Radioactivity of Nuclear Power Plant

  Components of a nuclear reactor


Fuel

Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. The  commonly used fuels are Uranium, Plutonium or Thorium. It can be U-235, U-238, Pu-236 or Th-232. Uranium is mostly preferred as it has high melting point.

Moderator

Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water, graphite and Beryllium are also used.

Control rods

These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. These rods absorb neutrons and stop the chain reaction to proceed further. These are made up of steel containing a high percentage of material like cadmium or boron which can absorb neutrons. When control rods are completely inserted into the moderator block then all the neutrons is absorbed and reaction comes to halt.

Coolant

The coolant is substance in a pipe to the steam generator where water is boiled. This is where heat-exchange process occurs. Heat is absorbed by the coolant that is produced in the reactor. Typical coolants are water, carbon dioxide gas or liquid sodium. 

Shielding / Containment

Shielding prevents radiations to reach outside the reactor. Lead blocks and concrete enclosure that is strong enough of several meters thickness are used for shielding. 

Turbine

Steam produced in the boiler is now passes to a turbine. The force of the steam jet causes the turbine to rotate. Heat energy (steam) is converted to mechanical energy (moving turbine)              .   

Generator

The generator consists of coils that change the mechanical energy into electric energy. The turbine moves and the change in magnetic flux cause electricity. This is transmitted to substations for distribution of electric power.

Principal parts of a nuclear reactor

  Types of Nuclear Reactor


Pressurised Water Reactor (PWR) which has water at over 300°C under pressure in its primary cooling/heat transfer circuit, and generates steam in a secondary circuit. In a pressurized water reactor, the water is pumped into contact with the core and then kept under pressure, so that it can't turn into steam. That pressurized water then is brought into contact with a second supply of unpressurized water, which is what turns to steam to turn the turbines.

Boiling Water Reactor (BWR)

Pressurised heavy water reactor (PHWR)

Advanced Gas Cooled Reactor (AGCR)

These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as primary coolant. 

Boiling Water Reactor (BWR) makes steam in the primary circuit above the reactor core, at similar temperatures and pressure. With BWRs, the water that comes directly into contact with the reactor core is allowed to become steam for generating electricity

(Another type uses heavy water, with deuterium atoms, as moderator. Heavy water contains a larger than normal amount of the hydrogen isotope deuterium 2H2O)

Deuterium is an isotope of hydrogen whose nucleus comprises both a neutron and a proton; the nucleus of a protium (regular hydrogen) atom consists of just a proton. The additional neutron makes a deuterium atom roughly twice as heavy as a protium atom.

  Nuclear Reactor in India


  Nuclear power plants in commercial operation


  Fuelling a nuclear power reactor 


Refuelling

Most reactors need to be shut down for refuelling, so that the pressure vessel can be opened up. In this case refuelling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refuelled under load by disconnecting individual pressure tubes.

If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium.

The moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.

During operation, some of the U-238 is changed to plutonium, and Pu-239 ends up providing about one third of the energy from the fuel.


In most reactors the fuel is ceramic uranium oxide (UO2with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and transparent to neutrons.Numerous rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5 to 4 metres long.

  Nuclear Accidents


 In the event of an accident, the primary concern is that the support structure (core) containing the fuel and the fission products may become damaged and allow radioactive elements to escape into the environment. 

The reactor core and even the fuel itself can partially or completely melt. Elevated temperatures and pressures can result in explosions within the reactor, dispersing radioactive material.

In most plants, the potential effects of a cooling-system failure are minimized by surrounding the reactor core with a steel-walled vessel, which in turn is surrounded by an airtight, steel-reinforced concrete containment structure that is designed to contain the radioactive material indefinitely

Nuclear Accidents and Disasters

 

In the partial meltdown at Three Mile Island, the plant's containment structure fulfilled its purpose, and a minimal amount of radiation was released. However, there was no such containment structure in place at the Chernobyl reactor — the explosions and the subsequent fire sent a giant plume of radioactive material into the atmosphere.

Although the Three Mile Island accident has not yet led to identifiable health effects, the Chernobyl accident resulted in 28 deaths related to radiation exposure in the year after the accident.

Fukushima Nuclear Disaster - Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. All three cores largely melted in the first three days. Ranked 7 due due to high radioactive releases over days 4 to 6, eventually a total of some 940 PBq . (Peta 1015 nucleus decays per second). There have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people were evacuated from their homes to ensure this.

Cold Shutdown - The term used to define a reactor coolant system at atmospheric pressure and at a temperature below 200 degrees Fahrenheit following a reactor cool down.

  Types of Radiation Exposure - Plant


Human radiation exposure as a result of reactor accidents is generally characterized in three ways:

  • total or partial body exposure as a result of close proximity to a radiation source, external contamination,
  • and internal contamination

Total or partial body exposure occurs when an external source irradiates the body either superficially to the skin or deeply into internal organs, with the depth depending on the type and energy of the radiation involved. In previous reactor accidents, only plant workers and emergency personnel who were involved in the aftermath had substantial total or partial body exposure.

External contamination occurs when the fission products settle on human beings, thereby exposing skin or internal organs. Populations living near a reactor accident may be advised to remain indoors for a period to minimize the risk of external contamination.

Internal contamination occurs when fission products are ingested or inhaled or enter the body through open wounds. This is the primary mechanism through which large populations around a reactor accident can be exposed to radiation. After Chernobyl, approximately 5 million people in the region may have had excess radiation exposure, primarily through internal contamination.

The release of radioactive water into the sea at the Fukushima plant has resulted in an additional route whereby the food chain may be affected, through contaminated seafood. Although the radioactivity in seawater close to the plant may be transiently higher than usual by several orders of magnitude, it diffuses rapidly with distance and decays over time, according to half-life, both before and after ingestion by marine life.

  Clinical consequences of radiation exposure


At a molecular level, the primary consequence of radiation exposure is DNA damage. This damage will be fully repaired or innocuous or will result in dysfunction, carcinogenesis, or cell death.

The clinical effect of radiation exposure will depend on numerous variables, including

  • the type of exposure (total or partial body exposure vs. internal or external contamination),
  • the type of tissue exposed (tissue that is sensitive to radiation vs. tissue that is insensitive),
  • the type of radiation (e.g., gamma vs. beta),
  • the depth of penetration of radiation in the body (low vs. high energy),
  • the total absorbed dose,
  • and the period over which the dose is absorbed (dose rate).

Acute Radiation Sickness

When most or all of the human body is exposed to a single dose of more than 1 Gy of radiation, acute radiation sickness can occur. Plant workers or members of the emergency response team.

Much of the short-term morbidity and mortality associated with a high total or near-total body dose is due to hematologic, gastrointestinal, or cutaneous sequelae. In the Chernobyl accident, all 134 patients with acute radiation sickness had bone marrow depression, 19 had widespread radiation dermatitis, and 15 had severe gastrointestinal complications. 

Hematologic and gastrointestinal complications are common because bone marrow and intestinal epithelium are especially radiosensitive as a result of their high intrinsic replication rate.

Cutaneous toxic effects are common because external low-energy gamma radiation and beta radiation are chiefly absorbed in the skin. In Chernobyl, estimated skin doses in some patients were 10 to 30 times the bone marrow doses. If total body doses are extremely high (>20 Gy), severe acute neurovascular compromise can occur. At Chernobyl, the highest absorbed dose in a worker was 16 Gy

Increased Long-Term Cancer Risks

  • In the region around Chernobyl, more than 5 million people may have been exposed to excess radiation, mainly through contamination by iodine-131 and cesium isotopes. Although exposure to nuclear-reactor fallout does not cause acute illness, it may elevate long-term cancer risks.
  • Studies of the Japanese atomic-bomb survivors showed clearly elevated rates of leukemia and solid cancers, even at relatively low total body doses.However, there are important differences between the type of radiation and dose rate associated with atomic-bomb exposure and those associated with a reactor accident. 
  • Alternatively, small increases in the risks of leukemia and non-thyroid solid cancers may become more apparent with improved cancer registries or longer follow-up
  • There is strong evidence of an increased rate of secondary thyroid cancers among children who have ingested iodine-131. Factors that increase the carcinogenic effect of iodine-131 include a young age and iodine deficiency at the time of exposure.
  • In accidents in which iodine-131 is released, persons in affected areas should attempt to minimize their consumption of locally grown produce and groundwater. However, since the half-life of iodine-131 is only 8 days, these local resources should not contain substantial amounts of iodine-131 after 2 to 3 months, area residents may take potassium iodide to block the uptake of iodine-131 in the thyroid

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