ML14133A481
| ML14133A481 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 01/26/1976 |
| From: | Parczewski K Office of Nuclear Reactor Regulation |
| To: | Baer R, Novak T Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML14133A482 | List: |
| References | |
| NUDOCS 8106290354 | |
| Download: ML14133A481 (12) | |
Text
Oo ATTACHMENT 1 0
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 JANf 2 j.1 75 Thomas M.. Novak, Chief, Reactor Systems Branch, SS THRU:
Robert L. Baer, Chief, Reactor Safety Branch, OR /%'-
CONCENTRATION OF BORIC ACID IN REACTOR VESSEL DURING LONG TERM4 COOLING - MEHOD FOR REVIEWING APPENDIX K SUBMITALS Attached is a mrenmorandum entitled: "Concentration of Boric Acid in Reactor Vessel during Long Term Cooling - Method for Reviewing Appendix K Submittals". The memorandum is intended for the reviewers of Appendix K submittals. It describes the methods used in *reviewing the calculations of boric acid buildup during a post-LOCA long term cooling.
K. I. Parczewski Reactor Satety Branch Division of Operating Reactors Atta chment:
As stated cc: 0. Ross D. Eisenhut Reactor Systems Branch Members Reactor Safety Branch Members 8106290
Enc:losure 1 CONCENTPATION OF BORIC ACID IN REACTOR VESSEL DURING LONG TERM COOLING -
METHOD FOR REVIEWING APPENDIX K SUBMITTALS I. Descriotion of the Problem Following a LOCA, boric acid solution is introduced into the reactor vessel by two modes of injection. In the initial injectign-mode, borated water is provided from the accumulators from the refueling water storage tank and from the boron injection tank (Westinghouse plants only). After this initial period, which may last somewhere between 20-60 minutes, the ECC system is realigned for the recirculation mode. In this mode borated water is provided from the containment sump. It is recirculated from the sump to the reactor vessel and back to the sump through the break. A portion of the water introduced into the reactor vessel is converted into steam by the decay heat generated in the core. Since the steam contains virtually no impurities, the boric acid content in the water that was vaporized remains in-the vessel.
The concentration of boric acid in the core region will therefore continuously increase, unless a dilution flow is provided through the core. Without the dilution flow the concentration of boric acid will eventually reach the saturation limit and any further increase in boric acid inventory will cause its precipitation. Boric acid deposited in the core may clog flow passages and.seriously compromise the performance of the ECC system.
To prevent this from happening, the ECCS should be designed and operated in such a manner that a sufficient throughflow is provided before the concentration of boric acid will reach its saturation limit.
The general performance criteria for the boron dilution systems are given in Appendix I to this memo.
There are two situations when ECC system cannot provide sufficient diluting flow through the core. It occurs during:
(a) cold leg injection with cold leg break (b) hot leg injection with hot leg break In both these cases the injected fluid does not have enough head to pass through the core. Since it is not possible for an operator to distinguish between cold and hot leg breaks,. the only way of assuring dilution flow through the core is-to provide one of the following modes of operation for the ECC system:
(a) alternate injecticro to cold and.hot legs.
(b) simultaneous injection to cold and hot legs.
(c) simultaneous cold leg injection and hot leg suction.
II. Methods Procosed by the Vendors for Preventino Boric Acid Concentration
- 1. Sabcock and Wilcox The B&W plants are unique in that they possess internal vent valves between the upper plenum and the downcomer.
The existence of these vent valves allows natural circulation to take place in the reactor vessel as long as the level of the two-phase mixture is high enough to reach the vent valves and keep them open. B&W has performed an analysis indicating that regardless of hot or cold leg break, the natural circulation flow through the-core could be maintained for over 30 days after a LOCA and no operator action is needed during that period of time (References 1, 2 and 3).
After 30 days B&W proposes three different modes of operation, aiming at establishing diluting flow through the.core. They are as follows:
(a) injection into the downcomer and suction or drainage from the hot leg through the decay heat line.
(b) simultaneous injection into the downcomer throuch the injection line-and. into the hot lea through the decay heat line.
(c) simultaneous injection into the downcomer through the injection line and into the hot leg through the'auxiliary pressurizer spray line.
B&W claims that any of these methods, if initiated within 30 days,,
will prevent boric acid concentration from reachina the sa.turation limit.
The NRC staff has reviewed the claims made in the B&W analysis and found that, indeed, in most plants natural circulation can provide dilution flow through the core well in excess of 30 days.
The modes of operation,. proposed by B&W for maintaining dilution flow beyond this time have to be reviewed individually for each plant because their performance depends on the parameters of each individual plant.
-3
- 2.
Combustion Engineering In the Combustion Engineering plants the initial injection of borated water is through the cold legs.
After some specified tire, cold leg injection is replaced either by simultaneous hot and cold leg injection, or by simultaneous hot leg suction and cold leg injection. The time when the switchover must take place is determined by the concentration of boric acid in the reactor vessel.
In the analysis (Reference 4) Combustion Engineering makes two assumptions.
It assumes ideal mixing of boric acid solution in the reactor vessel which produces uniform concentration buildup in the whole vessel and it assumes that in all cases there is a residual pressure of at least 20 psia in the reactor vessel.
This pressure causes higher boiling liquid temperatures with the resulting increase of 4 1/2 weight percent in boric acid solubility.
Both these assumptions are non-conservative and Combustion Engineering does not provide satisfactory justification for including them in the analysis.
In the procedure provided. by Combustion Engineering, hot leg.
suction is accomplished-by use of the shutdown cooling suction line and the hot leg injection can be made either through the shutdown cooling suction line or through the auxiliary pressurizer spray line. In most plants these three alternatives provide a system that can withstand a single active failure.
The procedure proposed by Combustion Engineering has certain limitations. If the steam produced in the vessel cannot be freely vented through the hot leg, it can depress the level of the liquid in the upper plenum below the bottom of hot leg nozzles and impede the suction through the shutdown cooling line. This case will occur when all loop seals, formed by the U bend in cold legs, are filled with water. On the other hand, if the steam leaving the vessel reaches velocities too high it may carry the. injected. fluid into the steam generator and.prevent it from reaching the core.
Both these problems have-been addressed by Combustion Engineering (References 5 and 6) and it was demonstrated that, for the: plants considered, these effects did not seriously affect the performance of ECC system.
0
- 3. Westirghouse After an initial period of cold leg injection, Westinghouse recomends hot leg injection as means for providing dilution flow throuch the core. The generic analysis performed by lWestinghouse (References 7 and 8) assumes that the fluid injected throuch the hot le. to the upper plenum mixes with the fluid in the reactor vessel.
Hot leg injection, therefore, provides a continuous dilution flow through the core for both hot and cold leg breaks.
The Westinchouse analysis, which forms the basis for these assumptions, is not complete.
Until more information is obtained, it is recomended that the initial cold leg injection should be replaced either with simultaneous hot and cold leg injection, or alternate periods of hot and cold leg injection, so that adequate dilution flow is provided for either a hot leg or cold leg break.
III.
NRC Review
- 1. General Description The NRC review of the submitted procedures includes independent evaluation of basic parameters (switchover time, minimum flow rates, etc.).
The independent evaluations have been performed for the following reasons:
(a) In some cases the information provided by the applicants were incomplete and it was not possible to check the soundness of their initial assumptions or their methods of analysis.
(b) In order to assure that under all operating conditions the procedures provided by the applicant will guarantee the maintenance of safe boric. acid concentrations, the NRC staff proposed a series of conservative'assumptions.
It is necessary to determine how this additional conservatism would affect the procedures proposed by the applicants.
One *of the most important assumptions introduced by the staff, was the four weigh percent margin in the saturation concentration of boric acid in the core region. This safety margin was introduced to take care of inaccuracies in predicting saturation concentrations -in the core.
The NRC review consists of two parts:
(a) analysis (calculation) of boric acid concentrations
- (b) hardware review.
.-5
- 2.
Analytical Methods The following methods were used in calculatino different performance parameters for the systems reviewed:
(a) Switchover time from cold leg to hot leg (or simultaneous hot and cold leg) injection.
In order to determine the switchover time the following conservative assumptions were made:
(1) During the initial cold leg injection, boric acid L:-dees not leave the core.
(2) The maximum concentration of boric acid in the core region (Cc) should not exceed 23.5 w/o (4 w/o bel.ow the saturation concentration at 212 0F).
(3)The initial injection period lasts for 20 minutes after
- a.
LOCA.
At this time the concentration of boric acid (Co) is uniform in the whole plant.
(4) The effective volume. in which boric acid buildup occurs consists of a volume of liquid in the core and. in the upper plenum up to the height of the bottom of the cold leg nozzle.
The rate of boric. acid buildup is expressed by the following differential equation:
dC
= A. t
. (CC) dt Where: C - boric acid concentration in core region C0,- maximum boric acid. concentration if all boric acid were concentrated in core region t-time A - group depending.on several plant parameter n -
constant.
0
-6 Solving the equation for the following boundary conditions:
t =t C =c 0,
0 t =t C = C and rearranging the terms, the following expression for the switchover time (ts) is obtained:
t 1-n n C,-
Cc 1/(1-n)
A. t 1-n C,-
C 0
Where:
A = K. P
("f +Ahf). Ms for: ts <4 x 10 sec (46 days): K = 0.156 and n = 0.283 M-mass of water in the containment sump, lb P - reactor operating power, Btu/sec.
C -
0.3077 boric acid/water In the case of alternate hot and cold leg injections, the subsequent switchover times could be obtained by substituting ts 1 s2, ts3 etc for to in the above equation.
(b) Minimum.cold leg injection The minimum cold leg injection required: during recirculation (prior to switchover to dilution mode) is determined.by the rate of boil-off in the core-at the very beginning of the long term cooling mode.
It is determined from the following expression:
.Qc 449 H_
gpm in"(
fh +
Ah ffg)
-7 Where:
H - decay heat generated at the beginning of recirculation 0
period, Stu/sec fin -
density of injected fluid, lb/ft3 Ah Lsubcooling enthalpy, Btu/lb f -
vaporization enthalpy, Btu/lb (c) Minimum hot leg injection The minimum injection rate, after the switchover from cold to hot leg or to simultaneous hot and cold leg injections, is determined by making conservative assumptions that the minimum injection should provide enough flow to replace the boiled-off liquid and to maintain sufficiently high dilution flow through the core.
The bdiloff rate is calculated using the decay heat generation rate at the switchover time from cold to hot leg injection. The dilution flow is determined for the maximum allowable concentration of boric acid.
Expression for the minimum injection rate is given below:
b =
449 H s Fin LAhf + (1 - 3.25 Co) Lb J Where: H - decay heat. generated at switchover time, Btu/sec..
The deca heat is calculated using the methods given in Reference 9.
(d) Steam binding When the switchover from cold to hot leg injection occurs shortly after a LOCA, large volume of steam leaving the reactor vessel may induce high steam velocities in outlet nozzles. These velocities impede the injected boric acid solution from reaching the core. In order to assure that hot leg injection is, not compromised, steam velocities have to be estimated and the rate of entrainment of the injected liquid calculated. The rate of entrainment. can be calculated using the methods given in Reference 10.
- 3.
Procedure In reviewing the boric acid submittals, the following procedure was used:
Steo 1 Review of the submittal and identification of any special features the applicant may be taking credit for.
In most cases the applicants follow the generic analyses developed by the vendors.
However, occasionally the applicant may take credit for the phenomena which are not considered in the generic studies.
For example, no consideration was given in the generic studies to the possible increase in the solubility of boric acid when sodium hydroxide is present. However, some applicant may use this phenomenon to increase the allowable so.lubility limits.
In such cases the reviewer should identify and individually evaluate such features.
Step 2 Determination of the basis parameters identified in Section 111-2 of this memo.
The information needed for the determination of these parameters are either provided in the original submittal or can be found in FSAR or PSAR.
Steo 3 Hardware review Using the P&I diagrams or special schematic diagrams provided by the applicant,. the systems are reviewed to assure that they meet the requirements identified i~n the analytical part of the.
review, without violating the single failure criteria.
IV. Conclusions The procedure for reviewing nuclear plant for boric acid precipitation is outlined in this memorandum. It should be mentioned that although this procedure may apply to the majority of plants, reviewed, there may be some cases where the reviewer may be forced to use completely different approach. He should be, therefore, flexible in choosing his procedures, guided by the considerations described in Appendix I. Note the Appendix I is intended to apply fully only to CP applications.
V. References
- 2.
Babcock and Wilcox, Topical Report BAW-10103, ECCS Analysis of B&W's 177--A Lowered-Loop NSS, June 19.75.
- 3.
Babcock and Wilcox, Topical Report BA-10105, ECCS Evaluation of B&W's 177-FA Raised-Loop NSS, June 1975.
- 4. Switzer, D. C.,
(.IZCO) letter to 0. D. Parr (NRC),
dated June 27, 1975, attachment A.
- 5. Switzer, D. C. (NNTECO) letter to 0. D. Parr (NRC),
dated September 25, 1975, attachent1.
- 6. Telecon, J. Longo (CE) and RSB personnel (NRC), October 30, 1975.
- 7. Caso, C. L. (Westinghouse) letter CLC-NS-309 to T. M. Novak (NRC),
dated April 1, 1975.
8.- Cermak,-J. 0. (Westinghouse) letter JOC-NS-369 to T. M. Novak (NRC), dated August.15, 1975.
- 9.
ANS, Decay Energy Release Rates Following Shutdownm of Uranium - Fueled Therzal Reactors, (Proposed AS Standard),
October 1971.
- 10. Wallis, G. B., One-dimensional Two-phase Flow, McGraw-Eill Book Company, 1969, Section 12.10.
APcendix I PWR BORON DILUTION SYSTEIMS FOR CP APPLICATIONS WHICH MUST MEET 10 CFR 50.46 CRITERIA FOR ACCEPTANCE
- 1. The boron dilution function shall not be vulnerable to a single failure. A sincle active failure postulated to occur during the long term cooling period can be assumed failure would then be in 1ieu of a single active failure during the short term cooling period.
- 2. The spurious operation of any motor operated valve (open or closed) shall not compromise the boron dilution function nor shall it jeopardize. the ability to remove decay heat from the primary system.
- 3. All components of the system which are within containment shall be designed to seismic Category 1 requirements and classified Quality Group B.
- 4. The primary mode for maintaininq acccptable levels of boron in the vessel should be established. Should a sinale failure disable the primary mode, certain manual actions outside the control room would be allowed, depending on the nature of the action and the time available to establish back-up mode.
- 5. The average boric aci.d-concentration in any region of the reactor
- vessel should not exceed the level of 4 weight percent below the solubility limits at the temperature of the solution.
- 6. During the post-LOCA long term cooling, the ECC system normally operates in two modes:
the initial cold leg injection mode,.
followed by the dilution mode. The actual operating time in the cold leg injection mode will depend on plant design and steam binding considerations,but, in general, the switchover to the dilution mode.should be made between 12 and 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after LOCA.
- 7. The dilution mode can be accomplished. by any of the following means:
(a) Simultaneous cold leg injection and hot leg suction (b) Simultaneous hot and' cold leg injections (c) Alternate hot and.cold: leg inj.ections..
- 8. In the alternate hot and cold leg.injection mode, the operating time at hot and cold leg injection should be sufficiently short to prevent excessive boric acid buildup.
-2
- 9.
The minimum ECCS flow rate delivered to the vessel during the dilution mode shall be sufficient to accommodate the boil-off due to fission product decay heat and possible liquid entrainment in the steam discharged to the containment and still provide sufficient liouid flow through the core to prevent further increases in-boric acid concentration.
- 10. All dilution modes shall maintain testability comparable to other ECCS modes of operation (HPI-short term, LPI-short term, etc).
The current criteria for levels of ECCS testability shall be used as guidelines (i.e., Regulatory Guides 1.68, 1.79, GDC 37)..