Be reflector

Mann, H. M. and Martin, A. B., Further Critical Exper iments on a Small Reactor of Enriched U-235/Al-H O Mo ierator and Be Reflector, ORNL-79, September 16, 1948. 

The most practical neutron source for NAA is a nuclear reactor, which produces neutrons via the nuclear fission process (see Chap. 57 in Vol. 6). Many research reactors are equipped with irradiation facilities that provide a stable, well-tailored, isotropic neutron field with sufficiently high flux. Low-energy (thermal) neutrons comprise the most important part of the reactor spectrum hence the degree of moderation is an important parameter. The irradiation channels are usually created in moderator layers, such as a thermal column or a Be reflector blanket. 

Miscellaneous measurenients have been completed indicating a 4-in.-thick section of reactor grade BeO is worth 2.0%AkA less than the same thickness of Be reflector within the DjO region. (The differences between Be and BeO are very dependent on few ppm boron impurity.) The worth of hydrogen (N = 2x 10 atoms/ cm ), when inserted as polystyrene, is (-12%Ak) when uniformly dispersed between fuel and reflector moderator. 

The honeycomb critical assembly is a universal split table machine containing a 1.83-m ( ft>cubical matrix of 76-mm (3 in.)-square aluminum tub It is deagned to serve as a flexible system for initial mbckup studies for basic critical parameter investigations. Fuel inventory consists of various Assile species such as 330 kg of O.OS-mm-thick U(93) foils with widths and lengths appropriate to the aluminum matrix tubes. Control and safety rods utilize sections of the core or reflector materials for their flmction and major disassembly is provided by the movable section of the table. Honeycomb is presently stacked with a UOrMo (core). Be (reflector) mockup of a space powa reactor. 

After extensive analysis it was concluded that the main reason for the reduction of the excess reactivity was the poisoning of the Be reflectors by He due to the He reaction. The... 

After the reactor re-start operation the amount of the He in the Be reflectors decreased. However, for the WWR-M Kiev reactor the reactivity increase resulting from the reduction of He was less than the reactivity decrease due to fuel bum-up. In this way the net excess reactivity was always decreasing for any power and at any period of reactor operation. 

Assuming the baseline heat generation rates modeled are accurate for a 1MWth gas-cooled space reactor, the calculated maximum working temperature of the Be reflector cladding is exceeded in all but the first case, implying that another material should be examined as a possible alternative for this component. 

Secondly, a short pulse duration is required in order to achieve a good axial resolution, i.e. two signals close together should be detected without interference. The task can be, for example, to detect a small reflector close to the surface or back wall of the test object, as the inspection has to cover the total volume as complete as possible, including the near-surface regions. 

The function h(t) to be restored is the impulse response of the medium x(t) is the transmitted pulse measured by reflection on a perfect plane reflector, for example the interface between air and water and y(t) is the observed signal. 

With the help of the DGS-diagram any defect echo can be evaluated, if the echo of a known reference reflector is available. 

With the reference block method the distance law of a model reflector is established experimentally prior to each ultrasonic test. The reference reflectors, mostly bore holes, are drilled into the reference block at different distances, e.g. ASME block. Prior to the test, the reference reflectors are scanned, and their maximised echo amplitudes are marked on the screen of the flaw detector. Finally all amplitude points are connected by a curve. This Distance Amplitude Curve (DAC) serves as the registration level and exactly shows the amplitude-over-distance behaviour" of the reference reflector for the probe in use. Also the individual characteristics of the material are automatically considered. However, this curve may only be applied for defect evaluation, in case the reference block and the test object are made of the same material and have undergone the same heat treatment. As with the DGS-Method, the value of any defect evaluation does not consider the shape and orientation of the defect. The reference block method is safe and easy to apply, and the operator need not to have a deep understanding about the theory of distance laws. 

Side drilled holes are widely used as reference reflectors, especially when angle beam probes are used (e.g. for weld testing). However, the distance law of side drilled holes is different to that of a flat bottomed hole. In the literature [2] a conversion formula is given which allows to convert the diameter of a side drilled hole into the diameter of a flat bottomed hole and vice versa, valid in the far field only, and for diameters greater than 1.5 times the wave length. In practical application this formula can be used down to approximately one nearfield length, without making big mistakes. Fig. 2 shows curves recorded from real flat bottomed holes, and the uncorrected and corrected DGS curves. 

In the case the element are focused in the elevation plane (our inner tube probe), it may be more convenient (or easy) to use a flat reflector at the focus plane or at its equivalent focus plane in water, for instance. 

As any conventional probe, acoustic beam pattern of ultrasound array probes can be characterized either in water tank with reflector tip, hydrophone receiver, or using steel blocks with side-drilled holes or spherical holes, etc. Nevertheless, in case of longitudinal waves probes, we prefer acoustic beam evaluation in water tank because of the great versatility of equipment. Also, the use of an hydrophone receiver, when it is possible, yields a great sensitivity and a large signal to noise ratio. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

The acoustical device component is placed in water and is configured like a conventional impulse echo equipment. The ultrasound wave passed the delay path and enters the specimen container through a very thin plastic window. The backside of the container is a steel plate and will also be used as a reference reflector to measure pn. 

Beryllium has a high x-ray permeabiUty approximately seventeen times greater than that of aluminum. Natural beryUium contains 100% of the Be isotope. The principal isotopes and respective half-life are Be, 0.4 s Be, 53 d Be, 10 5 Be, stable Be, 2.5 x 10 yr. Beryllium can serve as a neutron source through either the (Oi,n) or (n,2n) reactions. Beryllium has alow (9 x 10 ° m°) absorption cross-section and a high (6 x 10 ° m°) scatter cross-section for thermal neutrons making it useful as a moderator and reflector in nuclear reactors (qv). Such appHcation has been limited, however, because of gas-producing reactions and the reactivity of beryUium toward high temperature water. 

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