Extra Bytes: ‘Core Catcher’, recently installed at Kudankulam nuclear plant

‘Core Catcher’ at Kudankulam Nuclear Plant

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In News:

  • The Rosatom State Corporation Engineering Division of Russia said that it had installed a core melt localization device (CMLD) or “core catcher” at Unit 3 of Tamil Nadu’s Kudankulam Nuclear Power Plant (KKNPP).

News Summary:

  • A “core catcher” or a core melt localization device (CMLD) was installed at Unit 3 of Kudankulam Nuclear Power Plant (KKNPP).
  • The device is designed to localize and cool the molten core material in case of a meltdown accident.

Why is “core catcher” needed?

  • A meltdown accident happens in a nuclear reaction when the nuclear fission reaction taking place inside a reactor is not sufficiently cooled, and the buildup of heat causes fuel rods to melt down.
  • In the past, meltdown accidents have occurred at Chernobyl in Russia in 1986 and at Fukushima in Japan in 2011.
  • In such a situation molten core material, or Corium, gets formed in the core of a nuclear reactor.
  • The Corium so formed can remain radioactive for several decades, even centuries.

Core Catcher at Kudankulam Nuclear
Core Catcher at Kudankulam Nuclear Plant

How it works?

  • The core catcher is a cone shaped metal structure, and is installed at the bottom of the nuclear station’s protective shell.
  • The structure is double walled, with the gap between the two walls filled with FAOG (ferric and aluminum oxide granules).
  • The core catcher is filled with a ceramic mixture also including ferric oxide and aluminum oxide, called ‘sacrificial material’.
  • The sacrificial material prevents the Corium from trickling through and also acts as a cooling mechanism.

In brief: Nuclear technology

  • Nuclear reactors work under the principle of nuclear fission reactions.
  • Fission:
    1. When a neutron hits a nucleus of a radioactive atom (U-235, Plutonium-239), it triggers a breakup of that nucleus into two large pieces called fission fragments.
    2. In addition to the two large fragments, two neutrons are usually released which in turn hits the nucleus of other atoms in the reactor setting up what is called a chain reaction.
    3. The chain reaction of fission generates heat which is then used to move a turbine to produce electricity.
  • Moderator:
    1. The neutron released in the chain reaction move at extremely faster speed and may lead to uncontrolled chain reaction leading to explosion.
    2. Thus in order to slow down the neutron, the reactors use moderator.
    3. Moderators are generally atoms with lighter nuclei and does not absorb neutron.
    4. Generally ordinary water, heavy water and graphite rods are used as moderators.
  • Control rods:
    1. In a nuclear reactor, generally only one of the neutrons released from uranium fission is used to produce another fission reaction.
    2. The other is absorbed using control rods.
    3. Control rods absorb neutrons but do not release energy in the process.
    4. Plutonium (Pu-239) is sometimes manufactured using U-238 by hitting the neutron so released in the reactor.
  • Critical mass:
    1. The minimum amount of uranium-235 atoms required to set up the chain reaction is called the critical mass.
    2. Other fuels that could be used in nuclear reactors are MAGNOX, TRISO, CerMet fuel.
  • Enrichment:
    1. About 99% of the uranium found on earth is U-238 which is not suitable for fission as they are stable.
    2. As a result U-238 is converted to U-235 and this process is called enrichment.
  • Fuels used in nuclear reactors: Fissile material
    1. Only U-235, Pu-239 and U-233 are fissile material, meaning they can set up chain reactions.
      • U-235 releases 2 neutron when the atom is split.
      • Plutonium-239: releases 3 neutrons.
      • U-233: released a few neutrons (the average is 2.48 neutrons per fission for thermal fission).
    2. U-238 is generally used in ‘Breeder reactors’ where U-238 is converted to Plutonium-239 due to absorption of a neutron.
    3. Thorium 232 used to produce U-233.

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