RCCA bank

Jack8rkin

Hello everyone!

Just wanted to clarify about the "bank" in the term RCCA bank.

RCCA is Rod Cluster Control Assembly. It's basically several control rods on a spider.
So, "bank" means a group of such RCCAs that are intended for some purpose (i.e. reactivity compensation, reactor scram etc.).

Am I right saying so?

Thank you.

gmax137

Yes, at least for PWRs. Typically the banks are withdrawn or inserted gradually and sequentially in a pre-determined way.

Jack8rkin

Thank you. Exactly, I was asking about PWRs.
But there are also scram rods. I believe they are shot down into the guide thimbles and reach the core bottom very fast. Is it correct?

Astronuc

There are shutdown banks that have a fair amount of worth, but all rods in a PWR are simply dropped into the core under gravity when the magnetic coupling ceases.

Some PWRs use a control bank or grey bank with lesser reactivity for load following or frequency control capability, or axial power shaping. They use an Inconel absorber, or less (fewer rods/fingers) of the primary neutron absorber.

BWRs also have banks of control rods (control blades) which are hydraulically inserted into the core from below. Some more modern designs use a fine motion screw, but they are also hyraulically scrammed into the core when necessary.

Jack8rkin

Thank you!

QuantumPion

A single control rod is like a single fuel rod - just one piece. An RCCA is, like you said, a group of 20 or 24 control rods that insert into a single fuel assembly. A control bank is a group of 4 or 8 RCCA's which are located in radially symmetric positions around the core. The RCCA's are divided and arranged into banks specifically so that they suppress neutrons in high flux regions as other banks are inserted in sequence. Of the control banks, typically two banks are designated as "shutdown banks" and are not used for reactivity control - they are either all the way in while shut down or all the way out while critical. The purpose of the shutdown banks is to provide sufficient shutdown margin.

Jack8rkin

Thank you so much for such a detailed response!

rmattila

The terminology may vary form plant to plant - the AREVA terminology used with the EPR is to have individual RCCA:s assigned to fixed groups of 4 symmetrical RCCAs each, and the control banks are created by joining one or more groups together. The idea between this "double grouping" is that you can design and control the power and axial offset by controlling always the same banks, but you can change the actual physical RCCAs (=groups) that belong to these banks everry few weeks to prevent history effects caused by extended insertion of a control rod in one position.

Jack8rkin

Did you mean 4 symmetrical RCCAs each? (Symmetrical with respect to the core center, right?)
Or rods from a certain RCCA could be assigned to two diferent groups (which seems to be unlikely)?

rmattila

That's right, I should've been more clear in my expression. I made a correction in the text.