Comparing various benchmarks experiments with calculated gamma heating showed a systematic underestimation for the main fuel isotopes U and Pu. Prompt neutron lifetime, l , is the average time from a prompt neutron emission to its absorption fission or radiative capture or its escape from the system.
This parameter is defined in multiplying or also in non-multiplying systems. In both systems the prompt neutron lifetimes depend strongly on:. In an infinite reactor without escape , prompt neutron lifetime is the sum of the slowing downtime and the diffusion time.
Generally, the longer neutron lifetimes occur in systems where the neutrons must be thermalized to be absorbed. Most neutrons are absorbed in higher energies, and the neutron thermalization is suppressed e. This time is known as the prompt neutron generation time. The prompt neutron generation time is designated as:.
In power reactors, the prompt generation time changes with the fuel burnup. In LWRs increases with the fuel burnup. It is simple. This causes significant excess of reactivity, and this excess must be compensated via chemical shim in case of PWRs or burnable absorbers.
Due to these factors high probability of absorption in fuel and high probability of absorption in moderator , the prompt neutron lives much shorter, and the prompt neutron lifetime is low. An equation governing the neutron kinetics of the system without source and with the absence of delayed neutrons is the point kinetics equation in a certain form. This equation states that the time change of the neutron population is equal to the excess of neutron production by fission minus neutron loss by absorption in one prompt neutron lifetime.
The role of prompt neutron lifetime is evident. Shorter lifetimes give simply faster responses to multiplying systems. It must be noted such reactivity insertion 10pcm is very small in case of LWRs. The reactivity insertions of the order of one pcm are for LWRs practically unrealizable. In this case the reactor period will be:.
This is a very short period. Furthermore, in the case of fast reactors in which prompt neutron lifetimes are of the order of 10 -7 seconds , the response of such a small reactivity insertion will be even more unimaginable. In the case of 10 -7 , the period will be:. Reactors with such kinetics would be very difficult to control. Fortunately, this behavior is not observed in any multiplying system. Actual reactor periods are observed to be considerably longer than computed above, and therefore the nuclear chain reaction can be controlled more easily.
The longer periods are observed due to the presence of the delayed neutrons. Look at the reactivity insertion you need to insert to stabilize the system of the order to a tenth of pcm. The prompt neutron lifetime belongs to key neutron-physical characteristics of the reactor core. Its value depends especially on the type of the moderator and the energy of the neutrons causing fission.
Its importance for nuclear reactor safety has been well known for a long time. The longer prompt neutron lifetimes can substantially improve the kinetic response of the reactor the longer prompt neutron lifetime gives simply a slower power increase.
Product Type:. Publication Date newest to oldest Publication Date oldest to newest Relevance. ETDE Web. Prompt fission neutron logging for uranium. Full Record Journal Article:. Abstract A direct uranium logging technique using prompt fission neutrons PFN has been developed. Epithermal neutrons and thermal neutrons returning from the formation following fission are counted separately in detectors in the logging tool. The time-gated ratio of epithermal to thermal neutron counts provides a measure of uranium content which is free of perturbations from source output variation and borehole and formation effects common to both signals.
Logs obtained with the technique have shown good agreement with core measurements, and they have provided a major data source for delineation and exploitation of uranium mineralization. The PFN technique is also suitable for detecting other thermal neutron fissionable material, such as plutonium, which can occur as a result of waste disposal. The thermal and epi-thermal neutrons returning to the tool from the formation are counted in separate detector channels to provide a measure of U free from variations in neutron output and borehole factors common to both channels.
The tool also contains a standard scintillation gross gamma ray counter. The tool has no electric logs resistivity and self-potential. The lowest practical grade measurement is about. The tool must be calibrated by taking measurements in environments of known grade and porosity. An acceptable test pit is a 1 meter diameter and 1 meter deep polyethylene tank, usually installed in an excavation so only the top of the tank is exposed, filled with a specific grain size sand into which is poured a solution of uranium dissolved in nitric acid.
Multiple pits are required to establish a calibration curve. The minimum is three grade pits high, medium, and low and one barren pit.
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