Nuclear Fission: Principle Of Nuclear Reactor

nciple of Nuclear Reac

NUCLEAR FISSION

Fission Process U+n

235

U*

236

(A1,Z1) + (A2,Z2) + Nn + Υ

• Z1 + Z2 = 92, A1 + A2 + N = 236 • A1 = A2, symmetric fission rare (~0.01%) • Capture of neutron by 235U forms compound nucleus • ~2.4 prompt neutrons released per fission event • Delayed neutrons account for ~0.65% of fission neutrons • Probability of fission occuring is dependant upon the crosssection for that reaction

Reactor Kinetics 

Criticality factor k=1=ηFfpεPL Chain reaction

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Reactivity ρ= k-1/k ρ>0 represents a runaway chain reaction

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That is bad

Power ∝ neutron flux, Φ d Φ =(k-1) Φ(t) = Φ(t) dt ι T

Φ(t)= Φ(0)et/T Where T= ι/k-1 Flux grows exponentially!

Delayed n’s (β=0.0065) increases the mean neutron lifetime by a factor of ~1000!! Rate of n buildup is slowed ρ≥ β 0> ρ > β

prompt critical delayed critical

The Nuclear Reactor

 To build a nuclear reactor, what you need is some mildly enriched uranium. Typically, the uranium is formed into pellets . The pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are then typically submerged in water inside a pressure vessel. The water acts as a coolant. In order for the reactor to work, the bundle, submerged in water, must be slightly supercritical. That means that, left to its own devices, the uranium would eventually overheat and melt.  To prevent this, control rods made of a material that absorbs neutrons are inserted into the bundle using a mechanism that can raise or lower the control rods. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the rods are raised out of the uranium bundle. To create less heat, the rods are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.

The Lead-Cooled Fast Reactor (LFR)

 The Lead-Cooled Fast Reactor (LFR) system features a fast-spectrum lead or lead/bismuth eutectic liquid metalcooled reactor and a closed fuel cycle for efficient conversion of fertile uranium and management of actinides.  The system has a full actinide recycle fuel cycle with central or regional fuel cycle facilities. Options include a range of plant ratings, including a battery of 50-150 MWe that features a very long refueling interval, a modular system rated at 300-400 MWe, and a large monolithic plant option at 1200 MWe. The term battery refers to the longlife, factory fabricated core, not to any provision for electrochemical energy conversion.  The fuel is metal or nitride-based, containing fertile uranium and transuranics. The LFR is cooled by natural convection with a reactor outlet coolant temperature of 550 degrees C, possibly ranging up to 800 degrees C with advanced materials. The higher temperature enables the production of hydrogen by thermochemical processes.

The Molten Salt Reactor (MSR)

 In the MSR system, the fuel is a circulating liquid mixture of sodium, zirconium, and uranium fluorides. The molten salt fuel flows through graphite core channels, producing an epithermal spectrum. The heat generated in the molten salt is transferred to a secondary coolant system through an intermediate heat exchanger, and then through a tertiary heat exchanger to the power conversion system. The reference plant has a power level of 1,000 MWe. The system has a coolant outlet temperature of 700 degrees Celsius, possibly ranging up to 800 degrees Celsius, affording improved thermal efficiency.

 The closed fuel cycle can be tailored to the efficient burn up of plutonium and minor actinides. The MSR's liquid fuel allows addition of actinides such as plutonium and avoids the need for fuel fabrication. Actinides - and most fission products - form fluorinides in the liquid coolant. Molten fluoride salts have excellent heat transfer characteristics and a very low vapor pressure, which reduce stresses on the vessel and piping.

The SupercriticalWater-Cooled Reactor (SCWR)

 The supercritical water coolant enables a thermal efficiency about one-third higher than current light-water reactors, as well as simplification in the balance of plant. The balance of plant is considerably simplified because the coolant does not change phase in the reactor and is directly coupled to the energy conversion equipment. The reference system is 1,700 MWe with an operating pressure of 25 MPa, and a reactor outlet temperature of 510 degrees Celsius, possibly ranging up to 550 degrees Celsius. The fuel is uranium oxide. Passive safety features are incorporated similar to those of simplified boiling water reactors.  The SCWR system is primarily designed for efficient electricity production, with an option for actinide management based on two options in the core design: the SCWR may have a thermal or fast-spectrum reactor; the second is a closed cycle with a fast-spectrum reactor and full actinide recycle based on advanced aqueous processing at a central location.

The Sodium-Cooled Fast Reactor (SFR)

 The fuel cycle employs a full actinide recycle with two major options: One is an intermediate size (150 to 600 MWe) sodiumcooled reactor with uranium-plutonium-minor-actinide-zirconium metal alloy fuel, supported by a fuel cycle based on pyrometallurgical processing in facilities integrated with the reactor. The second is a medium to large (500 to 1,500 MWe) sodium-cooled reactor with mixed uranium-plutonium oxide fuel, supported by a fuel cycle based upon advanced aqueous processing at a central location serving a number of reactors. The outlet temperature is approximately 550 degrees Celsius for both.  The SFR is designed for management of high-level wastes and, in particular, management of plutonium and other actinides. Important safety features of the system include a long thermal response time, a large margin to coolant boiling, a primary system that operates near atmospheric pressure, and intermediate sodium system between the radioactive sodium in the primary system and the water and steam in the power plant. With innovations to reduce capital cost, the SFR can serve markets for electricity.

Advantages Of Nuclear energy over other forms Economical aspect Environmental friendly Nuclear Energy – The Future…

Economics 

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~90% capacity Reliable and available Fuel cost relatively low Decreasing capital investment with NGR Social, health, and environmental costs are lower 1Kg U yields ~20 000 times more energy than same amount of coal

Average Capacity Factor by Energy Source, 2002

Environmental Friendly

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Reduced SO2 and CO2 emissions Solidified high-level waste is geologically disposed

THE FUTURE    

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440 reactors currently operating worldwide 30 being built and 33 in planning Another 69 are being proposed Generation lV reactor designs are being developed U.S. investing $410 million into research Enrollment in nuclear engineering is up

Conclusions 

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Solution to energy crisis… more construction on the way. Better for the environment, more economical, and more abundant than other energy sources. Less dependance on foreign oil. Nuclear power can prove disastrous … but mankind has to work on its positive aspect and our earth can become a heaven to live in.