LudSable t1_jc2pnng wrote

After the Chernobyl disaster all the current active RMBK were fitted with safety features and actual shielding in the walls. Better late than ever...

>Following the accident at Chernobyl, all remaining RBMK reactors were retrofitted with a number of updates for safety. The largest of these updates fixed the RBMK control rod design. The control rods have 4.5-metre (14 ft 9 in) graphite displacers, which prevent coolant water from entering the space vacated as the rods are withdrawn. In the original design, those displacers, being shorter than the height of the core, left 1.25-metre (4.1 ft) columns of water at the bottom (and 1.25 metres [4.1 ft] at the top) when the rods were fully extracted.[3] During insertion, the graphite would first displace that lower water, locally increasing reactivity. Also, when the rods were in their uppermost position, the absorber ends were outside the core, requiring a relatively large displacement before achieving a significant reduction in reactivity.[40] These design flaws were likely the final trigger of the first explosion of the Chernobyl accident, causing the lower part of the core to become prompt critical when the operators tried to shut down the highly destabilized reactor by reinserting the rods. The updates are:

>* An increase in fuel enrichment from 2% to 2.4% to compensate for control rod modifications and the introduction of additional absorbers.

  • Manual control rod count increased from 30 to 45.
  • 80 additional absorbers inhibit operation at low power, where the RBMK design is most dangerous.
  • AZ-5 (emergency reactor shutdown or SCRAM) sequence reduced from 18 to 12 seconds.
  • Addition of the БАЗ or BAZ* system,[41] (rapid reactor emergency protection) which would insert 24 uniformly distributed rods into the reactor core via a modified drive mechanism within 1.8 to 2.5 seconds.
  • Precautions against unauthorized access to emergency safety systems.

>In addition, RELAP5-3D models of RBMK-1500 reactors were developed for use in integrated thermal-hydraulics-neutronics calculations for the analysis of specific transients in which the neutronic response of the core is important.[42]

>*BAZ button is intended as a preemptive measure to bring down reactivity before AZ-5 is activated, to enable the safe and stable emergency shutdown of a RBMK.


LudSable t1_j4asrn5 wrote