Auxiliary Systems K.S. Rajan Professor, School of Chemical & Biotechnology SASTRA University

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1 Auxiliary Systems K.S. Rajan Professor, School of Chemical & Biotechnology SASTRA University Joint Initiative of IITs and IISc Funded by MHRD Page 1 of 6

2 Table of Contents 1 VENTILATION SYSTEM ANNULUS GAS SYSTEM PROCESS WATER SYSTEMS FIRE WATER SYSTEMS REFERENCES/ADDITIONAL READING... 6 Joint Initiative of IITs and IISc Funded by MHRD Page 2 of 6

3 In this lecture, we shall discuss the auxiliary systems like ventilation, annulus gas system, process and fire water systems of a typical pressurized heavy water reactor. At the end of this lecture, the learners will be able to (i) (ii) (iii) (iv) list the components of ventilation system list the role and desired characteristics of annulus gas understand the working of annulus gas system understand the role of process water and fire water systems 1 Ventilation system The ventilation system in a nuclear power plant has important role during both the normal operating condition and in emergency situations. A typical ventilation system consists of ducts that provide flow path for air, and the dampers that control flow rate of air passing through the ducts. Chillers, fan cooler units and air conditioning units cool the air passing through the duct. Fans ensure circulation of air while filters remove impurities in air. Thus the important components of ventilation system are (i) Containment Emergency Cooling Systems: This system removes heat from the containment in emergency circumstances during which containment gets heated. The hot air during postulated accident is drawn by a fan and directed towards coils. These coils cool the hot air using water. The cooled air is directed towards the top of the containment to cool the containment. (ii) Filtration Systems: Fans draw air from the regions that are contaminated. This is passed through a filtration system consisting of PAC filters. P refers to the particulate filters that are responsible for removal of larger particulate matter in the contaminated air. A refers to the absolute filters capable of removing particles with more than 99.5 % efficiency. C refers to the charcoal filters in which the radioactive iodine is adsorbed on the surface of charcoal. The air after passing through PAC filters is directed to exhaust stack. (iii) Pressure Differential Systems: This system maintains appropriate pressure difference between containment and reactor building & between reactor building and turbine building. The pressure in the reactor building/auxiliary building is higher than that in the containment. This ensures that there are no leakages from containment to the reactor building/auxiliary building. Similarly, pressure in the turbine building is greater than that in the reactor building. These pressures are maintained by controlling Joint Initiative of IITs and IISc Funded by MHRD Page 3 of 6

4 the flow rate of air entering and leaving each building through control of appropriate fans. (iv) Cooling systems comprising air conditioning units and fans 2 Annulus Gas System Annulus gas is used in pressurized heavy water reactors (CANDU reactors). The annulus gas passes through the annular region between the coolant and pressure tubes in the calandria. The annulus gas must have low thermal conductivity and be less prone to corrosion. Irradiation of annulus gas should preferably not produce radioactive nucleotides. The low thermal conductivity for annulus gas is required to minimize the heat loss from the coolant in the coolant tubes to the moderator surrounding the pressure tubes. CO 2 is preferred as annular gas due to its lower thermal conductivity when compared to air and nitrogen, which were used earlier as annulus gas. Annular gas system is designed to (i) detect the leakage of heavy water from pressure tubes or coolant tubes into the annular region (ii) to provide an assessment of leakage rate of heavy water (iii) to identify the source of leakage and (iv) to locate the source of leakage with minimal exposure of hazardous environment to plant personnel. This system maintains Deuterium level below 0.1 vol % in annulus gas. The use of CO 2 as annulus gas is not without problems. The issues of chemical reaction and polymer formation still exist with CO 2 as annulus gas. Deuterium dissociates from heavy water due to radiation. Deuterium then enters the annulus gas through diffusion via stainless steel plugs at the end of the tube. Deuterium reacts with CO 2 producing D 2 O and CO. The presence of D 2 O in annulus gas increases the dew point over a period of time. Measurement of dew point of annulus gas is performed to ascertain the leakage of heavy water. However, heavy water formed due to reaction between deuterium and CO 2 may increase the dew point. This interferes with dew point measurement meant for detection of heavy water leakage. Hence the sensitivity of dew point meters for detection of heavy water leakage is compromised. The presence of CO in annulus gas due to reaction of CO 2 with deuterium leads to the formation of several organic compounds that constrict the flow passage for annulus gas and hence reduce the flow of annulus gas. This is another factor that compromises the detection of heavy water leakages. Transmutation reactions with carbon represent another challenge. 14 C is the major isotope produced which requires radiological protection to be ensured to personnel Joint Initiative of IITs and IISc Funded by MHRD Page 4 of 6

5 during maintenances. Gamma radiation monitors are installed to alert plant personnel in the event of high radiation levels. 3 Process water systems Process water systems of PHWR comprise two systems: (i) Active Process Water System and (ii) Process Water Cooling System. The Active Process Water System (APWS) acts as a barrier between radiation producing heat sources and the Process Water Cooling System (PWCS). APWS is a part of secondary and intermediate cooling system related to the plant s safety. Among several components and systems cooled by APWS, the important systems are (i) (ii) (iii) moderator system safety related equipment in reactor and service building pumps for shutdown cooling system, moderator pumps, ECCS heat exchangers, containment coolers etc. The Process Water Cooling System (PWCS) is a system that is directly connected to Ultimate Heat Sink (UHS). A large source of water such as lake, river or sea may be used as USH. In case, the plant is located away from a large water source, air may be used as ultimate heat sink to which the water would reject heat in a cooling tower. The primary objective of PWCS is to remove heat from APWS through process water heat exchangers. Apart from this, cooling to diesel generators, compressors etc. are also provided by this system. 4 Fire water systems This system is designed with the primary objective of providing water meant for fire fighting. However, if required, the system may also be used as backup cooling for minimizing the effects of certain design basis events, which include station blackout, and failure of Active Process Water System or Process Water Cooling System. To facilitate the use of fire water system for purposes other than fire fighting stated above, this system is provided with dedicated pumps driven by diesel generators. During station blackout, these pumps supply water to steam generators. Joint Initiative of IITs and IISc Funded by MHRD Page 5 of 6

6 5 References/Additional Reading S.S. Balaji, A.R. Gore, The Indian PHWR, Nuclear Engineering and Design, 236 (7-8) (2006), pp Joint Initiative of IITs and IISc Funded by MHRD Page 6 of 6

Comparison of characteristics of fast and thermal reactors, Role of fast reactors in Indian Nuclear Programme

Comparison of characteristics of fast and thermal reactors, Role of fast reactors in Indian Nuclear Programme K.S. Rajan Professor, School of Chemical & Biotechnology SASTRA University Joint Initiative