Re: Design of Nuclear Power Plants

["Cuttler, Jerry" <CuttlerJ@spkb.candu.aecl.ca>]

The "root cause" of the Chernobyl power pulse was the "less than adequate" 
design of the control rods.  If the rods had not inserted a large amount of 
reactivity into the bottom part of the core, when the reactor was tripped, 
the power pulse would not have occurred.  This was first identified by 
reactor physicists at AECL (Sheridan Park) immediately after the event.  The 
analysis is described in report AECL-9426 (1987).  Adi Dastur can supply 
more details.

Positive void coefficient aggravated the effect of the power pulse, but the 
analysis shows clearly that the power rise would have been quite slow, if 
the rods had not inserted reactivity during the reactor trip (scram).

I was told that this design weakness had been identified at the Ignalina 
plant, long before the Chernobyl event, but no corrective action was taken 
at that time.  I understand that the control rods in all RBMK reactors were 
modified, after the disaster, by lengthening the tie rod (by 1 metre) 
between the boron absorber section and the graphite "displacer" section to 
eliminate the water volume below the graphite section, when the rod is fully 
withdrawn.

Continuing to operate the reactor at 50% full power for an additional 9 
hours put the reactor in a very (axially) unstable state.  The top part 
became "neutronically decoupled" from the bottom part of the core.  So a 
"contributing cause" was the continued operation that put the reactor in an 
unstable state.  The operating procedures did not forbid such operation. 
 Had this mode of operation been analyzed in advanced, this weakness would 
have been identified.  Hindsight is 20/20.

Jerry Cuttler
Canadian Nuclear Society
 ----------
From: radsafe
To: Multiple recipients of list
Subject: Re: Design of Nuclear Power Plants
Date: Wednesday, May 01, 1996 11:01AM


Although the positive void coefficient that Bob Flood and Professor Cohen
have described was and is a siginificant design difference between the
"Chernobyl style" reactors and US LWRs, perhaps even more important in the
accident scenario that occured was the control rod design.  This caused
the operator initiated scram of the reactor to initially insert positive
reactivity.  Without this gross design flaw the accident would not have
been as severe.

Michael Baker ... baker@nucst11.neep.wisc.edu