Chapter 4. Power From Fission 1.Introduction 2.Characteristics of Fission 3. General Features 4. Commercial Reactors 5. Nuclear Reactor Safety 6. Nuclear Reactor Accidents Key elements: fuel, neutron moderator, control rod, neutron detector and radioactivity detectors, products

Source: IEA (2007)

Delivery Construction times for nuclear plants – Global average 66 months in mid-1970s 116 months (nearly 10y) in late 1990s 82 months (nearly 7y) during

Energy Estimate the energy released by the fission of 1.0 kg of 235 U. (3.15e-11 J) 1000 g = 8.06e13 J (per kg). Discussion This is a large amount of energy, and it is equivalent to the energy produced by burning tones of coal or oil. 1 mol 235 g 6.023e23 1 mol 235 U 92  142 Nd Zr n + Q Q = ( x ) = amu ( MeV / 1 amu ) = MeV per fission( e-13 J / 1 MeV ) = 3.15e-11 J This amount of energy is equivalent to 2.2×10 10 kilowatt-hour, or 22 giga-watt- hour. This amount of energy keeps a 100-watt light bulb lit for 25,000 years. 2. Characteristics of Fission

7 Fission Energy Budget Kinetic energy of fission fragments Prompt (< 10 –6 s) gamma (  ) ray energy Kinetic energy of fission neutrons Gamma (  ) ray energy from fission products Beta (  ) decay energy of fission products Energy as antineutrinos ( v e ) 168 MeV Energy (MeV) distribution in fission reactions

Fission neutron energy spectrum

The total and fission cross section for 235 U based on NJOY-processed ENDF/B (version V) data. Neutron interactions

The fast fission cross section for three fissionable uranium isotopes based on NJOY processed ENDF/B (version V) data

The Cyclotron and Fission Research 7 Li (p, n) 7 Be 3 T (p, n) 3 He 1 H (t, n) 3 He 2 D (d, n) 3 He 2 D (t, n) 4 He 3 T (d, n) 4 He Fusion reactions studied using the cyclotron

12 The Cyclotron and Fission Research Threshold* Energy range (keV) Reactionenergy(keV) narrow-energy neutron 51 V (p, n) 51 Cr Sc (p, n) 45 Ti Fe (p, n) 57 Co __________________________________ * The threshold energy is the minimum energy of proton required for the reaction. Neutrons of desirable energy is required for fission research.

Nuclear Fission13 The Cyclotron and Fission Research For neutron sources from the cyclotron, energy can be varied. Energy dependence of neutron induced fission studied. The cross section data enabled nuclear reactor design. fast neutrons - 10 MeV to 10 KeV) slow neutrons to eV for neutron induced fission

Nuclear Fission 14 Fission Products nuclides produced in nuclear fission Data on fission products are required for reactor design, operation, and accident responses. The study of fission products requires the separation, identification, and quantitative determination of various elements and isotopes. Fission products emit  particles until they are stable, n also. Mass number range: Elements range: all the elements in the 4 th, 5 th, and 6 th periods. including the lanthanides. 2. Characteristics of Fission

Nuclear Fission 15 Fission Products Fission yield is the relative amounts of nuclides formed in fission reactions. The fission yield curve shown here shows most fission reactions split fission atoms into two unequal fragments.

Nuclear Fission Products Fission-product and their decay data are needed for social and environmental concerns, and for the management of used fuel. Fission nuclides usually have very short half lives. Typical medium-life fission products: 85 K 10.7 y, 90 Sr 29 y, 137 Cs 30 y, Typical long-life fission products: 126 Sn 1.0e5 y, 126 Tc 2.1e5 y, 91 Tc 1.9e6 y, 135 Cs 3.0e6 y, 107 Pd 6.5e6 y, and 129 Tc 1.6e7 y. Xenon poisoning: 115 Xe,  c = 2,640,000 b, and t 1/2 = 9.2 h

Chapter 4. Power From Fission 1.Introduction 2.Characteristics of Fission 3. General Features 4. Commercial Reactors 5. Nuclear Reactor Safety 6. Nuclear Reactor Accidents Key elements: fuel, neutron moderator, control rod, neutron detector and radioactivity detectors, products

Simplified schematic layout of a typical reactor power plant. 3.1 A nuclear power plant

Control rods, containing neutron-absorbing elements (boron or cadmium) pressure vessels must be capable of withstanding internal pressures up to 160 bar. A biological shield, normally several feet of concrete, surrounds the entire system. Its purpose is to attenuate the intensity and neutron radiations to levels that are safe for humans outside the plant The coolant is pumped through the core inside the pressure vessel and through heat exchangers outside, where steam is generated and used to drive turbines for generating electric power. The melting point of uranium is 1403 K, The melting point of UO 2 is 3138 K

fastabso rp. 3.2 The neutron cycle

subcritical supercritical self-sustainingcritical

3.3 Moderator Properties of materials used as moderators

3.4 Optimizing the design f is a decreasing function and p an increasing function of moderator-to-fuel ratio N M / N F Uranium ~ graphite assemblies

Diffusion length the root-mean-square distance a neutron will diffuse in the medium before being absorbed Diffusion and slowing-down constants for moderators.

R, The reaction probability per unit time for N nuclei; M is the mass of fissile material if each fission liberates an amount E of recoverable energy, the power output is Reactor power and fuel consumption

example calculate the power output, rating and fuel consumption for a thermal reactor containing 150 tonnes of natural uranium operating with a neutron flux of energy per fission E = 200 MeV

Fuel consumption, leading to a loss of 235 U, depends on the total 235 U absorption cross section = 5.9 x /year one-fifth of the initial amount of 235 U refueling

Key Reactor Power Terms Availability – Fraction of time over a reporting period that the plant is operational – If a reactor is down for maintenance 1 week and refueling 2 weeks every year, the availability factor of the reactor would be (365-3 * 7) / 365 = 0.94

Key Reactor Power Terms Capacity – Fraction of total electric power that could be produced – If reactor with a maximum thermal power rating of 1000 MWt only operates at 900 MWt, the capacity factor would be 0.90 Efficiency – Electrical energy output per thermal energy output of the reactor Eff=W/Q R (MWe/MWt) ~33% Carnot efficiency,

5.2 Reactor Kinetics A Simple Reactor Kinetics Model Consider a core in which the neutron cycle takes l' seconds to complete The change Δn in the total number of thermal neutrons in one cycle at time t or

=1.001 Uncontrollable !

Revised Simplified Reactor Kinetics Models Consider a thermal reactor fueled with 235 U The average or effective generation time required for all the neutrons produced in a single neutron cycle is thus Delayed neutron average lifetime is A fraction β of the fission neutrons requires a cycle time of while a fraction (1 -β) is the prompt-neutron fraction and requires a cycle time of only =0.083 s

-> s controllable !

Chapter 4. Power From Fission 1.Introduction 2.Characteristics of Fission 3. General Features 4. Commercial Reactors 5. Nuclear Reactor Safety 6. Nuclear Reactor Accidents Key elements: fuel, neutron moderator, control rod, neutron detector and radioactivity detectors, products