Fission resonances

Subbarrier Fission Resonances and Transitional States Observed in. 311... 

In the second step of the analysis, the proton energy spectrum below 5.2 MeV was analyzed. In the U(t,pf) reaction. Backet al. (1974) observed a weak, narrow resonance at 5.0 MeV and a distinct shoulder (or resonance) around 5.15 MeV. Goldstone et al. (1975) and Just et al. (1979) reported the first clear observation of a series of narrow subbarrier fission resonances in produced in the (d,pf) reaction. In their analysis, the underlying states of these resonances were assumed to originate from the second well, close to the top of the inner barrier. 

In contrast to the isotopes, where sharp HD fission resonances have been identified... 

However, according to the calculations on the height of the fission barriers and on the depth of the third potential well (Cwiok et al. 1994 Moller et al. 1972), the appearance of fission resonances representing HD states is expected also in the case of... 

The orbackground concept for calculating resonance self-shielding the capture and fission resonances may be in error by 0.1 to 0.2% in kg the errors due to an improved correction for anisotropic down scattering are about as large. These relatively small corrections cannot account for the large discrepancies observed for Pu-fueled assemblies. The recent and probably more reliable results for o-Pu will give only a small improvement. 

Eliminating from the reactor prevents the production of more Pu and accelerates plutonium destruction. A major disadvantage of removing jg, e reduction or elimination of a prompt negative Doppler reactivity coefficient. As the plutonium fuel temperature ri.ses, the plu< tonium fission resonances broaden increasing the core reactivity. Without adequate control, this can lead to even higher fuel temperatures and cause a reactor accident. In addition, the delayed neutron fraction for - Pu (0.0022) is much smaller than that of (0.0069). This also makes a pure plutonium reactor more difficult to control. Removal of eliminates the primary reactivity holddown mechanism in LWRs. 

The reverse reactivity is noted in the acid-catalyzed hydrolysis of the esters. Pyrrole-3-carboxylic esters are hydrolyzed upon dissolution in concentrated sulfuric acid and subsequent dilution with ice. Evidence has been presented indicating that unimolecular acyl—O fission forms the resonance-stabilized pyrrolyl acylium ion (B-77MI30505). 

In nuclear reactors one has neutrons with energies ranging from thermal (0.025 eV) to several MeV. There are a series of sharp peaks in the total cross section for neutrons with energies between 0.2 and 3000 eV that are called resonances. These resonances correspond to exciting a specific isolated level in the compound nucleus that can decay by fission. The situation is particularly interesting for the neutron irradiation of even-even nuclei, such as 240Pu at subthreshold energies... 

Fig. 11.8). The resonances associated with fission appear to cluster in bunches. Not all resonances in the compound nucleus lead to fission. We can understand this situation with the help of Figure 11.9. The normal resonances correspond to excitation of levels in the compound nucleus, which are levels in the first minimum in Figure 11.9. When one of these metastable levels exactly corresponds to a level in the second minimum, then there will be an enhanced tunneling through the fission barrier and an enhanced fission cross section. 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

The H-l NMR spectrum of the cyclometallated hydride (Structure XI) shows no hydride resonance at room temperature, but on cooling a broad resonance starts to appear, and at -62°C it shows resolved coupling to 31P (19 Hz) and 103Rh (55 Hz). We ascribe this temperature-dependent behavior to the occurrence of a rapid, reversible C-H/Rh-H fission such as shown in Scheme II. Alternatively there may be rapid, reversible oxidative additions, Structure XII Structure XI, as in Scheme I. 

Benzocyclobutene-l,2-dione (74) undergoes base-catalysed ring fission between the carbonyls to give 2-formy I benzoate (75). Rate constants, activation parameters, isotope effects, and substituent effects have been measured in water.107 Rapid reversible addition of hydroxide to one carbonyl is followed by intramolecular nucleophilic attack on die otiier, giving a resonance-stabilized carbanionic intermediate (76a)-o-(76b). 

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