Comparative characteristics of absorber cluster assemblies of VVÉR-1000 and PWR reactors

Abstract

The control rods of the VV/~R-100 and PWR reactors are similar as to design and operating conditions. This is of unquestionable interest when comparing their serviceability, service life characteristics, absorbent and structural materials, and paths of further development. An objective evaluation of the level of work being done on new-generation control rod systems in Russia and the West can be made by considering those matters. The urgency of such analysis also stems from foreign publications [1] indicating the need to provide VVI~R-1000 reactors with absorber rods based on the alloy 80% Ag-15% In-5% Cd, which is used in PWRs.Design of Cluster Assemblies. A VVI~R-1000 cluster assembly consists of 18 absorber rods, which are f'tlled with boron carbide powder of density 1.7 g/cm 3, contain natural t~ and liB, and are suspended from a crossarm (Fig. 1). The absorber rod cladding is made of 06Khl8N10T (6% Cr 18% Ni 10% Ti) steel with an inside diameter of 8.2 mm and thickness 0.6 mm and with ends sealed by argon-arc welding. The column of absorber is 3800-3900 mm long and the entire absorber rod, 4222 + 3 mm. Above the rod is a gas compensator, filled with helium at atmospheric pressure. Various designs of clusters assemblies, containing 16, 20, or 24 absorber rods, are used in Westinghouse PWRs (USA) [2]. The absorber material used in them generally is the alloy 80% Ag--15% In-5% Cd and the cladding is made of 304L stainless steel (18-19% Cr, 8-12% Ni, 0.03 % C (max)). The absorber rod diameter is 10 or 11 mm and the cladding wall thickness is 0.47-0.49 mm. The overall height of the entire cluster assembly is 4000 or 4100 mm and its diameter is 151 mm (Fig. 2)..Working Conditions (Table 1). The cluster assemblies for VVI~R-1000 reactors and'~'WRs are unified, i.e., can operate in the automatic control and scram modes. Their purpose is to control the reactivity as working members of the control rod system over the entire reactor power range and scram. The VVI~R-1000 has 61 cluster assemblies, ten of which operate in the automatic control mode and the other 51, in the scram mode [3]. The absorber rods move inside the 06Khl8NIOT (6% Cr 18% Ni 10% Ti) steel channel tubes of the fuel assemblies. As the reactor is shut down they "free fall" under their own mass at a speed of about 1 m/sec in under 4 see.PWR absorber rods also move in the channel tubes of the fuel assemblies. A fuel assembly of 17 x 17 configuration has 25 guide channels, one of which is earmarked for in-core sensors and the remaining 24, for absorber rods [I]. A guide channel is made of zirconium alloy, is 3895 mm long, and has a wall thickness of 0.4 mm. The absorber rods should descend in less than 2.2 see.During operation cluster assemblies in the scram mode are raised to the top, with the bottom ends of the absorber rods roughly 100 mm from the upper edge of the reactor core. In the automatic control mode the absorber rods as a rule are inserted only 300-500 mm into the reactor core. The bottom part of the rods, therefore, is exposed most to radiati...

Materials Science Analysis of RCCA Structural Materials and Calculations for Lifetime Extension

“…The available literature data on the change in the diameter of the control rod cladding with neutron-absorbing material B 4 C under its operation in a reactor core were generalized [13,14]. The generalized data are presented as a dependence of the diameter change on the burnup of 10 B (Figure 7).…”