Question to the class: What are the pros, cons, and uncertainties of using nuclear power?

Transcription

1 Energy and Society Week 11 Section Handout Section Outline: 1. Rough sketch of nuclear power (15 minutes) 2. Radioactive decay (10 minutes) 3. Nuclear practice problems or a discussion of the appropriate role of nuclear power in our energy future (25 minutes) I. Nuclear Power Basics (15 minutes) If our ultimate goal is evaluating any technology is to decide whether it should be a part of our energy future, we should have a sense of the upsides, downsides, and unknowns of that technology. Question to the class: What are the pros, cons, and uncertainties of using nuclear power? Pros: Low lifecycle carbon emissions (about 60 g CO 2-eq/kWh, compared to 1050 for coal, 950 for natural gas, 140 for PV, and 10 for wind) High reliability (cf = 0.90) Some domestic fuel supplies (about 5-10% of world stock; 31% in Australia) Can serve as a baseload generator (and therefore as a replacement for coal) Cons Waste generation and disposal (how do we plan to keep waste safe for 10,000 years)? Hard path (centralized, high capital cost, technologically complex) Uranium mining is pretty environmentally damaging National security concern Nuclear weapons proliferation (particularly when recycling nuclear waste) Historically, a forgetting curve Uncertainties Levelized costs Hard to tell what the true risks of various outcomes (e.g., meltdowns) are Size of fuel resource reserves (with current technology and usage rate, we ll be out of uranium in ~100 years) Many speculative reactor designs (thorium reactors), some of which could deliver marked benefits in terms of safety and cost You were irradiated with information last week, and many of you are likely wondering what we d like you to internalize from the nuclear lectures and readings. We don t expect you to return to your textbooks and learn or re-learn atomic physics. What we would like is to make sure that you: 1) Have a basic, qualitative understanding of the different pieces that make up the lifecycle of nuclear power generation, including: a. The nuclear fuel cycle what are the major steps and what happens at each step? b. Nuclear reactions what happens during the reaction and why do you get energy out? c. Different reactor types what are the basic differences between pressurized water reactors and boiling water reactors? Light water reactors and heavy water reactors?

2 d. Nuclear waste and storage what are the main risks posed by nuclear waste? 2) Be able to work quantitatively with basic policy-relevant questions that involve nuclear power, including: a. Fuel characteristics and requirements b. Waste and exponential decay Nuclear Fission Basics Fission splits an atom into 2 (or more) daughter products. The mass of an atom is less than the sum of its constituent parts (e.g., protons, neutrons); the extra mass of the parts becomes the binding energy of an atom, the energy required to hold the atom together. E = mc 2 describes the equivalence of mass and energy. Fissioning 1 U-235 atom liberates about 200 MeV of energy. We can purposefully promote fission reactions by creating a system in which neutrons are free to slam into readily fissionable atoms (like U-235). This fission reaction liberates energy, and this energy can be used to boil water that is used to turn a turbine and create electricity. The fission of one U-235 atom with a neutron releases, on average, 2.5 neutrons. These neutrons can, in turn, fission other U-235 atoms in a chain reaction. Since more neutrons are liberated by a fission reaction than are required to start it, a nuclear reactor might see an exponential increase in the number of neutrons in the system and a corresponding exponential increase in fission events. Various mechanisms (e.g., control rods) are in place to control the population of neutrons to prevent supercriticality, or increasing power levels. II. Radioactivity (10 minutes) Isotopes = different versions of the same element same # of protons, different neutrons also nuclides. U-235 had 92 protons and 143 neutrons. Many (most) isotopes are stable, some undergo radioactive decay these are radioisotopes or radionuclide. Radionuclides naturally undergo nuclear fission. In such an event, a part of the atom is spontaneously emitted. This emission changes the atom, creating an isotope of a new element (e.g., C-14 into N-14). The rate at which they decay is an intrinsic property; some decay rapidly, while isotopes can take billions of years to undergo decay. A common way to characterize the rate at which a given isotope will decay is by its half-life, the amount of time it takes for half of a given stock of a radionuclide (say, 1 g or 100 atoms) to undergo decay. Half-lives can range from the very short ( for Po-214) to the very long (4.5 billion years for U-238). Radioactive decay in an exponential process, meaning that the number of decays at any given moment is a function of the size of the stock of radionuclides. (More radionuclides, more decays/second.) To describe the change in the size of the stock of radioactive isotopes, we call on the exponential growth equation: I = I 0e -t Where: is the decay constant and is equal to ln(2)/half-life. Example Problem 1 Plutonium (Pu) is created by the decay of 239U into 239Pu and ultimately to other Pu isotopes. 239Pu has a half life of 24,000 years. A large nuclear power plant produces 0.50 tons of reactor grade Pu per year, about 60% of which is 239P. How much of the 239Pu produced in 2009 will still be around in the year 10,000 CE? This is an exponential decay problem. To solve these, you can apply the formula C(t) = C0e λt

3 where C is the concentration at time t, C0 is the concentration at time zero, t is the time measured from time zero (10, = 7,991 in this case), and λ is equal to λ = ln2 t1/2 t1/2 is the half life here. Solving for λ gives you λ = ln2 = ,000 yrs yr And plugging λ, t, and C0 back into the original equation gives you C(t) = 0.30 tons es ( ) yr 7991 yr = 0.24 tons In other words, the Pu-239 will be our children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s children s (you get the point; this ends up being more than 250 generations) problem. As mentioned in lecture, Pu is an alpha emitter, which means that it poses little risk if exposure is outside the body, but can be very dangerous if exposure is internal (e.g., through drinking water). See The long lives of many radioactive byproducts of fission are what make longer-term waste storage such a challenge. III. Practice Problems or Discussion (25 minutes) Practice Problem 2 What mass of uranium ore (in kg) enriched to 3% 235 U is required to produce 6132 GWh of electricity (equivalent to a 1 GW power plant running at 70% capacity factor)? Assume that each fission of 235 U produces 200 MeV (3.2 x J), that all neutrons absorbed by 235 U cause fission, and that the nuclear power plant has a thermal efficiency of 33%. Start with the energy requirement. Based on the efficiency, you know you ll need The fission of each atom (nucleus) of 235 U generates 3.2 x J, so we know that we ll need 235 U accounts for 3% of the total U, so

4 Assuming that the uranium ore is 238 U (i.e., having a molecular weight of 238 g/mol), we can calculate the mass of uranium required. IV. Supplemental Information Sources of nuclear fuel o Uranium Supplies. Canada and Australia account for 40 percent of global uranium production; the United States accounts for 3 percent. According to a 2010 Organization for Economic Cooperation and Development (OECD) and International Atomic Energy Agency (IAEA) report, uranium resources are adequate to meet nuclear energy needs for at least the next 100 years at present consumption levels. More efficient fast reactors could extend that period to more than 2,500 years. [Beware of forecasts in both directions!] o Mining and Milling. Uranium miners use several techniques to obtain uranium: surface (open pit), underground mining and in-situ recovery. Uranium also is a byproduct of other mineral processing operations. Solvents remove the uranium from mined ore or in-situ leaching, and the resulting uranium oxide called yellowcake undergoes filtering and drying. (Source: Nuclear Energy Institute) Processing o Uranium Conversion. The yellowcake then goes to a conversion plant, where chemical processes convert it to uranium hexafluoride. The uranium hexafluoride is heated to become a gas and loaded into cylinders. When it cools, it condenses into a solid. o Uranium Enrichment. Uranium hexafluoride contains two types of uranium, U-238 and U-235. The percentage of U-235, which is the type of uranium that fissions easily, is less than 1 percent. To make the uranium usable as a fuel, its U-235 content is increased to between 3 percent and 5 percent. This process is called enrichment. The concentration of U-235 is so low in enriched uranium that an explosion is impossible. o Fuel Fabrication. After the uranium hexafluoride is enriched, a fuel fabricator converts it into uranium dioxide powder and presses the powder into fuel pellets. The fabricator loads the ceramic pellets into long tubes made of a noncorrosive material, usually a zirconium alloy. Once grouped together into a bundle, these tubes form a fuel assembly. (Source: Nuclear Energy Institute) Fuel consumption o Fission. In a nuclear reactor, fission occurs when the nuclei of atoms are split through the capture of neutrons. This fissile process can release new neutrons (on average more than one), some of which get absorbed by non-fissile material, some of which leak through the reactor core, and some of which get absorbed by fissile material. Re-absorption by fissile material is what continues the chain reaction. o Nuclear Reaction. The goal for a reactor operator is to maintain a balance between the rate of neutron production and neutron absorption/loss. When the rates are equal, we have a critical reaction. When there is a net productivity of neutrons, the reaction is super-critical the nuclear operator must increase the rate of neutron absorption to bring the reaction back into balance (e.g., by inserting a control rod; or, to shut down a reactor, by inserting safety rods). When there is a net loss in neutrons, the reaction is sub-critical. In comparison, a nuclear weapon is designed to achieve a super-critical reaction.

5 Used fuel processing and disposal o Interim Storage. Following use in the reactor, the fuel assembly becomes highly radioactive and must be removed and stored under water in a spent fuel pool at the reactor for several years. Even though the fission reaction has stopped, the spent fuel continues to give off heat from the decay of radioactive elements that were created when the uranium atoms were split apart. The water in the pool serves to both cool the fuel and shield the operators from any radiation. As of 2002, there were over 165,000 spent fuel assemblies stored in about 70 interim storage pools throughout the United States. After cooling a few years in the pool, the spent fuel element may be moved to a dry cask storage container for further on-site storage. An increasing number of reactor operators now store their older spent fuel in these special outdoor concrete or steel containers with air cooling. o Reprocessing. Less than 4% of the uranium loaded into the reactor is consumed in nuclear reactions. The rest of the uranium remains unchanged. Chemical processing of the spent fuel material to recover the remaining portion of fissionable products for use in fresh fuel assemblies is technically feasible. Some countries, such as France, reprocess spent nuclear fuel, but it is not permitted in the United States. o Final Disposal. The final step in the nuclear fuel cycle is the collection of spent fuel assemblies from the interim storage sites or future reprocessing facilities, and the disposal of any remaining high-level nuclear waste in a permanent underground repository. The United States currently has no such repository. 1 o Nuclear Waste. The main environmental concerns for nuclear power are radioactive wastes such as uranium mill tailings, spent (used) reactor fuel, and other radioactive wastes. These materials can remain radioactive and dangerous to human health for thousands of years. Radioactive wastes are classified as low-level and high-level. The radioactivity in these wastes can range from just above natural background levels, as in mill tailings, to much higher levels, such as in spent reactor fuel or the parts inside a nuclear reactor. o Half-life. The radioactivity of nuclear waste decreases with the passage of time through a process called radioactive decay. The amount of time necessary to decrease the radioactivity of radioactive material to one-half the original level is called the radioactive half-life of the material. Radioactive waste with a short half-life is often stored temporarily before disposal in order to reduce potential radiation doses to workers who handle and transport the waste, as well as to reduce the radiation levels at disposal sites. o Low-level Waste. By volume, most of the waste related to the nuclear power industry has a relatively low-level of radioactivity. Uranium mill tailings contain the radioactive element radium, which decays to produce radon, a radioactive gas. The other types of low level radioactive waste are the tools, protective clothing, wiping cloths, and other disposable items that get contaminated with small amounts of radioactive dust or particles at nuclear fuel processing facilities and power plants. o High-level Waste. High-level radioactive waste consists of irradiated or used nuclear reactor fuel (i.e., fuel that has been used in a reactor to produce electricity). The used reactor fuel is in a solid form consisting of small fuel pellets in long metal tubes. (Source: US Energy Information Administration) o Note: A typical 1000 MWe light water reactor will generate (directly and indirectly) m3 low- and intermediate-level waste per year. It will also discharge about 20 m 3 (27 tonnes) of used fuel per year, which corresponds to a 75 m 3 disposal volume following encapsulation if it is treated as waste. Where that used fuel is reprocessed, only 3 m 3 of vitrified waste (glass) is produced, which is equivalent to a 28 m 3 disposal volume following placement in a disposal canister. (World Nuclear Association) 1 Brief document about Yucca Mountain:

6 Types of Reactors For economic reasons, a few reactor types have dominated the commercial market. Most commercial reactors in the world use low-enriched uranium, e.g., 90% are light-water reactors (LWRs) where the fresh fuel is UO2 (the U is enriched to about 4-5% U-235), and heavy-water reactors (mostly in or supplied by Canada) where the U is natural uranium (0.711% U-235); and a few reactors use MOX fuel (a blend of uranium and plutonium), e.g, some fast reactors and some LWRs. There are two dominant subtypes of light-water reactors: boiling water reactors (BWR) and pressurized water reactors (PWR). The primary difference is that in BWRs, the water that contacts the core is converted directly to steam, which then passes through the turbine. The PWRs use a primary closed loop of cooling water that flows through the reactor core and passes through a heat exchanger to generate steam in a secondary loop of water flow. In response to the difficulties in achieving sustainability, a sufficiently high degree of safety and a competitive economic basis for nuclear power, the U.S. Department of Energy initiated the Generation IV program in Generation IV refers to the broad division of nuclear designs into four categories: early prototype reactors (Generation I), the large central station nuclear power plants of today (Generation II), the advanced light-water reactors and other systems with inherent safety features that have been designed in recent years (Generation III), and the next-generation systems to be designed and built two decades from now (Generation IV). These next-generation systems are based on three general classes of reactors: gas-cooled, water-cooled and fast-spectrum.

7 (Source: Lake 2002) Types of Fuel Processing The current once-through, or open, nuclear fuel cycle uses freshly mined uranium, burns it a single time in a reactor and then discharges it as waste. This approach results in only about 1 percent of the energy content of the uranium being converted to electricity. It also produces large volumes of spent nuclear fuel that must be disposed of in a safe fashion. Both these drawbacks can be avoided by recycling the spent fuel that is, recovering the useful materials from it. Most other countries with large nuclear power programs including France, Japan and the U.K. employ what is called a closed nuclear fuel cycle. In these countries, used fuel is recycled to recover uranium and plutonium (produced during irradiation in reactors) and reprocess it into new fuel. This effort doubles the amount of energy recovered from the fuel and removes most of the long-lived radioactive elements from the waste that must be permanently stored. It should be noted, though, that recycled fuel is today more expensive than newly mined fuel. Current recycling technology also leads to the separation of plutonium, which could potentially be diverted into weapons.

8 (Source: Lake 2002)