From Current Literature

Reactors for Research with Neutrons

At the end of 1951, articles were published in American journals on two heavy-water reactors intended for experimental purposes. The first is the Norwegian reactor, recently built at Kjeller near Oslo*), and the second is an American one, under the name NRX,

Fig. 1. Schematic section of the Norwegian reactor: 1 — outlet pipe of the ventilation system; 2 — graphite reflector surrounding the reactor tank; 3 — first stage of the refrigerator; 4 — upper shielding: layers of wood, cadmium, and lead; 5 — reactor tank; 6 — opening for a thermal column; 7 — openings leading to counters; 8 — irradiation tubes; 9 — heavy-water reservoir in the basement room; 10 — wall between the basement and the well.

) Dahl and Randers, Nucleonics 9, No. 5, 5 (1951); Scientific American 185*, No. 6, 31 (1951).

FROM CURRENT LITERATURE

in the system of the Chalk River nuclear center on the Ottawa River, 180 km from the city of the same name, and which had already been operating for 4 years*).

The present note gives a description of these reactors on the basis of data published in the articles cited.

The first of the reactors described was started up at the beginning of August 1951. The heavy water used in it is of Norwegian manufacture; the uranium was supplied by Holland from prewar stocks. The operating power of the reactor is 100 kW, although it can also operate at a somewhat higher power. At the 100 kW level the neutron-flux intensity is \(3 \cdot 10^{11}\) neutrons/\(\text{cm}^2\cdot\text{sec}\). The principal features of the design of this

Fig. 2. View of the upper plate through an opening in the shield. The photograph was taken in the summer of 1951 during trial tests.

reactor are analogous to the corresponding reactors available in Canada, France, and the USA. The reactor diagram is shown in Fig. 1.

Heavy water is enclosed in a thin-walled aluminum tank surrounded by a graphite reflector and concrete shielding. The uranium is contained in aluminum tubes arranged vertically inside the reactor tank. The heat released in the fission of uranium is removed by circulating heavy water, which passes through a refrigerator located outside the concrete shielding. To control the operation of the reactor, use is made—

) Lewis, Physics Today 4*, No. 11, 12 (1951).

are used cadmium plates sliding between the reflector and the reactor tank.

The reactor tank, containing 7 tons of heavy water, has a diameter of 2 m and is welded (in an argon arc) from sheets of 99.5-percent-purity aluminum. The tank itself is placed in the upper slab, in which the uranium columns are secured (see below). The bottom of the tank is not supported from below; it rests, by the edge of the upper plate, on the rim of a circular opening in the upper part of the concrete shielding. The weight of the tank with all its contents is approximately 11 tons. The upper slab is double; its outer and inner surfaces are connected by aluminum tubes with a total number of 76, which are distributed uniformly over the slab with a distance between centers of 180 mm. A view from the side of the upper opening in the shielding is given in Fig. 2. Some of these openings are used for control purposes: the insertion

Fig. 3. Interior of the reactor tank. Visible are the intersecting tubes for irradiation, part of the perforated tube, which is connected to the four thin tubes leading out through the bottom, and the filling opening in the center of the bottom.

of measuring instruments, in particular resistance thermometers for measuring the temperature and its distribution in the reactor tank. Most of the holes serve for the insertion of uranium rods and the fastening of uranium columns.

The material to be irradiated can be introduced into the reactor tank by using any of the six horizontal tubes passing-

the tank through a tank and brought out through the reflector and the shield. Two systems of such tubes (three tubes in each) are arranged perpendicular to each other and to the axis of the tank (Figs. 3 and 4). The distance between the centers of two adjacent tubes is 250 mm, the inside diameter is 63 mm, and the wall thickness is 3.5 mm. At their outer ends the tubes are closed with brass plugs with cadmium tips. For irradiation the sample is placed in an aluminum capsule, which is inserted into a short cylindrical sleeve of graphite of approximately the same diameter as the tube. This sleeve can be pushed along the tube to the middle of the tank and, after irradiation, pushed out from the other side.

The inlet opening for filling the tank with heavy water is located in the center of the bottom (Fig. 3). The drain for regulating the level passes out through a round perforated tube fastened inside

Fig. 4. Opening in the shield, occupied by the thermal column; visible are the reactor tank, the irradiation tubes, and one of the cadmium plates.

the tank along the wall, somewhat below the working level of the heavy water. This tube is connected with four outlet tubes coming out of the tank in each quadrant (Fig. 3). Under the bottom of the tank these four tubes join into one outlet. The inlet and outlet tubes are connected with the cooling system through the pump unit.

At present the reactor tank contains 2.2 t of natural metallic uranium in the form of rods 2.54 cm in diameter and 30 cm long. These rods, placed in paired aluminum tubes with a wall thickness of 2 mm (the distance between centers is 40 mm), make up the so-called uranium columns. The total effective length of the uranium rods in a column is 1.9 m. In the reactor tank the upper rods of all the uranium columns are approximately 10 cm below the level of the heavy

water. The space in the tube above the uranium rods, as well as the lower part of the tube, is filled with graphite. Each column contains 35.5 kg of uranium and weighs 43 kg. The upper part of each uranium column is provided with a special head for rigid attachment to the corresponding place on the inner surface of the upper plate.

The reactor reflector consists of graphite blocks measuring \(70 \times 14 \times 14\) cm, arranged in eight prisms with a cross section of \(70 \times 70\) cm. These prisms surround the reactor tank over its entire height, from the floor to the ceiling of the reactor room. The edges of the prisms nearly touch, and the eight 45-degree sectors between adjacent prisms are likewise filled with graphite blocks cut in the appropriate manner. As a result, the minimum thickness of the reflector is 70 cm. The space beneath the reactor tank is also filled with graphite blocks. In all, the reflector required 32.8 tons of graphite with a specific gravity of 1.68. Since the friction between the blocks is small, to prevent slipping during assembly the horizontal surfaces are coated with a thin layer of some binding substance.

The reactor tank and the graphite reflector are surrounded by reinforced-concrete walls, whose thickness nowhere amounts to less than 2 m. The upper edge of the concrete walls is approximately 5 m above the ceiling of the reactor room, the walls of which, on the inside, are covered with a thin layer of cadmium for additional protection.

In the ceiling part of the concrete shielding there is a round opening (see Fig. 2), 3.5 m in diameter and 2.9 m deep; originally, during operation, it was intended to shield this opening with a special tank of water 4 m high, which, of course, would fully provide protection; however, should it become necessary to replace uranium rods partially, this enormous tank would have to be removed.

It was therefore replaced by several layers of wood, cadmium, and lead; in them, directly above the tubings of the upper plate, openings were made, closed by steel-concrete plugs.

In the concrete shielding there is also a wide vertical opening for the thermal column (Fig. 4), measuring \(1.8 \times 1.8\) m with an effective length of 2 m; its outer side is shielded with lead 10 cm thick; in addition, between the lead and the graphite there is a cadmium interlayer. On the side of the shielding opposite the column there are two openings \(20 \times 20\) cm for experiments with fast neutrons; approximately in the middle of the wall thickness the opening has a considerably larger cross section. There are three through-holes for six neutron counters with boron trifluoride, which are the principal monitoring and control instruments. Finally, there are openings for irradiation tubes, slots for cables, into which cadmium rods are inserted, and channels for lowering and laying graphite blocks of the reflector. All openings in the shielding are well insulated. A fan continuously drives fresh air through the reactor room and thus removes radioactive dust and gases. This ventilation system is, to a certain extent, also a cooling system, since the heat released by the reactor is equivalent to approximately 6 kW, which somewhat reduces the load on the refrigerator.

The concrete walls of the shielding are lowered somewhat below the reactor room. Still lower is a basement \(5 \times 4.5\) m and 3.8 m high, where a reservoir for heavy water and the necessary equipment are located. The basement can be entered through a wide shaft with a removable wooden ceiling; one side of the shaft borders the basement. Here are the refrigerator and the circulation pump. The shaft and the basement are separated by a wall of concrete blocks 80 cm thick, which provides complete protection from radiation. For passage from the shaft to the basement

a shielding door has been made in the wall. All this can be seen in Figs. 1 and 5.

The reservoir located in the basement is connected to the bottom of the reactor tank by an aluminum pipe 14 cm in diameter. This pipe has a specially designed valve, controlled by a relay, which automatically opens the valve if the neutron-flux intensity exceeds a specified value; then the heavy water flows out of the reactor tank into the reservoir within 45 sec., and thus the chain reaction is stopped.

Fig. 5. The cooler, where heat exchange takes place between heavy water and ordinary tap water (first stage). This room adjoins the basement, where the reservoir with heavy water is located.

The heat released in the reactor is removed by a two-stage cooler. In the first stage, heat exchange takes place between the heated heavy water and ordinary tap water. In the second stage this water circulates through a radiator cooled by air, which is discharged outside by a ventilation system. With this cooling system, the average temperature of the heavy water at a power level of 100 kW does not exceed 40°C. Circulation of the heavy water is complicated by the requirement of maintaining strict hermetic sealing of the entire system containing the heavy water, and by the need to maintain the water level in the reactor tank within very narrow limits. This is ensured by a system of pumps and valves, the operation of which is controlled by an automatic indicator of the heavy-water level.

The course of the chain reaction is regulated by cadmium plates. In the non-operating position the plates are located in grooves of the shielding at the bottom of the reactor. For control of the reactor it is usually sufficient

of two plates; the two other “reserve” ones are used only in critical cases. All four plates are identical: the size of each plate is \(1.3 \times 0.35\) m. A cadmium layer \(1.7\) mm thick is clamped between two aluminum sheets \(4\) mm thick. The guides for the plates are 8-mm steel rails on spring anchor braces. The plates are suspended on steel cables with a counterweight, which for the reserve plates is 9 kg heavier than the plate weight, and for the plate weight is 17 kg. The plates themselves are located below the level of the bottom of the reactor tank. When the stop is released, the reserve plates rise in 1.7 sec.

To obtain data on the operation of the reactor and to control it, the readings of six neutron counters are used. Each of the six

Fig. 6. Exterior view of the NRX reactor.

counters has its own functions. One is connected directly to a galvanometer and shows the magnitude of the neutron flux even when the main power source is disconnected. Usually, observation of the magnitude of the neutron flux is carried out according to the readings of another counter. Three counters control the reserve plates: if the power rises too rapidly, one of these counters switches on the mechanism raising the cadmium plates, and then the relay controlling the valve that drains heavy water into the reservoir. The other two may be adjusted so as to raise the plates at some specified power level below the maximum.

Finally, the sixth counter, through the corresponding automatic device, regulates the movement of the control plates. These plates can be moved manually as well. The readings of all monitoring instruments, the control panel, and data on the state of all reactor units and devices are concentrated in the control room, where the central place is occupied by instruments showing the power level and the position of the cadmium plates.

In conclusion, it may be of interest to report information on the cost of the reactor and some of its parts (without uranium).

The NRX reactor, like the one described above, operates on heavy water and natural uranium and is intended for experimental investigations with neutrons, but its power is considerably higher, namely 10,000 kW. It differs from other reactors of similar type by the great intensity of the neutron flux, reaching \(6 \cdot 10^{13}\) neutrons/\(\text{cm}^2\) sec. This value is approximately 10–20 times greater than in graphite reactors of analogous power. A photograph of the external view of the reactor is given in Fig. 6.

The reactor tank is a cylindrical reservoir 2.4 m in diameter and 3 m high. Inside, tubes are arranged vertically, containing a total of 176 uranium rods. The cooling system is ordinary river water.

Heavy water is pumped into the reactor tank from a reserve reservoir until a certain critical level is reached and the chain reaction begins. Gradually a definite power is established, the magnitude of which can be controlled by the intensity of the neutron flux, measured by some suitable counter. When the power reaches the required value, it is necessary to adjust the height of the heavy-water level so that the power remains constant. This adjustment must be fairly precise, since, for example, when the reactor tank is filled to a height of 250 cm, raising the level by 2 cm doubles the power within two minutes. Therefore, in addition to level control, cadmium rods are also used. Thanks to automation of the control, the required power level is maintained rather stably, with an accuracy of up to 0.5%.

Fig. 7. Schematic cross-section of the NRX reactor (working section):
1 — holes for irradiation; 2 — concrete shielding; 3 — steel shielding; 4 — lead shield of the thermal column; 5 — thermal column; 6 — graphite reflector; 7 — reactor tank; 8 — 4-inch holes; 9 — 12-inch holes.

Because of the great complexity of all the reactor’s components, unforeseen disturbances of its normal operating regime are, of course, possible, which may make it necessary to terminate the chain reaction quickly. For this purpose special rods made of neutron-absorbing materials are used; by means of pneumatic devices they can, at the required moment, be rapidly inserted into those of the vertical tubes that are deliberately left unfilled with uranium rods.

Since the NRX reactor is intended for research purposes and therefore must meet the requirements of many experimenters with different interests, it can be started at any required power or stopped, if necessary.

In Fig. 7 (see p. 137) a schematic section of the reactor is given. In the upper part of the figure one of the openings with an automatic—

Fig. 8.

—device is shown schematically; it permits specimens to be introduced into, and removed from, the reactor for irradiation without stopping the latter. Figure 8 shows a photograph of this side of the reactor. Each of the large tubes visible in the photograph serves as the protective casing for a rod, at the end of which there is a holder for the specimen.

On both sides, through the concrete shielding, run graphite columns, which make it possible to obtain thermal neutrons. These thermal columns are useful for physical experiments with gamma rays and decay products formed when neutrons are captured, and for studying various nuclear reactions accompanied by the liberation of fast neutrons. The intensity of the thermal-neutron flux at a distance of 2.5 m from the column is \(10^9\) neutrons/\(\mathrm{cm}^2\) sec, while the density of thermal neutrons is 25,000 times less than the density of neutrons entering the ther—

ical column. The maximum value of the so-called cadmium ratio for the column is estimated at approximately 90,000.

The lower part of Fig. 7 shows the openings that are used to obtain collimated beams of neutrons and gamma rays for various research purposes. This side of the reactor is shown in Fig. 9. The covers closing the four-, five-, and six-inch openings are clearly visible in the upper row. The twelve-inch openings are located below. To the right is a gamma spectrometer for studying the formation of pairs. Further, from right to left, there is a neutron spectrometer. Then, at one of the twelve-inch openings, there is Robson’s apparatus,^1 on which he studied the radioactive decay of free neutrons. Below is a massive table on which another neutron spectrometer is mounted. Finally, at the far left is apparatus for studying the action of radiations on various materials. The installations are equipped with recorders for continuous recording of the results; some of these recorders are also visible in the photograph.

Fig. 9.

Figure 10 gives a diagram of the above-mentioned gamma spectrometer of the type proposed by Yoker and Macdaniel.^2 The first part of the installation is placed inside the concrete shielding and begins with a thick bismuth screen that closes the opening on the inner side of the reactor. The bismuth screen absorbs gamma rays from the reactor, but lets neutrons pass through. Near it is the target; the gamma rays produced in it are scattered in all directions, but some of them (approximately \(1/20\,000\) of the total number) pass through the lead diaphragms, emerge outside, and finally strike the radiator. Neutrons that have passed through the target are absorbed by a plug of boron and paraffin. The radiator is a co-

foil made of some heavy element, in which gamma quanta of the corresponding energy are converted into an electron–positron pair. Most pairs move in the same direction as the original beam of gamma quanta. Passing through the magnetic field, the electron and positron are deflected in different directions.

Fig. 10. Diagram of a gamma spectrometer: 1 — bismuth screen; 2 — target; 3 — internal lead diaphragms; 4 — cadmium screen; 5 — tube made of boron and paraffin; 6 — lead entrance diaphragm; 7 — magnetometer coil; 8 — electromagnet pole; 9 — lead casing; 10 — radiator; 11 — counters.

Fig. 11. Diagram of an apparatus for studying neutron decay: 1 — winding of the beta-spectrometer electromagnet; 2 — high-voltage electrode; 3 — windings of the proton spectrometer electromagnet; 4 — trap for absorbing the transmitted beam.

and the positron are registered in two counters arranged in the appropriate manner. The sensitivity of the apparatus is such that, for \(10^{13}\) quanta formed during neutron capture in the target, at a quantum energy of \(2.75\) MeV, one pair is registered. If the quantum energy is \(7.4\) MeV,

the sensitivity will be approximately 40 times greater. The data presented are intended to emphasize the importance of the magnitude of the neutron flux, which near the target is approximately \(7 \cdot 10^{12}\) neutrons/\(\text{cm}^2\) sec.

The layout of Robson’s installation for studying the beta rays in the radioactive decay of free neutrons is shown in Fig. 11. The neutron beam passes through a vacuum chamber and exits through thin aluminum windows. In neutron decay the reaction

\[ n \to p + e + \nu + 783\ \text{keV} \]

takes place. From conservation of momentum it follows that most of the energy passes to the light particles, namely to the electron and the neutrino, while the proton will have an energy of the order of several hundred electron-volts. Thus the electron can pass through a thin metal layer of the high-voltage electrode into the beta spectrometer, whereas the proton is captured by the high potential and enters the proton spectrometer. To eliminate the background, the electron-recording scheme is adjusted so that the pulse from the electron is recorded with a delay of \(0.9\ \mu\text{sec}\); this lag corresponds to the time of flight of the proton from the place of formation to the proton spectrometer. A careful analysis of the experimental conditions made it possible to establish that the observed protons and electrons are produced as a result of the decay of free neutrons. The half-life of free neutrons was estimated at \(12.8 \pm 2.5\) min from measurements of the intensity of the initial neutron beam \(\left(2 \cdot 10^9\ \text{neutrons}/\text{cm}^2\ \text{sec}\right)\) and from the number of recorded protons and electrons that are products of its decay.

The neutron spectrometers of this reactor can be used not only to study nuclear reactions with neutrons, but also to investigate the structure of molecules containing hydrogen and light atoms, and for a number of other purposes.

In conclusion, the author of the note indicates that at present a new NRU reactor is being built at Chalk River, which will be more advanced than the one described.

G. A.

CITED LITERATURE

1. Robson, Phys. Rev. 78, 311 (1950), 83, 349 (1951).
2. Walker and MacDaniel, Phys. Rev. 74, 315 (1948).