Decay heat

From ArticleWorld

Decay heat is the result of energy released by radioactive decay from fission products. Fission is the splitting of the nucleus of certain atoms, with the release of energy.

Fission reaction

187 Me V of energy is released swiftly, in what is known as kinetic energy from fission products.

• The kinetic energy(characterized by movement) from the fission neutrons are released the gamma rays quickly capture the neutrons.
• The additional energy of 23 Me V are discharged sometime after fission from the beta decay of fission products.
• 10 Me V of the energy released from the beta decay of fission products is in the form of neutron. The neutrons does not interact very well, thus, 10 Me V of energy will not be deposited in the reactor’s core.

Results

The result of the 10 Me V energy not being deposited in the reactor core is that 13 Me V (6.5% of total fission energy) is then deposited in the reactor core after any given fission reaction has occurred.

• When a nuclear reactor is shutdown it is then thought of as a nuclear fission not happening on a large scale.
• When a nuclear reactor is shutdown the major source of heat production will be done to the beta decay of these fission fragments.
• As a result of heat production being due to the beta decay the reactor shutdown, the decay heat will be about 6.5% of the previous core strength, but only if the reactor has had a steady power history.
• An hour after shutdown, the decay heat will be about 1% of the previous core power.
• Therefore, the decay heat production rate is dependent upon half-lives of the numerous fission products in the core, it will decrease slowly over time.

Removal

• Failure to remove decay heat may cause the reaction core temperature to rise to dangerous levels and cause nuclear accidents. An example of this is Three Mile Island. That is why it is important to remove decay heat, especially shortly after shutdown.
• There are several redundant and diverse systems to achieve heat removal.
• Heat is often dissipated to something called the “ultimate heat sink” which has a large capacity and requires no power. This method is typically used after decay heat has reduced to a very small value.
• Effective passive cooling for spent nuclear fuel is required for a couple of years, even after a year the typical spent nuclear fuel generates about 10kW of decay heat per ton, decreasing to about 1kW/t after ten years.