Rod cluster control

Fig. 21.25. Cutaway of PWR fuel element with the control rod cluster assembly. Element contains about 1200 lb U02 in up to about 264 rods. (Courtesy USDOE.)...

ONTROL ROD DRIVE SHAFT CONTROL ROD CLUSTER (WITHDRAWN)... 

Reactivity Control System actuated control rod clusters... 

Additional passive Control Rod Clusters Passive Additional, independent, passive type (no ATWS)... 

Control and protection system (CPS) X 13 control rod cluster assemblies and 6 shut-down rod cluster assemblies... 

The Package-Reactor is equipped with tube-type control clusters to shut down the core. Figure VIII-5 shows a schematic of the control rod cluster. The cluster consists of about 60 absorber rods, each of which appears as a stainless steel tube of about 20 mm diameter containing natural B4C powder. When the core is shut down, all control rod clusters are inserted. 

Reactivity compensation by burnable poisons and active control rod clusters. 

Secondary shutdown system Mechanical 12 independent control rod clusters of the reactor control system ... 

The emergency shutdown system consists of 12 independent control rod clusters, with each 6 being capable to shut down the reactor. 

The normal operation reactivity control system consists of 12 independent control rod clusters. Only 3 of them are simultaneously in use, providing a maximum reactivity margin of less then 0.8 Pefr The drop of reactivity control clusters also ensures the reactor shutdown. 

In the current design, all reactivity control rod clusters are built as active. Passive shutdown rods actuated by flow decrease beyond a certain limit (the same as proposed for the BN-800) are under investigation. 

The chemical and volume control system is so structured so as to limit Ae maximum boron dilution ratio after recognizing abnormal conditions Arough an alarm "High Neutrol Flux during Stoppage of Reactor in Neutron Source Domain" or an alarm "Control Rod Cluster Insertion Limit. Thus Ae operators can have enough time to take appropriate correction measures. 

For PWRs, practically, two systems of diesel generators are provided as emergency on-site power systems in addition to the external power system which is firmly connected to the power distribution grid. Therefore, a change of loss of power source is considered extremely limited even for a short duration. Even if total loss of power sources occurs for a short duration, the reactor could be brought to a safe shutdown through actuation of the safety protection systems and the control rod clusters. 

The core is equipped with two different control rod systems. The first is an active type, quite similar to classic PWR control rod systems and is divided into four different banks. At the beginning of cycle (BOC), with maximum boron concentration (711 ppm), with all active control rod clusters in, the multiplication factor is 0.90435. This figure changes to 0.99335 without boron. 

The second control rod system foreseen in the MARS reactor is a passive type and causes control rods insertion into the core when the core coolant temperature reaches a selected set value. The operation of this system (called ATSS, Figure IV-6) is based on the differential thermal expansion of a bimetallic sensor located inside the fuel assembly the differential displacement, due to coolant temperature increase, causes the release of a traditional-type control rod cluster. 

Clusters are in the form of a bundle of 18 absorber rods joined by a common traverse, moving inside the E-635 zirconium-alloy guide tubes of a 12.6 mm outer diameter and a 0.6 mm wall thickness. Each cluster has its own drive. On the total, there are 72 control rod clusters of the reactor control and protection system (CPS). The core map is shown in Fig. VII-10. 

Increase in the reactivity by 0.1 resulting from withdrawal of a control rod cluster. 

A schematic drawing of the top stmcture of the square fuel assembly is shown in Fig. 2.36 [9]. At the top, the control rod cluster guide tube branches off to the water rods. The coolant flows through the gap between the outer water rods to the outlet of the core. Such a stmcture should distribute the moderator to each fuel assembly and allow the insertion of the cluster type control rods into the assembly from the top of the core. 

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