ML20198A366
| ML20198A366 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 12/08/1998 |
| From: | ENTERGY OPERATIONS, INC. |
| To: | |
| Shared Package | |
| ML20198A356 | List: |
| References | |
| NUDOCS 9812160244 | |
| Download: ML20198A366 (2) | |
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CONTAINMENT SYSTEMS BASES
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.J/4.6.2 DEPRESSURIEATION, COOLING, AND pH CONTROL SYSTEMS l
.3/4.6.2.1 CONTAINMMT SPRAY SYSTEM The OPERABILITY of the containment spray system ensures that l
containment depressurization and cooling capability will be available in the event of a LOCA. The pressure reduction and resultant lower containment leakage rate are consistent with the assumptions used in the accident analyses.
1 The containment spray system and the containment cooling system are
-redundant to each other in providing post accident cooling of the containment atmosphere. However, the containment spray system also provides a mechanism for removing iodine from the containment atmosphere and therefore the time requirements for restoring an inoperable spray system to OPERABLE status have been maintained consistent with that assigned other inoperable ESF equipment.
1 3/4.6.2.2 TRISODIUM PHOSPHATE (TSP) l-A hydrated form of granular trisodium phosphate (TSP) is employed as a i
_ passive form of pH control for post LOCA containment spray and core cooling water to ensure that iodine, which may be dissolved in the recirculated reactor cooling water following a loss of coolant accident (LOCA), remains in solution. TSP also helps inhibit stress corrosion cracking (SCC) of austenitic stainless steel components in containment during the recirculation phase following an accident.
Baskets of TSP are placed on the floor of the containment building to dissolve from released reactor coolant water and containment sprays after a LOCA.
Recirculation of the water for core cooling C
and containment sprays then provides mixing to achieve a uniform solution pH.
Fuel that is damaged during a LOCA will release lodine in several chemical forms to the reactor coolant and to the containment atmosphere. A l
portion of the iodine in the containment atmosphere is washed to the sump by containment sprays. The emergency core cooling water is borated for reactivity control. This borated water causes the sump solution to be acidic.
In a l
low pH (acidic) solution, dissolved iodine will be converted to a volatile form. The volatile iodine will evolve out of solution into the containment atmosphere, significantly increasing the levels of airborne iodine. The increased levels of airborne iodine in containment contribute to the radiological releases and increase the consequences from the accident due to containment atmosphere leakage.
l After a LOCA, the components of the core cooling and containment spray systems will be exposed to high temperature borated water.
Prolonged exposure to the core cooling water combined with stresses imposed on the components can cause SCC.
The SCC is a function of stress, oxygen and chloride concentrations, pH, temperature, and alloy composition of the components. High temperatures j
and low pH, which would be present after a LOCA, tend to promote SCC.
This can lead to the failure of necessary safety systems or components.
Adjusting the pH of the recirculation solution to levels above 7.0 prevents a significant fraction of the dissolved iodine from converting to a volatile form. The higher pH thus decreases the level of airborne iodine in containment and reduces the radiological consequences from containment atmosphere _ leakage following a LOCA. Maintaining the solution pH above 7.0 also reduces the occurrence of SCC of austenitic stainless steel components i
in containment.
Reducing SCC reduces the probability of failure of components.
I ARKANSAS - UNIT 2 B 3/4 6-3 Amendment No. SG l
9812160244 981208 7
PDR ADOCK05000368Q P
CONTAINMENT SYSTEMS BASES A hydrated form of TSP is used becaase of the high humidity in the containment building during normal operation.
Since the TSP is hydrated, it is less likely to absorb large amounts of water from the humid atmosphere and will undergo less physical and chemical change than the anhydrous form of TSP.
The LOCA radiological consequences analysis takes credit for iodine retention in the sump solution based on the recirculation water pH being 2 7.0.
The radionuclide releases from the containment atmosphere and the consequences of a LOCA would be increased if the pH of the recirculation water were not adjusted to 7.0 or above.
The required amount of TSP is based upon the extreme cases of water volume and pH possible in the containment sump after a large break LOCA.
The minimum required volume is the volume of TSP that will achieve a sump solution pH of 2 7.0 when taking into consideration the maximum posrible sump water volume and the minimum possible pH.
The amount of TSP needed in the containment building is based on the mass of TSP required to achieve the desired pH.
However, a required volume is specified, rather than mass, since it is not feasible to weigh the entire amount of TSP in containment.
The minimum required volume is based on the manufactured density of TSP dodecahydrate.
Since TSP can have a tendency to agglomerate from high humidity in the containment building, the density may increase and the volume decrease during normal plant operation.
Due to possible agglomeration and increase in density, estimating the minimum volume of TSP in containment is conservative with respect to achieving a minimum required pH.
Sufficient TSP is required to be available in MODES 1, 2, and 3, because the RCS is at elevated temperature and pressure, providing an energy potential for a LOCA.
The potential for a LOCA results in a need for the ability to control the pH of the recirculated coolant.
If it is discovered that the TSP in the containment building is not within limits, action must be taken to restore the TSP to within limits.
During plant operation the containment sump is not accessible and corrections may not be possible. 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is allowed for restoring the TSP within limits, where possible, because 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> is the same time allowed for restoration of other ECCS components.
If the TSP cannot be restored within limits within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, the plant must be brought to a MODE in which the LCO does not apply. The specified Allowed Outage Times for reaching HOT STANDBY and HOT SHUTDOWN were chosen to allow reaching the specified conditions from full power in an orderly manner and without challenging plant systems.
The SR 4.6.2.2.a periodic determination of the volume of TSP in containment must be performed due to the possibility of leaking valves and components in the containment building that could cause dissolution of the TSP during norual operation. A Frequency of 18 months is required to determine visually that combined a minimum of 278 cubic feet is contained in the TSP baskets.
This requirement ensures that there is an adequate volume of TSP to adjust the pH of the post LOCA sump solution to a value 2 7.0.
The periodic verification is required every 18 months, since access to the TSP baskets is only feasible during outages, and normal fuel cycles are scheduled for 18 months. Operating experience has shown this Surveillance Frequency acceptable due to the margin in the volume of TSP placed in the containment building.
ARKANSAS - UNIT 2 B 3/4 6-4 Amendment No. M,M7,M4