ML20052A061
| ML20052A061 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 03/25/1982 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | Mattimoe J BABCOCK & WILCOX OPERATING PLANTS OWNERS GROUP |
| References | |
| TASK-2.K.3.30, TASK-TM TAC-45815, TAC-45817, TAC-45859, NUDOCS 8204270005 | |
| Download: ML20052A061 (6) | |
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File DEisenhut Gray File SMiner MPadovan HDenton ADe Agazio L PDR JStolz RMinogue, RES RJacobs Docket No. 50-312 TNovak D.Ross, RES OELD S.Bassett, RES TSpels q
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IE SHanauer Mr. J. J. !!attinoe RSB Section Leaders <;
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B&ll Owners Group WJe.nsen
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4pp2 6201 S Street WLyon, RES 4
P. O. Box 15830 DMcPherson, RES I
LA 8 Sacramento, California 95813 RMattson GVissing g
Dear !!r. Mattimoe:
PWagner
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On December 17, 1981, the staff net with representatives of B&W and ie D&W Otmers Group in crder for the ownen to present their plan for addressing staff concerns on the BfR small break LOCA analysis model, and to solicit staff concents on this plan. The purpose of this letter is to summarize the staff views that resulted from that neeting, and to provide you with additional information for planning your verification program.
At the conclusion of the meeting, the staff advised the Owners Group that while all of the identified elenents of you' proposed verification plan were acceptable and would contribute to rodel verification, there remained substantial concerns as to their sufficiency. This was because the verification plan program elements involved did not obtain applicable thernal hydraulic data from a facility geometrically similar to the B&W NSSS; and did not compare the conputer rodel against such data. Moreover, the proposed pmgran did not appear to address how the individual verift-cation elements could and would be extrapolated to the B&W geonetry under the conditions of interest. As you know, the need for model verification against experimental data from a facility geonetrically similar to the B&W NSSS has been the major point of contention between the B&W Owners and the staff.
Ilhile the staff continues to endorse the need for model verification against integral system data, we have agreed to work with the Bal Owners Gmup over a six-nonth period, ending in June 1932, in order for the owners to prepare and present a program to the staff that will provide acceptable snall break model verification, including all of the thennal-hydraulic phenonena of interest, without the need for a new test facility. In order to help facilitate your planning of this progran we are providing a general description of our concerns in the enclosure to this letter.
i tie are prepared to meet with you to discuss your progran, and how you believe it meets our concerns. Ile also believe it would be very useful l
at this meeting if you " walked through" sone of the small break scenarios of concern described in the enclosure; and show how the operator would be given proper guidance using existing plant-specific procedures and the future ATOG procedures, omcc) suancE)......... 8.2 04.g.y..g..g DATE )
Nnc FORM 318 00-80pRCM Ono OFFICIAL RECORD COPY usom mi-m.*
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I would also like to ask that, if possible. _you pmvide the staff with copies of the meeting presentation material as.far in advance of the neeting as possible. We have found that mom fruitful discussions can be achieved in this manner.
In. order to arrange the aforementioned meeting to discuss your program, please contact ifr. John Stolz of ry staff.
Sincerely [nni signed by Orig Darre'1 "
nhat Darrell G. Eisenhut. Director..
Division of Licensing Office of Nuclear Reactor Regulation
Enclosure:
Staff Concerns Ilith The B&il Small Break flodel cc w/ enclosure:
B&ll Owners D. Roy, B&!!
L. Lanese, GPU W. Itall, Consumers Power Company,,
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ENCLOSLRE STAFF CONCERNS WITH THE B&W SMALL BREAK MODEL During transients and accidents, including small break LOCAs, the primary loop ctnfiguration of the B&W NSSS is predicted to produce themal-hyraulic behavior unlike that predicted for an NSSS with inverted U-tube steam generators.
The major reason for this difference is that the B&W NSSS has a hot leg geome-try which requires coolant to rise vertically and flow over an inverted U-bend prior to entering the steam generator.
Under two-phase flow conditions, this geometry is predicted to produce fluctuating thennal-hydraulic behavior.
i The specific concerns that the staff will require the B&W owners group veri-fication program to specifically and acceptably address are described s
as follows:
1.
Interruption of Natural Circulation Steam entrapment at the top of hot legs during small breaks is predicted to interrupt natural circulation flow. The ability of the code to correctly I
P predict and calculate this phenomenon should be addressed. Specific areas to
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be addressed are:
o branch flow - The time of interruption of natural circulation, and t
and subsequent behavior, may depend on how steam flow, exiting the l
core, distributes between the upper vessel and the hot legs.
l o hot leg flow regime - The ability to calculate the interruption of natural circulation is dependent upon what assumptions are made regard-ing the hot leg flow regimes. We are not aware of any flow regime maps i
l that can be demonstrated to be applicable to the B&W hot leg geometry (i.e., short horizontal run, vertical upflow, inverted bend.) Justification i
for the flow regimes assumed is required.
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2.
Cold L'Jg Thermal Shock The cyclic interruption of natural circulation could produce a stagnant flow condition in the cold legs as well. This could reduce if not eliminate mixing of injected cold ECC water.
i Lack of adequate ECC mixing could result in unacceptable rates of change of coolant temperatures at the primary system pressure boundary. Staff calculations with the TRAC code have shown sig-nificant cyclic temperature variations (Figure 1) in the vicinity of the cold leg ECC injection (B & W and the B & W owners have previously been provided the LANL report describing,these calcula-tions). The potential for significant themal cycling in the vicinity of the ECC injection, and its resultant impact on pri-mary pressure boundary integrity should be addressed, and veri-fication of this aspect of the code preottions provided.
3.
Hydraulic Stability Following Accident Recovery Following a SBLOCA, the system will contain a large volume of steam in the high points and will refill with cool ECC water.
As the system refills, the steam condensing surface of the OTSG will become liquid-covered and two-phase natural circulation will cease due to loss of the condensing surface.
If the steam trapped in the high points (upper vessel and top of hot legs) does not condense because of non-equilibrium considerations, repressurization may occur.
Unless a flow path and heat sink can be restored, the repressurization will proceed until the break flow exceeds
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the ECC flow, and the system drains down to reexpose a condensing i'
surface. This could be a problem to the operator, who is trying to regain pressure control. The primary phenomenon that must be addressed in the verification program is the trapped
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l stcam bubble behavior in' th? vessel high points, particularly the condensation rate.
The areas just described address our major concerns regarding the dynamic thermal-hydraulic behavior of the B & W NSSS under accident conditions.
We require that each of these areas be addressed by the verification pro-gram. For all experimental data utilized that was not obtained from a fa-cility geometrically similar to the B & W NSSS, you should provide justifi-cation of the applicability of the data to the B & W NSSS (why does the lack of geometric similarity not affect the applicability?)
Sensitivity studies can be presented to justify that the phenomena of interest are relatively unaffected by variations in key input parameters.
If the phenomena of interest vary significantly as a result of variations in the key input parameters, then other means of verification should be provided.
We do not believe reliance on operating procedures is an adequate defense against large uncertainties in thermal-hydraulic behavior.
Other concerns which have resulted from uncertainties in the thennal-hydraulic behavior should also be addressed by the B & W owners as part of this program.
These are described as follows:
A.
Cooldown and Depressuritation following a SBLOCA l
Following recovery from a SBLOCA, and presuming single phase natural circulation can be restored, the upper head of the vessel may still contain a large quantity of stea'm. During depressurization, this steam bubble will expand. Once the ex-l pansion reaches the hot legs, the steam could enter the hot legs and collect at the top of the hot legs, possibly interrupting natural circulation. Presently, we understand operators will be l
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l trained to us2 the high point VInts to remove any steaa bubbics.
A depressurization rate, based on the volume expansion of vessel steam into the hot legs, has been calculated. This maneuver appears, at best, complicated. What provisions are there for
" hands-on" simulator training of the operators for this maneuver?
Address the potential for loop-to-loop instaoilities.
Provide verification of the codes' ability to calculate the depressuri-zation rate. What are the consequences if the depressurization f
proceeds too rapidly and.is there a net steam accumulation in the top of the hot legs?
I B.
Break Isolation During a SBLOCA which produces repressurization, break isolation f
could result in failure to achieve a condensing surface. How does the plant respond and is there adequate information available
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to the operator to diagnosis and manage the plant response?
f C.
Steam Generator Tube Rupture In the event of a OTSG tube rupture with a loss of offsite power, or loss of RCPs and forced convection, depressurization of the pri-mary system with the non-faulted generator could produce a low re-verse or stagnant flow in the primary loop with the faulted steam i
generator. With the faulted generator isolated, the faulted generator l
would become a heat source to the primary system. Could the reverse i
heat transfer boil primary coolant in the faulted loop and produce a steam bubble at the top of the hot leg inverted U-bend? What are the l
consequences of this regarding the continued ability to depressurize l
l the faulted generator?
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