Fission chambers

Four other types of radiation detectors are the self-powered neutron detector, wide range fission chamber, flux wire, and photographic film. 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U235. The neutrons interact with the U235, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. 

A description of how self-powered neutron detectors, wide range fission chambers, flux wires, and photographic film detect radiation is summarized below. 

The main equation for calculation of Keff with the fission chamber count rate is determined by the formula ... 

The operation principle of pulse channel for measuring the coimt rate is based on the neutron registration by the detector of ionization fission chamber or neutron counter, pulse transmission from the detector via the communication lines to the preamplifier inlet and their following amplification, formation and processing by means of the auxiliary electronic equipment. 

Highly sensitive He-3 detectors and fission chambers with moderator. Neutron measurements. 

Fission chambers are gas counters that detect the fragments produced by fission. The fission fragments, being massive charged particles with Z +20e and kinetic energy 60-100 MeV, have a short range even in a gas. They produce such an intense ionization that gas multiplication is not necessary. Thus, fission chambers operate in the ionization region. 

Another technique for neutron detection uses a fission chamber. One design contains a stack of alternate anodes and cathodes, one of the electrodes being covered by a thin layer of uranium enriched in The fission fragments produce large ionization even though the gas multiplication is quite low. This detector is more sensitive to fast neutrons than the BF3 counter, and can be used for fast neutron fluxes up to 10 n s with a backgroimd of a few cps. 

The ex-vessel neutron detection equipment consists of fission chamber neutron detectors mounted in six equally spaced vertical wells located just outside the reactor vessel as illustrated in Figure 4.3-4. The signals from these detectors are supplied to the nuclear instrumentation cabinet and Safety Protection Subsystem equipment located primarily in the reactor building. These data are used by the automatic control systems to operate the control rod drives or the reserve shutdown equipment, thereby changing the neutron flux levels within the reactor core. 

The startup detector assemblies (SDA) are fission chambers with the appropriate cabling and support structure. The SDAs are inserted into vertical channels in the reflector elements near the bottom of the core through three equally spaced penetrations in the bottom head of the reactor vessel. The SDA locations are shown in the plan view and vertical section of reactor core, Figures 4.3-5 and 4.3-6. The SDAs are interchangeable in any of the assigned locations. 

Each well contains three neutron detectors. Two neutron detectors provide neutron flux signals to the PPIS for use in the reactor trip circuitry. The third neutron detector provides a neutron flux signal for use by the NCS and Rod Control Systems for reactivity control during plant operation. The detectors used are fission chambers. The ranges covered are shown in Figure 4.3-11. 

VG-8 and VG-15 are pebble zone holes reaching to elevation 98 ft 4 in. which are to be used for fission chambers. Since the fission chambers will be most useful near the centerline of the reactor during start-up, but above the thermal shield during high-power operation, winches are provided above VG-8 and VG-15 to raise and lower the chambers. Total motion of approximately 12 ft is provided by this means. 

The parallel circular plate (PCP) and compensated ionization chambers and the fission chamber are of ORNL design and manufacture. The air wall and water monitor chambers are made by General Electric Company, and the. boron thermopiles are made by Nuclear Instrument and Chemical Corporation. 

In an early attempt to cover-still lower neutron intensities, the modulated ionization chamber. -was proposed. This device utilized the periodic motion of an absorber placed before the ionization chamber which resulted in a modulated signal proportional to neutrons only.r This was abandoned in favor of the fission chamber and counting rate meter which,. by reposi -tioning the fission chamber, is capable of covering a range of 10. Since this device has a long integrating time, it was felt th.at the resultant time delays would make it an unsatisfactory instrument for automatic control, and it is therefore used as an adjunct to manual control.-... 

Panel Q also carries a number of..knobs fof adjustment of zero, calibration, and adjustment of.balance voltage for the galvanometers There are, moreover, two additional items which deserve -particular mention. One is the set of three (out, stop, and in) push button controls in the lower left corner for movement of the fission chamber. < The other, is the pair of alarm lights and reset buttons at the upper r.ight -hand corner-which work from phototube alarm circuits when any of the galvanometers go off scale. 

The fission chamber was first placed in a graphite column containing an Sh-Be source at a position where the thermal-neutron flux and t he Cd ratio were known from standard indium foil measurements. By exposing the chamber... 

The fission chamber used to measure the flux of fast neutrons has... 

Twenty-four in-core fission chambers located in the fuel channels monitor neutron flux and are used to... 

In order to do this, a new type of detector (high-temperature, high-sensitivity fission chamber) was developed in France and tested in PHENIX. 

The sensitivity of this activation ratio method between 13 MeV and 15 MeV, on average, is 50%/MeV and 64%/MeV for Zr and Cr, respectively. This method is also applied to determine the mean neutron energy for an extended sample. In this case, the Zr and Nb foils are placed back-to-back in different positions inside the sample. The angular yield of D-D neutrons can be measured either by the U(n,f) using a depleted U layer in a fission chamber or by the In(n,n ) In reaction. The curve is relatively well known and its change between... 

SFNC Fission chamber Presence of spent fuel Attribute... 

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