A look back at Chernobyl disaster, in lieu of Japan’s disaster..

We all know that Japan is in the middle of recovering from a disaster…  And has more issues that they are still dealing with, mainly the Nuclear Power plant issues…

But, what do we know about Nuclear Reactors?  And what about the last great nuclear disaster?  The Chernobyl disaster? (The Chernobyl link is a great review of all the human and mechanical causes of the Chernobyl disaster.)

The issue in Japan, appears to be related to the Plant’s power requirements. Even when not actively generating power, nuclear power reactors require cooling, typically provided by coolant flow, to remove decay heat. Pressurized water reactors use water flow at high pressure to remove waste heat. Following an emergency shutdown (scram), the core still generates a significant amount of residual heat, which is initially about seven percent of the total thermal output of the plant. If not removed by coolant systems, the heat could lead to core damage. Even following an emergency shutdown, reactor cooling is still required to keep the temperature in the reactor core low enough to avoid fuel damage.

To place this into perspective, Chernobyl required a water flow of 28 metric tons (~7,400 Gallons) per hour to be able to cool the reactor safely. To generate that much water flow, Chernobyl required 5.5 MegaWatts to run one of the main water pumps.

The Japanese issue(s) appear to be completely related to the failure of Cooling system power, or a Coolant related leak… There have been reports of additional coolant being delivered to the plants, which suggest a leak…

Compare this to Chernobyl…

  • The roof of the reactor building and turbine hall at Chernobyl were constructed of a combustible material (bitumen)
  • The Chernobyl power generation tests were being run at a unsafe power level (far lower than was specified)

The Chernobyl reactor had several unknown design defects:

  • The reactor was very unstable at low power levels, and prone to sudden increases in energy production.  Often to a dangerous level.
  • The control rod system had a significant flaw.  The rods fit into a shaft that ended with Hollow channels that were filled with water.  As the rods were inserted, the coolant would be displaced, and thus the reactor power output would be momentarily increased.
While at this time, we do not know the exact issues and the final fate of the small number of Japanese nuclear plants that have been damaged…  We need to realize that the issues at the Japanese plants offers a chance to review the causes, and to solve these problems in the future.