ML20203G088

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General Description & Background of Offgas Sys
ML20203G088
Person / Time
Site: Perry  FirstEnergy icon.png
Issue date: 07/29/1986
From:
CLEVELAND ELECTRIC ILLUMINATING CO.
To:
Shared Package
ML20203G077 List:
References
NUDOCS 8607310317
Download: ML20203G088 (8)
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GENERAL DESCRIPTION AND BACKGROUND OF OFF-GAS PROCESSING SYSTEM System Purpose

'Ihe purpose of the Off-Gas System is to process and control the release of gaseous radioactive effluents from the condenser to the site environs.

The Off-Gas System performs its function by reducing the off-gas volume. This is accomplished by:

recombining hydrogen and oxygen into water, delaying the passage of Krypton and Xenon, and O -

physical adsorption

System Description

The Off-Gas System is non-safety class and is made up of three primary segments. The first segment consists of preheaters, catalytic recombiners, an off-gas condenser and a water separator. These components are used to j recombine the hydrogen and oxygen and to remove moisture from the off-gas process stream. The second segment consists of a hold-up line which allows any short-lived activity in the off-gas process stream to decay. The third segment consists of cooler condensers, moisture separators, prefilters, desiccant-type gas dryers, gas coolers, charcoal adsorber trains and C\

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afterfilters which remove trace moisture and delays the passage of Krypton and Xenon. The Off-Gas System recei'/es discharge steam, air, and noncondensible gases from the main condenser via the steam jet air ejectors (SJAE'st.

Hydrogen is removed in the first segment of the system by a catalytic recombiner which requires an inlet temperature of 350*F to begin recombination. The off-gas is teated from approximately 228'F to 350'F by the preheater to place the gas and recombiner in thermal equilibrium. Once recombination of hydrogen with oxygen to produce water vapor begins, the recombination reaction produces heat and raises the off-gas temperature to approximately 830'F. There are two 100 percent capacity preheaters each with an associated catalytic recombiner. The standby recombiner is kept hot by heaters which cycle on and off to maintain the recombiner at 350'F. After the off-gas passes tMough the recombiner, it passes through the off-gas condenser which cools the process stream to condense and remove the water vapor. The off-gas passes through the condenser shell while condensate flows

through the tubes to reduce the off-gas temperature from 830'F to approximately 140'F. The off-gas then passes through a water separator in which water droplets in the off-gas impinge on a wire mesh screen in the separator, drain to the bottom of the water separator and then drain back to the off-gas condenser. Level in the off-gas condenser is maintained by two level controllers and two level control valves. The condensate from the off-gas condenser drains to the low pressure condenser via the Main, Reheat, Extraction, and Miscellaneous Drains System.

The second segment of the Off-Gas System allows time for short-lived activity to decay. This is accomp13shed by the holdup line which is a series of loops

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.f in the off-gas piping which requires the off-gas approximately 10 minutes to pass through. Any condensate which collects in the holdup line is drained via a loop seal to the Radwaste System.

n e third segment provides for the decay of long-lived activity, primarily Krypton and Xenon. Krypton and Xenon are each members of the noble gases which vary in their holdup time and level of physical adsorption. 'Ihe charcoal adsorbers are more efficient when operated at low temperatures. For this reason, the adsorbers are located in a vault room kept at a temperature of approximately O'F.

Prior to entering the charcoal adsorbers, the off-gas is dried to prevent formation of ice which would plug the adsorbers. The moisture is removed by cooling the off-gas to 45'F in the cooler condenser which contains 35'F glycol on the secondary side. Additionally, moisture is removed by a moisture separator and the condensate drains via a loop seal to the Radwaste System.

We off-gas passes through the prefilter to remove any particulates before it enters the desiccant dryer. There are two cooler condensers, two moisture A

separators, and two prefilters, each of which has 100 percent capacity.

loop seal drain is located on the prefilter inlet to remove any accumulated moisture. The desiccant dryer is the final stage of moisture removal and it further reduces the moisture content.

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r i There are four desiccant dryers in this system arranged as two dryers per train. Each train has its own regeneration system to dry the desiccant once it becomes saturated and regeneration is accomplished automatically by timers.

shifting from one dryer to another in the same train is automatic while shifting between trains requires operator action.

After being dried, the off-gas is cooled to O'F by the gas coolers, and then enters the charcoal adsorbers. The gas coolers are cooled by refrigerated air from the Off-Gas Vault Refrigeration System and normal flow for the off-gas is through the A and B gas coolers. If the first charcoal adsorber in the train becomes plugged, it can be bypassed by shifting flow to the C and D gas coolers. Gas coolers C and D are piped to the inlet of the second charcoal adsorber in each train.

O There are two trains of charcoal adsorbers with four adsorbers per train. The pressure envelope of the system is designed to be explosion resistant and the pressure vessels are designed to withstand 350 psig static pressure.

Channeling in the charcoal adsorber is inhibited by using a flow distributor on the inlet and outlet piping in each vessel and by having long columns of charcoal. Both trains of the system are normally in service during plant operations. After passing through the charcoal adsorbers, the off-gas passes through either of two (2) 100% afterfilters which remove any particulates from the process stream. The off-gas is then discharged to the environs via the Off-Gas Building Exhaust System.

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p Initial Charcoal Loading and Previous Testing Initial loading of charcoal into the adsorber bed vessels began April- 5,1985 in accordance with General Electric's procedure GEP-IP-0040, and was performed by General Electric personnel. Upon completion of the initial loading, all adsorber bed vessels were inerted with a nitrogen (N 2

) blanket to inhibit vessel and piping corrosion and to maintain a dry environment for the carbon adsorbent.

Testing of the Off-Gas Vault Refrigeration System began in late May, 1985.

During this testing, portable heaters were installed in the adsorber vessel vaults. The vaults were heated and then stabilized at a temperature of 150*F and held at that temperature for approximately 4-1/2 hours. 'Ihe vault heat-up process took approximately 33-1/2 hours. The Off-Gas Vault Refrigeration System was then started in the pull-down mode and the acceptance criteria of i

O'F was successfully achieved in less than 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br />. Adsorber bed charcoal temperature indications were not required to be monitored during the heat-up and cool-down process, although charcoal was installed in the adsorber bed vessels.

Preoperational testing of the Off-Gas Processing System began on July 2, 1985 and was completed in October, 1985, with test exceptions remaining open. One l

of the exceptions identified during testing concerned the ability of the Off-Gas Vault Refrigeration System to maintain vault and charcoal adsorber temperatures at O'F +2'F. Prior to retesting the Off-Gas Processing System to close these test exceptions, it was decided to gather additional information to verify the cool down capability of the Off-Gas Vault Refrigeration System.

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O etest er the ort-ces v e1e errieer tien svste ce-eeced en sene 18.1eee and on June 20, 1986 the first Unusual Event was declared due to combustion

activity in vessels D014A and D014B. A detailed chronology of both combustion events is contained in the recovery plan included in this report.

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