ML20207T172
| ML20207T172 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 03/18/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20207T168 | List: |
| References | |
| NUDOCS 8703230277 | |
| Download: ML20207T172 (3) | |
Text
..
o nog
/
'o, UNITED STATES E'
'i NUCLEAR REGULATORY COMMISSION g,
t WASHINGTON, D. C. 20555
/
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO THE DELETION OF CHARCOAL AD50RPER C00LDOWN MODE PHILADELPHIA ELECTRIC COMPANY LIMERICK GENERATING STATION, UNIT NO. 2 DOCKET NO. 50-353
1.0 INTRODUCTION
With their letter dated December 29, 1986, Philadelphia Electric Company submitted a proposal to modify the filter train configuration within the Reactor Enclosure Recirculation System (RERS) for Limerick Unit No. 2.
Specifically, the licensee proposed to delete the cooldown air flow path to each of the charcoal adsorbers in the RERS filter trains. The cooldown air is provided upstream of each charcoal adsorber to limit excessive charcoal temperature rises due to potential radioactive decay heat buildup in the adsorbers during and following an accident.
It should be noted that the separate water deluge system, which is provided within the charcoal adsorbers for fire protection, is not affected by this proposal.
2.0 EVALUATION The RERS is a redundant engineered safety feature system. Each system is provided with a filtration train that has a design air flow capacity of 60,000 cfm. Each filtration train consists of a prefilter, an upstream HEPA, a filtration of contaminated air (todines and particulates) gned to provide a charcoal adsorber and a downstream HEPA. The RERS is desiin the reactor enclosure building following a postulated accident or an abnormal release of high airborne radioactivity into the reactor enclosure. A cooldown air path, equipped with an isolation valve, is provided upstream of each charcoal adsorber. The cooldown air is supplied only to a charcoal adsorber that (1) is not in operation and (2) is indicating an excessive charcoal temperature rise due to radioactive decay heat.
In addition, a separate water deluge system is provided within each charcoal adsorber for fire protection.
The licensee stated in the referenced letter that the maximum calculated temperature rise due to radioactive decay heat buildup following a design basis loss of coolant accident (LOCA) is 3.2'F.
The staff considers the LOCA to be the bounding case. The licensee further stated that this temperature rise will result in a cumulative charcoal bed temperature of 153.2*F. This value was estimated on the basis of the conservative assumption that the Reactor Enclosure Secondary Containment post-LOCA atmosphere temperature is 150*F.
We have reviewed the following licensee's assumptions which were used to calculate the temperature rise:
0703230277 070310 ADoCK0500g3 PDH
I' I
(1) 25% of the core iodine inventory is irerediately available for leakage i
from the primary reactor containment. This is consistent with the Regulatory Position in Regulatory Guide 1.3.
(2) 50% of the core iodine inventory is in the suppression pool water. This is consistent with the Regulatory Position in Regulatory Guide 1.7.
(3) The primary containment atmosphere leaks to the reactor building at a rate of 0.5 percent per day, witk additional steam leakage of 11.5 standard cubic feet per hour througn each main steam isolation valve.
Five gpn of suppression pool water is also assumed to be leaking into the Reactor Building through equipment leakaoe. These assumed values are consistent with those used in recently issued BWR technical specifications.
(4) An air mixing efficiency of 50 percent within the secondary containment.
This assumption is consistent with the acceptance criteria in SRP Section 6.5.3.
(5) A decontamination factor of 10 was used for iodine which becomes airborne after partitioning from the suppression pool water leakage. This assumption is consistent with the assumption used in NUREG-0016.
(6) A secondary containment post-LOCA environmental temperature of 150*F.
The staff's Regulatory Position is 180*F, as delineated in Regulatory Guide 1.52. This is addressed in the paragraph at the end of this section.
(7) A 100 percent removal efficiency for radioactive iodines by the charcoal adsorber. The staff considers this conservative for purposes of decay heat calculations.
(8) All of the iodine beta energies and 50 percent of gamma enerates for each isotope are adsorbed in the charcoal. The staff considers this assumption conservative.
(9) No iodine removal mechanism other than the charcoal adsorption. The staff considers this assumption conservative.
(10) No unfiltered leakage to the environment during the initial system 2 minute 15 second drandown time after an accident. The staff considers this assumption conservative.
(11) No decay heat load from noble gases. Because of the transient time for such isotopes, the staff finds this assumption acceptable.
(12)Themaximumheatloadduetotheheatofiodineoxidationisapproximately 42 watts (3 percent of the decay heat load). The staff finds this assumption acceptable.
2 (13)A1,520ft filter front surface area for natural convective heat loss.
The staff finds this assumption acceptable.
(14) A 820 ft outside steel surface area for radioactive heat loss. The staff finds this assumption acceptable.
(15)Theminimumcharcoalignitiontemperatureof627'F. The staff's regulatory position in Regulatory Guide 1.52 stipulates that the minimum charcoal ignition temperature is 627'F (330'C) at a 100 feet per minute
-air flow rate through the adsorber. With the adsorber isolated (no air flow), the staff estimates that the minimum charcoal ignition temperature could be as low as 450*F in a 2 inch adsorber bed.
Using the above assumptions, the licensee has calculated a cumulative charcoal temperatureof153.2*Fatthetimeofmaximumiodineloadingduringand following an accident. The staff has reviewed the licensee s assumptions and, with two exceptions, finds them to be acceptable. Our acceptance is based on either(1)conforrancewithapplicableRegulatoryGuidesandotherstaff guidance, or (2) sufficient conservatism to account for uncertainties in the analysis. The two exceptions are with respect to the licensee's assumptions regarding the secondary containment post-LOCA environment temperature (150'F) and the minimum charcoal ignition temperature (627'F). Using a 180*F secondary containment post LOCA environment temperature, the staff estimates that the charcoal temperature may reach up to 250'F with gradual radioactive iodine loading into the charcoal adsorber during and following an accident.
Also, the charcoal ignition temperature may be as low as 450*F with no air flow through the 2 inch thick adsorber.
In view of these considerations, the staff conc' udes that the maximum expected charcoal temperature rise in the adsorber still will be well below the minimum charcoal tonition temperature.
Therefore, the staff accepts the licensee's request to delete the cooldown air paths to the charcoal adsorbers.
3.0 CONCLUSION
On the basis of the above evaluation, the staff concludes that the licensee's request to delete the cooldown air flow paths to the charcoal adsorbers is acceptable. The bases for acceptance are that (1) the expected maximum charcoal temperature rise in the adsorber due to potential radioactive decay heat buildup during and following an accident is well below the minimum charcoal ignition temperature, and (2) a separate water deluge system within each adsorber is provided with temperature alarm set points. Hence, the requested change does not affect significantly the risk of a charcoal fire following an accident, and the safety function of the filters will be preserved.