Reactor startup neutron

Signals from the ex-vessel neutron detectors in conjunction with the in-reactor startup neutron detectors are utilized to derive neutron flux... 

The need for startup neutron detectors in-vessel is dictated by the low neutron flux at the ex-vessel detector location at startup and to ensure a controlled startup. Therefore, in-vessel startup detectors are used for flux monitoring while the reactor is brought to a critical configuration and during reactor shutdown periods. Three startup neutron detectors are installed to ensure adequate neutron flux measurements during these low power intervals. 

The nuclear instrumentation must be operable prior to reactor startup. The automatic rod control during startup will not operate if more than one of the three ex-vessel wide-range channels is out of service. The Safety Protection Subsystem requires at least three of its four nuclear input channels operating. The power range neutron flux control will not operate automatically with more than two of the six input channels out of service. 

The design of the reactor internals has not been addressed yet, but they likely will be made of graphite or carbon composites to accommodate the high-core outlet temperature required by the NGNP (1000°C). It is possible that carbon-insulated metallic alloy will be used for the core support structure, although this has not been evaluated yet. Control rods will be required to provide for reactor startup, normal operation, and shutdown. The munber and placement of control rods has not been evaluated yet, but the rods will be constructed from carbon composites for the drive shafts and absorber casing and boron carbide or other high-temperature absorber for the neutron absorber. The control rod drive mechanisms will be located above the reactor enclosure head. 

U-235(n,f) (many different fission products] Two reactions that produce neutrons as reaction products are listed here because most neutron sources for reactor startup are based on them ... 

UWNR uses a Pu-Be source for a "bugging" and calibration source, while the reactor startup source is a special Ra-Be source. The special part is that it has been irradiated to give a higher output of particles, and, thus, neutrons. It actually contains 100 milligrams (100 millicuries) of Ra, but it emits the same number of neutrons as an linirradiated Ra source of 2 curies ( 2 x lO n/sec). ... 

During a startup of the reactor observed neutron count rate doubles. How has this affected the... 

Two different neutron sources were used during two reactor startups. The source used in the first startup emits ten times as many neutrons as the source used in the second startup. Assume all other factors are the same for the second startup. Which ONE of the following states the expected result at criticality ... 

Prior to reactor startup, it is necessary to have a supply of neutrons in the core to provide positive, accurate power level and period instrument indications in the source range and to control the neutron multiplication rate as positive reactivity is added to the core. 

Neutron sources are of two types (1) a primary source, which is active for initial reactor startup and startup early in the life of the first core and (2) a secondary source, used for later startup of the reactor and activated during the operation of the reactor. 

Reduces neutron leakage provides reactor startup/shutdown/control... 

Operational techniques and special procedures. Reactor startup. As the size of homogeneous reactors increases, the use of control-rod neutron absorption to perform a startup becomes progressively less attractive c.g., to maintain criticality in lIIlE-2 while heating from 20 to 280°C requires a reactivity increase of more than 25% Ak because of the large negative temperature coefficient. It is much more convenient to provide a means of varying the amount of fuel in the core as required to ov ercome temperature and power coefficients. 

The fission-chamber signals are fed to conventional preamplifiers, A-1 linear amplifiers, logarithmic count-rate meters and a dual-pen recorder. For initial reactor startup before gamma-neutron reactions provided a sizable neutron source, the fi.ssion-chamber output was u.scd to drive a low-range pulse counter. 

Of what importance is the artificial neutron source to reactor startup and operation ... 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

In the startup of a reactor, it is necessary to have a source of neutrons other than those from fission. Otherwise, it might be possible for the critical condition to be reached without any visual or audible signal. Two types of sources are used to supply neutrons. The first, appHcable when fuel is fresh, is califomium-252 [13981-174-Jwhich undergoes fission spontaneously, emitting on average three neutrons, and has a half-life of 2.6 yr. The second, which is effective during operation, is a capsule of antimony and beryUium. Antimony-123 [14119-16-5] is continually made radioactive by neutron... 

The BF3 proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced. 

The determination of rate change of the logarithm of the neutron level, as in the source range, is accomplished by the differentiator. The differentiator measures reactor period or startup rate. Startup rate in the intermediate range is more stable because the neutron level signal is subject to less sudden large variations. For this reason, intermediate-range startup rate is often used as an input to the reactor protection system. 

The annular reactor core consists of fuel elements, graphite reflector elements, plenum elements, reactivity control material, and neutron startup sources. Each of these components is described below. 

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 detectors provide signals to the Safety Protection Subsystem, the NSSS Control Subsystem, and the rod drive control equipment from the startup range to as high as 200 percent power. Two detectors in each of six wells feed the Safety Protection Subsystem and one from each well feeds the NSSS Control Subsystem and rod drive control equipment. The NCS and rod drive control equipment use the signals to control reactor power through the flux controllers while the Safety Protection Subsystem signals are used to provide protection for abnormal plant conditions. 

Normal Operation Operation of the reactor under static power levels or startup conditions when neutron flux and heat distribution are relatively stable with normal operating limits in effect ... 

A set of pre-start up channels with high sensitivity BF3 proportional counters was commissioned. These channels along with startup channels help in smooth start up of the reactor with inherent neutron source, thus eliminating the need for high strength auxiliary... 

The chemical shim system uses the soluble neutron absorber boron (in the form of boric acid), which is inserted in the reactor coolant during cold shutdown, partially removed at startup, and adjusted in concentration during core lifetime to compensate for such effects as fuel consumption and accumulation of fission products which tend to slow the nuclear chain reaction. The control system allows the plant to accept step... 

Instrumentation needs for adeqrrate monitoring of the neutron flux for reactor power levels, including startup and shutdown conditions, shotdd be stated. These may... 

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. 

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