ML17252B180
| ML17252B180 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 02/22/1972 |
| From: | Commonwealth Edison Co |
| To: | US Atomic Energy Commission (AEC) |
| References | |
| 14 | |
| Download: ML17252B180 (10) | |
Text
I
- ,1 * '
~la:tory' Fil-a Cy.
~~ DRESDEN NUCLEAR POWER STATION UNIT3 I
SUPPLEMENT TO SPECIAL REPORT NO. 14 Containment Flammability Control in a Boiling '1V ater Reactor
. I Commonwealth* Edison Company 947
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DRESDEN NUCLEAR POWER STATION UNIT 3 SUPPLEMENT TO SPECIAL REPORT NO. 14 Containment Flammability Control in a Boiling Water Reactor COMMONWEAL TH EDISON COMPANY
D-3 TABLE OF CONTENTS
- 1. INTRODUCTION 1
- 2. PRINCIPLE OF CONTAINMENT ATMOSPHERE DILUTION OPERATION 1 2.1 Containment Atmosphere Dilution . . . . 1 2.2 Measuring Equipment . . . . . . . 2 2.3 Capability of Purging Containment as Backup 2
- 3. ANALYSES ............. . 2 3.1 Containment Atmosphere Dilution Operation (CADO) Requirements 2 3.2 Containment Pressure Response . ,* 2 3.3 Containment Atmosphere Mixing 3
- 4. RESULTING EXPOSURE 3
- 5. CONCLUSIONS . . . . 3
I D-3 I
I . ------------ - - - - - - - - - - - - - - - - -~---- -9RES9EN-3-SPEGIAl:--REPORT I (SUPPLEMENT TO SPECIAL REPORT NO. 14)
- 1. INTRODUCTION Following a loss-of-coolant accident (LOCAi, hydrogen will be evolved within the primary containment from metal-water reactions and from radiolysis. A metal-water reaction releases hydrogen only whereas radiolysis releases both hydrogen and oxygen. In March 1971, the AEC published Safety Guide 7, which established design parameters for "Control of Combustible Gas Concentrations in Containment Following a Loss of Coolant Accident." Our position with respect to the parameters in Safety Guide 7 was stated in Dresden 3 Special Report No. 14 and in amendments on other.dockets. In compliance with the letter from P.A. Morris, dated August 3, 1971, the following report describes a post-LOCA containment atmosphere control system which meets the requirements of Safety Guide 7. This report includes only the values and limits as established in Safety Guide 7.
Since Safety Guide 7 was published, variou~ solutions have been considered which hopefully would resolve the problem of hydrogen control. Controlled venting of the containment was considered as a solution but was rejected by the AEC because of the associated off-site doses. This report proposes a solution to controlling the combustible gases within the containment by simply diluting the combustible gases. Thus, the combustible gases will always remain below their flammability limit. We have determined, in accordance with the parameters provided in Safety Guide 7, that this proposed containment atmosphere dilution operation is an acceptable method of controlling the combustible gas concentratiom. within the containment following a loss-of-coolant accident.
- 2. PRINCIPLE OF CONTAINMENT ATMOSPHERE DILUTION OPERATION AEC Safety Guide 7 specifies the flammability limit for hydrogen and oxygen as 4% and 5% by volume, respectively (i.e., the hydrogen concentration should not exceed 4 vol % if more than 5 vol % oxygen is present, and the oxygen concentration should not exceed 5 vol% if more than 4 vol% hydrogen is present). The hydrogen concentration or the oxygen concentration must therefore be controlled to within these flammability limits. Further use of Safety Guide 7 for a metal-water reaction event shows that inerting of the containment will be required to prevent a flammable mixture as the rapid metal-water reaction occurs. The more slowly evolved hydrogen and oxygen from radiolysis can be limited to less than flammable mixture by limiting the oxygen concentration. Limiting the oxygen concentration will be accomplished by adding nitrogen to the containment atmosphere, thus diluting the oxygen concentration to less than the flammability limit.
2.1 Containment Atmosphere Dilution The oxygen level in the inerted containment during normal operation is less than 5% by volume. Following a loss-of-coolant accident, radiolysis will begin to form more oxygen. By adding nitrogen to the containment as the radiolytic formation of oxygen occurs, the oxygen is diluted and will remain below the flammability concentration of 5% by volume. Since the radiolysis rate decreases with time as a result of fission product decay. the required nitrogen addition rate will also decrease with time.
Thus, the containment atmosphere dilution (CAD) consists of simply adding nitrogen to the containment to dilute the oxygen concentration to below the oxygen flammability limit. The containment pressure will increase due to the addition of nitrogen but will be limited or controlled by containment leakage. The CAD operation is controlled completely from the control room. The nitrogen addition rate is *controlled and monitored from the control room.
Additional nitrogen for long-term operation can be obtained and connection with the present nitrogen makeup system can be readily accomplished. Technical Specification additions associated with this system are attached to this report.
D-3 2.2 Measuring Equipment TnefiYCJrogen and oxygen concentrations will be continuously monitored with hydrogen and oxygen analyzers.
Both of these samplers and the associated instrumentation for readout in the control room wili meeHhe req~irements of IEEE 279. The radioactivity level of the containment atmosphere will also be monitored and will be displayed in the control room. Such an atmosphere radioactive monitoring system will also meet the requirements of IEEE 279. The hydrogen, oxygen, and radiation monitoring equipment was presented iri Dresden Special Report No. 14. The monitoring capability for other parameters such as containment pressure are presently installed.
2.3 Capability of Purging Containment as Backup The reactor presently employs a containment purging system. This system provides capability for purging the containment through the Standby Gas Treatment System (SGTS) with final discharge out of the main *stack, thus limiting the potential release of radioactive iodine and other radioactive materials to the environment. This containment purging system will provide a method for controlling combustible gas concentrations in the containment in the unlikely event that the CAD operation does not perform properly.
- 3. ANALYSES As presented in the previous sections, the oxygen concentration will be limited to less than the flammability limit by dilution with nitrogen. The analyses for containment atmosphere dilution requirements and pressure response are performed using the parameters included in AEC Safety Guide 7. Th!! analysis for radiation exposure to the public is presented in Section 4.
3.1 Containment Atmosphere Dilution Operation (CADO) Requiremenu The minimum actuation time for the CADO can be defined {IS the time when the hydrogen and the oxygen in the containment atmosphere reaches a flammable concentration. For the initially inert containment and with the large metal-water reaction defined by Safety Guide 7, oxygen concentration is the parameter to limit. This is demonstrated in Figure 1, where the percentage concentrations of hydrogen ancl Qxygen following a LOCA are shown. The hydrogen concentration increases very rapidly above the 4% limit due to the large amount of metal-water reaction assumed to occur. Therefore, flammability is dependent upon the time the oxygen concentration reaches a 5% concentration. Note that, although oxygen concentration was assumed to be at 5% initially,* the concentration drops off immediately due to the additions of steam from the reactor and hydrogen from the metal-water reaction in accordance with Safety
. Guide 7 following the accident. The oxygen concentration gradually* increases as the steam is condensed and! as radiolysis adds oxygen to the containment. The time when the oxygen concentration reaches 5% is noted on Figure 1 as 1.1 days and is defined as the minimum actuation time of the CADO,
. Figure 1 also shows the maximum expected oxygen and hydrogen concentrations as a functiqn of time using more realistic parameter values (though still conservative) than those presented in Safety Guide 7. Note that the minimum actuation time for the CADO using these assumptions would be 17 days and would be based on controlling hydrogen concentration.
As discussed in Section 2, the amount of nitrogen that must be added to the containment is dependent on the amount of oxygen that is generated due to radiolysis. Figure 2 shows the amount of nitrogen required as a function of time after the LOCA. The addition rate decreases with time because fission product energy, hence radiolysis, decays with time.
3.2 Containment Pressure Response Since nitrogen gas is being added to the containment, which is a fixed volume system, some increase in pressure proportional to the amount of gas added can be expected. This pressure rise is limited by the diminishing need for nitrogen as a function of time plus the normal containment leakage rate. Figure 3 shows the containment pressure as a function of time. The "zero" containment leakage case is shown as a reference only since some containment leakage is expected. Also, the operator will have *sufficient time available to establish some small leakage rate if the containment
- This is the Technical Specification limit for oxygen during operation. Actual oxygen concentrations are expected to be less than this
~~. .
D-3 leakage rate proves to be too small. Figure 3 also shows the "normal" post-LOCA pressure, which is simply a
_ _ _ _ _ _ _ _c_crntin~~ion of the con~Lnment resRQ!l_se cu_ry_e_shown_in_Eigure.5.3.-1-1-of-the F-SAR.- ----
It can be seen that the containment pressure can be controlled if necessary (and it would be necessary only for the extreme conditions defined above) by establishing containment leakage rates which are less than the Technical Specification containment leakage rate. And, as discussed in Section 4 below, no increase in radiation exposure over that shown in Section 14 of the FSAR will occur.
3.3 Containment Atmosphere Mixing With the containment inerted by maintaining the oxygen concentration below 5%, the only factor tending to de-inert is the radiolytic formation of oxygen. Using the assumptions of Safety Guide 7, the formation rate.of oxygen does not exceed 3 scfm. In a containment with a free air volume greater than 200,000 ft 3 (Dresden 3 containment free volume is approximately 280,000 ft3). the addition of this small amount of oxygen from a dispersed source cannot cause local nonuniformity of oxygen concentration greater than a small fractional percentage. There are many driving sources for mixing within the containment. The one driving force for mixing that can be precisely calculated (Le.,
diffusion) is sufficient to ensure thorough mixing. Where the local oxygen concentration exceeds the average oxygen concentration by more than 0.1%, diffusion will occ1,1r readily in volumes larger than 10 ft 3
- Other driving forces for mixing (natural and forced convection) will also exist in the containment; therefore, absolute mixing can be assured.
- 4. RESULTING EXPOSURE.
The containment atmosphere dilution (CAD) operation does not result in any increased exposure to the public over that calculated and presented in the FSAR. Section 14 of the FSAR presented off-site radiation exposure to the public as a result of various accidents. These radiation exposures assumed a containment leakage rate of 1.6%/day, which remains constant over the 30-day period following the accident. If this leakage is present during the CAD operation, the resulting exposure is the same as that presented in Section 14 of the FSAR. Should the containment leakage rate be less than that assumed in the FSAR, the resulting exposure will be less than that presented in Section 14 of the FSAR.
Figure 3 presented the containment pressure vs time curve for various assumed containment leakage rates. Even if one postulates that no containment leakage were to occur, then some time following the accident (say, 30 days after a LOCAi. the operator will detect from containment pressure that no containment leakage is present. The operator will then induce a controlled leakage by purging into the SGTS at a rate less than 1.6%/day. The resulting radiation exposure to the public would then be less than those presented in Section 14 of the FSAR because of the 30-day time interval before a containment leakage rate commenced.
It can be concluded that for any operation resulting from *containment atmosphere dilution th_e resulting radiation exposure to the public is no greater than that presented in Section 14 of the FSAR.
- 5. CONCLUSIONS The containment atmosphere dilution operation is shown to meet the requirements of AEC Safety Guide 7.
Specifically, a comparison to AEC Safety Guide 7 Section C "Regulatory Position" shows that:
- 1. The plant will have monitoring equipment for detecting and measuring the hydrogen and oxygen concentrations in the containment. Mixing of the containment atmosphere is not required as discussed in this report. The combustible gas concentrations are controlled without reliance on purging of the containment atmosphere.
- 2. The equipment for measuring the hydrogen and oxygen concentrations will meet the criteria of IEEE-279. This equipment in itself does not introduce safety problems which affect containment integrity.
- 3. The plant has the installed capability for controlled purge of the containment atmosphere through the SGTS and discharge from the main stack.
D-3
- 4. The parameter values listed in Table 1 of Safety Guide 7 are used for the purpose of calculating hydrogen and
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1------ . oxygen gas concen!!'~_!ions in_!b.lL_c_ontainment_and-for-evaluating-the-design-capability-to-contfcffanC:I to purge
combustible gases evolved in the course of a loss-of-coolant accident.
- 5. Materials within the containment will not be subjected to corrosion from either the emergency cooling systems or containment spray system, since no corrosive solutions are employed in these systems.
Thus, following a loss-of-coolant accident, sampling and control of combustible gases resuiting from the postulated metal-water reaction. radiolysis and corrosion are accomplished and do not necessarily involve releases of radioactive materials to the environment. The capability for purging the containment through filters to limit the potential release of radioactive iodine and other radioactive materials should the containment atmosphere dilution operation not perform properly is presently installed. The equipment employed for the containment atmosphere dilution operation will be designed and constructed to the standards of the engineered safety features. Finally, this design does not introduce any safety problems that may affect containment integrity such as might be present should a flame recombiner or similar device be employed for the control of the combustible gases.
3.5 LIMITING CONDITION FOR OPERATION 4.5 SURVEILLANCE REQUIREMENT I. Nitrogen I. Nitrogen
- 1.
- There shall be a minimum of 200,000 ft 3 of 1. Once a month the quantity of nitrogen avail-nitrogen on site. If this minimum volume of able shall be logged.
nitrogen requirement cannot be met, an orderly shutdown of the reactor shall be initiated. 2. Once each month the valves in the nitrogen makeup system shall be actuated.
Bases: Bases:
3.5 4.5 I. The nitrogen supply of 200,000 ft 3 will supply the 1. The nitrogen quantity must be checked to ensure primary containment with nitrogen sufficient to continuous operation of the nitrogen makeup dilute and control the containment oxygen concen- system over a period of seven days.
tration to less* than 5% per volume in the unlikely event of a LOCA for a period of seven days. Addi-tional nitrogen can be obtained and delivered to the site within a 24-hour period; thus, a seven-day supply provides adequate margin.
SAFETY GUIDE 7 \
ASSUMPTIONS MORE REALISTIC ASSUMPTIONS GH 2 = 0.2, 5% 'Y AB'_ 15% MWI 10
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oi...____________..._________....1.__________..._____________...__________.__________....._____________.._____+'--..... .I 0.2 o.5 1.0 2.0 5.0 10 20 50 SAFETY GUIDE 7 \
ASSUMPTIONS \
0
<fl MORE REALISTIC ASSUMPTIONS w (GH 2
- G.2, 5" 'YAW 15" MW) 20 N
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0.2 0.5 1.0 2.0 6.0 10 20 &O TIME AFTER LOCA (day)
FIGURE 1. DRESDEN 2, 3- UNCONTROLLED H2 AND o2 CONCENTRATIONS FOLLOWING A LOCA NO CONTAINMENT LEAKAGE
.-- 11 C ,eft.A 0.8 l/:j-I 0
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~ 0.6 I-zw
- E z
cI-z 0
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0
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0.2 0 ....________....______________________________________.....__________________.....________....________....____....,__~
0 20 40 60 80 100 120 140 180 180 I 200 TIME AFTER LOCA (day)
FIGURE 2. DRESDEN 2, 3 - MAXIMUM NITROGEN REQUIRED FOR DILUTION - AEC SAFETY GUIDE 7 ASSUMPTIONS: NO CONTAINMENT LEAKAGE
DESIGN P = 56 psig - - - - , - - - - - -
50 0.5% vol/day LEAKAGE I DESIGN I 40 a,
161 a:
- > 1.6% vol/day LEAKAGE
!a161 (TECH SPEC LIMITI a:
CL.
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0 w
...z 8 N 20 ADDl~ION BEGINS 10 -- -- ..... .............. ~
....... "NORMAL" POST-LOCA PRESSURE
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0 .._____....!.__.....!__......................l.d......____.....________....._................i........_.......oi;a,__.......................................______..___. _.........!__!.o!."""-ll.all 0.1 1 10 100 \uKJO TIME AFTER LOCA (dayl FIGURE 3. ORES.DEN 2, 3 - CONTAINMENT PRESSURE AS A FUNCTION OF TIME FOLLOWING LOCA WITH DILUTION (MAXIMUM NITROGEN ADDED PER FIGURE 2)