Absorber materials control elements

Tlte control element assembly, shown in Fig. 15. consists of 0.8-inch (2-ceiuimeler) outside diameter Inconel tubes containing boron carbide pellets as the neutron absorbing material. A gas plenum is provided in order to limit the maximum stress due to generation of internal gas pressure. 

A water inlet aperture 104 is disposed in the sleeve 100, so that water may enter into the sleeve 100, flow through the channel 98, through the sleeve 92 and out of the aperture 94, thus cooling the control element 82. A neutron 5 absorbing liner 106 is also disposed within the sleeve 100. The liner 106 should be constructed of a material having a neutron capture cross section of at least 100 barns. 

Due to its neutron-absorbing efficiency, boron carbide is attractive as a neutron absorber material, and is used both in powdered and solid forms to control the rate of fission in nuclear reactors (Figure 4.19b)[530j. B4C mixed with other materials, such as aluminum metal or polyethylene plastic, is applied to protect it against oxidation in the reactor environment. AI-B4C metal-matrix composite plates (e.g., Boral, Bortec) have wide applications as isolators in spent fuel element racks, in the inner sections of reactor shields as shutdown control rods and neutron curtains, as shutters for thermal columns, and as shipping containers. 

Variant 1 of the SVBR-75/100 core load, which has maximum reactivity margin, uses the reactivity control system composed of 37 control rods. The channel of each control rod displaces 19 fuel elements in the core the absorbing material is enriched boron carbide. 

The control system for the reactor incorporates the rod cluster control (RCC) concept, which has replaced the cruciform control elements used on the earlier PWRs. The control rod poison is distributed uniformly in the form of small-diameter rods which are inserted into the sheaths located within the fuel clusters. The 24-rod assembly is coupled to a drive shaft which is actuated by a drive mechanism mounted on the reactor vessel head. Compared with the large cruciform assemblies, the RCC system gives an increased reactivity worth per unit weight of absorber, coupled with a more uniform power distribution and a greatly reduced flux perturbation effect due to the water gaps which are created by movement of the control rods out of the core. The absorber material in the control rods is silver-indium-cadmium in the form of extruded rods which are sealed within stainless steel tubes. 

Each absorbing element (AE) consists of a cluster of rods linked by a structural element (namely, spider ), so the cluster moves as a single unit. Absorber rods fit into the guide tubes. The absorber material is the commonly used Ag-In-Cd alloy. Absorbing elements (AE) are used for reactivity control during normal operation (adjust and control system) and to produce a sudden interruption of the nuclear chain reaction when required (fast shutdown system). 

In the reactor core there are 90 control members of the control and protection system. Each control member includes 3 absorber assemblies. The absorber assemblies are located in the central tubes of 270 fuel assemblies. The absorber assemblies consist of 16 absorber elements placed in the circle at regular intervals. The absorber material is vibro-packed powder of boron carbide (B4C). By using a special cross-bar, the absorber assemblies in three adjacent fuel assemblies are united in the control member of the CPS. Each control member of the CPS can be moved inside the reactor by an individual drive, which is independent in terms of the control action. The travel distance of the CPS control member is 2600 mm. 

Absorber rods in the shutdown systems are standardized in terms of absorber element diameter and number in the shroud tube and also in terms of the dimensions of the shroud tube with rods developed for the BN-800 reactor. The effective density of the absorber material, boron carbide, is also standardized. Five rods are intended for reactor emergency protection, five rods for compensation of the reactivity effects (all rods with B enrichment of 60%) and two rods with natural boron carbide for power control. The reactivity balance during reactor refuelling is given in Table XXI-3. 

CZTS is composed of abundant and non-toxic elements and has a 1.45-1.51 eV band gap with a high optical absorption coefficient (> 104 cm ), which makes it suitable for solar cell absorber layers. CZTS also shows promising thermoelectric properties, with ZT values of up to 0.36 at 700 K. Control of the materials composition has been shown to be fundamental for optimization of its functional properties. Solution processed CZTS absorber layers have provided photovoltaic efficiencies much higher than those obtained by vacuum-deposition techniques. This may be attributed to the better control of the composition and crystal-phase homogeneity by solution processing. Therefore, solution based routes for the preparation of solar absorber materials and solar cells are moving more and more into focus of scientific and industrial research. 

Some of their characteristics (temperature, thermal volume power, neutron spectra, cooling media, etc.), which strongly impact the neutron absorber material choice and the Control Element Assemblies (CEA) design, are reported in Table 15.2. The relations between the systems characteristics and the CEA choice and design are as following ... 

Besides principles and mixts, Stahl made much use of the idea of chemical instruments, by which he meant those mechanical agents that made mixts possible but were not their material cause, that is, were not the stuff of which mixts were made. Stahl s instruments included fire or heat, which was a necessary cause of so many chemical reactions. The controlled use of sources of heat had been one of the key skills for alchemists and chemists since their disciplines first took shape. Note that heat here is an instrument and not an element, as it had been for Aristotelians and others. Water could operate as a solvent, without entering into a mixt, and in that case it too could function as a mechanical agent, one of Stahl s instruments. Air, for example, when it absorbed phlogiston, could similarly be an instrument. Chemists used instruments as tools to produce or analyze mixts. 

---