ML20137B542
| ML20137B542 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 12/24/1985 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20137B520 | List: |
| References | |
| TAC-57558, TAC-65453, NUDOCS 8601150555 | |
| Download: ML20137B542 (20) | |
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UNITED STATES
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SAFETY EVALUATI0t! BY THE OFCICE OF f'UCLEAR. REACTOR REGULATIOi!
EXEMPTI0iv RE0bEST FROF THE REACTOR %ESSEL 4EAD VENT PEOUIREMENT OF 10 CFR 50.44t c H3)(iii)
SACRAMENTO MUNICIPAL UTILITY DISTRICT RANCHO SECO NUCLEAF: GENERATING STATION
. DOCKET E0. 50-312
1.0 INTRODUCTION
During the accident at THI-2, significant quantities of noncondensible gases (hydrogen and fission gases) resulted from fuel cladding oxidation due to low reactor vessel water level and inadequate core cooling conditions.
The collection of noncondensibles in the reactor coolant system (RCS) high points and vessel head impaired the ability ~
to achieve natural circulation cooling and hampered efforts to achieve a stable long-term cooling mode.
Based upon its review of the TMI-2 secuence, the NRCfstaff initiated a wide range of actions designed to improve the caoability of oower reactors to achieve natural circulation cooling. Included among these items was the requirement of NUREG-0737, Item II.B.1 (Reference
- 1) that each licensee shall install reactor coolant system and i
reactor vessel head high point vents. As explained by ?lUREG-0737,
.Q the purpose of the vent system is to enhance core cooling caoability for beyond-design-basis events, b ss.. istentially increasing the k
e6 ability to deal with large (Ja M*s'. of. noncondensible gases which W -
could interfere with core cooling. '
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et occ The rer' cement for the installation of RCS high point 'and reactor vessel
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head vents was subsequently codified in the Comission's regulations at 10 CFR 50.44(c)(3)(iii) on December 2,1981 as part of the Final Rule. .
2 Interim Requirements Related to Hydrogen Control.
Consistent with the earlier staff position, the regulation reoutred the installation of the RCS high point vents, including a reactor vessel head vent, in order to provide improved operational capability to maintain adequate core cooling in the event of an accumulation of noncondensible gases in the RCS or reactor vessel head.
On July 2, 1982, the licensee,. Sacramento Municipal Utility District (SMUD), requested (Reference 2) an exemption from the requirement o 10 CFR 50.44(c)(3)(iii) to install a reactor ve:sel head vent.
Installation of high point vents at the top'of the hot leg U-bends and the pressurizer for Rancho Seco was accomplished during the 1983 refueling outage.
The case presented by the licensee, in Peference 3, as .iustification for the exemption reouest was based upon the ability to perform a plant depressurization to cold shutdown (following a small-break LOCA) even with a gas bubble in the RV he ,
without interrupting natural circulation.
In a letter dated March
~- 15,1983 (Reference 4), SMUD committed to implementing procedures an training for-use of the high point vents in venting noncondensible gases trapped in the reactor vessel- head.
j On July 25,1983, the NRC staff issued an[ interim exemption, until December 31, 1985, to defer the implementation date for installation of a reactor vessel head vent. At that time, the staff was unable to conclude that noncondensible gases could be safe _1y vented by the hot leg high point vents alone. The primary reason for the staff's
3 conclusion was the lack of integral system test data which would demonstrate th? feasibility of the proposed venting procedure. The interim exempticn was granted in order to allow the licensee, as committed by Reference 4, to perform the necessary integral system testing.
On April 12, 1985, '
the licensee provided the results of testing performed in the Once-Through Integral Sys' tem (UTIS) test facility which was performed to demonstrate that a reactor vessel head vent is not necessary to ensure adequate core cooling in the presence of significant ouantities of noncondensible gases. Based upon these test results, the licensee requested, in Reference 5, a permanent exemption from the aouirement to install a reactor vessel head vent.
This safety evaluation provides our evaluation of the licensee's exemption request.
Section 2 provides a discussion of the OTIS test facility and the testing which was performed.
The methods employed at Rancho Seco to vent noncondensible gases from the reactor vessel head a discussed in Section 3. L'ithin Section 4 the relationship of the OTIS test'results to Rancho Seco is provided.
Finally, Section 5 presents the conclusion of the EhC staff's review of the ' licensee's exemption reques't. '
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4 2.0 OTIS FACILITY AND TEST RESULTS 2.1 Facility Description *
' The OTIS facil.ity is an exper cental test facility at B&W's Alliance Research Center in Alliance, Ohio. The OTIS facility was designed to "
evaluate the thermal / hydraulic conditions in the reactor coolant
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system and steam generator of a raised-loop B&W reactor during the ;
natural circulation phases of a small-break LOCA. 'The DTIS facility !
is a portion of the integral Systems Test Program sponsored by the "
Nuclear Regulatory Commission, EPRI, B&W Owners Group and B&W (Reference 6).
A summary of the OTIS facility is provided in the Y
subsequent paragraphs.
t The DTIS test facility is a scaled 1x1 (one hot leg , one cold leg) electrically heated loop which simulates the important features of a B&W plant. (
The locp consists of one 19-tube Once Tnrough Steam !
Generator (OTSG), a simulated reactor (consisting o# an external downcomer and a reactor vessel), a pressurizer, a single hot leg and 3
a single cold leg. Reactor decay heat following a scram is simulated with electrical heaters which are capable of simulated decay heat levels up to 5% scaled power. Reactor coolant pumps are not simulated in the facility. Other key features simulated are a reactor vessel vent valve, a pressurizer power operated relief vaive (PORV), a hot leg vent, high pressure injection, and auxiliary feedwater. .
The loco was specifically designed to minimize leakage .
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\ Guard heaters were employed on the hot leg piping, pressurizer surge line and reactor vessel hea.d to minimize heat loss from the loop. -
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The four scaling criteria, in order of priority , utilized to
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configure the,0 TIS facility were:
o Full Elevation o
Post-Small Break LOCA Flow Phenomena o Volume o
irrecoverable Pressure loss Characteristics Use of this scaling philosophy resulted in a full (approximately 95 feet) height facility which has a power to volume scaling of 1:1686 .
The hot leg diameter was scaled to preserve Froude nu b m er. This criterion was chosen to preserve two-phase flow regines. Flow restrictors were utilized to preserve irrecoverable pressure drop .
The DTIS instrumentation consists of approximately 250 channels of data processed by a high speed data acquisition system . The instrumentation includes pressure and differential pressure measurements; thermocouple and resistance thermocouple measurement
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of fluid, metal and insulation temperatures; and pitot tubes and flowmeters for measurement of flowrates in the loop.
Measurements are also provided for leak, HPI, hot leg vent, PORV, and secondary system feed and steam flows.
Noncondensible gas injections are controlled and metered.
Noncondensible gas discharge with two-phase primary effluent streams are also measured. ~
The foregoing is a brief description of the DTIS facility . More detailed information is provided in Reference 7
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2.2 OTIS Test Results Two tests. OTIS. test 240100 and 240200, were funded by the Florida Power Corporation and the Sacramento Municipal Utility District to demonstrate that a reactor vessel head vent is unnecessary to a adequate core cooling following the release of noncondensible gase into the reactor vessel. The test descriptions and results are presented in References 8 and 9.
A summary of the test conduct and results is provided below.
2.2.1 Test Initialization Both tests were initialized in a similar msrner. The objective of the test initialization is to obtain a hot and noncondensible -
en gas lad i
primary system.
The OTIS loop was initially pressurized to 1700 psi with a low, 5 foot, level in the pressurizer.
Natural circulation was established with a secondary side steam generator pressure of 1200 psia and a level of feet; core power was set at 1% scaled power.
The secondary side conditions were chosen to provide elevated primary system tempe .
Several noncondensible gas additions were made to the reactor ve head.
These gas additions were made to saturatecoolant the loop with noncondensible gas and to create a large noncondensible gas bub reactor vessel head. $
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Following these gas additions, core power was increased to 2% scaled power to achie.ve the desired loop fluid conditions for testing. As the loop approached a stable natural circulation condition, a final, large gas addition was made at the top of the U-bend in order to interrupt natural circulation. The testing was then initiated from this condition.
2.2.2
- Test conduct and Results 1
2.2.2.1 OTIS Test 240100 (Reference 8) -
The purpose of OTIS test 240100 was to examine the effectiveness of the hot leg high point vent to remove a noncondensible gas bubble from the reactor vessel head during a natural circulation cooldown ,
i Of specific interest was whether, during the primary system depressurization, the gas expanding from the reactor vessel head into the hot leg would be removed from the system, via the hot leg high point vent, and natural circulation would be maintained.
The specific testing procedure utilized for OTIS test 240100 was based f upon the Rancho Seco operating procedures.
In sumary form, the testing procedure specified was:
Restore natural circulation by opening the hot leg '
high point vent, i1
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8 Verify natural circulation.
Depressurize the steam generator secondary side t6 achieve a cooldown rate of approximately 100*F/hr.
Depressurize the primary system in steps not to exceed 70 to 100 psi while remaining within the P T envelope.
Basically, this results in maintaining the primary system
, subcooling between 50 to 100'F.
Maintain the hot leg high point vent open throughout the plant cooldown.-
Continue the primary system cooldown and depressurization until primary system temperatures and pressure are less i
than 280*F and 250 psia, respectively. These are typical decay heat removal system actuation conditions.
The relationship of the DTIS test conduct to the plant specific oRanch
. Seco procedures will be discussed in section 4.
Within 5 minutes of opening the hot le point vent, the '
- ^ noncondensible gas in the hot leg U-bend was removed and natural circulation was restored.
Following the recovery of natural circulation , 1 the DTIS test operator proceeded to cool the primary system to achieve .
80*F subcooling.
A natural circulation cooldown was then performed which lowered the primary system pressures and temperatures from appro' f
1750 psia and 545'F to 180 psia and 300*F, respectively. '
Throughout the cooldown process, natural circulation was continuously maintained. Thus, the test indicates that-the hot leg high point vent was effective in removing the noncondensible gas in the reactor vessel head.
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9 2.2.2.2 0715 Test 240200 (Reference 9) -
Tne purpose of. OTIS test 240200 was to demonstrate that, even ff natural circulation cooling was interrupted by collection of ncncendensible gases in the hot leg U-bend, an alternate cooling technioue, feed and bleed using the HPI and PORV, could be utilized to cool the core.
The specific testing procedure utilized for OTIS test 240000 can be summarized as follows P.estore natural circulation by opening the hot leg ver.t.
After natural circulation is recovered, close the hot leg vent and proceed with a natural circulation cooldown and depressurization.
After natural circulation is interrup ed by the nencondensible gas in the reactor vessel head expanding and collecting in tne nst les U-bend, open the PORV and actuate UPI and pre:end to ecol the core with feed and bleed cooling.
Continue the plant cocicown within the F T envelope until the i
system pressure and temperature is lowered to 284 psig and 280*F.
As with CT!S test 240100, openino c' tre hot leg high peint vent recovered natural circulation within 5 minutes. At that time, the operator closed the hot leg vent and inititted a natural circulation
- cocidown.
Approximately 30 minutes after the start of the plant cooldown, sufficient gas collected in the het leg U-bend toir.tt- ):
10 natural circulation. '
The' operator then initiated HPI and* opened the PORY to establish feed and bleed cooling. This mode of cooling was tiaintained for approximately an additional four hours . At that time, the loop achieved a quasi-equilibrium condition and the operator was unable to depressurize the plant further. The operator then proceeded to open the hot leg vent, while maintaining feed and bleed cooling, in order to refill the primary loop. _Ultimately, the primary loop was refilled; the operator closed the PORY and continued with a natural circulation cooldown, with the hot leg vent open, similar to that performed in OTIS test 240100.
Although the system was unable to be fully cooled and depressurized using feed and bleed cooling, it should be noted that core cooling l ,
was maintained throughout the test. Outlet core fluid temperatures were generally maintained between approximately 40 and 85'F subcooled. c The test also illustrated that if a natural circulation cooldown was interrupted, via gas accumulation in the hot leg U-b natural circulation could be regained by opening the hot leg vent.
3.0
- RANCHO SECO VENTING PROCEDURES In order to ensure adequate core cooling following the generation of a significant amount of noncondensible gases caused by.an inadequ core cooling (ICC) event, the licensee has made several changes t6 their operating procedures. ,
These procedures are documented in Reference 10.
.A description of the venting aspects of,these i
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procedures and their bases is provided below. Specialemhhasisis placed on the procedure for a subsequent plant cooldown following an ICC event.
If core conditions during an ICC event become significantly degraded, such that cladding temperatures in excess of 1400'F are inferred from -
the core exit thermocouples, the Rancho Seco procedures require that the hot leg vents be opened. These vents are opened to ensure that '
some of the noncondensible gases which may be created by cladding rupture and/or the zircaloy cladding metal water reaction can be vented from the primary system. The procedures also specify several other actions, such as ensuring HPI flow and depressurizing the steam generators, that the operator should take in order to recover core cooling and mitigate the extent of potential core damage.
I Assuming that systems conditions are such that the operator actions specified in the ICC procedure are sufficient to return the core to a coolable condition, the primary system will ultimately refill and natural circulation would be recovered. At that time, the reactor vessel head will be filled with noncondensible gases and the operator will p'roceed with a natural circulation cooldown.
In order to prevent an interruption of natural circulation due to an t
accumulation of noncondensible gases in the hot leg U-bends during the subsequent plant cooldown and depressurization, the Rancho Seco procedures specify that the hot leg vents remain open throughout the '
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12 natural circulation phase. In addition, the procedure limits the o
plant cooldown rate to 100'F/hr and limits the plant deprissurization to steps not to exceed 70-100 psi. In addition, the operator is directed to utilize'the pressurizer vent, if available, to further limit the depressurization rate.
These actions are specified in order to limit the rate of gas
' expansion from the reactor vessel head into the hot leg to less than the venting capability of the hot leg vents.
It should also be noted that the Rancho Seco procedures specify operator actions should natural circulation be interrupted. The procedures require that feed and bleed cooling be established while leaving the hot leg high point vents open. In this manner, core cooling will be maintained while the gases in the hot leg U-bends are vented and natural circulation is recovered. Following the recovery of natural circulation, feed and bleed cooling would be terminated and the natural circulation cooldown would be centinued.
1 Once the primary system is depressurized to the decay heat removal system conditions, the plant would be placed in the decay heat removal mode and the hot leg high point vents will be closed. At this time, the plant will be in a cold shutdown condition with a noncondensible gas bubble still remaining in the reactor vessel upper head.
The licensee outlined several methods for removing the gas
' bubble from the reactor vessel head, depending on plant. status and equipment availability.
The methods outlined rely on degassing the i
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primary system fluid and thereby allow the reactor vessel-head bub to be absorbed by the primary system fluid.
The specific means to degas the primary system include (1) use of the pressurizer spray pressurizer vent, (2) use of the pressurizer heaters and pressurizer vent, and (3) use of the makeup and purification system. As the gas bubble will not interfere with core cooling, we find this approach
'accepta bl e.
4.0 APPLICABILITY OF OTIS TEST RESULTS TO RANCHO SECO Application of smaller scaled integral system test results to infer expected full scale plant performance should be performed with caution.
Typically, the results of scaled tests are used to verif y computer codes.
1 These verified codes are then used to calculate plant performance with increased confidence.
In this case, the licensee views the OTIS tests as " proof of principle" tests related to the ability of the hot leg high point vents to vent noncondensibl e gases from the reactor vessel head during a plant cooldown and depressurization following'an ICC condition.
This approach requires a careful review of OTIS scaling to assure that the facility prope simulates expected plant thermohydraulic performance.
In addition, test conduct must be examined to assure that the impact of plant procedures on the system response is properly reflected.
4.1 OTIS Scaling As described in section 2. the DTIS facility is a 1x1, full height ,
full pressure simulation of a 205 FA plant.
The facility scale
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factor is 1:1686 with rt:pect to power-to-volume scaling.* As such, the facility has several atypicalities, such as excessive metal mass ,
which are expected in smaller scaled facilities. The hot leg vents were simulated using a scaling factor of 1:1686 with an adjustment for the scaled power level of 3600 Mwt, used as the basis for overall OTIS scaling, and the Rancho Seco power level of 2772 Mwt. Using these scaling factors, an ideal scaled vent area of 2.5 cm2was derived.
The actual scaled vent area chosen was 2.11 cmtto in order arrive at a' vent area intermediate to that for the Crystal River 3 and the Rancho Seco plants.
In addition, the DTIS tests were perfonned using nitrogen as the noncondensible gas.
The licensee has evaluated the scaling compromises utilized n the i
OTIS tests.
The licensee has concluded that the rate of evolutio gases from the RV head and from the coolant depends primarily upon the physical properties of noncondensible gases in a depressurizati and cooldown environment.
The licensee concluded that the overall
~- geometrical scaling used for OTIS is sufficient to demonstrate the ability of the high point vents to remove gases trapped within the reactor vessel upper head.
Relative to the use of nitrogen versus hydrogen, which is expected be the predominant gas species following an ICC event, the license has concluded that this should result in a conservative tes simulation.
The licensee has examined the use of nitrogen gas on Loth the expansion rate from the RV head and on the venting rate o the hot leg vent.
Based on relative densities between nitrogen and
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hydrogen, the license concluded that the relatively dense ' nitrogen would tend to ,more readily gravitate towards the lower voided s
elevation in the RV head and would be more readily swept out of the RV head.
With vapor in the hot leg U-bend, the relatively dense nitrogen would tend to be segregated towards the lower elevation and
! be less readily vented. Thus it was concluded that the use of nitrogen would conservatively simulate expected' plant performance i
with hydrogen gas trapped within the RV head.
i We have reviewed the licensee's conclusions relative to OTIS scaling 1
I and generally agree with their assessment. However, we do not j '
totally agree with the scaling rationale utilized to size the hot leg
- vent in the OTIS facility. As stated in Reference 7, the core vessel portion of the reactor vessel contained excess volume. As a result, non-flow lengths, specifically the reactor vessel head, were shortened in order to maintain overall reactor vessel power-to-volume scaling. Thus, the DTIS head volume is undersized. If the scale
} factor is chosen based upon the ratio of the hot leg venting areas in
{ OTIS and Rancho Seco, i.e.
b Scale Factor = Rancho Seco Vent Area OTIS Vent Area i
we get a scale factor of 1528.
! Applying this scale factor to the reactor j vessel head, it is found that the DTIS head volume is appro '
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" ideal" scaled Rancho Seco head Asvolume. a result, given similar
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16 depressurization rates, it appears that the OTIS tests may underestimate .the gas expansion rate from the reactor vessel head While we have concerns with respect to the scaling of the hot leg i vent, we also note that the use of nitrogen gas tends to compensate ;
for this effect.
If only the gas was to be vented through the hot i
leg vent, the volumetric venting rate would be approximately 3 7 1
times less for nitrogen than for hydrogen.
As a result, it appears that the overall relhtionship between the hot leg venting rate and i
gas expansion rate is conservatively simulated.
L in addition, as will be discussed further below, the depressurizatioi rates utilized in the DTIS tests were greater than those allowed b t i
the Rancho Seco procedures.
As a result, we have concluded that OTIS conservatively reflects the expected Rancho Seco performan .
While we believe the DTIS tests are sufficient for examining the effectiveness of venting trapped noncondensible gases from the i reactor vessel via the hot leg vents, we also recognize that the OTIS .
facility is only a 1x1 representation of a B&W plant. :
j To confirm t that multiloop behavior does not affect this mode of gas removal ,
tests are currently scheduled in the MIST facility. The MIST facility is a 2x4 representation of a B&W 177 FA lowered loop pl ,
l t i These tests are expected to be performed in 1986 ,
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, j 17 4.2 Test Conduct .
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Specified test conduct for the DTIS tests were described in Section 2; the applicable Rancho Seco procedures were discussed in Section 3.
! As was described, the OTI3 tests were constructed to represent the i
specific Rancho Seco procedures.
. t i We reviewed the OTIS test conduct and noted that, in general, the j.
depressurization steps utilized were in excess of that allowed by the .
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' Rancho Seco procedures (maximur 100 psi depressurization step). 4 Thus, the gas expansion rate from the RV head experienced in the 07I5 .
test exceeded that expected at Rancho Seco even when a scaling factor based on equivalent hot leg venting areas is utilized. in addition, i i' j due to the use of nitrogen gas in the DTIS test, the high point vent !
i relieving rate is less than that expected for Rancho Seco. Thus, we !
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have concluded that the actual OTIS test conduct should boundi :
expected Rancho Seco performance. !
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l 5.0 CONCLUSIONS Based upon the foregoing, we have concluded: !
- t OTIS test 240100 :
, demonstrates that the hot leg vent can be
- i utilized to remove noncondensible gases from the reactor !
vessel head, during a plant cooldown and depressurization, without interrupting natural circulation. -
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071S test 240200 demonstrates that even if nonconbens e gases in the reactor vessel head expanded eginto the ho s
U-bend and interrupted natural circulation
, feed and bleed cooling can be used to assure core cooling. In addition, opening of the hot leg vent will also allow the n restor of natural circulation.
The Rancho 5eco venting procedures have b een developed to limit gas expansion rates from the reactor ead in vessel h order to ensure that natural circulation continuously it maintained during a plant cooldown.
In addition, the Rancho Seco procedures specify in ation of feed and bleed cooling of the core, should nat ural circulation
. become interrupted, while leavingvents the open hot to leg i
allow recovery of natural circulation.
The reactor vessel head gas bubble which would remain following the plant cooldown to the decay removal heat system will not interfere with core cooling.
The licensee has developed various means to remove u e in the this gas b longer term if it should be desirable.
- The OTIS test were performed in a manner which the results bound the expected Rancho Seco pla t n performance.
F. "
19 Based upo' n these conclusions, we recommend that the Itcensee 'be granted a permanent exemption from the requirement of 10 CFR 50.44(c)(3)(iii) to install a reactor vessel head event at Rancho Seco.
Principal Contributor: R. Jones Dated: December 24, 1985 e
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6.0 ' REFERENCES ' .
t 1.
NUREG-0737. Clarification of TMI Action Plan, Item II.8.1 .
2.
Letter, J. J. Mattimoe (SMUD) to J. F. Stolz (NRC), " Request for Exemption,10CFR50.44(c)(3)(iii),NUREG-0737.ItemII.E.1,"DocketNo 50-312, July 2, 1982. .
3.
NUREG-0737." Docket No. 50-312, July 1,1981. Letter, J. ,
4.
Request - Vessel Head Vents," Docket No. 50-312
- 5. .
Letter, R. J. Rodriguez (SMUD) to J. F. Stolz (NRC), " Exemption Request April - 10 12, 1985 CFR 50.44(c)(3)(iii). Vessel Head Vents," Docket No
. 50-312, ,
6.
NRC-04-83-168/RP2399-1, " Contract Among the Babcock ny r 'd W and Electric Comission." Power Research Institute and the U. S. NL' lear Reg 7.
Once-Through Specification Integral System Test Program OTIS Loop Functional 1
Wilcox Company ROD:. 84:4091-24:01; prepared by the Babcock and 8.
OTIS Hot Leg High Point Vent Test #240100, No. 12-1152307-00, July 1984 (provided withB&W Occument
- 9. Reference 5).
OTIS Hot Leg High Point Vent Test #240200, B&W Occument No.12-1152308-00, July 1984(providedwithReference5) . '
i' 10.
Letter, R. J. Rodriguez (SMUD) to H. 't. Thompson, Jr. (NRC)
" Exemption j Request19,- 1985.
September 10 CFR 50.44(c)(3)(iii), Vessel Head Vents " Docket 50 31
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