Prompt neutron

The products of nuclear fission reactions are radioactive and disintegrate according to their own time scales. Often disintegration leads to other radioactive products. A few of these secondary products emit neutrons that add to the pool of neutrons produced by nuclear fission. Very importantly, neutrons from nuclear fission occur before those from radioactive decay. The neutrons from nuclear fission are termed prompt. Those from radioacth e decay arc termed delayed. A nuclear bomb must function on only prompt neutrons and in so doing requires nearly 100 percent pure (or Pu) fuel. Although reactor... 

Prompt neutrons are ihose newrons released coinrideni with a fission process. 

As noted earlier, the prompt neutrons are emitted from the fully accelerated fragments after scission. The number of these neutrons, Vy, (= 2.4 in the case of 235U) as... 

Prompt y-ray emission competes with or follows the last stages of prompt neutron emission. These photons are emitted in times from 10 15-10 7s. Typical y-ray multiplicities of 7-10 photons/fission are observed. These photons, as indicated earlier, cany away 7.5 MeV. This y-ray yield is considerably larger than one would predict if y-ray emission followed neutron emission instead of competing with it. Because of the significant angular momentum of the fission fragments ( 7-10 h) even in spontaneous fission, photon emission can compete with neutron emission. The emitted y rays are mostly dipole radiation with some significant admixture of quadrupole radiation, due to stretched El transitions (J/= 7, — 2). 

The prompt neutrons emitted in fission are available for fission in other nuclei - hence the chain reaction. The fission fragments formed initially are rich in neutrons. For example the heaviest stable isotopes of krypton and barium are 86Kr and 138Ba. Excess neutrons are emitted from the fission fragments as delayed neutrons or converted to protons by beta decays. For example... 

The MMKFK-2 code system can be used for nuclear safety analysis, namely, for calculation of NS LMR SRP s Keir, L, a and Pen parameters (where Keff is an effective breeding factor L is a prompt neutron lifetime in the multiplication system under calculation Peff is an effective fraction of delayed neutrons a is a damping coefficient). 

For the self-consistent calculation of Keff, Pefr, L and a by the Monte-Carlo method in the MMKFK-2 code package there is a code called MCDENSP [5]. By means of the perturbation theory this code allows the calculation of prompt neutron lifetime L as a... 

Tarasova, O.B. and Polevoi, V.B. Determination of Prompt Neutron Lifetime by the Monte-Carlo... 

Bezhunov, G.M., et al. (1989) Experimental and analytical study of prompt neutron lifetime in fast reactors with moderation zones in the reflector, in Neutronic problems of nuclear power system safety paper theses VI AU-Union Workshop on Reactor Physics, Tsniiatominform Publishers, Moscow, pp.48-50 (in Russian). 

Daruga, V.K. and Polevoi, V.B. (1989) Experimental Testing of the Results on the Prompt Neutron Lifetime as a Function of Reactivity of Fast Reactor with a Moderating Reflector, Calculated by the Monter-Carlo Method, IPPE preprint 2027, Obninsk (in Russian). 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

The excitation energy of the primary fission fragments is given off by emission of prompt neutrons with energies varying between about 0 and lOMeV (mean value... 

MeV) and of prompt y-ray photons. The number of prompt neutrons emitted by the primary fission fragments depends mainly on their excitation energy. It increases with the mass number of the fissioning nuclei (Table 8.2). In Fig. 8.17 this number is plotted as a function of the mass of the fission fragments. It is relatively low for fragments with filled neutron shells N = 50, = 82). 

For operation of nuclear reactors, the delayed neutrons (section 8.9) play an important role, because they cause an increase in the time available for control. The multiplication factor due to the prompt neutrons alone is kesi - P), P being the contribution of the delayed neutrons, and as long as / eff(l P) < 1, the delayed neutrons are necessary to keep the chain reaction going. In the fission of 0.65% of the fission neutrons are emitted as delayed neutrons from some neutron-rich fission fragments such as or Xe. 

Six groups of fission products are particularly important to the control engineer. These decay to form daughter nuclei that are in a sufficiently excited state to throw off a neutron on formation. Such neutrons are called delayed neutrons, and the six groups of fission products are known as delayed neutron precursors. Delayed neutrons make up less than 1% of the total number of neutrons liberated by fission, but the fact that they are released on a much slower timescale than the prompt neutrons liberated at the time of fission renders the control of nuclear reactors relatively easy. 

Since the total number of neutrons in the next generation will be proportional to k and the number of next-generation prompt neutrons will be proportional to kp, it follows that the fraction of prompt neutrons in the next generation will be kp/k. Similarly the fraction of next-generation delayed neutrons will be kdi/k, for I = 1 to 6. The delayed neutron fraction for group i is given the symbol ft, so that... 

From Figure 21.3 we may see that an absorption event occurs every Si seconds, at which point no neutrons (delayed and prompt) are absorbed and k no prompt neutrons are produced. Let the decrease in neutrons in an absorption event be called Sn , where... 

Turning now to the prompt neutrons formed, let us name the increase in prompt neutrons immediately following an absorption event Snp, so that we have... 

Figure 21.3 Neutron absorption and the production of prompt neutrons and delayed-neutron precursors.

Fission cross sections are denoted by For fissionable isotopes of thorium and elements of higher atomic number, the average number of neutrons produced per fission is listed in the same row as the fission cross section, in the same column as the mass, to conserve space in the table. The average number of prompt and delayed neutrons produced by fission with a thermal neutron is denoted by V. The average number of prompt neutrons produced by fission with a thermal neutron is denoted by Vp. The average number of neutrons emitted per spontaneous fission is denoted by t jp. ... 

Glendenin (87) attempted to explain the high yields of the 133 and 134 mass chains by assuming that nuclei with 83 neutrons (which has already emitted the usual number of prompt neutrons) would have a high probability of boiling off a prompt neutron to form the most stable 82 neutron configuration rather than emit or y rays as in the ordinary case. According to this scheme, the 134 mass chain would be increased by the reaction... 

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