US. NAVAL RESEARCH LABORATORY

NAVAL RESEARCH LABORATORY NUCLEAR REACTOR

ACKNOWLEDGEMENT This article i s yeprinted front a pamphlet prepared by the Naval Research Laboratory.

BACKGROUND

The Naval Research Laboratory Nuclear Reactor (Figure 1) and its associated facilities for research were designed to meet the needs of a wide variety of investigations in the physical sciences. Its princi- pal function is to provide a powerful source of neu- trons, required in many aspects of the Laboratory’s research programs in such fields as nuclear physics, solid state physics, metallurgy, chemistry, mechanics, electronics, and reactor technology.

Initial planning began late in 1952. In the summer of 1954 the Congress appropriated $996,000 for con- struction, which was begun in mid-1955. The reactor was brought critical for the first time on September 17, 1956.

Pursuant to a stipulation of the Congress when it authorized construction of the NlU Reactor, the in- stallation was built and is operated under license from the U. S. Atomic Energy Commission in accord- ance with provisions of the Atomic Energy Act of 1954. The men who operate the controls of the reactor are individually licensed by the A.E.C. as operators.

Neutrons-small sub-atomic particles or fragments of matter which are given off when the reactor is op- erated-are extremely useful in numerous investiga- tions. They may be used to study the structure of matter, including such practical applications as the structure of new alloys. They may be used to create new substances; to make chemicals radioactive for direct study, or after radioactive decay for study of the new decay products; and to produce chemical changes and biological changes.

In addition, the NRL reactor is suitable for inves- tigating many problems arising in connection with the design and operation of nuclear reactors whose heat is used to propel ships or other vehicles or to drive electrical generators.

UTILIZATION FOR RESEARCH

DESIGN AND OPERATION

The pool type reactor design was chosen because of its flexibility for research and its inherent safety characteristics. Reactors of this general type have been operated safely at other locations for several years.

The pool is located approximately in the center of a structure 60 ft wide and 210 ft long (Figure 2). At each end of the building are located laboratories and offices, as well as a large “hot lab” where work can be done by remote control on highly radioactive ma- terials, and a shielded room for accurate measure- ment of radioactive materials. The pool is 40 ft long, 26 ft wide, and 20% ft. deep. One end of the pool con- sists of a heavy concrete radiation shield with a number of openings or “beam ports” in it providing access to neutrons.

The fissionable material in which the nuclear chain reaction or “burning” occurs is uranium 235. It is contained in fuel assemblies which are suspended in the water on an aluminum framework mounted at the middle of a movable platform or bridge which spans the pool. The array of fuel assemblies is called the “core” of the reactor (Figures 3 and 4) . The bridge rides on rails making it possible to locate the core at any point on the center line of the pool.

The water serves three purposes in this type of reactor. First, it slows down the fast-moving neutrons (average speed about 10,000 miles per second) which are released when a uranium 235 atom splits in half or “fissions”. The slower-moving thermal neutrons .(average speed about one mile per second) are more reactive with other uranium 235 atoms, causing them to split and yield additional neutrons; thus the pres- ence of the water reduces the amount of uranium 235 necessary to sustain a nuclear chain reaction. Sec- ond, the water serves as a coolant to remove the heat formed in the uranium by the reaction, and, finally,

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Figure 1. One end of the pool terminates in a special radiation shield of lead, concrete and other materials. A number of open- ings or beam ports penetrate this shield permitting neutrons from the core to emerge under controlled conditions for use in physics research.

it is a radiation shield which protects personnel from the harmful neutrons and gamma rays generated in the uranium fuel. The gamma radiation from the core after 8 hours operation at 100 kilowatts is roughly equivalent to that from a ton of radium.

The first several months of operation of the re- actor were devoted to a series of tests and calibra- tions at low power levels. The entire control system was given thorough testing. The control and safety rods and various instruments were calibrated. The minimum amounts or “critical masses” of uranium 235 required for operation in different positions in the pool and for various core loading configurations were determined, and a number of other operating characteristics of the reactor were measured. The re- actor was formally opened for research on January 15, 1957.

FUEL ELEMENTS AND REACTOR CORE The uranium fuel of the NRL reactor has been en-

riched to 90% uranium 235, the remainder being, uranium 238. The basic fuel unit is a plate 60 thou- sandths of an inch (about 1/16 inch) thick. Each plate is in reality a sandwich; the center 20 thousandths is uranium-aluminum alloy, and a layer of aluminum cladding 20 thousandths inch thick is bonded to each side. The cladding seals in the radioactive product ele- ments formed by the splitting of uranium 235 atoms,

558 A.S.N.E. Journal. August 1957

preventing the former from contaminating the pool and air. A fuel assembly consists of 18 of these plates spaced about 1/9 inch apart, and contains a total of 140 grams (about 5 ounces) of uranium 235. Each assembly is open at top and bottom. Water flows up through the assembly by convection, passing between the fuel plates and removing the heat formed during operation.

If the reactor were operated regularly for 40 hours per week at a power level of 100 kilowatts, a set of fuel elements containing about 11 pounds of U-235 would last for approximately 40 years. At the end of that time only about 10 per cent of the uranium would have been burned up; however, certain of the product elements of fission formed in the fuel plates absorb neutrons necessitating replacement of the fuel assemblies. The spent units will be shipped in lead containers to special fuel processing plants where the fuel plates will be dissolved in acid and the unburned uranium separated chemically and re- covered for further use.

The useful life of the fuel assemblies is propor- tionately shortened by operating the reactor at higher power levels or for longer periods each week.

In several special fuel assemblies half of the plates have been omitted to provide an open channel for insertion of control and safety rods (Figures 3 and 5).

“US. NAV. RES. LAB.” NUCLEAR REACTORS

Figure 2. The uranium core of the reactor is suspended in the pool from the center of the reactor bridge.

A control or regulating rod of s tadess steel is moved up and down in one of these channels to regulate the power of the reactor. Two or three other rods con- taining boron, a highly absorbing material for neu- trons, are inserted in other fuel assemblies in the core as a safety measure to provide for rapid quench- ing of the nuclear reaction if needed. These safety rods are also sometimes called shim rods, since by raising or lowering them slightly coarse adjustments are made in the power level.

The core of the reactor consists of a group of fuel assemblies arranged side by side in a generally square array. As few as 17 or as many as 30 fuel assemblies may be required to sustain a chain reac- tion, the exact number depending on the configura- tion used and the materials immediately adjacent to the fuel in the pool.

HOW THE NUCLEAR REACTION IS STARTED

In starting the reactor, the safety rods are with- drawn from the core first and held in reserve for emergency shutdown, then the control rod is with- drawn far enough to permit the neutron chain fission reaction to become self-sustaining.

The initial neutrons required to start the reaction

are supplied by a “neutron source,” a small sealed capsule about the size of a thimble containing radio- active polonium and the element beryllium. This is located at the edge of the core adjacent to the fueI assemblies. As the reaction is started, the concentration or neu-

trons is measured by a fission chamber located ad- jacent to the fuel, and the information is displayed in the control room. When the neutron concentration or “flux” has increased somewhat, other detectors take over the measurement function and the sensi- tive fission chamber is withdrawn from the core. When the reaction becomes self-sustaining the neu- tron source is also withdrawn from the region of the fuel.

The neutron flux is permitted to increase gradually until the desired operating power level has been reached, at which time the control rod is moved in slightly, then continuously adjusted either manually or automatically, to maintain that level.

Each safety rod is coupled to its drive rod by an electromagnet. If an undesirable condition should de- velop, the current to the magnet is interrupted and the rod falls by gravity into the core, stopping the nuclear reaction.

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Figure 3. Closeup of reactor core shows fuel assemblies (center) mounted on grid plate, which has 7 x 7 array of holes permitting variations in fuel arrangement to meet research requirements Control rod is in center of fuel array. with two safety rods behind it. Slender tube at left contains movable neutron source for startup. Fuel assembly at right is mounted on elevator which runs up to water surface. Recess in shield and “rabbit” tube are visible in background.

THE REACTOR CONTROL SYSTEM The eyes and ears of the control system are the

radiation detectors located on each side of the core. The hands of the system are the safety and control rods. The remainder of the system consists of motors to drive the rods up and down, instruments to display and record the signals from the detectors, a control console, and a complex electrical and electronic sys- tem connecting these and including many interlock- ing features to provide maximum safety in operating the reactor. The control system was designed in its entirety by NRL scientists and was constructed and assembled in the special shops of the Laboratory. The detectors were designed and constructed at NRL, as was the control console.

THE SHIELD The shield has several unique construction features

(Figures 6 and 7) . It was desirable to keep its thick- ness to a minimum so that the beam ports would be as short as possible in order to provide more intense neutron flux at the outer ends of the ports. Special concrete was used to reduce the shield thickness re-

Figure 4. The heart of the reactor is the uranium core 10- cated in the pool at the bottom of the trusswork attached to the bridge. The cylinders on each side of the fuel are radia- tion detectors which measure the power level of the reactor. Except for the fuel units, the entire bridge and core assembly, including the detectors, was fabricated by the Laboratory.

quired. The heavy aggregate on the side toward the core is barytes, a barium sulfate rock, barium being a strong neutron absorber. In sections of the shield behind the barytes portion, the heavy aggregate is iron ore and iron punchings, a good gamma ray shield. The aggregates were placed dry in the forms. A cement-water mixture was then pumped up through the mass in a single operation.

In the center of the shield just behind the reactor core recess is a stack of graphite blocks called a thermal column, roughly 3% f t x 4 ft and 9% f t high. Graphite is a dense form of the element carbon, which is a good medium for slowing down neutrons to thermal velocities-somewhat better than water, in fact. Two openings near the top of this column pro- vide sources of thermal neutrons where these are re- quired in particular experiments.

The seven beam ports which penetrate the concrete and carbon provide sources of fast or fission-energy neutrons. In addition to these openings there is a “through-hole” passing alongside the core, which is convenient for certain types of cxpcriments. These holes penetrate the graphite blocks at various angles. The blocks in this region of the thermal column were

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“U.S. NAV. RES. LAB.” NUCLEAR REACTORS

Figure 5. A single fuel plate (A) is essentially a thin num sandwich containing 8 grams, about 1h 02. of uranium 235 in a layer through the center.

A fuel assembly (B) is made up of 18 plates spaced a small distance apart to permit water to flow between them for cool- ing and neutron moderation. It contains 140 grams, or about 5 02. of uranium 235.

In several special fuel assemblies (C) 9 of the center plates have been replaced by an open channel in which the neutron- absorbing control or safety rods are inserted.

Movement of the stainless steel control rod (D) up or down in the array of elements comprising the core controls the nuclear reaction.

fastened in a jig and the holes bored through the en- tire assembly on a large boring mill in the NRL shops.

The graphite region extends out on each side of the core recess or niche. The graphite immediately sur- rounding the core is sometimes called the “reflector” because many of the neutrons which are slowed down in this region diffuse back into the core. When the core is in the niche it is graphite-reflected on three sides and water-reflected on the remaining three sur- faces. Since graphite is a better reflector than water, less fuel is required to constitute a critical mass in the niche than out in the pool. Because the neutron

density for a given power is greater around a smaller core, this arrangement also increases the available neutron flux at the ends of the beam ports.

The water barrier in front of the graphite and the beam ports is a sheet of aluminum half an inch thick, which is essentially transparent to neutrons and gamma rays. The edges of this aluminum skin are recessed in the concrete. The recessed portion is coated with zinc to prevent corrosion of the alumi- num by the concrete.

The aluminum skin has up to 20 feet of water head against it, requiring that it be strengthened from be- hind to withstand the pressure at this depth. The use of metal reinforcing plates would have replacqd gra- phite where it was most needed close to the core as a reflector. Tests of the graphite bars showed that they had more than enough strength to be used also for mechanical reinforcement, and a row of them is bolted to the back of the aluminum plate to prevent it from buckling.

THE “RABBITS”

A special facility available for the irradiation of small quantities of materials is a system of pneumatic tubes in which small carriers called “rabbits” may be moved quickly by compressed air. Two rabbit termi- nals are provided near the core in the niche position. Depending upon research requirements, samples may be irradiated here for a few seconds if desired, or for longer times. The containers may then be returned to lead-walled receiving boxes outside the shield or directly to remote receivers near the laboratories in the facility.

THE POOL

High purity water is used in the pool to minimize corrosion of the thin aluminum cladding on the fuel elements. City water is further purified by passing it through a special treatment plant in the reactor building.

The pool may be divided in half by positioning a movable water gate across its center, making it pos- sible to drain half the pool while the reactor is being operated, or the radioactive core is stored, in the other half. This increases flexibility of operation. The half of the pool farthest from the shield may be used for investigations aimed at improving methods of calculating radiation shielding requirements, opti- mizing shield arrangements and materials, etc. It may also be used as a gamma ray irradiation facility using radioactive fuel elements as a gamma source.

The pool, which holds 150,000 gallons, drains to an underground tank of 170,000 gallon capacity. Con- tents of this holdup tank are carefully analyzed by health physicists to assure that any radioactive ma- terials are below tolerance levels prescribed by the AEC before discharge to the Potomac River.

THE HIGH RADIATION LABORATORY OR HOT CELL

A room heavily shielded by concrete called a “hot cell” adjoins the pool at the end opposite the shield. Any of a number of metallurgical, chemical, or me- chanical operations can be conducted on highly radio-

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NUCLEAR REACTORS “U.S. NAV. RES. LAB.”

MrnAL THERMAL COLUMN dF4-INCH SQUARE GRAFIUTE BARS (ACCESS ABOVE)

REEMO\(PBLE BARYTES CONCRETE BLOCKS

CORE

- - _.-___ ___ (BARYTES -- - - _. -

__ - _- -- - - AGGREGATE) - __ ~- .. -

I CONCRETE (IRON PUNCHlNGSAND __ - _- LIMONITE AGGREGATE) - - - - -

Figure 6. Horizontal cross section through the shield shows location of beam ports and graphite thermal column.

active specimens in the kilocurie range in this room by scientists working with special manipulators from an adjoining room. Objects made radioactive by su- spending them close to the reactor core in the pool may be transferred directly into the hot cell through one of three underwater transfer chambers.

The cell is equipped with three special windows each three feet thick for direct viewing of operations. Each of these consists of two sheets of one-inch thick plate glass with the intervening space Wed with a saturated solution of zinc bromide, a dense liquid with good optical properties and an excellent shield against radiation. In front of each window is a pair of master-slave hand manipulators, and the cell also has a heavy duty powered manipulator operated electrically from a console.

OTHER SPECIALPURPOSE LABORATORIES

The facility includes a shielded room approxi- mately 12 x 30 ft used for measuring or “counting” the radioactivity in samples which have been exposed in the reactor. The walls and roof of this room are

of two-foot thick concrete to reduce background radiation which would interfere with accurate meas- urements.

A laboratory for film development work also has been provided.

TKE BUILDING

The reactor facility was constructed in a portion of an existing building on the grounds of the Labora- tory to conserve space and reduce construction costs.

In the remote event that the interior of the building might be contaminated with radioactive materials, the entire building may be sealed. All of the exterior walls are sealed with a layer of aluminum foil in- stalled behind the plaster, all exterior doors are a heavy, refrigerator type with gaskets, and all the ventilation ducts may be closed instantly by a cen- tral control unit. The roof is of concrete slab con- struction covered with tar.

The building has no exterior windows for reason of containment. A high light level and a variety of

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“US. NAV. RES. LAB.” NUCLEAR REACTORS

GEAR BOX ASSEMBLY -

4

WATER LEVEL 36.5’ _ _ ~ _ _ .

CORE TRUSS’

ZINC

DFXL OF WATER SEAL

SPECIAL FUEL ELEMENT (SECTION. S W I N G SAFETY ROO IN POSITION) ONLY ONE FUEL ELEMENT SHOWN

\ REACTOR CORE-

CORE GRID PLATE-

R SAFETY ROD DRIVI: ASSEMBLY

BORAL

LEAD BRICK - +*‘ALUMINUM WATER

BARRIER

COUPLING MAGNET ,+I -SAFETY ROD ‘ci I

-LEA0 PLATE - BARYTES CONCRETE

REMOVABLE CONCRETE BLOCKS

HORIZONTAL THERMAL

GRAPHITE THERMAL COLUMN REMOVABLE PLATFORU

- COLUMN ACCESS

t-- - --r I I

BEAM PORT

~NGENT I AL THROUGH: I I I

.

Figure 7. Vertical cm&’sectioa through shield, reactor core and bridge.

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NUCLEAR REACTORS "US. NAV. RES. LAB."

colors have been provided in the building to reduce personnel fatigue. The interior color scheme was planned by color specialists from the Navy Bureau of Yards and Docks.

SPECIFICATIONS

Materials Fuel-Uranium enriched to 9Oc; U-235, as U-A1 alloy,

Critical mass in pool-approximately 3400 grams Critical mass in shield position- approximately 2700

Total inventory- 5000 grams Fuel cladding-0.020 in. aluminum Moderator -Water-0.117 in. film Reflector Coolant -Water-O.117 in. film Shielding

in TSF type assemblies

grams

-Water, or water and graphite

In pool-Water In shield position- graphite, boral, lead, and barytes

concrete po0 r

Dimensions-26 ft x 40 ft x 20% ft deep Capacity -150,000 gallons Water -conductivity 0.15 micromhos; supplied by

mixed bed ion exchange unit, output 30 galimin

Core Dimensions Up to 21 in. x 21 in. x 24 in. high

Cooli.ng Water, natural convection

Operating Conditions Power rating-1000 KW (initial operation at 100 KW) Heat flux max. at 100 KW- 1000 BTU/hr ft' Average power density at 100 KW- 1.1 KW/liter Average specific power at 100 KW-30 KW/kg of U-235

Controls Regulator- one stainless steel vertical rod Safety/shim- three boron-lead vertical rods Reactivity change at 100 KW-0.08

Neutron Flux at Core Surface at 100 KW Average fast -3.5 x 10" n/cm? sec Average epithermal-5 x 10" n/cm? sec Maximum thermal -1.3 x 10" n:'cm'sec

Experimental Facilities 6 round beam ports, 6 in. diameter 1 rectangular beam port, 10 in. x 12 in. 1 square tangential through-hole, 4 in. x 4 in. 1 thermal column, 3% ft x 4 ft x 9% ft 2 pneumatic rabbits

Observations and studies conducted by the Woods Hole Oceanographic

Institution indicate the existence of a thermally driven deep current flow-

ing to the south under the Gulf Stream. The exploration of this current is

being conducted by the use of an electronic signalling device encased in

an aluminum pipe which can be made neutrally buoyant and tracked at

the desired depth. Confirmation of this current will add substantially to

knowledge of the ocean's circulation.

564 A.S.N.E. Journal. Auqurt 1951