Discharge pit

Figure 3.8.4 Sea water system flow diagram

fuel pit heat exchangers. A bypass clean-up line on each cooling loop between the outlet of the spent fuel pit pump and downstream from the spent fuel pit heat exchanger is used to send a small bypass flow to the spent fuel pit demineralizers and filters which clean up the flow. In addition to these cooling and clean-up loops, a separate loop consisting of skimmer pumps and skimmer filters cleans up the surface water of the spent fuel pit.

The spent fuel pit cooling and clean-up system removes the decay heat from the spent fuel assemblies which have already been stored in the spent fuel pit(s), and it also removes the decay heat from the spent fuel assemblies which are being transferred from the reactor core to the pit(s) during the plant refueling operation, and cleans up the pit water, when necessary. Finally, this system is also used to clean up the refueling water storage tank water.

2. Functions

The spent fuel pit cooling and clean-up system carry out two functions. The first is to remove decay heat from spent fuel assemblies stored in the spent fuel pit Hie second is to clean up spent fuel pit water.

(2) Spent fuel pit heat exchangers

The two spent fuel pit heat exchangers are a conventional shell and U-tube type. To remove the decay heat generated in spent fuel, pit water flows through tubes of the heat exchangers, while the component cooling water flows through their shell sides. The total design capacity of the spent fuel pit heat exchangers is based on an assumption that one full core load of fuel assemblies is unloaded into the fuel pit, in addition to the 1/3 the core fuel assemblies already stored in the pit. Under these conditions, the two spent fuel pit heat exchangers must be able to maintain the pit water temperature below approximately 52°C .

4. Fuel Handling System

A fuel handling system is used for fuel handling and storage operations, from the off-loading of new fuel assemblies to the shipping of spent fuel out of the plant (cf. Section 3.2.2 (4)).

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Component cooling water

Spent fuel pit pump

Figure 3.8.5 Spent fuel pit cooling and clean-up system flow diagram

8. Radioactive Waste Disposal System

A radioactive waste disposal system (WDS) has functions to properly and safely treat and storage radioactive wastes generated during the plant operation, keeping the amount of radioactivity released to the environment as low as practically achievable. The main types of wastes generated in PWR plants are listed below.

• Gaseous wastes containing radioactive noble gases, such as krypton and xenon, originated from the degassing operation of the reactor coolant in the volume control tank.

• Liquid wastes containing fission products and radioactive corrosion products, such as the letdown water transferred to the WDS from the reactor coolant system to adjust the boron concentration in the reactor coolant, various equipment drains, etc.

•Solid wastes, containing radioactive materials of the same level radioactivity as the liquid wastes, such as evaporator concentrates generated in the reactor coolant clean-up operations and the liquid waste disposal system operations, and spent resins, spent filters, etc.

Hiese radioactive wastes are collected and stored in tanks, and processed by a waste gas treatment system, a liquid waste treatment system, or a solid waste treatment system, and then they are allowed to decay, or they are separated or concentrated, dependently on their nature, before being reused, stored or released.

1. Gaseous Waste Disposal System

(1) System composition and functions

The, gaseous waste disposal system treats waste gases according to their nature, stores the treated gases in waste gas decay tanks to allow their radioactivities to decay for given periods, and then after their radioactivities have decayed, releases them to the environment from the plant ventilation exhaust stack with monitoring of their radioactivities. Due to the different chemical natures of hydrogen and nitrogen waste gases, the gaseous waste disposal system is divided into a nitrogen waste gas treatment system and a hydrogen waste gas treatment system. A schematic flow diagram of a typical gaseous waste disposal system is shown in

Figure 3.9.1.

The nitrogen and hydrogen waste gas treatment systems are described in the next subsections.

1. Nitrogen waste gas treatment system

The waste nitrogen gas containing small amounts of radioactive fission gases such as krypton and xenon is originated from several processes including the degassing process of reactor coolant in the volume control tank during plant cold shutdown operations, the cover gas discharging process from various tanks as their water levels increase, and the gas venting process from various components. The waste gas mixture is compressed and stored in waste gas decay tanks for a given period of time to allow the radioactive decay of krypton and xenon which are included in the mixture. After the decay of these two gases, the waste gas is discharged to the atmosphere through the plant ventilation exhaust stack.

2. Hydrogen waste gas treatment system

Much of the gas to be treated by the waste hydrogen gas treatment system originates from the degassing of reactor coolant discharged to the volume control tank. Depending on the plant design, this waste gas mixture, mainly composed of hydrogen, is treated by either a hydrogen re-combiner unit or a hydrogen separator unit installed in the hydrogen waste gas treatment system. In the hydrogen re-combiner unit, hydrogen gas in the waste gas reacts with oxygen gas to become water vapor, and the remaining waste gas is stored in hydrogen re-combiner gas decay tanks. In the plants using the hydrogen separator unit, hydrogen gas is separated from the waste gas by palladium alloy membrane cells and reused in the plant, and the remaining gas is stored in hydrogen waste gas decay tanks.

A new waste gas treatment method is being applied to the latest plant designs, and differs from the above methods using gas decay tanks to reduce radioactivities of gases released to the environment The new system employs a charcoal­bed noble gas holdup system, in which the waste gases pass through a charcoal-bed column, where radioactive krypton and xenon gases are selectively adsorbed by charcoal beds, resulting in an effective reduction in radioactivities of waste

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Figure 3.9.1 Gaseous waste disposal system

gases discharged to the environment. The flow diagram of the charcoal-bed waste gas treatment system is shown in Figure 3.9.2.

(2) Components and functions

1. Waste gas decay tanks; hydrogen re-combiner gas decay tanks; hydrogen waste gas decay tanks

The waste gas decay tanks receive and store nitrogen waste gas to allow radioactive gases contained in the gas to decay, or, if possible, store the nitrogen gas until it can be reused. The waste gases are released to the atmosphere after the minimum storage period of 30 days, or reused as the cover gas for recycle holdup tanks in the CVCS. The numbers and the capacities of the waste gas decay tanks are determined based on such parameters as the displaced volumes of cover gases, the storage periods, the different operation modes of discharge or reuse, and the estimated volumes of waste gases generated

during the plant normal operation.

The hydrogen re-combiner gas decay tank stores waste gases separated from hydrogen gas by the hydrogen re-combiner while the hydrogen waste gas decay tank receives and stores waste gases separated from hydrogen gas by the hydrogen separator, for some time during which they decay radio actively. The minimum storage period of gases varies from 10 to 30 years depending on plant designs.

2. Waste gas compressors; hydrogen re-combiner gas compressors; hydrogen waste gas compressors

A waste gas compressor is used to compress and send the nitrogen waste gas containing radioactive fission gases to the waste gas decay tanks, and a hydrogen re-combiner gas compressor, or a hydrogen waste gas compressor is used to compress and send the hydrogen waste gas to the hydrogen re-combiner gas decay tank, or the hydrogen waste gas decay tank.

Purge

8^aS Gas compressor

■ Vent header ^as

TYTYl surge

Equipment vent ^Compressor separator

Figure 3.9.2 Charcoal bed noble gas holdup system

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