Neutron multiplication

A critical assembly is a split bed on which fissionable material used to mock up up a separated reactor core that is stacked half on each half. One half is on roller guides so that the two halves may be quickly pulled apart if the neutron multiplication gets too high. Use the Preliminary Hazards Analysis method described in section 3,2.1 to identify the possible accidents that may occur and the qualitative probabilities and consequences. List the initiators in a matrix to systematically investigate the whole process. Don t forget human error. 

More advanced applications of neutron counting were based on the expectation that spontaneous fission events of superheavy nuclei should be accompanied by the emission of about ten neutrons [41,42], distinctly more than two to four observed for any other spontaneous fission decay. Such neutron bursts can be recognized by recording neutron multiplicities - events with several neutrons in coincidence - with 3He-filled counting tubes [43,44] or large tanks filled with a liquid scintillator sensitive to neutrons [45],... 

Fig. 8. Spontaneous fission activity in hot spring water at the Cheleken peninsula after concentration by ion exchange and precipitation methods. Shown is the measured neutron multiplicity distribution (dots) compared with measured distributions for 238U, 246Cm and 252Cf spontaneous fission and calculated distributions for sets of v and a2, the average number of neutrons per fission and its variance. Reproduced from G.N. Flerov et al. [48], Fig. 1, copyright (2002), with permission from Springer-Verlag.

No indications of spontaneous fission activities on the moon surface were obtained by neutron multiplicity counting of 3 kg of lunar rocks [45]. 

Stimulated by these studies, samples of primitive meteorites were inspected by neutron multiplicity counting. In the Allende meteorite available in large quantities, a weak fission activity at the 1014 g/g level was reported [71-73] but could not be chemically enriched [74-76]. 

For use in nuclear weapons, the concentration of °Pu in the plutonium should be low, because the presence of this nuclide leads to the production of appreciable amounts of neutrons by spontaneous fission if the concentration of °Pu is too high the neutron multiplication would start too early with a relatively small multiplication factor, and the energy release would be relatively low. Higher concentrations of " Pu also interfere, because of its transmutation into " Am with a half-life of only 14.35 y. To minimize the formation of " °Pu and " Pu, Pu for use in weapons is, in general, produced in special reactors by low bum-up (<20 000 MWth d per ton). 

Transmutation. Recycling actinides to the LWRs will decrease the average material neutron multiplication factor by only 0.8 percent, provided that they are of high purity [C2], Recycling to LMFBRs, however, will be preferred. There will be less neutron capture in impurities, such as lanthanides, and the average fission-to-capture ratio of the actinides should be higher in a fast spectrum than in a thermal one. 

These 112 thermal neutrons constitute the second generation of neutrons which according to our definition of neutron multiplication factor is so that... 

In order for these devices to function efficiently, the neutron multiplication must not begin until the critical size has been well exceeded. Furthermore, the number of neutrons in the initial generation must be sufficiently high in order to reach a very high n-flux in the short lifetime of the supercritical configuration. Therefore, a few microseconds before maximum... 

Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient) than will a U-fueled reactor. 

This section will cover some simple calculations related to the reactor. The reactor had a cold, beginning of life, neutron multiplication factor of 1.037 0.001, which corresponds to an excess positive reactivity of 5.7 based on a delayed neutron fraction of 0.0065. The burnup for the reactor was determined using a fairly simple set of equations. The consumption over 10 years at a power level of 200 kWth was 0.8 kgs of and at 400 kWth, 1.6 kgs of would be consumed. Given that the fuel loading is 186 kgs of the burnup is -0.86% for the upper end of the uranium consumed. This burnup results in a loss of 1 reactivity. 

The primary goal of this study was to ensure that there was sufficient excess reactivity in the neutron multiplication factor to keep the reactor critical for the 10 year lifespan while ensuring that the reactor would be subcritical during major accident scenarios. The position of the reflector can be used to set the multiplication factor of the reactor. Burnup in the reactor causes a proliferation of additional materials to absorb neutrons and reduces the density of fissile materials, lowering the neutron multiplication of the reactor. This can be offset by closing the reflector, which decreases the neutron leakage of the system. This is shown in Figure 5-1. 

The change in neutron multiplication vs reflector position is nearly linear for an extended region. Beyond 22 cm it starts curving, asymptotically approaching the neutron multiplication factor of the core without reflectors. The combined worth of the moveable reflectors is roughly 26.5 from full open to full closed position. Figure 5-2 shows how the reflectors open up on the core and where the centerline of the reactor is. It also indicates the length of the reflectors. 

The neutron multiplication factor (k-effective) was 0.964 0.001, well below the desired value of 0.985. 

The Virus House was finished in October. Besides a laboratory the structure contained a special brick-lined pit, six feet deep, a variant of Fermi s water tank for neutron-multiplication studies. By December Heisenberg and von WeizsScker had prepared the first of several such experiments. With water in the pit to serve as both reflector and radiation shield they lowered down a large aluminum canister packed with alternating layers of uranium oxide and paraffin. A radium-beryllium source in the center of the canister supplied neutrons, but the German physicists were able to measure no neutron multiplication at all. The experiment confirmed what Fermi and Szilard had already demonstrated that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction. 

The pile at Haigerloch had served for the KWI s final round of neutron-multiplication studies. One and a half tons of carefully husbanded Norsk-Hydro heavy water moderated it its fuel consisted of 664 cubes of metallic uranium attached to 78 chains that hung down into the water from the metal shield Pash describes. With this elegant arrangement and a central neutron source the KWI team in March had achieved nearly sevenfold neutron multiplicatioi Heisenberg had calculated at the time that a 50 percent increase in the size of the reactor would produce a sustained chain reaction. 

Peierls, Rudolf. 1939. Critical conditions in neutron multiplication. Proc. Camb. Phil. Soc. 35 610. 

Beyond the peak the cross section falls to some low value. The yield of neutrons at higher energies has been measured by Jones and Terwilliger2 from about I3 Mev to 320 Mev. The slow rise in the neutron yield found by these authors can be attributed, in part, to an increase in the neutron multiplicity as successively more reactions become energetically possible. 

However, if absorption predominates over multiplication, the right side of (3) is larger than the left side while the reverse is true if the contribution of the neutrons of energy E to neutron multiplication predominates. Thus, by suspending the validity of (3), the expression (lb) can accoimt also for absorption and for the multiplicative processes such as fission. 

Reactivity Functional parameter of a nuclear reactor, which expresses an instantaneous balance of the neutron multiplication processes and represents an index of the tendency to the variation of the power generated in the core at a certain instant. If reactivity is zero, then the power stays constant if the reactivity is positive, the power increases and the contrary happens if the reactivity is negative. Release (of fission products) Dispersion of radioactive contamination outside one or more design barriers (s). 

The goal is to obtain the maximum aedible neutron multiplication such that subcriticality is assured. 

Neutron absorbers are sometimes employed in the packaging to reduce the effect of moderation and the contribution to the neutron multiplication resulting from interaction among packages (see para. 501.8). Typical neutron absorbing materials used for criticality control are most effective when a neutron moderator is present... 

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