Neutron flux monitors

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

The classical calibration of NAA methods involves comparison of sample activities with those of co-irradiated standards of the same element. Especially at multi-element analysis, the need for a large number of standards has limited the sample throughput capacity. To circumvent this, alternative calibration techniques have been elaborated. The single comparator method makes multi-element determinations possible, by use of a single element standard (neutron flux monitor). The mass of the analyte is calculated by use of an experimentally determined element-specific factor (k-value), valid for the analytical equipment in question (Girardi et al., 1965 Linekin, 1973 Simonits et al., 1975). Later, a more generalized standardization method, based on accurately determined constants for the active compound nuclei (ko-factors), and applicable to various analytical equipments, has been proposed (Moens et al., 1984 De Code et al., 1987). 

The Neutron Flux Monitor System comnonly referred to as the "Beckmans"I is composed of four picoammetersi ion chambers and controllers for setting safety circuit trip points. The four ion chambers are located in instrument risers beneath the reactor and are positioned for equilibrium power level calibration ... 

Ihe present Beckmai s will be retained as the bottom neutron flux monitors in the any two-out-of-four tripplng network. TOie level trips of the Intermediate Range Monitor will be in a two-out< of-three network at the B, D, DR, F, and H Reactors where three channels will be installed. An "any-two-out-of-four" tripping network should be used at C and the K Reactors. 

Because of the addition of the top neutron flux monitoring system in the form of the level trip of the Intermediate Range Monitor, adequate protection can be still maintained with a reduced Zone Temperature Monitor (especially when considering... 

The surveillance Charpy and tensile samples were exposed together with neutron flux monitors (Ni and Co wires for fast and thermal neutrons. 

In the early 1970s, Simonits proposed the development of a standardization method using universal fc-factors. In this method, the essential information for a gamma ray emitted by any nuclide produced by neutron activation would be contained in a universal constant, the ko factor, and all factors depending on the specific irradiation and counting conditions would be calculated by models. The inventors of the ko method envisioned that for each sample analyzed at least one neutron flux monitor would be co-irradiated and counted, and all other parameters of the models would be measured once and only remeasmed when irradiation conditions changed. Thus, multielement analysis could be performed with the same amount of work needed for single-element analysis. 

Inadvertent dilution can in principle be detected using nuclear flux measurements and boron concentration monitoring. Neutron flux monitoring is difficult during reactor shutdown or startup phase, because neutron level in the ionization chambers is very low (10 to 10 n/ctn xs). The continuous boron concentration monitoring is based on boron meters which have limited sampling points in the RCS. 

There are two sets of ionization chambers (10 chambers in each set) for neutron flux monitoring which are located in a concrete wall of the reactor well. Two additional ionization chamber suspensions were introduced (1976) in the immediate vicinity of the guard vessel wall, that significantly enhanced reactor subcriticality control during refuelling operations. 

Neutron flux monitoring To cover the whole range of flux from shutdown to full power, 3 sets of detectors with overlapping ranges are provided. One set is placed within the core and is effective from mW to 100 W. The other two sets are located below the reactor. While the second set responds to the range from tens of watts to few MW, the third set covers the range from a few hundred kW to beyond rated power. 

Administrative barriers are typically used for a prevention of (slow) boron dilution. Most of the plants would have a Technical Specification requirement on the boron concentration and its verification during the shutdown operation. In addition, there is an example of administrative requirement on the minimum permissible redundancy of neutron flux monitoring system detectors. 

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