Neutron flux level

Fission counters are used extensively for both out-of-core and in-core measurements of neutron flux in nuclear flux in nuclear reactors. In out-of-core situations, they monitor the neutron population during the early stages of power ascension when the neutron flux level is very low. For in-core measurements, fission counters are used for flux mapping (and consequently, determination of the core power distribution). They are manufactured as long thin q lindrical probes that can be driven in and out of the core with the reactor in power. Typical commercial fission counters for in-core use have diameters of about 1.5 mm (0.06 in), use uranium enriched to at least 90 percent in as the sensitive material, and can be used to measure neutron fluxes up to 10 neutrons/(m s) [10 neutrons/(cm s)]. 

Signals to the Plant Protection and Instrumentation System (PPIS) and the NSSS Control Subsystem (NCS) are supplied by neutron detectors. During power operation, the neutron flux levels are monitored by detectors located in wells between the reactor vessel and the concrete cavity wall. These detectors are distributed symmetrically around the reactor vessel at about the core midplane. During low power operation, starting up, shutting down, and while shut down, the neutron flux levels are monitored by source-range detectors, located in selected side reflector elements near the bottom of the active core. 

In the refueling mode, the reactor vessel is depressurized. All control rods in the inner and outer reflectors are fully inserted except for two inner and two outer rods which may be removed for refueling a 60 degree sector of the core. The neutron flux level is continuously monitored by the source range detectors. 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

Small neutron sources in the fuel handling hole in selected fuel elements provide adequate neutron flux levels to ensure a controlled startup. 

The Neutron Control Subsystem (NCSS) consists of the drive mechanisms for positioning the control rods, the rod controls, the reserve shutdown control equipment (RSCE) with its controls, and the instruments for measuring neutron flux levels within the reactor vessel (i.e., in-vessel flux mapping units and startup detectors) and around the perimeter of the reactor outside the vessel (i.e., ex-vessel flux detectors). The control rods and the reserve shutdown material are part of the Reactor Core Subsystem (Section 4.2). Most of this equipment is configured into assemblies which are normally installed in penetrations in the top or bottom of the reactor vessel. These assemblies are periodically removed either to provide access to the core for refueling or for maintenance of the equipment. 

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. 

Each zone shall be so defined as to provide information representative of the neutron flux levels sdong the total socleil direction of the reactor within the flattened portion of the reactor.. Figure 2, Division of Reactor Into Zones, shows an example of a two, four, nine and sixteen zone reactor. A single high level system may be used If It divides the reactor Into nine or more zones and has a response time of seconds or less with fixed or percentage level trips. 

The JOYO operators control the reactor power, i.e. neutron flux level, by adjusting the position of the control rod subassemblies in the core. This is a manual operation performed from the central control room. To improve operational reliability as well as to reduce the mental load on the operators, an automatic control rod operation system [16] has been developed. This system has the following capabilities ... 

Boron Thermopiles. The boron thermopiles are used for an additional check on neutron flux level. They are located, in the graphite. (VG-27, 28, 42, 44, 56 see Fig.- 3.F) all at the same horizontal level. These instruments consist of a large number of thermocouple junctions in series, with alternate junctions coated with boron. The coated junctions become warm owing to absorption of neutrons, thus yielding a net voltage which is a measure of neutron flux. The instrument case is about H in. in diameter by 6 in. long. The approximate sensitivity is 6 mv in a flux of 10. Internal resistance is of the order of 3 to 4 ohms. By proper mounting and thermal insulation of the thermocouples the instruments may be made quite insensitive to ambient temperature (say, 40 /xv/°C). For transient conditions the time constant is about 1 sec. 

The neutron flux level for the PWR surveillance program is approximately 10 n/cm s ( > 1 MeV). The highest fluence of the PWR surveillance data is about 6 x 10 n/cm (E > IMeV) as of 2012. The transition temperature shifts, ARTndtS, of all the PWR surveillance data are plotted in Fig. 4.12 for base metals and Fig. 4.13 for weld metals as a function of neutron fluence. In general, ARTndt increases with fluence, but the values of the shift are not very high. The highest ARTndt values for base metal and weld metal are 88 °C and 131 C, respectively. The transition temperature shift depends mainly on the copper content of steel, and it increases with the copper content. 

The reactors will be scrammed automatically by hi neutron flux level signals set as defined in Tables I - III. 

High neutron flux level 2 of 3 Msnual No. 1 Variable 3... 

NUclear Instrumentation measures the levels the distribution and the rate of change of neutron flux density in the reactor. Monitoring data and safety circuit trip signals are provided as appropriate over the full range of neutron flux levels from those existing during sub-critical shutdown conditions to those characteristic of full production level operation. 

Since the above-core calculations represented an infinite array of fuel channels, they did not include those parts of the pressure vessel in the vicinity of the hot gas duct penetration and therefore no account was taken in the above-core calculation of the neutrons wMch scatter around the top comer of the graphite stack and enter the hot gas duct penetration area. The neutron fluxes in the vicinity of the hot duct entrances were therefore takai to be equal to the fluxes calculated for the side shield model with a contribution added to account for the top-core leakage component. This component was scaled fi om the sub-core results and is considered to probably result in pessimistically high flux values for the hot gas duct pen ration since the neutron flux levels below the core are generally higher than those above. 

FIG. 19. Section of MCBEND Computer model used to predict the cold and hot gas duct Neutron flux levels... 

Beckman - A micro-microammeter made by Beckman Instruments, Inc. These are used with an ionization chamber to monitor neutron flux level and/or other types of radiation in various parts of the reactor. 

Sub-Critical Monitor - An electronic system for monitoring the neutron flux level while the Keff is less than unity. The system consists of a fission chamber in the active zone of the reactor connected to an amplifier-scaler, etc. 

Figure 4 shows the buildup of Xenon poisoning as a function of time after reaching full power for various neutron flux levels, In all cases, the reactivity loss (from a clean start) due to Xenon, will reach 90% of equilibrium value in about 24 hours and full equilibrium level in 40 to 50 hours. 

Often we want to indicate or represent a quantity that can vary over a very large range, such as neutron flux level, reactor power level, or activity of a decaying source of radioactive material. We can do this easily by using meters or graph paper that is laid out according to the log or In of the number represented. In the table below are some numbers and their logs. 

However, It Is now well established that zirconium alloys under stress at temperatures around 500°C will creep more rapidly In a neutron flux than In out-of-reactor tests (refs 12, 15)- A typical result from UKAEA work which indicates the magnitude of this difference between in-plle and out-of-plle creep rates under conditions approaching those relevant to SGHWR pressure tubes Is given In Pig. 4, and over recent months considerable amounts of experimental work in Canada, the UK and the US have been devoted to obtaining such curves. It has been found that the Irradiation enhancement Increases with Increase of neutron flux level. 

As described in Sect. 4.2.1.1, unfueled samples of the graphite foam were irradiated in the HFIR at a neutron flux level on the order of lO neutrons/(cm2-s). From Fig. 5.2, the flux in the SSR is on the order of 2 X I0l2 So a single cycle of 25 d in the HFIR would correspond to a fluence of 12,400 d or 34 years for the SSR—far beyond the design lifetime of 10 years. An examination of fueled specimens would be the next logical step in a fuel qualification procedure, but ihe cost of such an experiment exceeded fimds available for this project. 

Excore source-range detectors indicated increasing neutron flux levels. 

The staff found that in 1968 a study was done by SRL to determine xenon oscillation dependence on various reactor parameters (Reference 16). This study indicated that xenon oscillation depends primarily on the neutron flux level, spectrum, and shape the neutron migration area reactor dimenstons saturation xenon worth and the temperature coefficient of reactivity. Of these parameters, only the neutron flux level and shape and the temperature coefficient were found to represent viable design variables. 

BF3 proportional counters are highly sensitive to neutrons, and usually are used in a pulse countii mode to monitor the neutron flux and the reactor period (rate at which the neutron flux changes) in the source (start up) range. Even it is not usual, BF3 detectors can also be used as compensated ionization chambers, to monitor the neutron flux level and the reactor period in the intermediate range. 

Nuclear channels for research reactor applications can be modular or integrated. In integrated channels, with exception of the pre-amplifier, all electronic circuits necessary to measure and monitor the neutron flux level and period are incorporated in only one cabinet, in such a way that a single failure will require the complete channel to be serviced. Modular channels are made up of independent modular instruments, like amplifiers, power supplies and ratemeters, and any single failure will require only the replacement of the failed modulus. One family of standard modules widely used in research reactors instrumentation systems is the Nuclear Instruments Modules (NIM) which can be used for control and safety applications. 

The protection system was designed to avoid any unsafe condition. It was subdivided into two subsystems, the nuclear detection subsystem and the interlock subsystem. The nuclear detection subsystem is used to monitor neutron flux level and period. It is composed of 8 nuclear channels to monitor the neutron flux from start up to 100% of full power (100 watts), including comparators and isolation ampliBers. Three channels are used in the start-up region, and the others in the intermediate and power regions. In each region we have three measurements of the neutron flux (power) and three measurements of the period. The nuclear channels are complemented by two linear channels, used (alternatively) to control the reactor in automatic mode. Figure 5 shows the relative location of the detectors, and the operational interval of them. 

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