Neutron emitters, delayed

The simplest model of time-dependent behavior of a neutron population in a reactor consists of the point kinetics differential equations, where the space-dependence of neutrons is disregarded. The safety of reactors is greatly enhanced inherently by the existence of delayed neutrons, which come from radioactive decay rather than fission. The differential equations for the neutron population, n, and delayed neutron emitters, are... 

Example Problem An important delayed neutron emitter in nuclear fission is 137I. This nuclide decays with a half-life of 25 s and emits neutrons with an average energy of 0.56 MeV and a total probability of approximately 6%. Estimate the energy of an excited state in 137Xe that would emit a 0.56-MeV neutron. 

The fact that neutrons can be detected with reasonably high efficiency and with minimal interferences from other radiations permits the practical determination of fissionable species such as isotopes of uranium and thorium by delayed neutron counting. The known delayed neutron emitter precursors are all short lived and the irradiated samples are counted with 10BF3-filled proportional counters immediately after irradiation without any separation chemistry. 

In a study of 58-sec lodine-157 as a delayed neutron emitter in fission products, a novel target arrangement was used (255) This consisted of a hydrochloric acid solution (150 ml, containing 1 ml cone. HCl) of uranyl nitrate to which several milligrams of potassium iodide and bromide carriers,... 

In the case of a few light nuclei, the radioisotope formed is also unstable against neutron emission, and delayed neutron emitters have been found. 

The first technical problem of a reactor is its criticality. It was possible to think, at one time, that the calculation of the nuclear behavior of a reactor had been reduced to standard operations. Bight now, we do not know whether this is true. It is certain, however, that the standard operations of the present time are very different from the standard operations of five or ten years ago. The last five years or so brought an extension of the reactor theory to nonhnear cases, that is, to power densities high enough to affect the reactor s properties in periods comparable with the natural period of the reactor, that is comparable with the lifetimes of the delayed neutron emitters. If this situation prevails— and it was present in some reactors which were in operation more than ten years ago—the dynamic behavior of the reactor does not obey a linear equation any more. There are effects which are proportional to the square of the neutron density because they depend not only on the number of neutrons but also on the temperature increase created by these neutrons. A characteristic of such a behavior is the deviation from the exponential or sine-like time dependence of the neutron densities. 

In PHENIX and SUPERPHENIX, primary sodium is taken directly from the vessel and transported by an assembly of pipes and pumps to a module located on the reactor slab. The presence of any delayed neutron emitters is then revealed by neutron counters. 

The experiments are aimed at finding a system to measure delayed neutron emitters directly in the vessel. This offers the following advantages no sampling device is required, the system is simplified, costs are reduced, safety is enhanced through a reduction in response times and the system fully complies with the integrated concept. 

The investigation of safety and more particularly of severe accident conditions is important for accelerator driven systems (ADS). Subcritical ADS could be of particular interest for the actinide transmutation from the safety point of view, because fast reactors with Neptunium, Americium and Curium have a much smaller fraction of delayed neutron emitters (compared to the common fuels and U), a small Doppler effect and possibly a positive coolant void coefficient. This poses a particular problem of control since the fraction of delayed neutrons is essential for the operation of a nuclear reactor in the critical state. In addition, the IRC presented in the past a review of accelerator-driven sub-critical systems with emphasis on safety related power transients followed by a survey of thorium specific problems of chemistry, metallurgy, fuel fabrication and proliferation resistance. 

Equation (10) is a symbolic way of writing the Boltzmann equation for the kernel H however, in the case where delayed neutrons are considered, it can be used to determine those components of H which are associated with the delayed neutron emitters. In this respect (10) is more general than the Boltzmann equation which ordinarily deals only with the neutron fluxes, not the density of delayed neutron emitters. 

The same arguments may be used in the case that a neutron death is accompanied also by the prompt production of delayed neutron emitters. Each delayed neutron emitter decays according to the radioactive decay law with mean life r and emits a neutron coincident with its decay. The resulting equations are... 

Thus, N may be regarded as the total neutron importance in the reactor as in Equation (68), the Ct as the latent importance of the delayed neutron emitters as in Equation (69), and... 

An additional equation can be derived from (9) and (10) which is very important in experimental low power reactor physics. When a reactor undergoes a step-insertion of reactivity, which renders the reactor slightly supercritical, for example, and when transient disturbances die out, the neutron and delayed emitter populations will behave as e, where o) is the largest root of the inhour equation, obtained from (9) and (10) by taking fi(t) F(t) e ... 

The longest-lived example of such a delayed neutron emitter (or delayed neutron precursor) is the nuclide Br. It undergoes P decay with a half-life of 55 s. The daughter nucleus is the... 

In some cases the fission product nucleus decays by neutron emission. Examples of such delayed neutron emitters are the iodine isotopes 137, 138, 139 with halflives of 24, 6, and 2.7 seconds, respectively, and the bromine isotopes 87, 88, 89,90 with halflives of 55, 16, 4.5, and 1.6 seconds, respectively. As a consequence of the... 

Measuring the flux ratio directly is difficult the shortest-lived delayed neutron emitter group has a half-life of only 0.23 seconds, so determining the point at which the control element is full in is not easy. The computer-based acquisition system is, however, fast enough when sampling at 10 msec intervals, so use of the prompt drop approximation will be used as one method of measurement. 

Half-lives of delayed neutron emitters vary from 55.7 sec. to 0.23 sec the mean life ( ) of delayed neutron emitters is 12.7 seconds. 

Certain photon reactions of the light elements give rise to delayed neutron emitters, e.g. 

Reactivity effects. A problem uniciue to circulating fuel reactors is the loss of delayed neutrons in the external circuit. Since the time spent by the delayed-neutron emitters outside the reactor core is generally greater than that spent within the core, a considerable fraction of the delayed neutrons may be wasted. In addition, since most of the delayed-neutron emitters are produced as gases, they may be carried off during degassing operations. For the delayed neutron fraction in thermal fission is only... 

Shielding. Shielding of an LMFR is complicated by the necessity of shielding an external circuit in which the delayed neutron emitters and fission products decay. 

We present below the main answers available to the first question (the second question will be dealt with in Section V), considering Eqs. (1) and (18) separately, and beginning with the former. For the purpose of stating the boundedness results it is convenient to reformulate these equations as follows. Solving the m last equations of Eqs. (I) for the delayed neutron emitters and substituting into the first equation yields, after dividing by p t). 

Radioactive decay by proton emission is a very seldom observed decay mode for very neutron deficient nuclides because decay by /S or EC normally has a very much shorter partial half-life ( 4.14). Decay by p" has been observed for " 0 E 1.55 MeV, ti/ 0.25 s, -1.5%). However, jS decay sometimes leads to a proton-unstable excited state which immediately (< 10 s) emits a proton. Several 0 emitters from to Ti with N = Z — 3 have /S delayed proton emission with half-lives in the range 10 — 0.5 s. Also radioactive decay by simultaneous emission of two protons has been observed for a few proton rich nuclides, e.g. Ne, 6 10 ° s. 

Highlights. Treatment of the radioactive waste from nuclear reactors is one of the points that receive wide public attention and the disposal and burial of HLW in particular is a contentious issue due to the concerns about leakage to the environment. The technical solutions that are currently used to treat the waste that were listed earlier (concentrate-and-contain, dilute-and-disperse, and delay-and-decay) are not suitable for HLW, where safer solutions like vitrification or Synroc are sought. The characterization of the LLW and MLW waste is not as complicated as that of spent fuel but stiU greatly more complex than analysis of fresh fuel. Some of the procedures and methods used in other parts of the NFC are suitable for LLW and MLW. The composition of HLW must be determined in order to estimate the decay rate of the radioactivity and to classify the required protective measures that depend on the radionuclides and their products (emitters of alpha, beta, gamma, and neutrons). 

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