CHAPTER 09 final Rev 10.docx

CHAPTER 9 – TABLE OF CONTENTS

List of Figures

Chapter 9 – Record of Revisions

9 auxiliary systems

9.1 Handling and Storage of Reactor Fuel

9.1.1 Storage of Unirradiated Fuel

Unirradiated fuel elements may be stored in the reactor core and the fuel storage area, subject to the NBSR Physical Security Plan. Temporary storage of fuel is permitted. The principal issues associated with the storage of unirradiated fuel are those of security and inadvertent criticality. The former is addressed in the NBSR Physical Security Plan. Prevention of inadvertent criticality is ensured by proper design and use of each storage location. Specifically, fuel is placed into the reactor vessel in accordance with written procedures that require a core condition incompatible with criticality without deliberate additional actions by reactor operators. The fuel elements in the fuel storage area are stored in such a way that the physical geometry of the area prevents inadvertent criticality.

A criticality monitor is installed in the storage area in compliance with 10 CFR 70.24(a)(2) [2014]. A second monitor is installed to provide redundancy.

9.1.2 Storage Locations of Irradiated Fuel

Irradiated fuel may be stored in the reactor core and the Spent Fuel Storage Pool. Temporary storage of irradiated fuel is permitted.

Security for fuel storage is addressed in the NBSR Physical Security Plan.

9.1.3 Spent Fuel Storage Pool

The water volume of the storage area when empty is approximately 33,000 gallons (125,000 liters) of light water (H2O). Facilities are provided in the pool to receive and store irradiated fuel assemblies, load irradiated fuel assemblies in casks for shipment, cut non-fueled sections from irradiated fuel elements, and store these sections prior to disposal. The east end of the pool has an area reserved for the cutting of fuel elements and for shipping cask activities. The storage of irradiated fuel is at west end of the pool.

As fuel is discharged from the reactor to the pool, the elements are placed in storage racks designed to hold fuel elements. The racks are made of borated aluminum and the materials and physical geometry of the racks preclude achieving a keff of greater than 0.9.

For shipping, the fueled sections are cut out of the full fuel element to yield two sections per fuel element of about 13" (33 cm) each in length. These 13” (33 cm) sections are stored in the same racks as full length elements.

The room containing the spent fuel storage pool is equipped with an area radiation monitor that alarms both locally and in the control room in the event of an excessive radiation level. Tools and materials for handling irradiated fuel are stored in this room.

9.1.4 Defueling and Refueling

The NBSR fuel-handling system provides for the removal of fuel elements from the vessel, rearrangement of the fuel elements within the core, and insertion of fuel elements from C200 to the core or transfer of elements from the storage pool to the core. The reflector volume above the core is used to raise a fuel element out of the core, move the element horizontally, and then lower the element to a selected core position or to a transfer chute outside the core. From the chute, the fuel element can be lowered into a receiving mechanism anchored in a canal that is considered part of and leads to the fuel storage pool.

There are three phases of the routine refueling procedure; first, the spent fuel elements are removed from the core and transferred to the spent fuel pool; second, the remaining elements are rearranged as desired in the core; and thirdly, the new fuel elements are loaded into the core.

Removal of a fuel element from the core is delayed until the decay heat produced by the fuel element is insufficient to damage the cladding of the fuel. To remove an element from the vessel, the heavy water level in the vessel is lowered below the top of the transfer chute, the chute drained, and the element is lowered into the transfer chute and directly into a receiving mechanism extended from the storage pool. The transfer chute is secured and the remaining elements may be re-arranged with the water level below the transfer chute or with the water level above the transfer chute. Finally, the vessel water level is increased, which in conjunction with the design of the lower outer plug, minimizes tritiated water vapor release to the C200 area during fresh fuel element insertion through an opening in the plug. After passing through the plug opening, the fuel element is placed on a transfer mechanism directly below the opening, preparing the element for insertion into the core. Later sections provide detailed descriptions of fuel movements.

9.1.4.1 Transfer Mechanism

The transfer mechanism consists of a set of pickup tools and transfer arms, which penetrate the inner top shielding plug. The tools and transfer arms are hand operated. There is a pickup tool over each fuel element, and transfer arms are located such that every possible fuel element position can be reached by at least one transfer arm. Figures 9.2.a – 9.2.d show the interaction between transfer arm, pickup tool, and fuel element head, and provide scale for the same.

Wherever a transfer arm location might interfere with an experimental thimble, it is designed to rotate on a cylinder around the experiment. In this way all fuel element transfers can be made without interfering with any future vertical in-core experimental facilities. The following features provide the tool operator with tool and element position information:

· A plate with penetrations matching those of the inner top plug and mounted above the plug, serves as an index for pickup tool and transfer arm positions.

· The elevation of the pickup tool head, and so the elevation of the element in the core upper grid, is determined through the use of a bar gage deployed between the index plate and a cylinder fastened to the tool shaft above the index plate.

· The position of the transfer arm is shown by index plate marks to which the tool operator aligns a mark on the cylinder.

· There is a mechanical interlock between the shaft cylinder and the index plate. The interlock ensures the latching bar of the fuel element head is fully latched or fully unlatched prior to moving the element in a vertical direction. Vertical movement of the tool and rotation of the tool must be distinct from each other to ensure this design function. The interlock also assures the proper orientation of the fuel element rectangular dimensions to the upper grid plate rectangular penetrations prior to lowering the element into the core. No distinction is made by the system between the possible orientations 180° apart.

A typical transfer procedure is as follows. A pickup tool is lowered until it engages the head of the fuel element. It is then rotated under slight pressure until it slips over large pins (lead pin extensions) in the fuel element head. The tool is rotated about 45° at which point the pins will be above the J-slot in the tool. Then, it is pushed down and rotated clockwise an additional 45°. The two distinct motions disengage the fuel element latch bar from the top grid plate and allow the element to be lifted as the pickup tool is withdrawn. As the end of the pickup tool nears the inner top plug, it enters a mechanical maze, which allows the tool to be accurately located and supported while a transfer arm is rotated into place. When the arm is in place, the element is lowered into it, and rotated counter-clockwise until small pins (roll pin extensions) in the fuel element pickup head shaft engage grooves in the transfer arm upper surface, preventing the element head from rotating as the pickup tool is rotated further counter-clockwise, approximately 45°, to disengage it from the element. The tool is then raised fully into the inner top plug. The shoulder of the element head also sits in a cupped area of the transfer arm when the element is suspended from the arm, further securing the element to the transfer arm.

The transfer arm can then be rotated to a different position where another pickup tool can engage the element and lift it from the transfer arm, to allow the now empty arm to be rotated away. If the element is at a transfer position above the core, it can be placed in another transfer arm and moved to another spot. In this way, the element can be moved to any desired location or placed over the transfer chute for removal. Once in the desired location, the element is lowered into that grid position with the pickup tool. The element is locked in the top grid plate by pressing down and rotating the pickup tool counter-clockwise approximately 90°.

9.1.4.2 Transfer Chute and Telescoping Cylinder

This portion of the transfer system consists of assorted valves and a pivoting hydraulic telescoping cylinder called the refueling cannon, because of its shape and horizontal-to-vertical pivoting capability.

The transfer chute has connections for helium and drains. While decay heat will dry the element, the helium is available to expedite the removal of D2O from the element while it is in the transfer chute. The drain connections are explained below. Fresh D2O is drained after it has been used to flush as much tritiated D2O as possible from the element and the chute valves, thus minimizing the tritium concentration in the pool. Three remote operated valves in series separate the D2O/He atmosphere of the reactor vessel from the H2O in the pool canal and pool. A telescoping cylinder is the receiving mechanism, which is on a pivot, so it can swing both horizontally and vertically. A small hydraulic cylinder that is supplied with H2O from the Demineralized Water Experimental Cooling System controls this motion of the transfer cylinder.

In the normal transfer procedure, the D2O level is lowered below the top of the transfer chute. This lower level is achieved by opening a fuel transfer overflow line, the top of which drains the D2O in the reactor vessel to the desired level of approximately 70" (178 cm), preventing vessel D2O from flowing into the pool. Then, the D2O remaining in the transfer chute is drained to the D2O storage tank and the middle valve (FTV-2) of the transfer chute is opened. It is sometimes desirable to soak or flush the fuel element with fresh D2O to minimize tritium addition from the D2O to the pool H2O, and so fresh D2O is added to the dry chute and then drained prior to opening FTV-2. After sufficient time has been allowed for thorough draining, the bottom valve (FTV-3) is opened and the telescoping cylinder is raised so its receiving end is at an elevation above that of FTV-3. The upper valve (FTV-1) is opened just prior to the element being lowered from its position above the transfer chute. The fuel element is then lowered until the element nozzle engages the matching receiver atop the telescoping cylinder. The element is released in the same manner as when releasing an element to a transfer arm, and the pickup tool is withdrawn to a position above FTV-1. Since the element may be at a temperature above the boiling point of water, it will be necessary to close FTV-1 or FTV-2 to prevent any H2O steam that is generated by the hot element in the canal from contaminating the main D2O system. As soon as the valve is closed, the element is lowered into the canal by retracting the telescoping cylinder. When the cylinder is completely retracted within the shell of the cannon, the cannon/cylinder/element is tipped to a horizontal position and the cylinder/element extended so the element can be reached by conventional handling tools from the pool and placed in storage.

If necessary, it is also possible to return an element to the reactor by reversing the above procedure, as follows. Using the telescoping cylinder, the element is inserted into the transfer chute between valves FTV-2 and FTV-3 with FTV-1 closed. The element remains in this position for a few minutes to evaporate the H2O on its surface. Then, the top valve is opened and a pickup tool is lowered to lift the element above FTV-1.

9.1.4.3 Fresh Fuel Insertion to the Reactor Vessel

Fresh fuel elements are inserted through the lower outer plug of the reactor. The shield plug in the transfer chute is removed, after raising the vessel level approximately 6” (15.2 cm) above the normal water level, and a fresh element is inserted through the shield plug hole onto the transfer arm and placed in the core in the usual fashion.

The lower outer plug is designed to minimize the amount of tritiated water vapor diffusing up through the 5" (12.7 cm) diameter hole during the brief time it is open; a skirt, or cylinder, projects below the D2O surface from the bottom of the plug so only a very small surface area is exposed to the hole. In this way only a few cubic inches of D2O saturated helium is available to the hole instead of the whole gas volume over the surface of the vessel water. The presence of this cylinder also minimizes contamination of the helium cover gas by air.

9.1.4.4 Fuel Handling and Safety

Spent fuel handling after reactor shutdown may not occur until a specific period of time has passed. Experiments, measurements, and calculations have confirmed fuel cladding integrity is maintained for a fuel element in air if the element remains submerged for that specific period of time prior to removing it from the water. While compliance with the applicable technical specification ensures that the temperature of the fuel element cladding will not exceed the blister temperature, helium and water are also available to cool a fuel element in the transfer chute.

9.2 Storage Pool Cooling System

Figure 9.1 depicts the Fuel Storage Pool Cooling System. This system removes the decay heat from the spent fuel in the storage pool and provides a means of maintaining the water clarity and purity by removing any particulate matter that may be present. Demineralized water is circulated through the system by one of two storage pool circulating pumps. One pump is normally operated with the other in standby. The running pump takes its suction from a collection basin and provides 75 gpm (4.7 liters/sec) to the storage pool heat exchanger (HE-8). The storage pool water inlet temperature to HE-8 is approximately 72 °F (22 °C) and the outlet temperature is about 50 °F (10 °C). This can vary depending on the temperature of the chilled cooling water entering the secondary side of heat exchanger. The outlet flow is split with 65 gpm (4.1 liters/sec) returning directly to the storage pool and the other 10 gpm (0.6 liters/sec) sent through a cleanup loop, consisting of a booster pump, pre-filter, ion exchanger, and after-filter. This water is then returned to the storage pool. Priming of the pumps prior to pump startup to ensure adequate suction head for each pump is accomplished by filling each circulating pump volute from a demineralized water source from the water treatment system.

Reactor fuel elements and other reactor related components are manipulated and stored in the storage pool. Spent fuel and other irradiated material movements are conducted underwater to afford adequate shielding of personnel. All spent fuel remains in the pool until it is shipped off site. Measurements made without circulating pump flow have shown an inventory of spent fuel equal to 6 years of normal operation cannot generate sufficient heat to raise the pool water temperature above 100 °F (38 °C).

The storage pool contains demineralized H2O from the water treatment system. Water losses from the pool are automatically made up by a storage pool level control system. This level instrument senses the storage pool collection basin level and sends a signal to open valve WTV-l if the level is low, which in turn controls the makeup flow of demineralized water from the water treatment system. The same level instrument will send a signal to shut WTV-l on a high level indication. A float type level instrument in the collection basin acts as a backup to ensure closure of WTV-l.

9.2.1 Component Description

The storage pool is 18'2" (5.5 m) deep, 20' (6.1 m) long, and 10' (3 m) wide. When filled to a depth of 18 feet and with no submerged equipment or fuel, the pool and canal contain approximately 33,000 gallons (125,000 liters) of water. The canal extends from northeast quadrant of the pool to the sub-pile room for the purpose of passing spent fuel from the reactor vessel dropout chute to the storage pool. A collection basin is located at the other end of the storage pool, which provides a net positive suction head to the circulating pumps.

Two identical 75 gpm (4.7 liters/sec) centrifugal pumps provide for the circulation of the storage pool water. Both pumps are controlled from the control room by means of hand-operated switches. One pump is normally running and the other is in standby. Should the running pump trip, the standby pump will start automatically.

HE-8 is of plate-and-frame construction, and has a flow pattern that is single pass and counter-flow. The heat exchanger transfers heat from the storage pool to the NIST chilled water system.

There is one 10 gpm (0.6 liters/sec) centrifugal booster pump that circulates the storage pool water through the purification section of the system. The pump is normally running and manually controlled from the control room.

Two cellulose fiber cartridge filters are installed at the inlet and outlet of the ion exchanger. The filter housing and metal internals are made of 304 stainless steel.

One mixed bed ion exchanger maintains the purity and clarity of the storage pool water. This ion exchanger uses an HOH resin. The vessel is made of 304 stainless steel.

The storage pool piping system components are made of aluminum or stainless steel, and all materials are compatible with demineralized water.

9.2.2 Instrumentation

Storage pool system instrumentation provides the capability to monitor storage pool water level, collection basin water level, and conductivity of storage pool water. A low water level in the storage pool will alarm in the control room.

9.2.3 Design Considerations

The storage pool cooling system is designed to remove the heat generated by two full core loadings of spent fuel and an additional number of spent fuel sections that would exist from element cutting operations. Spent fuel is not placed into the pool until the cladding temperature of the fuel in air will remain below the blister temperature for the cladding. The temperature of the pool water is maintained at approximately 50 °F (10 °C).

Other than use of an ion exchanger resin, chemistry control of the pool water is passive; without the introduction of a large volume of a contaminant, the surface area of the pool and the materials of the stored materials are conducive to maintaining water conductivity favorable to the pool materials and objects stored in the pool. Impurities are removed from the water to provide the required optical clarity for manipulation of spent fuel or refueling operations. Since some of these impurities would be radioactive, their removal will reduce the radiation levels at the the pool. The filters remove suspended solids and the system operates with a turbidity not to exceed 2 ppm and a conductivity equivalent to a pH of 6 to 8. Principle metal components in contact with the pool water are made of stainless steel and aluminum.

Further information on system design may be found in Chapter 3 and through the provided links for components.

9.3 Primary Coolant Purification System

9.3.1 Design Basis and Functional Requirements

The Primary Coolant Purification System is designed to maintain the chemistry and purity of the primary coolant by removing both soluble and insoluble corrosion products and other foreign materials from a portion of the heavy water in the heavy water systems. The purification system assists in the maintenance of the primary coolant conductivity at less than 1 μmho; an approximately equivalent pH would be 5.5. Mechanical filtration of the coolant removes particles of 0.2 mils (5 microns) and larger. Maintaining proper chemistry of the heavy water ensures that the components in contact with the water are not degraded over the life of the D2O systems. Minimizing contaminants in the coolant minimizes the activity of the coolant, thereby decreasing the radiation exposure of the operations personnel.

The system also provides a means of transferring water from the D2O Storage Tank directly to the emergency tank and the reactor vessel, pathways routinely used during reactor shutdown period activities, e.g. increasing vessel level prior to refueling.

9.3.2 System Description

The Primary Coolant Purification System as shown in Figure 9.3 supplies heavy water to the D2O Emergency Cooling Tank and the D2O Experimental Cooling System. The purification system receives water from the reactor vessel overflow and heated water from the D2O Experimental Cooling System.

The D2O Storage Tank, sized to hold the entire plant inventory of heavy water, is the supply of water for the pumps in the purification system. The storage tank receives primary coolant from the aforementioned sources and the coolant collects in the tank sump. D2O Storage Tank Pumps DP-7 and DP-8 supply heavy water to D2O Purification Heat Exchanger HE-2. One pump is normally run, with the second in standby to maintain a flow rate of approximately 35 gpm (132 lpm) in the system.

HE-2 cools the D2O from approximately 100 °F (38 °C) to 90 °F (32°C) to prevent excessive temperatures from damaging the ion exchanger beds and mechanical filters. After being cooled, approximately 15 gpm (57 lpm) of heavy water is diverted through the purification train, consisting of a pre-filter, two ion exchangers and a post-filter, while the remaining 20 gpm (76 lpm) of heavy water bypasses the filter media. These two streams rejoin to supply heavy water to the D2O Emergency Cooling Tank at a flow rate of approximately 20 gpm (76 lpm), and to the D2O Experimental Cooling System at a flow rate of 15 gpm (57 lpm). Heavy water supplied to the Emergency Cooling Tank returns to the reactor vessel.

9.3.4.1 D2O Storage Tank

The D2O Storage Tank is located in a pit below the Process Room floor. The total capacity of the tank is 14,650 gallons (55,500 liters), with a sump capacity of 326 gallons (1,230 liters). The first 50 inches (127 cm) of indicated storage tank level reflects the sump level. During normal operation, the sump is filled with D2O to a level between 35 and 45 inches (89 and 114 cm). The tank is fabricated of aluminum.

The storage tank acts as the reservoir for the purification system. It can receive D2O from the moderator dump line through Moderator Dump Valve DWV-9, and from the fuel-transfer chute through Fuel Transfer Overflow Valve DWV-37. Heavy water from the D2O drain lines and traps located throughout the primary system drains by gravity to the storage tank. In an emergency, the Emergency Sump Pump can return any heavy water collected in the emergency sump to the storage tank.

9.3.4.2 D2O Storage Tank Pumps

The two storage tank pumps are mounted outside the D2O Storage Tank and each pump has a capacity of 75 gpm (285 lpm). Pump DP-7 is powered from Emergency Power MCC A-5, while pump DP-8 is powered from Emergency Power MCC B-6.

The reactor operator can remotely control the pumps from the Main Control Panel located in the Control Room. During normal power operation, one pump is run to supply flow to the purification system, while the second pump remains in standby. The pumps are interlocked with the D2O Purification Heat Exchanger Secondary Cooling Water Control Valve, SCV-12. To ensure greater pressure on the primary side of the heat exchanger than on its secondary side, one of the pumps must be running before SCV-12 will open.

9.3.4.3 D2O Purification Heat Exchanger

HE-2 is a 1.18x106 BTU/hr (0.35 MW) plate-and-frame type heat exchanger that transfers heat from the purification system to the secondary cooling water.

The heat exchanger is located in the Process Room.

9.3.4.4 Ion Exchanger

The IX vessels are designed to operate at 110 psig (0.8 MPa) at 110 °F (43 °C), with a flow of 25 gpm (95 lpm). The ion exchangers are located in the Process Room and the IX are a mixture of hydrogen and hydroxyl resins used for ion exchange of cations and anions. To protect personnel from exposure to any activated material that may become trapped in the resin bed, an engineered shield surrounds the columns.

9.3.4.5 Ion Exchanger Filters

The IX pre-filter and the after-filter are identical units which remove particles 5 microns and larger from the coolant. The after-filter also prevents any resin beads from the ion exchangers entering the Primary Coolant System. To protect personnel from exposure to activated material that may become trapped in the filter medium, lead bricks are stacked around the filter housing as shielding.

9.3.4.6 Valves

Several remotely controlled valves in the purification system piping allow control of the distribution of heavy water by the reactor operator at the Main Control Panel in the Control Room. These valves all have pneumatic positioners. The air to operate them is supplied by the air system, discussed in Section 9.11.

The Reactor Pump-Up Valve, DWV-11, can be used to add heavy water from the storage tank to the vessel through the reactor outlet piping and DWV-134, bypassing the purification system. DWV-11 and DWV-12 can be used to add water from the storage tank to the Emergency Tank, bypassing the purification system. These three valves are 3-inch (7.6-cm), pneumatically positioned valves.

The Bypass Control Valve, DWV-22, controls the amount of bypass around the ion exchangers and filters and is positioned to work in concert with other control valves to allow flows and tank levels throughout the primary system to come to acceptable equilibrium conditions, given any initial state for the heavy water systems.

The D2O Experimental Cooling System Inlet Isolation Valve, DWV-26, is a 2-inch (5-cm), pneumatically positioned valve used to control the flow of purified heavy water supplied by the purification system to the D2O Experimental Cooling System. It is located on the common inlet header to the system pumps.

The D2O Emergency Cooling Tank Inlet Isolation Valve, DWV- 40, is a 2-inch (5-cm), pneumatically positioned valve that controls the flow of purified heavy water supplied by the purification system to the D2O Emergency Cooling Tank. It is located in the tank inlet line.

The D2O Injection Control Valve, DWV-39, is used to inject purified heavy water from the purification system into the common reactor inlet header.

The Purification System to Emergency Sump Pump Discharge Cross-connect Isolation Valve, DWV-20, is a 1½-inch (3.8-cm), pneumatically positioned valve connecting the Emergency Sump Pump discharge line to the purification system discharge header downstream of the IX and filters. This allows heavy water collected in the Emergency Sump during an emergency to be used as a makeup source for the heavy water systems. Heavy water returned through this arrangement bypasses the purification system filters and ion exchangers.

9.3.4.7 Instrumentation

9.3.4.7.1 Flow

An ultrasonic flow element measures the inlet flow of water to HE-2 and the flow element is mounted on the exterior of the piping. An alarm unit alerts the operator to a low-flow condition. Indication is provided in the Control Room and the range of the channel is 0-120 gpm (0-455 lpm).

The flow of heavy water through the purification train is measured by a 1½-inch (3.8-cm) orifice in the piping on the outlet of the after-filter. Indication is provided in the Control Room and an alarm unit alerts the operator to a low-flow condition through the filters and ion exchanger beds. The channel range is 0-30 gpm (0-113 lpm).

9.3.4.7.2 Temperature

The temperature of the heavy water entering and leaving HE-2 is detected with thermocouples mounted in the inlet and outlet piping for the heat exchanger. Indication is provided in the Control Room and an alarm unit alerts the operator when the temperature of the water leaving the heat exchanger is high. The range of each channel is 50-125 °F (10-52 °C).

9.3.4.7.3 Pressure

Differential pressure between the primary- and secondary-sides of HE-2 Is sensed with a pressure transmitter. Indication is provided in the Control Room and an alarm alerts the operator to a low ΔP condition across the heat exchanger. The channel range is 0-50 psid (0-0.35 MPa).

9.3.4.7.4 Level

The level of the heavy water in the D2O Storage Tank Is sensed with a level transmitter and indication is provided in the Control Room. An alarm alerts the operator to an abnormal level. The range of the channel is 0-175 inches (0-445 cm).

9.3.4.7.5 Conductivity

The conductivity of the heavy water at the inlet and outlet of the ion exchangers is sensed with a conductivity cell mounted in the inlet piping for the ion exchanger columns. Indication is provided in the Control Room and alarms alert the operator to a high conductivity level on the inlet and the outlet water. The range for each channel is 0-2 μS.

9.4 D2O Experimental Cooling System

The D2O Experimental Cooling System distributes heavy water from the Primary Coolant Purification System to cool the cryostat for each cold neutron sources and pneumatic sample assemblies RT-1 and RT-2. The system utilizes cooled primary coolant from the discharge of HE-2 and returns the heated water from the heat loads to the D2O Storage Tank.

9.4.1 System Description

Figure 9.4 shows the D2O Experimental Cooling System.

The D2O Experimental Cooling System supplies heavy water at a flow rate of approximately 15 gpm (60 lpm). One of the two D2O Experimental Cooling Pumps can take suction on the purification system piping through valve DWV-26; normally, water discharged directly from a storage tank purification pump and around the D2O Experimental Cooling System pumps is sufficient to cool system heat loads. System piping is located in the Process Room and at the reactor face on the C100.

Backpressure regulating valve DWV-25 controls the experimental supply pressure, thus preventing over-pressure or under-pressure whenever flow is established or secured to a given component that could affect the cooling of the other components supplied by the system. The heated D2O is then returned to the D2O Storage Tank.

9.4.1.1 Heat Sources

The total heat load on the D2O Experimental Cooling System is approximately 1.71x105 BTU/hr (50 kW). Most of this heat load is due to the cryostats. The rabbit assembly tips contribute negligibly to the system heat load.

9.4.1.2 D2O Experimental Cooling Pumps

ECP-1 and -2 are single-stage, centrifugal units with a nominal capacity of 36 gpm (140 lpm). Pump ECP-1 is powered from Emergency Power MCC A-5, while pump ECP-2 is powered from Emergency Power MCC B-6.

The pumps can be remotely operated from the Control Room. Both pumps are normally are off, but the pumps can be configured for one pump in run and one pump in standby. The pumps can also be run together.

9.4.1.3 Valves

Pressure Regulating Valve DWV-25 is a 2-inch (5.08-cm), pneumatically positioned valve used to regulate the pressure at the common discharge header. An over-pressure condition will cause the valve to travel farther in the open direction and decrease the pressure by sending more heavy water to the D2O Storage Tank. An under pressure condition causes the valve to travel farther in the close direction and increase the pressure by sending less water to the storage tank.

9.4.1.4 Instrumentation

Installed instrumentation provides remote read-out of the flows and temperatures at appropriate points in the system. Local pressure gages and temperature indicators are mounted in the piping for local indication where appropriate.

9.4.1.4.1 Flow

Two channels monitor the flow of cooling water from the Unit 2 neutron cold source cryostat and two channels monitor the flow of cooling water from the BT-9 cryostat. Flow is measured by a 1½-inch (3.8-cm) venturi installed in the Unit 2 cooling water outlet piping and flow from the BT-9 cryostat is measured by a venturi in the outlet pipe of the cryostat. Indications are provided in the Control Room and on the display for each cryostat. Alarm units annunciate a low-flow condition in the Control Room and at each display.. The range of each of the Unit 2 channels is 0-25 gpm (0-95 lpm). The range of the BT-9 channels is 0-10 gpm (0-38 lpm).

9.4.1.4.2 Temperature

The temperature of the D2O cooling water entering each cryostat is sensed with a resistance temperature detector (RTD) mounted in a thermowell located in the piping. Indication is provided on the refrigerator displays. The range of the channel is 50-200 °F (10-93 °C).

The temperature of the D2O cooling water leaving each cryostat is sensed with a resistance temperature detector mounted in a thermowell in the piping. Indication is provided in the Control Room and on the refrigerator displays. Each of the four channels has a range of 50-200 °F (10-93 °C).

9.4.1.4.3 Pressure

Suction and discharge pressure of the pumps are indicated locally.

PIC-2 maintains a constant pressure on the discharge of the Experimental Cooling Pumps by varying the position of the Pressure Regulating Valve DWV-25. The pressure setpoint is selected in the Control Room and indication is available in the Control Room. The alarm unit has outputs that supply signals to annunciate a low-pressure condition. The range of the channel is 0-100 psig (0-0.7 MPa).

9.5 Thermal Column Tank Cooling System

9.5.1 General Description

The Thermal Column Tank Cooling System shown in Figure 9.5 provides cooling of the bismuth shield and when filled with heavy water enhances the thermalization of neutrons for use in the graphite thermal column. This cooling system is completely independent of the primary D2O system. The system consists of the main Thermal Column Tank, which is constructed to fit the outside curvature of the reactor vessel and contains an interior tank that is filled with approximately 7,600 pounds (3,447 kg) of bismuth and has a coolant volume of 240 gallons (908 liters). The tank is protected against overpressure by maintaining a volume of water in the system insufficient to fill the tank, the system piping, and the surge tank. Other system components are a surge tank, two pumps, a heat exchanger (HE-9), and a filter system. A flow controller provides both indication and control of the flow in the system. A low flow condition will cause an alarm and a rundown of the reactor shim arms. Coolant enters the thermal column tank and flows upwards and above the top of a bismuth shield. It leaves the tank at the bottom of this shield. The water flows into a surge tank that provides suction to the thermal column pumps. The surge tank has a low level alarm and a low level trip for the pumps that prevents cavitation due to low suction head from decreasing tank level. These pumps move the coolant through the filter system, HE-9, and back to the thermal column tank. The secondary cooling system provides secondary side cooling of HE-9. With normal flows and pressures, there is a 1" (2.4 cm) gas space between the top of the bismuth shield and the top of the thermal column tank. A small vacuum is maintained above the tank to minimize loss of coolant through a tank flaw on the front tank face. A helium bottle maintains a constant helium sweep of the system. Currently the system uses demineralized light water as coolant.

9.6 Thermal Shield Cooling System

9.6.1 General Description

The Thermal Shield Cooling System removes the heat generated by neutron and gamma interactions in the thermal shield. The heat is transferred from the circulating thermal shield water to the secondary system cooling water through a dedicated plate and frame heat exchanger in the Process Room. A circulating pump draws hot water from a storage tank, pumps the water through the heat exchanger and then to two supply headers from which cold water is delivered to embedded cooling tubes in the shield. The water in the tubes removes the heat from the shield and the water flows to two return headers, which discharge to the storage tank.

Supply header pressure for the majority of the cooling tubes is less than 0” of water and this low pressure is accomplished through the use of eductors and associated valves. Maintaining a vacuum at the inlet of the tubes ensures the cooling water drains through the tubes resulting in a minimum pressure differential across each tube wall, thus minimizing leakage out of the tubes through flaws in the tube walls. In addition to maintaining a minimum internal tube pressure, a positive pressure is maintained on the exterior of a majority of the tubes with pressurized CO2 provided to the areas adjacent to the tubes. The CO2 is supplied by the cavity purge system, described in Section 9.10.

It is important to keep leakage from the tubes to a minimum, because water leakage must be treated as radioactive waste product, with the accompanying disposal problems. Controlling water chemistry reduces the degradation of the cooling tubes and the activation of contaminants in the circulating water. CO2 leaks into the tubes and assists in keeping the pH of the water at the desired level. Chemicals are also added as necessary to maintain the quality of the water. Through chemistry controls and mechanical filtration, as many contaminants as possible are prevented from circulating through the high flux areas of the thermal shield, which reduces the radiation levels near thermal shield cooling system components that cannot be shielded. Reducing radiation levels is consistent with the radiation safety principles of the NBSR.

9.6.2 Component Description

As shown in Figure 9.6, one circulating pump circulates water through the heat exchanger and the cooling tubes at approximately 220 gpm. The hot water from the tubes joins other thermal shield cooling system flows and returns to the storage tank. A second pump is installed as a backup and is normally in a standby condition.

A single educator pump provides a combined water flow of 75 gpm to two eductors in the system. Eductors, or jet pumps, use fluid flow through a shaped pipe section to create a Venturi effect and lower the pressure in the suction of the eductor. Fluid is then drawn into the suction and joins the fluid flow from the inlet to the educator and is discharged to the system from the outlet of the eductor. One eductor has a suction point on the supply header for the cooling tubes in the upper section of the shield. The other eductor has a suction point for the supply header for the cooling tubes in the lower section of the shield, the lower section having much fewer tubes than the upper section. A second eductor pump serves as a backup and is normally in a standby condition. The flow from both eductors joins the hot water flows from the return headers.

HE-6 is a plate-and-frame heat exchanger that transfers heat from the thermal shield cooling water to the secondary cooling water. A temperature controller automatically throttles the amount of incoming secondary water through the heat exchanger to achieve a set outlet temperature, resulting in a differential thermal shield water temperature across the heat exchanger of approximately 7°.

The storage tank is constructed of copper and was modified in 2012. The tank is vented to the Irradiated Air System.

Filters and strainers provide mechanical filtration of insoluble material, reducing C100 radiation levels and reducing fouling of system components.

9.6.3 Instrumentation and Control

Primary instrumentation in this system includes the measurement of temperature, flow, pressure, and pH. A control interface of digital design is located in the Control Room and the interface allows operators to monitor system parameters and make changes from the Control Room to designated system components, e.g. control valves. A second control interface of greater capability is located outside of the Control Room in the Process Automation Controller (PAC) room and is intended for troubleshooting and software changes.

9.7 Experimental Demineralized Water System

The Experimental Demineralized Water System, shown in Drawing 60-006, provides low and high pressure demineralized water in the process room, C100 ring header area, C200 reactor top trench (lines are capped), and as a backup to the refueling system. The system consists of one air powered diaphragm pump. Pump suction is drawn from the water treatment system. Discharge pressure is control by a compressed air regulator.

The previous experimental demineralized water cooling system utilized a storage tank. The tank is now abandoned in place along with its level indication instrumentation. The storage tank has a capacity of 2,400 gallons (9,084 liters). The storage tank is vented to the irradiated air system.

9.8 Helium Sweep and Helium Supply System

A cover gas is used to prevent a degradation of heavy water isotopic purity and to minimize diffusion of tritiated heavy water vapor from the coolant systems to the Confinement Building atmosphere. The Helium Sweep Gas System shown in Figure 9.8 provides an inert helium atmosphere over all vessels and tanks that normally contain heavy water (D2O). These tanks are the reactor vessel, the D2O storage tank, the emergency cooling tank, and the purge tank. The system also has a recombiner to augment reactor recombination of any disassociated D2O, the products of which might be present in gaseous form, thereby minimizing the presence of heavy hydrogen in the helium system.

One of two helium blowers circulates helium at approximately 20 cfm (0.6 m3/min) through a chiller and a section of recombiner inlet line wrapped with 110 VAC tape heaters that goes into the recombiner. Heavy water vapor formed in the recombiner condenses and drains into the D2O storage tank. Dry helium is then distributed to various components of the D2O systems. A helium gasholder is provided to maintain a constant pressure on the system. Makeup is provided by six bulk helium tanks located outside, to the southwest of the confinement building. Two bottle banks consisting of four cylinders each serve as a backup supply. An installed cold trap condenser, using liquid nitrogen, is available as a heat sink for purging operations, thus recovering any D2O vapors.

The thermal column system also uses helium as a cover gas, but its cover gas system is independent of the main helium system. A helium gas bottle located in the process room supplies this cover gas.

9.8.1 Component Description

The helium chiller is constructed of aluminum plate and NIST site chilled water supplies the cooling medium that removes moisture from the helium gas as the helium temperature is lowered. After sufficient moisture has been removed, tape heaters are used on the inlet pipe to pre-heat the incoming helium.

The helium blowers are pumps that use a circular rotor within a D2O filled elliptical case. D2O is supplied from the D2O purification system. The blowers are electrically interlocked with a flow switch that prevents blower operation without this D2O supply. These blowers can pump 20 cfm (9.4 liters/sec) of helium at a discharge pressure of 5 inches of water.

The helium recombiner is a cylindrical vessel made of aluminum plate filled with alumina-palladium pellets. These pellets are held in place with mesh wire cloth basket and recombination of heavy hydrogen and oxygen occurs within the pellet bed. The recombiner has been shown during initial testing of the reactor to be unnecessary for maintaining deuterium gas at concentrations of no greater than 4%. The reactor recombines D2 and oxygen that have undergone radiolytic disassociation in the reactor. The recombiner is in service as an additional measure for controlling the concentration of deuterium gas in the helium sweep system.

The cold trap is a cylindrical vessel containing1-inch diameter tubes filled with liquid nitrogen when in use as a cold trap. Sweep gas enters from the side, passes across the tubes, and exits from the bottom. A carbon filter made of activated charcoal enclosed in an aluminum tank is used when samples are drawn from the cold trap.

The gasholder is a 6061 aluminum cylinder, inside of which is a traveling piston. The piston moves up and down, which moves an attached diaphragm to accommodate volume changes in the system. The piston has a rubber fabric seal to prevent helium leakage. The piston is weighted to obtain the desired pressure on the system. Mechanical linkage connects the piston to a pointer and micro-switch arrangement for level indication, high and low volume alarm, and operation of a relief valve. In addition, there is a backup relief valve that is mechanically connected to the gasholder diaphragm.

9.8.2 Instrumentation and Control

Instrumentation is provided to indicate the volume of gas contained in the gasholder and helium flow in the system. Temperature controls and indicators for heater and recombiner internal and outlet temperature are also provided. There is indication of recombiner inlet/outlet pressure on the Control Room console, and indicators of inlet pressure of the helium blowers, bulk tank pressure, bulk tank supply header pressure, and bottle bank header pressure.

9.9 CO2 Purge System

9.9.1 General Description

The CO2 Gas System (Figure 9.9) exists to minimize the amount of air coming into contact with the neutron flux produced by the NBSR. The minimization of this contact is necessary due to the small amount of argon in air. Although argon is slightly less than one percent of air, when contacted with neutrons Ar40 readily becomes Ar41. Ar41is radioactive with a half-life of 1.83 hours and emits both a strong beta and gamma particles. The prevention of this contact is made by establishing a purge of the voids with neutron flux with CO2.

There are four voids that must be purged of air. The largest is the cavity between the reactor vessel and the thermal shield. The second is the large thimble that houses the Unit 2 Cold Source. The third cavities are the rabbit tube thimbles and the associated blower, hoses, etc. that allow the rabbits to be sent and retrieved from the core. The fourth is no longer used. It is the flexo-rabbit system that sends rabbits to RT-5 and transfers rabbits from C-002 to the warm labs.

The CO2 gas is supplied from a bulk storage tank located in the South Yard. CO2 leaves the storage tank at ~250 psig and is routed to the A-wing basement. In the A-wing basement pressure is reduced to ~17 psig and then feed into two lines. One line runs to B-200 and it supplies all the needs of the flexo-rabbit system.

The other line of the tee is teed once again on B-2. These two lines go into confinement on B-1. One line is reduced to ~ 8” water pressure and supplies CO2 to a gasholder that supplies the blower for the rabbit system. The other line of the tee is teed once again with one line supplying the reactor vessel cavity purge and the other the cryogenic cavity purge. In both cases the pressure is reduced to ~5” water pressure and each purge has a separate flowmeter to set the rate of the purge.

When rabbits are being shot the make-up to the CO2 gasholder is placed in Auto. A recorder has been programmed to maintain the gasholder level between 10” and 30”. When rabbits are not being shot the make-up to the gasholder is turned off to conserve CO2 usage.

9.9.2 Component Description

The bulk storage tank supplies the CO2 gas for this system. When full the tank holds 26 tons of liquid CO2. The CO2 is at ~250 psig and ~ -10oF. As CO2 gas is removed from the tank the vaporization of the gas cools the liquid slowing the formation of CO2 gas, so heaters are installed that cycle as necessary to keep…