Generators, neutron flux

This paper presents calculation and experimental studies of a moderator with a thermal neutron extraction channel, based on an NG-400 pumped neutron generator produced by the All-Russia Automation Research Institute. The neutron generator provides a maximum 14-MeV neutron flux density of 5T0 n/cm -s on the outer surface of the target chamber. 

In practice the assumption of the uniform heat release per unit length of the rod is not valid since the neutron flux, and hence the heat generation rate varies along its length. In the simplest case where the neutron flux may be taken as zero at the ends of the fuel element, the heat flux may be represented by a sinusoidal function, and the conditions become as shown in Figure 9.20. 

Since the heat generated is proportional to the neutron flux, the heat dQ developed per unit time in a differential element of the fuel rod of length dx may be written as ... 

Polonium was considered a rarity. With beryllium, the alpha emitter generates a strong neutron flux. Used as the detonator in the first atomic bombs. 

The incorporation of associated alpha particle detection in a sealed tube neutron generator (STNG) appears to severely aggravate the concerns over the limited neutron flux and tube lifetime previously detailed for STNG FNA approaches. A mean time to failure of some APSTNGs at a neutron flux of lO n/s is about 200 h [24]. Work is continuing to improve this mean time to failure. 

In peaceful uses of nuclear reactions, electrical power plants can be driven by a nuclear reactor very close to criticality, with careful control of neutron flux excess heat from the well-shielded nuclear reactor is driven off by a liquid (H20, Na, or Hg), which in a secondary cycle or a tertiary cycle generates electricity by turning induction turbines. 

In order to control subcriticality of SRP s storage facilities by the stationary method, it is necessary to know the induced fission neutron generation rate per gram of fuel (Qind) and the rate of neutron generation by above-mentioned sources per gram of fuel (Qsp) in the maximum neutron flux points [17], If Qsp and Qmd are known, the system multiplication can be written in the following way ... 

In such systems (Fig. 7.5), spallation reactions induced by a high-intensity beam (10 to 250 mA) of GeV protons on a heavy target produce an intense neutron flux. These neutrons, after being more or less moderated, are used to drive a sub-critical blanket. The extra neutrons provided by the accelerator allow the maintenance of the chain reaction while burning the long-lived nuclear waste. The plant generates electricity, part of which is used to supply the accelerator. 

Early studies (1936-1950) of neutron scattering used radium-beryllium neutron sources but their low neutron flux prevented exploitation of neutron scattering as a spectroscopic technique [4]. Today neutrons are either extracted from a nuclear reactor or generated at a pulsed, accelerator-based spallation source. The exploitation of neutrons from nuclear reactors in structural studies and spectroscopy dates from the 1950s and from pulsed sources from the 1970s. A useful summary of the development of neutron sources is given in [5]. 

The cross section for this exothermic reaction peaks at a deuteron kinetic energy of about 120 keV with a value of about 5 b. The neutrons produced have an energy of about 14 MeV. (The neutron kinetic energy changes slightly with the direction of neutron emission.) The maximum neutron flux provided by a neutron generator is of the order of 10 neutrons/(m s). 

The cross section for this reaction peaks at about 2-MeV bombarding deuteron energy with a value of about 100 mb. At acceleration voltages normally used in neutron generators ( 150 kV), the cross section is about 30 mb. The (d, d) reaction offers neutron fluxes of the order of lO neutrons/(m s). It is important to note that both the (d, t) and the (d, d) reactions produce essentially monoenergetic neutrons. 

Unfortunately, the gamma fluxes generated by these reactions are very small, relative to neutron fluxes produced by reactors. 

For the determination of arsenic in biological material approx. 300 mg of dried matter was sealed in ultraclean silica vials, irradiated at a thermal neutron flux of approximately 9x1 o n cm sec . After irradiation the vials were washed with concentrated nitric acid, frozen in liquid nitrogen to reduce the internal pressure and crushed in a plastic container. The samples were then decomposed under pressure with a mixture of concentrated nitric and sulphuric acid, evaporated to near dryness after opening the vessels, taken up in hydrochloric acidic solution, made up to volume and after reduction with Kl subjected to an arsine generation step. The evolved arsines were trapped, the trap sealed into a polyethene vessel and the 560 keV gamma photopeak from As counted. The authors reported absolute detection limits of either 0.5 fig or 0.05 fig depending on the detector used (Orvini and Delfanti, 1979)... 

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