Required During MSLB 1.
Reactor Protection System (RPS) 2.
Safety Feature Actuation System (SFAS) 3.
Emergency Feedwater Initiation and control (EFIC) System 4.
Main feedwater isolation val'tes (MFIVs) 5.
Main feedwater control valves (MFCVs) 6.
Main feedwater start up control valves (MFSUCVs) 7.
Motor driven AFW pump 8.
Turbine driven AFW pump 9.
High pressure injection pumps
- 10. Core flooding tanks (CFTs)
- 11. Scrated water storage tank (BWST)
- 12. Condensate storage tank (CST)
- 13. At least one diesel generator (in case of loss of offsite power)
BASCOCK & WILC0X a McDermett Company
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