Lifetime, neutron

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... 

From the brief description of proposed pulse and stationary methods for SRP subcriticality calculation it is quite obvious that they require the introduction of calculation constants, i.e. the specified values of delayed neutron lifetime and effective fraction for the pulse method and the neutron source for the stationary one. Thus, it follows that for these experimental methods to be realized the SRP s parameters that characterize multiplication properties have to be calculated with a good accruacy. 

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). 

Tarasova, O.B. (1994) Variation ofKeff and Neutron Lifetime in Fast Critical Facilities at the Change over from BNAB-78 Nuclear Data System to the BNAB-90 one, IPPE preprint 2355, Obninsk (in Russian). 

The other particles, with the collective name hadrons, are divided into two groups, the mesons and the baryons. Mesons are composed of two quarks and baryons of three quarks. Some mesons and baryons are listed in Table 3.4. All mesons are rather unstable, with lifetimes up to about 10 s. The baryons are also very unstable, with the exception of the neutron (lifetime 890 10 s) and the proton, which is considered to be stable whereas theoretical considerations predict a certain instability (lifetime >10" ° s). 

In contrast to the other light nuclides, the primordial abundance of 4He (mass fraction Y) is relatively insensitive to the baryon density, but since virtually all neutrons available at BBN are incorporated in 4He, it does depend on the competition between the weak interaction rate (largely fixed by the accurately measured neutron lifetime) and the universal expansion rate (which depends on geff)- The higher the nucleon density, the earlier can the D-bottleneck be breached. At early times there are more neutrons and, therefore, more 4He will be synthesized. This latter effect is responsible for the very slow (logarithmic) increase in Y with rj. Given the standard model relation between time and temperature and the nuclear and weak cross sections and decay rates measured in the laboratory, the evolution of the light nuclide abundances may be calculated and the frozen-out relic abundances predicted as a function of the one free parameter, the nucleon density or rj. These are shown in Fig. 1. 

Figure 2. The BBN-predicted primordial 4He mass fraction Y as a function of the BBN-predicted primordial Deuterium abundance (by number relative to Hydrogen) for three choices of N . The width of the bands represents the theoretical uncertainty, largely due to that of the neutron lifetime rn.

The time between neutron generations is the time between a neutron being produced and the time it is absorbed, into either a fissile or non-fissile nucleus. In reality this time interval will vary between individual neutrons, but we will make the simplifying assumption that the lifetime of all neutrons may be characterized by the average neutron lifetime, /, which is typically 1 millisecond in a commercial thermal reactor. Neutrons are being bom continuously in a reactor, and we may assume that at any instant of time the neutrons have a uniform spread of all ages between 0 and I seconds. Let us divide the neutron lifetime. /, into a large number, M, of time intervals. Si, where... 

The resulting elemental abundances predicted by standard BBN are shown in Figure 4 as a function of tf [22]. The left plot shows the abundance of e by mass, Y, and die abundances of the other three isotopes by number. The curves indicate the central predictions from BBN, while die bands correspond to the uncertainty in the predicted abundances. This theoretical uncertainty is shown explicitly in the right panel as a function of rf. The uncertainty range in He reflects primarily the la uncertainty in the neutron lifetime. 

A proper analysis of the tine dependent behavior of.a reactor operating on thermal neutrons must take into account the important effects on its criticality, reactivity, and stability which arise from such factors as fission i products of high thermal-neutron capture cross-section, depletion, temperature, average neutron lifetime in the reactor, flux level, and reactor period. As has been seen in the requirements placed on the.reactor, considerable excess reactivity must be built into the active core before start-up. The control rods must keep the reactivity below the critical value before and during start-up. 

The neutron lifetime I and effective delay fraction pi for delay group may be computed directly from Equations (40) and (41) after solving for no and mo. Alternately, they may be evaluated without resorting to the calculation of the adjoint or to the integrations involved. Thus, a perturbation comparing the standard no reactor satisfying... 

Fig. 3. Influence of neutron lifetime on explosive energy release.

Q = peak energy density in the core (joules/gm) f = total delayed neutron fraction / = neutron lifetime (sec)... 

In a conventional point kinetics model, involving the concentration of xenon and the flux tending to bum it out, the problem is nonlinear. However, it seems reasonable to make the prompt jump approximation in which the neutron lifetime and the lifetime of the precursors are neglected in comparison with the periods of the iodine and xenon isotopes governing the problem. We can then regard the flux as a control variable that can be adjusted to different levels as required in an optimum program. 

The prompt neutron lifetime of the fourth assembly was measured by the Rossiet method and the noise analysis method. The resdlts were in good agreement, giving f= 14 microseconds for a/3 of 0.0085. 

C. W. Griffin, "Measurement of the SRE and KEWB Prompt Neutron Lifetime Using Random Noise and Reactor Osculation Techniques, NAA-SR-3765 (October 1959). 

Pulsed neutron techniques were used to measure neutron lifetimes on most of the cores. 

The experiments that have been performed are standard. Critical configurations have been determined for many different compositions. Reactivity coefficients have been determined by period measurements or by use of control rods calibrated by period measurements. Reactivity worths have also been determined analysis of the response to oscillator and rod drop experiments. Rossl-o measurements have been made on a number of assemblies to determine the ratio (rf the effective delayed neutron fractlqn to the prompt neutron lifetime and thus indirect to give information on the neutron spectrum. Detector responses, both as a function of detector material and as a function of position, have been made to determine data relevant to power distributions, bucklings, reflector savings, and neutron spectra. Spectrum measurements have been made by use of ehiulslon plates. 

Some results from the basic physics program were fission ratios at the core center of 0.029 for 0 U-238/of 0-235. 1.07 for o Pu-239/of U-235, and 0.23 for of U-234/of U7235, and a neutron lifetime of 4x lO sec from Rossi alpha measurement with a 0eff 0.0065. ... 

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