End effect

The CPS rods in RBMK are located in channels cooled by their independent cooling circuit. The main part of the rod containing a boron carbide neutron absorber is 7 meters long (reactor core height). Under the absorber is a graphite displacer connected to it by a telescopic rod. The displacer length is about 5 meters. When the rod is removed (moved to the upper position) from the zone, the graphite displacer replaces the water of the CPS channel, which avoids unnecessary absorption of neutrons by water (graphite has a significantly lower ability to absorb neutrons compared to light water ) and, thus, “save” neutrons, which, in turn, increases the efficiency of the reactor.

The RBMK core is 7 m high and it would probably be better to make a displacer of the same length, however, the channel height, which is below the core, was designed smaller and does not exceed 5 m (~ 4.5). Thus, if the rod is in its lowest position, there is no space left for placing a seven-meter displacer.

With a fully absorbed absorber, a 4.5-meter displacer is in the active zone, and the remaining space below it (1.25 meters) is filled with water from the CPS channel. Thus, graphite weakly absorbing neutrons is located in the central part of the active zone, where the number of thermal neutrons is maximum, and water, which is much stronger than graphite absorbing neutrons, is located on the periphery of the active zone (in its upper and lower parts), characterized by significantly lower thermal fluxes neutrons, where its ability to absorb neutrons is partially offset by the "small number" of the latter.

The development of the effect occurs when the rod moves into the active zone from its extreme upper position, when graphite, which weakly absorbs neutrons, at the first moment of time replaces water in the lower region of the CPS channels, which has a higher absorption capacity. As a result, conditions are created in the lower part of the core for the formation of positive reactivity and the growth of local power. It must be repeated that the described region is located at the bottom of the active zone (about 1 m), characterized by a low neutron flux (significantly lower than the average value in the reactor). At the same time, the local reactivity introduced due to the end effect is proportional to the square of the neutron flux (perturbation theory), therefore, with the nominal operating conditions of the reactor, the end effect is very small. In addition, the possibility of the manifestation of the end effect is suppressed by the introduction of negative reactivity in the upper part of the reactor core, accompanying the introduction of an absorbing rod. Thus, for the development of the end effect, which significantly affects the operation (controllability) of the reactor, a certain combination of specific factors must occur, and for this reason the effect has not been detected for a long time.

The end effect was discovered in 1983 during the physical start-up of the reactors of the 1st unit of Ignalina , as well as the 4th unit of the Chernobyl nuclear power plant . ^{[1] [2] The} studies showed that the end effect is observed when single rods from the upper limiters are immersed in the active zone. It was experimentally shown that the mass introduction of rods (more than 15-18 RR rods) excluded the end effect ^[1] (nevertheless, see ^[1] p 3.4).

The end effect could contribute to the catastrophic development of the Chernobyl accident on April 26, 1986, since it is known from the recorded data that immediately before the catastrophe the reactor had an unacceptably low operational reactivity margin , and, therefore, most of the CPS rods were located on the upper limit switches. In this case, the mass introduction of CPS rods into the core could lead to the introduction of uncompensated reactivity (according to various estimates, from 0.3 to 1.1 β). One way or another, the end effect prevented the reactor from being drowned by the CPS rods for the first seconds (up to 5-6) after the formation of the corresponding command.

After the Chernobyl accident, the RBMK reactors were modernized, including changes in the design of the CPS rods, eliminating the positive end effect. The upgraded control rods had a seven-meter displacer and absorber. The displacer consisted of two parts - the 5-meter old and 2-meter tape, which when folding the telescope is put on the displacer.

Currently, all RBMK reactors are introducing cluster regulatory bodies (CROs) with a fixed displacer (the so-called sleeve) made of a weakly absorbing neutron aluminum alloy. This displacer is cooled externally by the water of the CPS loop. In the inner part of the KRO sleeve, holes are provided in which the absorbing rods of the CPS are moved “on dry”.

• Chernobyl accident: addition to INSAG-1 . Safety Publication Series No. 75-INSAG-7. IAEA, Vienna, 1993.