CHAPTER 04 final Rev 11.docx

CHAPTER 4 – TABLE OF CONTENTS

List of Tables

List of Figures

Figure 4.2.5: Reserved

Figure 4.2.9: Reserved Figure 4.2.10 Reserved ……………………………………………………………………………

Chapter 4 – Record of Revisions

4 REACTOR DESCRIPTION

4.1 Summary Description

The NBSR is a heavy water (D2O) moderated and cooled, enriched fuel, tank-type reactor designed to operate at a thermal power level of 20 MW. The core is immersed in heavy water to thermalize fast neutrons to sustain the nuclear chain reaction, remove heat created by the reaction and serve as the first stage of shielding. Major reactor parameters are provided in Table 4.1.1.

All construction materials used in the reactor tank are either aluminum or stainless steel, with stainless steel present in only a few components. Aluminum and stainless steel are chemically compatible with the heavy water coolant and exhibit excellent resistance to corrosion and erosion. Aluminum has low induced radioactivity and is resistant to radiation damage.

The core is located in the lower section of an aluminum tank. Thirty-seven fuel element locations in addition to four semi-permanent irradiation thimble tubes are provided. Seven of the fuel element locations are specially adapted for thimble tubes, leaving only thirty positions available for fuel element assemblies. The fuel element is a MTR plate type element consisting of U3O8 mixed with aluminum powder contained in aluminum clad plates. Each fuel element contains an upper and lower fuel section separated by a gap resulting in a split core design. This “split-core” design, with uranium fuel placed above and below the mid-plane of the reactor using heavy water moderation, results in the thermal neutron flux reaching a peak in the center of the gap.

The large volume in the core provides very flexible capabilities for thermal neutron irradiation. Insertion of nine radial beam tubes and the cold neutron source large diameter vessel penetration into the gap allows high intensity beams, with low “background” from unwanted fast neutrons and gamma rays, to be extracted for thermal neutron scattering research. A pneumatic rabbit system provides researchers with the ability to automatically inject samples into the core region of the reactor while thimbles provide for their manual loading.

Routine operation of the reactor uses forced circulation of the primary coolant. Analysis has shown that natural convection cooling is adequate for operation of the NBSR at a power level of 500 kW, but power is administratively limited to 10 kW. Operations up to 10 kW without forced flow are permitted for any length of time since the heat generated by the core is insufficient to cause significant heating of the reactor coolant.

The Inner Reserve Tank ensures an adequate cooling water supply to the core in the event of a major rupture of the subpile piping. The inner reserve tank, located in the top reflector, can only be drained through two non-isolable pipes at the bottom of the tank. These pipes feed a distribution pan which routes emergency cooling water to the individual elements in the core. Lower D2O Reserve Panis located in the lower section of the reactor vessel, and extends upward from below the lower grid plate to a level above the lower fueled portions of the elements. This Pan helps stabilize the Beam tubes and provides some delay to the loss of coolant to the outside of the lower fueled section of the fuel elements in the case of one type of LOCA. The effect it has on maintaining cooling has not been relied on in the analyses of the LOCA scenarios and therefore not considered as an essential component of the Emergency Cooling System.

4.2 Reactor Core

The reactor core is located in the lower section of an aluminum tank, 7 feet (2.13 m) in diameter by 16 feet (4.87 m) in height. The fuel elements are held in place by upper and lower grid plates. The grid plates provide for 37 fuel element positions and four 2.5 inch (6.35 cm) semi-permanent irradiation thimbles. Seven of the fuel element locations are especially adapted for 3.5 inch (8.89 cm) experimental thimbles, leaving 30 positions for fuel element assemblies. The 37 positions are placed in 7 rows to form a hexagonal pattern with the rows oriented east to west in the core. The fuel is contained in three rings within this hexagonal pattern, with the inner two rings having six fuel elements each and the outer ring having the remaining 18 fuel elements. The 7 experimental thimble positions form a circular pattern about the center location. The fuel element assemblies are located on 6.9 inch (17.5 cm) centers in the NBSR core. Each of the thirty fuel elements fits into a unit cell; the cell locations are fixed by openings in the grid plates. Figures 4.2.1 and 4.2.2 provide cross-sectional views of the reactor, including the core.

Control of the reactor is achieved by four semaphore-type shim safety arms and one automatic regulating rod. Primary control of the reactor is accomplished by use of the four shim arms. They are used to attain criticality on start up, and may be used to make major changes in the power level of the reactor and compensate for reactivity changes that occur as a result of xenon, temperature, and fuel burnup. Fine control of the reactor is accomplished by the use of the regulating rod. The four-shim arms are mounted on hanger brackets just under the upper grid plate. The regulating rod is located in a 3.5-inch (8.89 cm) vertical thimble. The locations of the shim arms and the regulating rod are shown in Figure 4.2.1.

Normally, sufficient photoneutrons are available for startup. After extended shutdowns, the strength of this neutron source may be insufficient to provide indication on the nuclear instrument channels and for reactor startup. On these occasions, a nominal 2-curie Americium-Beryllium neutron source is inserted into the core region to provide sufficient source neutrons for reactor startup.

Heat generated by fission is removed from the core by means of the primary coolant system. Coolant enters through a plenum at the bottom of the fuel, passes up through the fuel and into the reactor vessel, and then out through two outlet pipes in the bottom of the vessel. The inner six fuel positions and the central thimble are fed by one plenum while the remaining fuel and thimbles are fed by a concentric plenum. The primary coolant passes out of the reactor and flows through pumps and plate-type heat exchangers before returning to the reactor vessel, in a closed loop. The helium blanket system keeps a small pressure of helium of about 4 inches of water (1 kPa) on the reactor and allows for the recovery of any D2O lost from the system due to evaporation. A detailed description of the primary coolant system and the helium sweep system are given in Sections 5.2 and 9.5 respectively.

4.2.1 Reactor Fuel

At the present, the NBSR reactor utilizes only the Materials Testing Reactor (MTR) plate-type fuel element. No plans exist for the use of another fuel element design, other than one which substitutes low enriched uranium for highly enriched uranium, a design not yet proven.

4.2.1.1 Fuel Plate

The nuclear fuel is a U3O8 plus aluminum powder dispersion fuel, enriched to approximately 93% 235U. The 235U content of each element is approximately 350 g, distributed equally in the 34 fuel plates that form the fueled section of a fuel element. Each fuel plate is approximately 13 inches in length by 2.7 inches in width by 0.05 inches in thickness (33 cm length by 7 cm width by 0.13 cm thick). The dimensions of the core, or fuel meat, in each plate is 11 inches in length by 2.4 inches in width by 0.02 inch thick (28 cm by 6 cm by 0.05 cm), and the cladding thickness is 0.015 inches (0.0381 cm). Each plate contains about 13 g of U3O8 and 19 g of Al in the 8.9 cm3 available for the fuel meat. The resulting volume fractions are approximately 18% U3O8, 78% Al, and about 4% void. Normally, each fuel plate is annealed prior to assembly of the fuel element and final curving of the plate is accomplished over a die. Adding curvature to the plate minimizes the effects of heating on the mechanical joints that hold the fuel plates in place in the fuel element assembly. No burnable poisons or neutron moderators are added to the fuel elements.

4.2.1.2 Fuel Element Description

A typical fuel element assembly is made up of fuel plates, curved and flat unfueled plates, and upper and lower cast pieces, those pieces serving as the handling and nozzle adapters. Each element has an upper section and a lower section with seventeen fuel plates per section. The two sections are separated by a 7 inch (17.78 cm) gap, i.e. a volume without fuel plates. Each fuel plate has an unfueled region at each end of the plate, which provides additional confidence in the cutting operations employed to remove the assembled fuel sections from the fuel element, described in Chapter 9, prior to shipping the irradiated fuel offsite. The overall length of the fuel element assembly is approximately 69 inches (175 cm).

Support for the fuel plates is provided by two unfueled curved outside plates and two unfueled flat side plates which form a box section for the full length of the assembly between the upper and the lower adapters. The thickness of the two unfueled outside plates is 0.065 inches (0.165 cm) (slightly thicker than a fuel plate). The thickness of a side plate is approximately 0.2 inches (0.5 cm). The side plates have 19 slots 0.095 inches (0.241 cm) deep to receive the 17 fuel plates and two unfueled plates. The fueled and unfueled curved plates are mechanically fastened between the flat side plates by swaged mechanical connections.

The bottom adapter serves as both an inlet nozzle and a check valve. Coolant enters the internal passage of the lower adapter, flows up through the internal conical transition section, through the 18 channels defined by the curved fuel plates and unfueled curved plates of the lower fuel section, into and through the gap, through the 18 channels of the upper fuel section, and then out the upper adapter. A small amount of coolant, 4% of the total volume, bypasses the internal flow during coolant pump operation, preventing bulk stagnation in the moderator. when the force of the water flow is greater than the spring force associated with the latching spring. A 0.012-inch (0.030 cm) gap is opened between the nozzle and the lower grid plate opening. This bypass flow is possible only when the exterior conical section of the lower adapter is lifted from a mating conical seat in the lower grid plate. The fuel element is spring loaded down by a latching mechanism. The lifting force necessary to achieve this bypass flow results from the hydraulic drag of the coolant on the fuel assembly. Should flow cease for any reason, the fuel elements will be forced down on the seats and retain a portion of the bulk coolant to the height of the fuel elements. Thus the bottom adapters act as check valves, allowing the upward bypass flow when the primary coolant pumps are operating, but preventing the draining of the outlet side of the fuel elements if there is no flow.

The upper adapter contains the spring loaded cross bar lock mechanism that locks the fuel elements into the grid plate structure. When the fuel element has been fully inserted through the upper grid plate into the lower grid plate, additional pressure on the handling head will compress the spring bringing the cross bar down inside the upper adapter, to a position just under the upper grid plate. Counter-clockwise rotation of the handling head rotates the cross bar such that the ends of the cross bar project through the side windows of the upper adapter and pass under the bottom surface of the upper grid plate. Release of the downward force allows the spring to pull the cross bar up into small notches in the bottom surface of the upper grid plate, thus locking the fuel assembly between the grid plates.

4.2.1.3 Fabrication

Fabrication of NBSR fuel elements is in accordance with the NIST specification for aluminum clad fuel elements. Prior to insertion into the core, new fuel element assemblies are subjected to stringent quality assurance. The manufacturer inspects the fuel assemblies in accordance with U.S. Department of Energy and NCNR fuel specification requirements.

Each fuel element is assigned a unique serial number. The serial number for the fuel element is engraved on both side plates, one each adjacent to each half of the split fuel core.

4.2.1.4 Development History of MTR Fuel

The enriched uranium fueled plate type element with aluminum for a structural cladding material has a long and trouble-free history in research and test reactor technology. All of the variations in the basic plate type element derive from the MTR design and development work done circa-1950. The MTR commenced operation in 1952. Since then, a variety of reactors using the same general type of element have been built and operated in this country and abroad. The NBSR has been operating since 1967. This basic plate type fuel element, operating at coolant conditions and power densities far more severe than those of the NBSR, has many hundreds of megawatt years of successful operating experience. There have been only two changes of significance to the original NBSR fuel element design: the elimination of unfueled interior plates, and step-wise increases in the U-235 fuel loading (170 g to the current 350 g).

The outer shell of the NBSR fuel element represents the only major variation from the classic MTR plate type fuel element. Since this outer shell controls the establishment of the proper hydraulic regime for heat transfer purposes, confirmation of the structural and hydraulic design objectives was accomplished on a hydraulic stand, using a fuel element assembly fitted with dummy plates. Flow rates of 12 ft/sec - 25 ft/sec (3.7 m/s - 7.6 m/s), which includes the normal operating range, were employed to measure flow conditions in each channel and across typical channels as well as the total pressure drop, drag forces, bypass flow around the lower nozzle, and the vibration characteristics of the spring loaded element lock. The predicted performance of the NBSR fuel element design was confirmed. The primary features of uniform flow delivered to all channels, lack of structural deformation and absence of vibration were all proven. Operating experience with NBSR fuel elements has been excellent. The design and manufacturing of the fuel element assemblies has been proven to be extremely reliable and durable.

The corrosion history of aluminum MTR type fuel elements has been studied extensively. Fuel plates of the same basic configuration and the same material as those used in the NBSR fuel elements have been operated at higher flows, higher temperatures and at much higher heat fluxes than are achieved in the NBSR. All of these factors generally increase the corrosion rate and yet corrosion of the fuel elements during lifetimes comparable to those in the NBSR has not been a problem from the standpoint of structural integrity. No NBSR element has exhibited significant signs of corrosion or symptoms of corrosion damage. The lifetime of the NBSR fuel element is typically one year (burn up limited).

The U3O8-Al dispersion fuels have been in widespread use for over forty years; extensive testing of fuel plates to determine the limits on fission density as a function of fuel loading has been performed. Since the fuel loading is about 10.3 g per plate, the 235U density is

3.0 x 1027 atoms/m3. Assuming all the fuel was consumed, the maximum possible fission density would be 2.6 x 1027 fissions/m3 (14% of the neutrons absorbed produce 236U). With a burnup of 73% in the 8-cycle fuel elements, the typical fission density is 1.9 x 1027 fissions/m3.

Figure 4.2.6 shows the results of several measurements of swelling in fuel plates (Snelgrove, 1994). The curve represents the maximum burnup for a given fuel loading; MTR type plates with U3O8 fuel mixtures below the curve had acceptable levels of swelling. Upon irradiation, some of the fuel meat is transformed into U4O9, and a mixed UAl4-Al2O3 phase. Though fission gas bubbles are observed in the Al2O3, so long as the reacted fuel particles remain largely isolated, as in a moderately loaded dispersion, swelling will be modest and predictable. NBSR fuel is moderately loaded at 18%, and the 8-cycle fission density is well below the curve. In general, dispersion fuels swell at a rate of 3% ± 1% per 1027 fissions/m3, so the expected swelling in the NBSR fuel plates would be between 4% and 9%.

Irradiated fuel plates have also been subjected to high temperatures in order to determine limits for fission product release. Blistering of the U3O8 plates occurs between 842 F and 1022 F (450 ºC and 550 ºC). Since breaking of the fuel particles often precedes blistering, fission gasses are first released through microcracks that develop as blisters form. The maximum allowable fuel clad temperature is therefore 842 F (450 ºC) (Snelgrove, 1994).

4.2.2 Control Rods

The NBSR has two types of control rods. A large change in reactor power is accomplished by use of four semaphore type shim safety arms. A small change in reactor power is accomplished by the use of a regulating rod. A detailed description of reactor control, including reactor shutdown processes, is given in Chapter 7. The location of the shim arms and the regulating rod are shown in Figures 4.2.1 and 4.2.2.

4.2.2.1 Shim Safety Arms

The design of the NBSR shim safety arms is shown in Figures 4.2.7 and 4.2.8. The shim safety arms contain 0.040 inch (0.102 cm) thick cadmium poison sheet clad with aluminum on both sides. Each shim arm is 1 inch thick by 5 inches wide (2.54 cm by 12.7 cm) with a 52 inch (132.1 cm) poison length. The hollow interior of a shim is backfilled with helium as a precaution against damage to the shim from radiation interactions with air. Each shim arm is mounted on hanger brackets just under the grid plate. The shim safety arms are located between the inner five rows of fuel elements with an east to west orientation symmetrical about the center row of the core (as shown in Figure 4.2.2). The drive shafts penetrate the reactor vessel below the water level and drive the shim arms directly. The vessel penetrations are sealed and made leak tight with rotating seals and the drive mechanisms are mounted in recesses in the biological shield. Figure 4.2.8 shows the shim safety arm assembly.

The blade portion of the NBSR shim safety arm is formed using extruded aluminum seamless hollow tubes with an “oval” or “racetrack” cross section which is the ultimate cross section of the shim safety arm. A tube designated to be an outer tube is of greater external and internal dimension than a tube designated as an inner tube. A cadmium sheet is formed into a U shape, the width of each flat surface of the sheet equal to one-half the width of the inner aluminum tube. An aluminum tube collar (used on both ends of the cadmium) is also formed into a U shape. Two sets of the cadmium and aluminum collar are required for the blade assembly with the sandwich cross section.

Two cadmium sheets and two aluminum collars are placed over the inner aluminum tube, the collars preventing the cadmium from moving along the length of the inner aluminum tube during the finishing of the assembled blade section. There is no bond at the longitudinal seam formed by the U-shaped sheets. The outer aluminum tube is pulled over the inner assembly to make the sandwich blade section. The section is then mounted on a draw bench where roughing and finishing dies are drawn through the inside of the section. Small inelastic strains occur which bring the section to its final dimension and more importantly, ensure contact between the cadmium and aluminum. Strict control of the clearances between the inner assembly and the outer aluminum tube results in an acceptable aluminum/cadmium bond after the roughing and finishing dies are drawn through the inside of the section. The void in the blade section is then repeatedly evacuated and purged with helium and seal welded shut. Helium at a pressure just slightly above atmospheric pressure is left in the void.

Each NBSR shim safety arm has an operational travel of 41 and a maximum travel of 50. The full-in position is when the blade centerline is 41 below horizontal. Full retraction of 41 brings the blade to its horizontal position in the top reflector above the core just below the upper grid plate. To prevent over travel during normal operation of the shim arm, installed upper and lower limit switches are set to approximately 41 and 2, respectively. This ensures that the shim arm is not driven over the full range of its travel and into the upper grid plate or the shim arm catcher.

The blades are supported by a hub-unit that rides on two ball bearings. These bearings are mounted in a hanger bracket. Each hanger bracket is inserted into one of two mounting brackets and bolted in place. The mounting brackets are the same ones used to support the grid plate. The hanger brackets are bolted to the reinforcing ring that is welded to the vessel. Beneath these hanger brackets, shim arm guides are welded to the vessel wall to position the shim arms at the proper angle for installation. Shim arm guide extensions, or ‘catchers’, were added to prevent an arm from falling out of the core in the event of a broken shim arm or shaft. The catchers are located just below the down positions of the arms.

The drive systems and shock absorbers are mounted on the biological shield. A stainless steel, splined shaft connects the drive units to the arm assemblies. The drive shafts are inserted into the hub of the shim arms from the side, through the vessel wall. An outer bearing assembly supports the shaft and an inner bearing-seal assembly, both of which are accessible from the shim arm cavities in the biological shield.

The shim arm drive essentially consists of a large compression spring that is compressed by a ball nut and screw jack when the shim arm is raised. The shim arm shaft is connected to the housing that holds the ball nut. As a result, the shim arm is raised or lowered as the nut rides up and down the screw.

The ball screw jack is driven, in turn, by an electric motor, through a high ratio gear case and finally through an electromagnetic clutch. When the arm is raised, the compressed spring is pushing on the very low friction ball nut, attempting to force it back down to its rest position. This would require the screw to turn, but it cannot turn because it is connected through the clutch to the output shaft of the high ratio gearbox. Should the clutch be disengaged, however, the screw is free to turn and the spring will ram the nut, and so the shim arm, back to the full-in position.

The reactor operator manually controls the drive motor. Digital position indication is provided to the operator on the reactor control panel by a potentiometer coupled to the shim arm drive shaft. The power to move the shim arm in or out comes from the Rod Drive Power. Electrical power for the clutches and shim indication comes from Critical Power Panel 1.

The energy of the shim safety arms due to a rapid return or scram is absorbed by a hydraulic shock absorber. This shock absorber is mounted on the biological shield, adjacent to the drive package. A mechanical stop, to prevent over travel of the arm should the shock absorber bottom out, is located on the linkage that connects the shim arm shaft lever to the shock absorber. All impact loads are, therefore, borne by the biological shield.

The shaft connecting the drive package and the shim arm must be sealed where it passes through the vessel wall. This is accomplished with a mechanical bellows type seal. The seal units are tested at no more than 15 psi (1.02 atmospheres). Since the seals are exposed to a maximum operating pressure of 3.5 psi (0.24 atmospheres), no leakage is expected.

The total reactivity worth of four new shim safety arms is approximately 27% Δρ and individual shim worth varies between approximately 6.5% and 8%. The maximum reactivity insertion rate using all four shim safety arms is 5 x 10-4 Δρ/sec.

The lifetime of the shim arms is affected by poison burnup, corrosion and radiation damage. Under normal operating conditions, shim safety arms have a lifetime of approximately 21,000 MW-days. The poison burnup rate is the limiting factor in shim lifetime. The poison in each shim arm consists of a total thickness of 0.080 inches (0.20 cm) of cadmium. The burnup rate during shutdown, when the shim arms are fully inserted, is negligible compared to the burnup rate during operation. In the presence of the shim arms, the flux will fall rapidly with distance above the core so the shim arms will burnup much more rapidly along their bottom edges and tips. Due to the effect of the shim arms, the flux is significant only to the bottom 2 inches (5.08 cm) of the arm. It will take in excess of 950 days at full power operation to reduce the shutdown margin below an acceptable level. Even a very thin section of cadmium is just as black to thermal neutrons as a thick sheet, so the shutdown margin is changed only by complete cadmium burnup in a large fraction of the arm.

The fact that corrosion does not limit the shim arm lifetime is demonstrated by the fact that no limiting corrosion has been observed over forty-five years of operation of the NBSR reactor.

The radiation damage to the shim safety arms is not significant during reactor operation since the shim arms are in the top reflector above the core where the fast neutron flux is relatively low. No radiation damage has been observed on the exterior of any of the twenty-eight shim arms replaced since the reactor first went critical

4.2.2.2 Regulating Rod

The regulating rod consists of a solid aluminum cylinder, 2.5 inches in diameter by 29 inches long (6.35 cm by 74 cm). It is located in a 3.5-inch (8.9 cm) vertical thimble (Figure 4.2.1 and 4.2.2). The rod is driven by a standard commercial design vertical drive mechanism mounted in the top plug. The regulating rod acts as a poison designed with a worth of approximately 0.58% Δρ. Although aluminum is not a poison, its macroscopic absorption cross-section is more than 400 times that of heavy water. Calculations show that displacement of heavy water contributes only 10%-20% of the negative reactivity inserted by the regulating rod. The low absorption cross-section of the aluminum ensures the long life of the absorbing atoms and the spreading out of the aluminum over a large volume minimizes the local thermal flux depression. These features combined with the location of the rod near the center of the reactor cause only a 1%-2% perturbation of the thermal flux at the beam holes as the rod is moved.

The drive train consists of two 2-phase electric servo motors in parallel which drive an extremely accurate lead-screw nut combination. An extension shaft mounted on the nut at one end carries the regulating rod at the other. As the screw revolves, the nut, and thus the regulating rod, moves up or down at a fixed rate. The nominal rate of movement, 29 inches (74 cm) per 15 seconds, is determined by the motor speed and the pitch of the lead screw. Drive power for the two servo motors comes from the flux controller card in the NC-5 channel of the nuclear instrumentation.

The total rod travel is 29 inches (74 cm). Limit switches mounted in the drive package are used to indicate and limit the extremes of travel as well as to signal the operator when the rod is within 7 inches (18 cm) of the fully inserted or the fully withdrawn positions. Knowing this, the operator can adjust the shim arms to keep within the lower and upper limits of the regulating rod travel. On a scram or rundown signal, the regulating rod is driven at normal speed to its fully inserted position.

Burnup presents no problem, because the half- life of an aluminum atom in a flux of 1014 n/cm2-sec is approximately 1000 years. Since the rod is made of the same material as the rest of the core structure, it suffers minimal corrosion (similar to other core components). Regulating rod lifetime is greater than 20 years.

The regulating rod operates in a shroud of approximately 3.5 inches I.D (8.9 cm). The shroud has the same configuration as the 3.5-inch (8.9-cm) experimental thimbles. A fixed orifice in the nozzle of the shroud delivers a coolant water flow of 8 gpm from the outer plenum. This flow passes up around the regulating rod and then out into the bulk coolant. At a calculated heating rate of 2.7 W/cm2 at 20 MW at 8 gpm the maximum regulating rod temperature is approximately 20 F (11 C) greater than the primary coolant inlet temperature.

4.2.3 Neutron Moderator and Reflector

The core is immersed in heavy water to thermalize fast neutrons to sustain the nuclear chain reaction, to remove heat created by the reaction and to serve as the first stage of shielding. No other material is used within or in the area immediately surrounding the core region to moderate the fast neutrons created by the fission process.

The side reflector is 20 inches (51 cm) thick and the top reflector thickness is normally maintained at 118 inches (3.0 m), the height of the 3-inch (7.6 cm) overflow pipe. During normal fuel transfers, water is maintained at a level slightly above the core at the height of the upper grid plate elevation by the 6-inch (15.2 cm) fuel transfer overflow pipe, concentric with the 3” overflow pipe. In the unlikely event that the shim safety arms cannot be inserted, the reactor operator can initiate a Moderator Dump to drop the water level to approximately one inch above the core to effect an emergency shutdown of the reactor. Reactivity measurements show the top reflector worth under normal operating conditions to be approximately 4% reactivity when shim arms are halfway to the full out position, and about 10% reactivity when shim arms are fully out. This provides a shutdown capability for the most reactive core configuration and provides a fully redundant backup to the shim safety arms.

4.2.4 Neutron Startup Source

The normal operating cycle for the NBSR is 7 weeks, with continuous operation at its licensed power for approximately 38 days. The remaining 11 days are used for maintenance activities and refueling. With the exception of occasional longer shutdowns, this schedule continues unabated. As a result, the power history of the heavy water NBSR is more than sufficient to maintain a strong photoneutron source for reactor startup.

After extended shutdowns, the strength of the photoneutron source may be insufficient to provide indication on the nuclear instrument channels. On these rare occasions, an encapsulated americium/beryllium neutron source of nominal strength 2 curies is inserted into the core region to provide sufficient source neutrons for reactor startup by utilizing any one of the vertical experimental thimbles. The source is positioned within the thimble to provide neutron indication prior to reactor startup. Use of a thimble to position the source allows for cooling of the source directly by the D2O in the thimble. The startup source is removed from the reactor after the reactor is critical and prior to raising the power level.

4.2.5 Core Support Structure

The core support structure is designed to ensure that all fuel elements, reactivity control devices and in-core experimental facilities are properly secured against all anticipated loads including both the buoyant force of the coolant and the hydraulic forces associated with the primary coolant flow. The principal feature for achieving this is the heavy grid structure that is positioned at the top of the core combined with ten hold down bolts fastening the upper grid to mounting brackets located on the vessel wall. This grid is designed to lock the fuel and other core components in place during reactor operation and to prevent movement of the core components by the hydraulic lifting force.

The internal structure of the reactor vessel supports the core, the shim safety arms and the inner reserve cooling tank. The upper and the lower grid plates are shown in Figures 4.2.11 and 4.2.12, and their relationship to other core components is shown on Figure 4.2.1. The grid plates are made of aluminum 6061-T6.

The 3.5 inch (8.89 cm) experimental thimbles are held down by poison tubes from the top plug. The function of the tubes is to reduce activation of the fuel element transfer mechanism, the bottom of the center shield plug, and the supports and connections for any experiments installed in the thimbles. The seven poison tubes extend from the bottom of the refueling plug to the 3.5 inch thimbles in the core. The center, 36 inch (91.4 cm) portion of each tube contains a 40 mil (1 mm) thick concentric layer of cadmium within the aluminum wall. This cadmium reduces the neutron flux from approximately 8x1012 n-cm-2-s-1 to 1x1012 n-cm-2-s-1, so that there are no material lifetime issues as a result of neutron irradiation. The 3.5 inch thimbles are designed to be moved in the same way as the fuel elements, using a simple adapter on the top of the thimble; the associated hold down tube must be removed prior to moving the thimble. All of the other core components are held down against the upward force of the water by the upper grid plate. The fuel elements are locked under the upper grid plate and the 2.5 inch (6.35 cm) experimental thimbles are held down directly by the upper grid plate. Thus, the upper grid plate must resist the upward force of the core components caused by the flow of the primary coolant. The lower grid plate is loaded by the hydraulic pressure in the two plenums, the weight of the core components when water is not flowing, and the thermal stresses resulting from radiation heating.

The lower grid plate is completely supported at the edges by the outer plenum flange plate, a 1 inch (2.54 cm) plate welded to the outer plenum directly beneath it. The outer plenum is welded to the vessel bottom, so the load of the lower grid plate is supported from below. The flange plate also has sections extending to the vessel wall where they are welded for further support. The grid plate is fastened to the flange plate by eighteen 1 inch (2.54 cm) diameter stainless steel bolts. The large number of bolts is employed to give a tight seal between the lower grid plate and its mounting surface. Their loading capacity far exceeds that required to handle the grid plate loading.

The upper grid plate is attached to four mounting brackets welded to the vessel wall. These brackets are further reinforced by quarter rings welded to them and to the vessel wall. Ten

0.750 inch diameter (1.91 cm) stainless steel bolts fix the grid plate to the mounting brackets. The upper grid plate mounting brackets also serve to support the inner reserve cooling tank. It stands on four legs, each resting on one bracket, and is bolted in place by one bolt passing through each leg into the mounting bracket. These features are also shown in Figure 4.3.1, a cut away view of the vessel internal structure.

4.3 Reactor Vessel

The NBSR reactor vessel contains the reactor core and its support structure, D2O, a D2O plenums and their connections to inlet and outlet piping, control devices, fuel element transfer chute, inner reserve tank (IRT) for emergency cooling, emergency cooling distribution pan, and D2O holdup pan. Figure 4.2.1 is the elevation of the reactor while Figure 4.2.2 shows a cross-sectional view.

4.3.1 Design

The reactor vessel is an aluminum-alloy vessel 7 feet (2 meter) in diameter and 16 feet (5 meter) in height and is designed in accordance with the ASME Boiler and Pressure Vessel Code for Unfired Pressure Vessels, 1959 Edition of Section VIII, including all revisions, addenda, and applicable code cases in effect at that time.

Basically, the vessel is a vertical cylinder with an elliptical bottom and a flange at the top. The reactor vessel flange rests on top of the Thermal Shield Shim Ring and is bolted to it by twenty-four 1-inch (2.5-cm) bolts. The thermal shield is an iron-lead light water cooled structure that protects the biological shield from excessive radiation heating. This shielding structure surrounds the reactor vessel and rests on a concrete foundation supporting the weight of the vessel; a nominal gap of 1-inch (2.5-cm) is maintained between the vessel and the shield. A major shield component, the lower outer plug, rests directly on the vessel flange. A stainless steel O-ring gasket forms a seal against helium or heavy water at the interface of the reactor lower outer plug and the top face of the reactor vessel flange. A second such gasket forms a seal against carbon dioxide at the interface of the bottom face of the reactor vessel flange and the Thermal Shield Shim Ring. Both seals are located beneath massive shields that are rarely moved. The helium and CO2 systems are at very low positive pressures, and these seals are just two of many inaccessible components of the system boundaries. Helium and CO2 leak rates have been measured, and the performance of the system boundaries is monitored via tritium and 41Ar monitoring, respectively, as well as the consumption rates of the gases.

Main coolant enters the vessel through two inlet pipes, at the center of the vessel bottom: The inner plenum is located within, and is concentric to, the outer plenum. Main coolant exits through two outlet pipes which are welded to the vessel bottom on either side of the outer plenum pipe. The lower grid plate is bolted to both the inner and the outer plenums forming a watertight seal. The D2O holdup pan surrounds the core to a height just above the lower fuel section of the core and is attached to the lower support plate.

The upper girth of the reactor vessel, made of 0.50 inches (1.3 cm) thick aluminum 6061-T6, extends down approximately 115.75 inches (294 cm) below the surface of the reactor vessel flange. The lower girth and the reactor vessel bottom are made of 0.875 inches (2.2 cm) thick aluminum 5052. The lower girth extends down from the upper girth to approximately 166 inches (422 cm) below the reactor vessel flange. The beam ports, through tubes, cold source port, and rabbit tube thimbles all attach to the reactor vessel in the lower girth.

The design temperature for the reactor vessel is 250 F (121 C), and the design pressure is 50 psig (345 kPa). The normal reactor outlet temperature is approximately 114 F (45.5 C) and the normal operating pressure is approximately 3 inches of water. After fabrication, the vessel was hydrostatically tested at a pressure of 75 psig (517 kPa). The maximum hydrostatic pressure, which occurs at the bottom of the reactor vessel, is approximately 7.5 psig (52 kPa). The hydrodynamic pressure at 8700 gpm (550 L/s) is minimal, as the water exits from the fuel elements at the hydrostatic pressure at that elevation. The only hydrodynamic forces are the upward force on the upper grid plate from the elements due to water flow though the elements.

During the design of the vessel, the loads resulting from constraining forces or members were considered, along with those from steady state and transient thermal conditions, including emergencies. The reactor vessel and its associated piping move freely under the influence of thermal expansion. Only the reactions from the bellows-type CO2 seals are transmitted to the vessel. Sliding pad-type pipe supports absorb the major portion of all reaction forces resulting from primary system flow in the external piping. Hence, the resulting loads on the vessel are small, and in conjunction with all other loadings do not cause any stress levels above the maximum allowable working stress for various reactor sections. No impact loads are transmitted to the vessel. The shim safety-arm drive and shock absorbing systems are mounted on the biological shield so that the impact of a scram is not transmitted to the vessel.

Pressure surges that might be generated in the vessel by reactor power transients are small and would not cause the vessel to exceed the design pressure. The vessel, hence the coolant system, is open to the helium cover gas system, which has a large expansion volume and multiple overpressure relief paths. In addition, a safety relief valve for the main coolant system provides overpressure protection.

The vessel low heating rates experienced since its operation and the excellent thermal conductivity of the aluminum combine to yield negligible stresses from internal temperature gradients. Areas of distinct interest from the standpoint of thermal expansion are the through tube to shell joints, and the through tube column reactions resulting from end restraints. Both of these areas were investigated and the resulting stresses considered in the vessel design. These loads do not exceed the code-allowable working stresses at any point. Stresses resulting from differential expansion between dissimilar materials are negligible due to the absence of dissimilar materials. The very small temperature differentials between the coolant and the vessel components generate insignificant thermal transient loads.

Irradiation damage studies run on various aluminum alloys show that the changes in the engineering properties of these materials are not significant for NBSR vessel design. The tips of the beam tubes are considered to be the area most susceptible to radiation damage. See Chapter 16 for more information.

The only mechanism identified for exerting pressures on the beam tube tips is from an experiment inserted into a beam tube. For all beam tubes, there is an aluminum diaphragm at the inner face of the thermal shield that prevents inadvertent insertion of any experiment into the thimble. All experiments are reviewed for safety, and no experiment or part of an experiment is allowed inside the biological shield unless it is surrounded by a container that can contain the maximum possible internal pressure or force that could be generated by any credible accident. In the case of cold sources, which actually are inserted beyond the inner boundary of the thermal shield into the thimble, the design basis requires a container that can withstand a maximum hypothetical accident. This accident is defined as a guillotine break of a vacuum line, followed by a moderator chamber rupture in the presence of an ignition source for the resulting oxygen-hydrogen gas combination, leading to detonation of the gas mixture. No damage to the reactor vessel or the confinement building occurs.

Since the vessel is an all-aluminum structure, and since the flow rates in the vessel and associated piping are small, only aluminum corrosion was considered in the design. The vessel has been filled with D2O for 45 years. There have never been any signs of corrosion on any vessel components or on the fuel elements removed each cycle. Rigorous attention to the primary water chemistry (see Section 9.3) assures that there will be no corrosion of the reactor vessel.