CHM 107 Notes Chapter 7 Nuclear Fission by decree


									                                       CHM 107 Notes
                                   Chapter 7 Nuclear Fission

 I.   Radiation background discussion
       A.    Radiation is not all bad, in fact most radiation is harmless and necessary for life. Radiation is
             simply energy in the form of waves or particles. We are familiar with most types of radiation
             (sunlight, heat, radio and TV waves...) ALL waves travel at the same speed, the speed of light
             which is 3.0 x 108 m/s. So long radio waves and small X rays all travel at the SAME speed.
       B. Types of radiation from high E (small λ ) to low E ( long λ ): gamma rays, X rays, UV rays,
             visible, IR rays (heat), microwaves, radio and TV.
       C.    Many materials naturally emit some kind of radiation: The sun emits all sorts of radiation (IR,
             UV and visible for starters), living things emit heat, rocks emit alpha and beta particles. Human
             made devices found everywhere emit or accept radiation: transmitters emit radio waves which
             our stereos and TV's receive, microwaves emit microwaves (imagine that), and on and on.
       D.    If the radiation is high energy we term it radioactivity. The materials that emit alpha and beta
             particles or gamma rays are termed radioactive. Thus our radios are not radioactive but smoke
             detectors are (They consist of alpha emitters and a sensor. When smoke gets in the way, the
             alpha particle doesn't reach the sensor and the alarm goes off) Note that alpha, beta and gamma
             radiation comes from the nucleus of a radioactive atom.
II.   Fission
       A.    Splitting of a large atom into smaller ones: 23592U + 10n      90
                                                                               38Sr +
                                                                                      143          1
                                                                                         54Xe + 3 0n,
             plutonium Pu has a similar reaction
               1.     Uranium is bombarded with neutrons and that initiate
                      the reaction. Since three more neutrons are products,
                      they in turn hit other U atoms and continue the reaction
                      building up times 3 each time. This is a chain reaction
                      (self propogating) and can result in an explosion if we
                      have enough U atoms = critical mass.
               2.     The energy released by 200 grams of U is about 2 x
                      1013Joules. Compare to 1 ton of coal which is about 5 x
                      107J. It would take one million tons of coal to create the same energy of a handful of U.
       B.    Uncontrolled fission can be used in bombs = atomic bombs. In WWII we dropped Little Boy
             and Fat Man on Hiroshima and Nagasaki - these were U and Pu bombs made with critical mass
             so they would result in a nuclear explosion.
               1.     Fission bombs are made with critical mass, but it is divided into pie like sections that are
                      separated and thus won’t explode or detonate prematurely.
               2.     When the bomb is “armed” the pie like sections are squished together to achieve critical
                      mass and thus it can now explode once neutrons hit it.
       C.    Controlled fission can be used to generate electricity. We control the rate by using Boron rods
             which can absorb neutrons to stop or slow the chain reaction.
               1.     Nuclear power plants use fission reactors to boil water that makes steam, which travels
                      down pipes and turns turbines. Check out diagrams in your text. The fission reaction is
                      hot, so that heat can boil water into steam. The steam then turns the turbines just like in a
                      coal plant. It is all about boiling water!!!
               2.     U mined from the ground is 0.7% Uranium. We need to enrich this ore to 3% Uranium
                      for the reactors. Note for bombs we need at least 20% uranium. So terrorists will not
     steal U from power plants for bombs – it is not enriched enough. Enriching is a rather
     dangerous process. You must be careful to avoid too much exposure.
3.   Worldwide there are 440 nuclear reactors in 31 countries which equal 16% of world
4.   20% of US power is nuclear. We have 103 nuclear plants in the USA. We use old-
     fashioned 1950’s style light water reactors in the US. They have problems: the hot water
     put into lakes raises the lake temperature (thermal pollution) and can ruin the ecosystem
     of the lake, U waste rods are still radioactive, the U ore must be enriched which requires
     expensive and dangerous processing, if the rods stick or the coolant is shut off meltdown
     can occur releasing radioactive material into the air = fallout. Chernobyl had a
     meltdown - as of Sept 2005, 56 people have died from the Chernobyl accident. This is
     terrible, but is the only nuclear power plant disaster in 50 years. Human error caused the
     meltdown. The workers stopped the water, the reactor heated up, they tried to insert the
     control rods to slow the reaction but the gears had melted. So the reaction got so hot it
     melted the container and the roof. Don't forget how many people die each year from coal
     mines and coal power plants. Basically all our current energy power plants have major
     downsides. This picture is an old style 1950’s type reactor.

5.   The worst-case scenario with a nuclear power plant is core meltdown, which results in a
     hot building and the roof melts. The wind then blows the radioactive dust away until it
     “falls out” of the air. The area where it blows is called “fallout” region. A nuclear
     explosion like a nuclear bomb is impossible at a power plant - they are nowhere near
     "critical mass" so no, a nuclear power plant can't explode like a nuclear bomb. The
                     problem is not the explosion, but the radioactive material being spread upon the wind to
                     the environment.
               6.    Heavy water reactors - use D2O instead of H2O. D is deuterium and is hydrogen with 1
                     proton and 1 neutron thus a mass of 2 so it is "heavy" compared to typical H atoms.
                     Good part is that the U ore does not need to be enriched. This is a huge advantage.
                     Canada uses these types of fission power plants and they are called CANDU reactors.
               7.    Breeder reactors - names so because they actually produce fuel (plutonium) for nuclear
                     power plants by adding other atoms to the U ore. So the waste is actually useable fuel -
                     this is a huge improvement. It takes about 10-20 years for a plant to produce enough Pu
                     fuel to start another plant. Russia and France use these new designs.
        D.   Using nuclear warheads for fuel – We can buy Soviet nuclear warheads, take out the 20%
             Uranium, reduce it to 3% uranium, and then use it in nuclear power plants. So far we have
             bought 6855 Soviet warheads!
        E. Pros and Cons of Nuclear Power Plants
               1.    Pros
                         a. No pollution
                         b. Less ore needed, less mining, less cost of transporting the ore to the plant, less
                              pollution from the transportation trucks, fewer mine related deaths, less rubber
                              needed for tires, less diesel fuel needed since less trucks needed
                         c. Does not lead to global warming
                         d. Newer designs are much better than the old style US 1950’s reactors
               2.    Cons
                         a. What to do with used Uranium rods that are still radioactive, where do we store
                              them? (Not as big a problem though with breeder reactors)
                         b. Enrichment process is dangerous (not needed though if use heavy water reactors
                              like CANDU)
        F.   Pros and Cons of Coal Plants
               1.    Pros
                         a. Coal is cheap and plentiful
                         b. Coal is not radioactive
               2.    Cons
                         a. Mining deaths every year way outnumber nuclear deaths
                         b. Transporting the coal ore to the plants is very expensive due to millions of trucks
                              per plant
                         c. Lots of rubber and diesel fuel needed for the trucks
                         d. Produces NOx and SOx and mercury ash
                         e. Contributes to global warming
III.   Nuclear Fusion (JUST FOR FUN, NOT ON EXAM)
        A.   Fuse small atoms together into larger atoms: 2 21D           3     1
                                                                            1H + 1H    Called thermonuclear
             reactions because require extremely high temperatures to get started.
        B.   98% of universe is Hydrogen and Helium
        C.   Stars are just giant fusion reactors: Yellow mainstream stars like our sun fuse H into He, Red
             giants fuse He into Li, N, O, F, up to Fe
        D.   The H bomb is the same reaction as stars - H fusion (aka thermonuclear warhead)
               1.    We've already made fusion bombs since the goal is to NOT have control. Never used in
                     war yet but have run test explosions.
               2.    Uses LiD and ignition of the fusion reaction is done with a tiny fission reaction starter.
        E. If we could control H fusion we would solve all energy problems.
               1.   Good facts - H is cheap and plentiful, it is not toxic, the waste is Helium, which is not
                    toxic, no meltdown potential.
              2.    Bad facts - We are not there technically yet, we can't control fusion yet, needs high
                    temperatures, laser ignition is one possibility being researched.
IV.   Biological Information (JUST FOR FUN) much of this info comes from a medical text.
       A.    Average exposure to humans - yes this means YOU!
              1. natural sources: cosmic rays, rocks, natural radiation inside body, Radon = 294 millirem
                    per year
              2.    artificial sources: X rays in medicine, meds, consumer goods, nuclear power plants = 65
                    millirem per year (note the power plants are only 2 millirem)
              3.    Radon is 200 millirem per year! Radon is a noble gas yet it is radioactive. Why is this a
                    problem? Radon being a gas is inhaled by us! Once inhaled, some is exhaled, but some
                    decays into Po. Po is a solid alpha emitter. Since Po is a solid, it is not exhaled thus it
                    just sits there in our lungs emitting alpha particles, which are most dangerous inside our
                    body. Causes lung cancer. Radon accumulates in basements. Problem in areas that have
                    lots of basements (Canada, Colorado...)
              4.    Levels of exposure
                        a. 25 rem just noticeable in blood (rems, not mrems)
                        b. 100 rem causes minor radiation sickness
                        c. 400 rem causes 50% chance of death
                        d. 600 rem fatal
                        e. 50,000 rem needed to kill bacteria
                        f. 1,000,000 rem needed to kill viruses (hardy aren't they?)
                        g. smaller but repeated doses over several years may cause cancer, especially
                        h. sunlight exposure can cause skin cancer
       B.    Therapy
              1.    destroys selected cells and tissues like cancer cells
              2.    used when cancer is spread out (not localized) thus surgery unlikely
              3.    used after surgery to get any left over cancer cells
              4.    used when cancer is moving (metastatic state) to other areas
              5.    X rays, gamma rays, and proton beams are all focused on the suspected area to minimize
                    the damage of normal cells
       C.    CT scans (computer assisted tomography)
              1.    X rays used, detected, and data compiled by computer to produce image
              2. used to detect brain tumors
              3. used to detect stroke damage
       D.    MRI (magnetic resonance imaging)
              1.    used on soft tissues that contain H atoms (water and fat)
              2.    uses radio waves so not radioactive
       E. PET scan (positron emission topography)
              1.    inject positron emitter (11C)
              2.    positrons collide with electrons around other atoms and produce gamma rays
              3.    detect the gamma rays and compile image
              4.    show brain changes when we think (brain stimulated)

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