CHAPTER 06 final Rev 08.docx

Chapter 6 – Table of contents

List of Figures

Chapter 6 – Record of Revisions

6 ENGINEERED SAFETY FEATURES

6.1 Summary Description

The concept of Engineered Safety Features (ESFs) evolved from the defense-in-depth philosophy of multiple layers of features designed to ensure safe operation of the reactor. ESFs are designed to prevent accidents or mitigate accidents by controlling the release to the environment of radioactive materials resulting from an accident. The ESFs at NBSR include the Emergency Cooling System, the Confinement Building, and the Ventilation Systems. ESFs can be automatically actuated by instrumentation that monitors various parameters during the reactor operation, or be manually actuated by the reactor operator. The principal accidents they protect against are overheating of the core should forced-flow of primary coolant be unavailable, and uncontrolled release of radioactive material to the surrounding environment.

Chapter 13anlayses show that the NBSR Confinement Building reduces to acceptable level the consequences of the most limiting accident scenario. Accordingly, a more extensive containment facility is not required and not included as part of the reactor ESF.

6.1.1 Emergency Cooling System

The Emergency Cooling System, shown in Figure 6.1, can provide cooling for the reactor core and experiments should primary coolant be lost through leakage, for example, from a pipe rupture in the Primary Coolant System. Using conservative assumptions with no operator action, a minimum of 28 minutes of coolant flow is always available to the core from the Inner Reserve Tank (IRT) located within the Reactor Vessel during normal operation of the reactor.

The D2O Emergency Cooling Tank, which is located about 37 feet (11 meters) above the top of the reactor core, acts as an overhead reservoir of emergency coolant. This tank receives a continuous flow of approximately 20 gpm (76 lpm) of primary coolant from the Primary Coolant Purification System. During normal operation, 3,000 gallons (11,350 liters) of heavy water in this tank is available for emergency cooling. This volume is determined by the height of an internal standpipe within the tank. Excess water normally overflows from the D2O Emergency Cooling Tank through this standpipe into the IRT within the reactor vessel. The IRT has a holdup volume of approximately 800 gallons (3,000 liters). During normal operation, the IRT is full and water from the IRT continuously replenishes the reactor vessel. Thus, heavy water flows continually from the D2O Emergency Cooling Tank to the IRT, out of the IRT and returns to the Primary Coolant System during normal operation of the reactor.

Should the water level in the reactor vessel fall below the top of the IRT for any reason, the IRT will start draining though the two nozzles near its bottom into the emergency cooling distribution pan. This flow requires no mechanical action; it results directly from the loss of water in the reactor vessel. Initially, the flow from the IRT is 40 gpm (150 lpm), but decreases as the water level in the tank falls. The tank takes approximately 28 minutes to drain, allowing time to assess the situation and determine which of the alternate flow paths to use to supply emergency coolant to the reactor core. There is sufficient heavy water in the Emergency Cooling Tank and the IRT to provide 2½ hours of cooling on a once-through basis. The IRT may be replenished from the Emergency Cooling Tank by manipulating control valves DWV-32 and DWV-33 (“top-feed”.) The coolant passes over the fuel, removing decay heat, and then drains out at the pipe rupture location. Heavy water can also be drained from the Emergency Cooling Tank to each reactor inlet plenum by manipulating control valves DWV-34 and DWV-35 (“bottom feed”.) With this mode the coolant passes upward through the core and exits through the normal reactor outlet lines, assuming that the pipe break is on the outlet side of the vessel. It then drains out of the system at the pipe break location, onto the floor of the Process Room or the Subpile Room. The Subpile Room has a floor drain to the Emergency Sump (Sump 4). The concrete curb and floor drains in the Process Room direct the heavy water to Sump 4 where it is either pumped to the hot waste system or to the Emergency Cooling Tank or to the D2O Storage Tank located in the Process Room. The storage tank is sized to hold the water inventory of the heavy water systems of the NBSR. While the location of the pipe break governs which path is selected, supplying emergency coolant to the IRT would typically be the initial path chosen.

In severe accidents domestic light water at a rate greater than 25 gpm can be added to the Emergency Cooling Tank through a spool-piece and double manual isolation valves. The availability of domestic light water is known by the absence of a loss of water alarm, the detector location for which is in the supply pipe to the spool-piece.

6.1.2 Confinement Building

The front or east wing of the NCNR consists of offices, cold laboratories, warm laboratories, shops, and other special purpose space, all of which are outside the Confinement Building. The north side of this wing has a cold basement area where various plant spaces for air-conditioning equipment, power distribution gear, demineralizing equipment, emergency power units, and storage are located. On the rear or west wing, the Confinement Building is a three-level structure, 90 feet x 90 feet (28 meter x 28 meter) in size and 600,000 ft3 (19,300 m3) in volume when empty. Both east and west wings are interconnected. The Cold Neutron Guide Hall is located on the north side of the Confinement Building. In the Confinement Building, the thermal neutron beams are accessed on the first floor. The control room and general access to fuel elements and in-core thimbles are from the second floor. The basement contains most of the process systems, fuel element storage pool, and radiochemical laboratories. The stack, guide penetrations, and some secondary equipment are located on the north side of the confinement building.

None of the accidents discussed in Chapter 13 lead to significant overpressure within the reactor building. Thus, the Confinement Building is not required to be a steel containment vessel, but rather a structure designed to meet the more normal building design requirements. The building is designed to withstand wind, snow, and seismic loadings. Internally, it is designed for the large dead weight loading due to the reactor itself and the heavy biological radiation shields. The heavy concrete walls and floors supporting the structural loads also serve as radiation shields.

The ventilation systems allow the Confinement Building atmosphere to be maintained at a slight under-pressure compared to external atmosphere pressure under both normal and emergency conditions, assuring that any leakage is into the Confinement Building rather than out.

The plan view of the first floor of the NBSR Confinement Building at elevation 428 feet (130 meters) is shown in Figure 6.6. The reactor (including biological shielding) is about 20 feet (6 meters) in diameter and is located in the center of the first floor. The biological shield runs up to the ceiling, and supports the inner rail of the annular crane that services the area. The experimental facilities that use the beam tubes and the thermal column are located on this floor, and are serviced by a 15-ton (13,600-kg) annular crane. This floor is at the same level as the main floor of the adjacent laboratories and office space. Personnel access to the Confinement Building from the adjacent building is through either of the two entrances located on the building east wall, from a door on the southwest of the confinement building, or from an entrance through the basement (B-2 level) adjacent to the confinement building. A sliding steel door at each of the three entrances to the Confinement Building closes automatically and associated gaskets inflate. The southwest door is normally closed and the gasket inflated.

The plan view of the second floor of the Confinement Building at elevation 450.4 feet (140 meters) is shown in Figure 6.7. The top of the reactor shielding is flush with the floor, and utility and access trenches under the floor provide access to the radiation facilities that go into the core and into the reflector from the top of the reactor. This area is serviced by a 20-ton (18,000-kg) crane to move heavy equipment throughout most of C200 and between the second floor, first floor, and lowest level of the building through the provided floor openings.

The Control Room is also located on the second floor and looks out over the reactor top. All relevant process and reactor instrumentation readout is located in the Control Room. Thus, reactor systems operation can be monitored from this single location.

Many experimental stations are located within the Confinement Building. Eleven neutron- guides, maintained under vacuum for their full length, supply a number of other experiments in the adjacent Cold Neutron Guide Hall. The guides are centered on the core mid-plane and radiate from the Cold Neutron Source to the experimental stations. For seven of the guides, at the point where each guide passes through the Confinement Building wall there are Neutron Guide Isolation Valves to close off the penetrations in an emergency, whenever a Major Scram signals a closure of the Confinement Building.

The remaining three guide penetrations (two guides share a single penetration) are sealed at the wall with a magnesium window and other engineered seals within C100; valve actuation is unnecessary to meet confinement requirements.

6.1.3 Ventilation Systems

A simplified version of the ventilation systems within the Confinement Building for both normal and emergency conditions is shown in Figure 6.8.

The air supply for the normal ventilation system includes fresh air from outside and recirculated air within the building. There are separate recirculation fans for the three main levels in the Confinement Building. Three exhaust fans move air to the intake of a fan at the base of the building stack, which is the point of release to the environment. Automatic Control Valves (ACV) operate with the fresh air fans, three exhaust fans, and the emergency exhaust fans. Each of the effluent pathways is monitored for radioactivity and the stack has a dedicated radiation monitor.

Fresh air is brought into the building with two fans, one dedicated to the Process Room and the other dedicated to the remainder of the building. The Air-Conditioning (AC) Fresh Air System is a conventional heating and cooling system and supplies conditioned air to the first and second floors, and to the basement laboratories. The first and second floor systems circulate a mixture of fresh and recirculated air, while the basement laboratories system uses fresh air only without recirculation. The Process Room Heating and Ventilating (H&V) Supply System maintains the process equipment area temperature by heating and ventilating with once-through fresh air.

Three separate exhaust systems operate during normal operation of the reactor. The Normal Air Exhaust System takes air from those areas supplied by the AC Fresh Air System and combines with the exhaust air from fume hoods from the basement laboratories. The Process Room Exhaust System draws air from the process equipment area. Finally, the Irradiated Air Exhaust System takes air from potentially contaminated areas. Air from each of these systems passes through similarly designed High Efficiency Particulate Air (HEPA) type filtering systems. The air is then released through the stack after being appropriately diluted and monitored for an acceptable level of radioactivity.

The locations of the exhaust ductwork ports are chosen to control air flow within the building experimental areas. Most air from the first and second floor areas is drawn into exhaust ducts embedded in the reactor biological shield, thereby controlling contamination levels within the building, since potential sources of contamination are closest to the reactor.

Under emergency conditions, the air interior to the Confinement Building can be recirculated after being filtered through a system consisting of both a HEPA and a charcoal filtering system. The Emergency Exhaust System would start and is designed to draw air at such a rate from the building that a pressure differential can be established across the building structure to assure that any leakage is into the building rather than out regardless of likely outside pressure variations due to wind or barometer changes. It consists of two redundant subsystems; each draws air from Normal Air Exhaust System ductwork. Air exhausted from the building in each subsystem passes through a filtering system consisting of both HEPA and charcoal filters before releasing through the stack.

HEPA filters are designed to remove particulate matter of a specified diameter. Charcoal or carbon filters are designed to remove radioactive iodine gas. Reduction of dose to building occupants and to the public is the function of the filters in an emergency situation that may have radioactive particulate material and fission gases as a hazard.

6.2 Detailed Description

6.2.1 Emergency Cooling System

The objective of the Emergency Cooling System is to provide an adequate amount of primary coolant to the reactor core to prevent exceeding the blister temperature of fuel elements, should primary coolant be lost through a pipe rupture in the Primary Coolant System.

6.2.1.1 Description of Components

6.2.1.1.1 D2O Emergency Cooling Tank

The D2O Emergency Cooling Tank is an aluminum cylindrical reservoir with a capacity of 3,300 gallons (12,500 liters). The tank normally contains 3,000 gallons (11,350 liters) because of an internal standpipe that limits overflow to the reactor vessel. It is equipped with two additional internal standpipes; one that overflows at 3,150 gallons (11,900 liters) to prevent overfilling and one at the 2,000-gallon (7,600-liter) level. Although the 2,000-gallon standpipe allows up to 1,000 gallons (3,800 liters) of D2O for emergency cooling to experiments, its isolation valves are gagged shut to reserve the normal full volume of the D2O Emergency Cooling Tank for the core cooling. The Helium Sweep Gas System sweeps this tank to remove any decomposed gases.

6.2.1.1.2 Inner Reserve Tank

A vertical cross section of the Reactor Vessel is shown in Figure 6.2. The Inner Reserve Tank (upper D2O reserve tank) is located within the Reactor Vessel above the core upper grid plate. It is made of concentric cylinders of approximately 6 feet 10 inches (210 cm) tall and 4 feet (120 cm) in diameter. There are five 5-inch (13-cm) diameter vertical thimble port penetrations, five top vent holes which go to the reactor vessel, one 5-inch (13-cm) diameter penetration for the reactor vessel overflow pipe, one 2½ inch (6.5 cm) diameter D2O inlet penetration, one 2½ inch (6.5 cm) helium inlet penetration, and four guide holes. Two nozzles which penetrate the inner cylindrical wall can drain the tank by directing the flow into the emergency cooling distribution pan as shown in Figure 6.3 and in Figure 6.4. Holes in the distribution piping are located near each element position and the cooling water flows through these holes.

The mounting brackets of the upper grid plate support the Inner Reserve Tank (IRT). Its four legs, each resting on one bracket, are bolted in place by one bolt passing through each leg into the mounting bracket.

6.2.1.1.3Lower D2O Reserve PanThe D2O Holdup Pan (Lower D2O Holdup Pan) extends upward from below the lower grid plate to above the lower fueled portions of the elements, as shown in Figure 6.5. The end fittings of the fuel elements and any other tubes inserted into the lower grid plate are conical to minimize leakage of water down through the fuel element seats. If the vessel water levels falls below the upper core, the pan keeps the lower core submerged in heavy water, for an unknown period of time, and also collects any of the heavy water from the IRT which splashes over the top of the distribution pan or runs down the outside of the fuel elements. Due to the unpredictability of the amount of leakage and length of time there is coolant in the pan it is not considered as part of the Emergency Cooling System (Section 6.1.1) and credit is not taken in any LOCA analysis.

6.2.1.1.3 Pumps

The Emergency Sump Pump is powered from MCC B-6. This pump is required to be operable for normal operation of the reactor. Primary coolant collected in the Emergency Sump (sump 4) within the curb area north of the subpile room may be pumped to the D2O Emergency Cooling Tank.

The Sump Pump to Hot Waste is powered from MCC A-5. This pump would be normally used to pump the accumulated liquid waste collected in the sump to liquid waste facilities.

Each of these pumps can be controlled from the Main Control Panel in the Control Room.

6.2.1.1.4 Control Valves

The Reactor Operator controls all valves discussed in this section from the Main Control Panel in the Control Room. All valves are pneumatic-operated diaphragm valves and, with the exception of DWV-40 and DWV-13, are normally closed. The electrical power for the solenoids is provided from panel DCP2, located in the Control Room.

Emergency Cooling to IRT valves DWV-32 and DWV-33 control the flow of emergency coolant to the IRT.

Emergency Cooling to Plenums valves DWV-34 and DWV-35 control the flow of emergency coolant to the Inner and the Outer Plenums.

D2O Experimental Emergency Cooling Valves DWV-29 and DWV-30, once enabled, can regulate the flow of emergency coolant to the D2O Experimental Cooling System.

Sump Pit to D2O Emergency Cooling Tank Valve DWV-20 controls the flow of heavy water from Sump 4 to the D2O Emergency Cooling Tank.

Sump Pit to D2O Storage Tank Valve DWV-21 controls the flow of heavy water from Sump 4 to the D2O Storage Tank.

D2O Emergency Cooling Tank Drain Valve DWV-13 controls the flow of heavy water to the D2O Storage Tank from the 3150 standpipe in the D2O Emergency Cooling Tank.

D2O can be returned to the D2O Emergency Cooling Tank: from the D2O Storage Tank through HE-2 and DWV-40; from the D2O Storage Tank through DWV-12; and from the sump through DWV-20 and DWV-40.

The design requirements for the piping and valves are the same as those specified for the Primary Coolant System discussed in Chapter 5, Reactor Coolant Systems.

6.2.1.1.5 Instrumentation

Instrumentation is provided to sense D2O Emergency Cooling Tank water level, emergency cooling water flow to the reactor vessel, sump level, and the discharge pressure of the emergency cooling sump pump.

Emergency Cooling Tank Level Indicator Alarm Channel LIA-2 measures the level of the emergency coolant in the D2O Emergency Cooling Tank. Level Transmitter, LT-2, sends a signal to Level Indicator, LI-2, and Level Alarm, LA-2, which is proportional to the level of emergency coolant in the D2O Emergency Cooling Tank. The alarm unit alerts the Reactor Operator to a low level condition in the tank (50”). The channel range is 0-60 inches (0-152 cm). Sump level is detected at three levels and alarms are provided in the Control Room for each level. Pump discharge pressure is detected and indicated locally, normally with a bourdon tube gage.

6.2.2 Confinement Building

The objectives of the Confinement Building and its associated Heating, Ventilation and Air Conditioning (HVAC) systems are to prevent the uncontrolled release of radioactivity to the surrounding environment.

The Confinement Building is designed to contain radioactive material released in an accident, e.g. iodine from a failed fuel element, so that it can be exhausted in a controlled manner through an Emergency Exhaust System that filters out radioactive materials before releasing the confinement air to the environment. The leakage through the building is limited to a leak rate of 24 cfm per inch (0.27 meter3 per minute per cm) of pressure differential across the confinement walls, ceiling, and slab. Release of radioactive material at a specified concentration inside the Confinement Building is detected in the normal exhaust systems by redundant instrumentation, which initiates closure of the building.

Closure of the building is achieved through closure of passageways and building penetrations that are open to the interior of the building. Each of the first floor eastern entrances consists of two sets of double doors separated by a short passageway and the entrance from the basement has a single set of double doors. A sliding steel door is located to the side of the innermost doorway for each passageway. During normal operations, this sliding door is fully open. Ingress into the Confinement Building can be controlled locally or remotely. Egress from the building is unrestricted. After a Major Scram signal is generated (see Chapter 7), door scram relays (DSR) are energized and start electric motors for each steel sliding door. These motors are powered from MCCA-5 and MCCB-6. The sliding steel door is driven closed by the motor, a limit switch near the door frame is engaged, turning the motor off and opening a solenoid valve to supply air to a gasket installed around the doorway. The gasket inflates to seal against the interior surface of the door. There is also a local override switch on the each side of the wall of the building near each door that will open the doors regardless of the presence of a major scram signal. If the major scram signal is present and the local control is used to open the door, the door will return to the inflated-seal condition shortly after the local control is used. Only after clearing the major scram condition can operation of the local controls return the doors to their normal condition.

Penetrations for normal ventilation, cold water, hot water, natural gas, and liquid waste are closed off with automatic valves that respond to a major scram signal.

6.2.2.1 Confinement Building Leakage Rate

6.2.2.1.1 Design Leakage Rate

Confinement, as opposed to containment, is a concept suitable to the situation wherein the worst hypothetical reactor incident results in negligible overpressure, as is the case for the NBSR. Here, the design concept is to confine or retain radioactive gases so that they can be filtered and passed up the stack at a reasonably low rate for subsequent atmospheric dispersion. The tighter the building, the lower is the airflow rate through the filter and up the stack.

6.2.2.1.2 Leakage Rate Tests

The building tightness specifications can be monitored by two different methods. The first method involves the measurement of the relaxation time from test differential pressure conditions (simulating an overpressure and under-pressure conditions), the value of which is over 64 minutes. The second method involves the measurement of the flow rate resulting from the same overpressure or under-pressure conditions, at less than 24 cfm per inch (0.27 meter3 per minute per cm) of water differential pressure across the wall. The second method is normally used as part of the surveillance tests to verify the tightness of the building. The Confinement Building components that isolate associated systems are shut and the building is pressurized to +6.0 and -2.0 inches (+16 and -5 cm) of H2O using a dedicated blower. Measurements are taken at each test pressure. The blower flow rate necessary to maintain the building at the test pressure is equivalent to the leakage rate through the building structure.

6.2.2.1.3 Inspection of Penetrations

Independent of or in conjunction with leakage rate tests, building penetrations that may affect the confinement integrity are inspected and tested for tightness. Door seals or other gaskets are periodically inspected for seal integrity.

6.2.3 Ventilation Systems

6.2.3.1 Ventilation System under Normal Conditions

6.2.3.1.1 Supply and Recirculation Systems

The two systems supplying conditioned air during normal operation are the Air-Conditioning (AC) Fresh Air System and the Process Room Heating and Ventilating (H&V) Supply System.

Fresh air is brought into the mezzanine equipment area through louvers in an airway in the south wall of the Confinement Building. The AC Fresh Air System uses supply fan SF-2 to draw fresh air from the intake via ACV-1. ACV-1 closes off the fresh air intakes in the event of a high radiation level within the confinement building ventilation system.

SF-2 directs air to the independent recirculating system for C100, the independent recirculating system for C200, the independent recirculating system for the basement laboratories, and the elevator machinery room. A relief valve, ACV-12, is in place to prevent excessive under-pressure relative to outside atmospheric pressure, i.e. protects the building structure against a large differential pressure across the building structure.

The Process Room H&V Supply System uses fan SF-11 to draw air from the aforementioned airway and through ACV-2 and direct the air to the Process Room. ACV-2 is the confinement isolation valve for the H&V system. The Process Room is maintained at a negative pressure relative to the rest of the confinement building through control of system air flow. The system flow is controlled by the speeds for SF-11 and exhaust fan EF-27, the speeds specified by a controller and a switch/timer on the Process Room Control Panel located on the B-1 level. The settings for the switch/timer are occupied mode and unoccupied mode. The occupied setting provides for three times the volume of fresh air than the flow for the unoccupied system setting, a measure that creates a more hospitable working environment.

6.2.3.1.2 Exhaust Systems

The exhaust systems for the Confinement Building during normal reactor operation consist of the Process Room Exhaust System, the Normal Air Exhaust System, and the Irradiated Air Exhaust System. The Process Room Exhaust System uses exhaust fan EF-27 to draw air from the Process Room and from the elevator machinery room. The exhaust air passes through two banks of HEPA filters and a holdup chamber. The volume of the holdup chamber is chosen to give time for the automatic closure valve ACV-3 to close before the air can flow the distance between a radiation detector that initiates the closure signal and the valve. The air is sent to the dilution chamber at the base of the stack and the chamber has a fresh air intake and contains exhaust fan EF-2. EF-2 blows the diluted air to the atmosphere via the stack.

The Normal Air Exhaust System uses exhaust fan EF-3 to draw air from the first and second floors, storage pool area, counting room, and radiological laboratories. A damper on the fan automatically adjusts the rate to maintain a pressure of -0.10 inches (-0.25 cm) of water in the building relative to that in the high-bay area (outside static pressure) on the first floor. Filter banks clean all effluent air driven by EF-3. The discharge of EF-3 combines with discharge air from EF-23, the fan which draws air from fume hoods in the radiological laboratories. The combined air flow passes through a holdup chamber, the volume of which allows time for valve ACV-7 to automatically close before the air can flow the distance between a radiation detector that initiates the closure signal and the valve. The exhaust air is sent to the dilution chamber at the base of the stack for release to the atmosphere by EF-2.

Irradiated Air Exhaust System fan EF-4 to draws air from all beam port shutter cavities in the biological shield around the reactor, along with irradiated air from various process systems, e.g. Process Room sumps. The system is designed to ensure that all leakage of air is into beam port cavities rather than out. Any potentially irradiated or contaminated air is directly exhausted rather than being recirculated. All irradiated air passes through a separate set of filters identical to those in the Normal Air Exhaust System. A holdup chamber is located between EF-4 and ACV-6. The air is then discharged to the dilution chamber and the suction of EF-2.

6.2.3.2 Ventilation System under Accident Conditions

6.2.3.2.1 Emergency Recirculation System

During emergency operation, the reactor building internal air can be recirculated and filtered by the Emergency Recirculation System. This system is normally off and can be operated from the Control Room or the Emergency Control Station, described in section 6.2.3.3. Air can be circulated by supply fan SF-19 in the second floor, the first floor, and the Process Room after ACV-11 is opened, at a rate of approximately 5,000 cfm (140 meter3 per minute). Process Room circulation is initiated only if the SF-19 control switch in the Control Room is selected to process room position. The circulating air passes through HEPA filters and charcoal filters before returning to the first floor, second floor, and the Process Room.

6.2.3.2.2 Emergency Exhaust System

The Emergency Exhaust System is designed to draw air from the building at such a rate as to establish a pressure differential across the confinement barrier, assuring that any leakage is into the building rather than out, regardless of likely variations in outside pressure due to wind or barometric changes. The system has two redundant trains to give maximum assurance of its operation and can be controlled from the Control Room or the Emergency Control Station on the B2 level outside the Confinement Building.

Each of the two redundant trains (or subsystems A and B) in the Emergency Exhaust System contains an exhaust fan and identical filters and controls. Either subsystem can draw 100 cfm (3 meter3 per minute) of air from the Normal Air Exhaust System ductwork. The two subsystems are isolated from each other by valves ACV-4, ACV-8, ACV-5, and ACV-9. These are the suction and discharge valves for Emergency Exhaust Fans EF-5 and EF-6; these valves are positioned through contacts operating in their respective fan-starter circuits. Hence, the operation of a fan places the entire subsystem in service. Since the Process Room does not exhaust to the Normal Air Exhaust System ductwork, during emergencies 35 cfm (1 meter3 per minute) of air can be ventilated by manually operating ACV-10, a special connection to the Process Room.

Fan motors are supplied from MCCA-5 or MCCB-6, with each fan coupled to an AC motor and a DC motor. The DC motors are normally configured for automatic operation and run only when AC power is not available. One AC fan motor is normally set for automatic operation, while the second AC fan motor is set for standby operation.

Upon closure of the Confinement Building, all automatic closure doors and valves shut, and associated door gaskets inflate. With AC power available, both subsystems will begin expelling air from the building until a building pressure of -0.10 inch (-0.25 cm) H2O is reached (relative to the pressure at the supply louver for the AC Fresh Air System.). At this pressure, the standby sub-system ceases continuous operation, while the subsystem in automatic continues operating until reaching a pressure of -0.25 inch (-0.64 cm) H2O. The automatic fan then cycles as necessary to maintain this pressure. Should the pressure return to approximately -0.10 inch (-0.25 cm) the standby fan will start cyclical operation to assist the now continuously running automatic fan in lowering the pressure. If AC power is lost, both subsystems automatically cycle to reach an approximate pressure of -0.25 inch (-0.64 cm) H2O. This mode of operation should minimize cycling of the motors, which will reduce the electrical load on the station battery.

Subsystem A and B each contain a charcoal filter and three HEPA filters. These filters clean all air exhausted from the building by fans EF-5 and EF-6. A dust filter downstream of the charcoal filters removes any carbon particulate that is released from the charcoal filters. Since one of the two trains is in operation during an emergency, 100 cfm (3 meter3 per minute) of clean air is released to the atmosphere directly through the stack.

6.2.3.3 Description of Components and System Controls

6.2.3.3.1 Emergency Control Station

This Station has an emergency panel containing the controls for all four fans in the emergency system, namely, SF-19, EF-5, and EF-6 (including both AC and DC controls for EF-5 and EF-6), the controls for valves ACV-10, -11, and -12, and indicators for valve positions of all valves ACV-1 through ACV-12. The building differential pressure can be monitored at this panel and there is flow rate indicator for exhaust air through the Emergency Exhaust System. Two area radiation monitors in this panel show radiation levels at the east wall of C100 and the east wall of the Process Room. Finally, indications for effluent radiation monitors for Irradiated Air, Normal Air, and stack exhaust are available.

6.2.3.3.2 Confinement Building Under-pressure Protection

The vacuum relief valve ACV-12 opens at -2.5” H2O.

6.2.3.3.3 Filter Description

Several types of filters are installed in the reactor building. High efficiency particulate filters are used in all reactor building exhaust systems to prevent particulate effluent from reaching the reactor stack and being discharged. Each filter is at least 99% efficient for a particulate size of 0.3 microns. The activated charcoal filters consist of at least two banks of filters in series, each of which consists of multiple perforated members, arranged in parallel to form a bed, one-inch (2.54-cm) thick, of granular activated charcoal. The filters at the rated flow have a leakage rate of no more than 1%. The filters were designed to remove greater than 99% of the Iodine from air at approximately 80°F (27°C).

6.2.3.3.4 Exhaust System and Stack

Dilution Exhaust Fan EF-2 is located in Room D02, which serves as the dilution chamber and suction plenum for the fan. This room is located at the base of the exhaust stack. EF-2 discharges air directly into the base of the “A” section of the Exhaust Stack. A velocity controller is designed to keep the discharge rate of air through the stack at a constant 30,000 cfm (850 meter3 per minute) regardless of the seasonal variations external to the Confinement Building. The controller accomplishes this by varying the position of automatic dampers in the fan room to add dilution air from the exterior of the building to the air exhausted from the Confinement Building. Exhaust Fans, EF-5 and EF-6, discharge directly into the exhaust stack.

The principal parameter of the stack is its height to facilitate dilution. Since under normal operation, 41Ar activity is produced within the confines of air spaces in the reactor system, it was deemed necessary to elevate the point of release so that the exhaust air would be appreciably diluted via atmospheric dispersion before it entered the unrestricted area surrounding the facility. The height of approximately 100 feet (30 meters) above grade level. The stack is a dual one, one side of which exhausts the Confinement Building, while the other side exhausts the Warm Laboratories.

6.2.3.3.5 Ventilation Fans

The operator normally remotely controls from the Control Room all of the ventilation fans discussed below. The controller for each normal ventilation fan and the emergency exhaust fans have redundant contacts operated by the Fans Scram Relays (FSR), the Door Scram Relays (DSR), or both. The redundancy ensures the normal ventilation is secured and the emergency ventilation starts upon the receipt of a major scram signal. The supply fans and EF-27 are powered from MCCA-3 or MCCB-4. The exhaust fans and SF-19 are powered from MCCA-5 or MCCB-6. EF-5 and EF 6 are powered from MCC DC. All fans and associated ACV are on the C01 mezzanine level, unless otherwise noted.

The controllers for fresh air fans SF-2 and SF-11 power ACV-1 and ACV-2, respectively.

SF-3 is the recirculation fan for C100.

SF-12 is the recirculation fan for C001, C-002, C-003, the B3 hallway, and C-004.

SF-1 is the recirculation fan for C200.

EF-2 is the C wing and warm lab wing exhaust fan to the building exterior and is located on the D01 level.

EF-3 is the C wing exhaust fan, excepting the Process Room and the EF-4 exhaust areas, and the fan controller supplies the power for ACV-7.

EF-4 draws from potentially contaminated and activated air areas and the fan controller supplies the power for ACV-6, which is located in C001.

Two separately powered motors drive EF-5. One is a DC motor and the other is an AC motor. The control circuitry prevents both motors from being energized simultaneously. The fan controller supplies the power for ACV-4, ACV-8, and ACV-10.

Two separately powered motors drive EF-6. The control circuitry prevents both motors from being energized simultaneously. The fan controller powers ACV-5 and ACV-9.

EF-23 is the common exhaust fan for the C001 and C002 hoods and has a common discharge with EF-3.

EF-27 draws primarily from the Process Room and the fan controller powers ACV-3.

SF-19 is the emergency recirculation fan.

6.3 References

American National Standards Institute/American Nuclear Society, (1977) ANSI/ANS-15.7, Research Reactor Site Evaluation. ANS: LaGrange Park, Illinois.

Atomic Safety and Licensing Appeal Board (May 18, 1972). In the Matter of Trustees of Columbia University in the City of New York.

Carew, J., Cheng, L., Hanson, A., Xu, J., Rorer, D., and Diamond, D., (2004). Physics and Safety Analysis for the NIST Research Reactor. BNL-NIST-0803, Brookhaven National Laboratory: Upton, New York.

NBSR 9 – Final Safety Analysis Report on the National Bureau Of Standards Reactor.

NBSR 9A – Supplement A of the Final Safety Analysis Report on the National Bureau Of Standards Reactor, October 1966.

NBSR 9B – Supplement B of the Final Safety Analysis Report on the National Bureau Of Standards Reactor, December 1966.

URS Group, Inc (March 2003). Geology, Seismology, Geotechnical Engineering, and Hydrology of the NIST Research Reactor Site. Gaithersburg, Maryland.

6-1

Figure 6.1 Emergency Cooling Water

Figure 6.2 Reactor Vessel Internals

Figure 6.3 Inner Reserve Tank and Emergency Cooling Distribution Pan

Figure 6.4 Emergency Cooling Distribution Pan

Figure 6.5 Holdup Pan and Welded Penetration

Figure 6.6 Confinement Building – First Floor

Figure 6.7 Confinement Building – Second Floor

Figure 6.8 Normal and Emergency Ventilation

Figure 6.9 Irradiated Air System image1.png image2.png image3.jpeg image4.jpeg image5.jpeg image6.png image7.png image8.png image9.png