Prompt fission

Equation (11a) is a transcendental equation for B, It can be simplified to some degree as follows (CP-2881). Let us denote the average age of prompt fission neutrons by... 

Measurements on time distribution of prompt fission neutrons using Rossi-alpha method (applicable for Peff measurements) were made on this assembly. Prompt neutron decay constants were measured for various arrangements of polyethylene moderator discs in the assembly reflector, including the case when there were no moderator discs in the reflector. It became possible to explain the presence of short decay periods owing to the corrected coefficient of cormection between the core and the polyethylene discs in the firamework of the two point model. 

Cylindrical reactor with source. An attempt is made here to describe the kinetics of a homogeneous cylindrical reactor with a point source of fast neutrons located at an arbitrary interior point on the axis of the reactor. The theory developed is quite general, at least to the extent that five delayed neutron groups are taken into consideration and appropriate ages are assigned to the source neutrons, the prompt fission neutrons, and each of five groups of delayed fission neutrons. Application of the theory is made to the problem of the determination of the power level in a so-called zero power critical assembly. 

The result of the analysis in the excitation-energy region between 5.05 and 5.2 MeV is shown in Fig. 5.15, where the prompt fission probability is displayed (panel a) together with the results of a fit by rotational bands similar to the procedure described above for the upper resonance region. Again the spin-dependent excitation probabilities of the rotational band members were based on DWBA calculations by Back et al. (1971) for E " = 5.1 MeV. 

In summary, study of the prompt fission probability of reveals the shape of the fission... 

With all the other parameters obtained as described above, Eq. (3) can then be solved for the age of prompt fission neutrons to thermal energy Tp. The nonlealB e probabilities Kp(B) and Kd(B) are then calculated by Eq. (2) and these quantities are used in Eq. (1) to obtain koo. The results, along with the parameters used in the calculation, are gdven in Table I. 

Upon neutron capture in the target nucleus A, a highly excited compound nucleus is generated which decays within about 10 s, forming the two fragments B and D. About 10" s later, the prompt fission neutrons n are emitted in the thermal... 

Answer Since the prompt fission neutrons are released almost instantaneously, the generation time as far as these neutrons are concerned, is one-thousandth of a second. However, when the delayed neutron time of rou ly one per cent of the neutrons is averaged with the lifetime of the 99 per cent prompt neutrons, the effective generation time is changed from one-thousandth of a second to one-tenth of a second. 

Prompt fission neutrons are emitted with the continuous energy spectrum shown in Figure 2.7. The spectrum spans an energy range of approximately 10 MeV, from less than 1 10 MeV. The average prompt neutron energy is about 2 MeV. 

In a neutron generation, the prompt fission neutrons in one generation, n f, in Figure 3.4, undergo multiplication to produce prompt fission neutrons in the succeeding generation. 

Fig. 2.5. Energy spectrum of prompt fission neutrons the function p(E) gives the relative probability of emission of a neutron of energy E.

The 1-D discrete ordinates model (XSDRNPM), used for the power density profile calculation shown in Fig. 5.1, was used to calculate the dose rate at the outside surface of the graphite reflector. The calculation was performed with the SCALE 27 neutron-18 gamma group library (ENDF/B-V). The dose rates were calculated at the axial midplane and 1 cm above the outside surface of the reflector. The neutron dose rate, as calculated with the American National Standards Institute (ANSI) standard flux-to-dose-rate factors, was 5200 rem/h, and the gamma dose rate (ANSI standard) was 250 rem/h. The gamma dose rate should be regarded as a minimum estimate. Software and data library developers are uncertain as to whether the prompt fission gammas were accurately represented in the calculation. 

Other Reactions.—A method which has been particularly stressed by Russian workers, e.g. Flerov et al., and which bears a superficial resemblance to the above, is what might be called the prompt fission reaction. Here two heavy nuclei are caused to fuse, giving transiently a very large mass which undergoes prompt fission to give a wide spectrum of products, including, it is hoped, some superheavy nuclei. 

Prompt Fission No high 2 U -b U fission products including 114... 

Since we are considering a critical reactor, we may assume that there is no primary source present. However, since we are interested in the effects of delayed neutrons, we must consider our source term to be composed of two parts the source of prompt fission neutrons and the source of delayed fission neutrons. If we let represent the fraction of the total number of fission neutrons that are delayed and the fraction that are associated with a single time of delay (a single precursor), then (1 - jS) is the fraction of fission neutrons that are emitted promptly. We can then write our prompt source term as before, except that it must be decreased by a factor (1 - jS). An additional source term must be included for the delay-neutron source. The rate of formation of delayed neutrons of the i h kind is equal to the rate of decay of its precursor, which is... 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

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