Texas A&M University, Department of Nuclear engineering, Ph.D. Qualifying Examination, Fall 2016

Transcription

1 Part 2 of points of the total exam worth of 200 points Research Area Specific Problems Select and answer any 4 problems from the provided 15 problems focusing on the topics of research tracks in the department: Computational Nuclear Engineering Health Physics Nuclear Materials Engineering Nuclear Security and Safeguards Power Engineering You may select and answer any of the given problems. Your problem selection may consist of problems from multiple tracks as you desire. Computational Nuclear Engineering Problem 1 of 15 (25 points) Given the probability density function e x, for x 0, write an algorithm for generating samples from a random variable that is governed by this probability distribution. Problem 2 of 15 (25 points) Consider the integral of a function f() x over the range 1, 1 using a two-point quadrature rule: Determine the values of 1 1 dx w f ( x ) w f ( x ) w i and x i so that the quadrature rule exactly integrates the four functions, 3 ax, 2 ax, f () x ax, and a. Problem 3 of 15 (25 points) Consider the differential equation dy yx ( ), dx with y(0) y0. Using a step size, h, write out the numerical solution after one time step using the forward Euler (explicit Euler) method and the backward Euler (implicit Euler) method. Show what the limits on h are so that the solution is stable. 1

2 Health Physics Problem 4 of 15 (25 points) How many days would it take for a 1000 MW e reactor, which is 30% efficient, to produce an amount of Cs-137 that is lethal to a human being at 1 m from that source in 30 s? To receive full credit, state any assumptions that you make. Problem 5 of 15 (25 points) Describe six types of common damage observed in chromatin irradiated by ionizing radiation in living cells or tissues. List them in order of ease of repair. Discuss how the spectrum of DNA damages may vary with changes in the LET of the incident radiation. How would you expect the spectrum of chromatin damage to change in the absence of oxygen? Problem 6 of 15 (25 points) A tissue equivalent ion chamber can be used to measure the absorbed dose due to all types of external radiation, including neutrons. In order to determine the dose due to neutrons it is necessary to have a second dosimeter that is relatively insensitive to neutrons but can be used to measure absorbed dose due to all other radiations. Two potential choices are listed below for the 2 nd dosimeter: Choice 1 - Graphite ion chamber filled with CO 2 Choice 2 - Geiger counter with stainless steel wall and halogen quenched gas. In each case, indicate if they have any sensitivity to neutrons over a wide range of energies, and describe their limitations as dosimeters for photons as a function of energy. Problem 7 of 15 (25 points) The plot of creep rate as a function of normalized temperature and shear stress is show on the right. Nuclear Materials Engineering What is creep? How is creep rate defined? Name at least one particular structural component in light water reactors (LWRs) where creep is likely to occur. Explain your selection. Regions A, B, and C in the figure represent creep driven by three different mechanisms. Describe each mechanisms and discuss their functional dependencies on stress and temperature. Which region (selected out of A, B, and C) is most affected by neutron damage? Explain. 2

3 Problem 8 of 15 (25 points) Use the uranium-iron (U-Fe) binary phase diagram to answer the following questions: Sketch the equilibrium microstructures expected when a liquid U-Fe mixture is cooled down from A1 to A3 and from B1 to B3, respectively. (Show all six structures: A1, A2, A3, B1, B2, and B3). For point B2, calculate the mass fraction of UFe 2 and U 6 Fe. If uranium metal or a uranium alloy is used as fuel and an iron based alloy (e.g., HT-9, ODS steel, or low alloy ferritic steels) is used as fuel cladding, fuel-clad interactions occur after the fuel swells and is forced into intimate contact with the cladding. Describe the nature of the fuel-clad interface after a prolonged time; include comments on mechanical and chemical changes. Sketch the interface structures and identify all phases and interface compounds assumed in your sketch. Considering uranium alloy fuel and iron-rich cladding and considering the possibility of fuelcladding interactions, what is the maximum operating temperature from the consideration of structural integrity and safety? Explain why. Problem 9 of 15 (25 points) Answer the following questions: The Hall-Petch effect describes the impact of grain size on the yield stress ( y ) of an alloy; for example, y increases with decreasing grain size (d) y ( ) 0 T k yd 1/2 where ( T ) is a temperature dependent variable and 0 k y is a temperature-independent constant. Derive this equation OR use schematic sketches to explain the mechanisms creating this grain size dependence. Radiation damage can also change the yield stress. Explain all possible mechanisms where radiation hardening produces an increase in. y 3

4 Nuclear Security and Safeguards Problem 10 of 15 (25 points) You are given the following adversary sequence diagram. Using the diagram, answer these questions: What is the minimum detection path? What is the minimum delay path? If the CDP for this diagram is at the Controlled Building (or physical area C), what is the most vulnerable path? Problem 11 of 15 (25 points) Explain the PUREX process (graphically if desired). From a nonproliferation perspective (i.e., you are a safeguards analyst), describe the primary concerns associated with PUREX process. From a proliferation perspective (i.e., you are the state proliferator), describe the primary concerns associated with PUREX process. Problem 12 of 15 (25 points) Briefly describe two methods followed by the IAEA safeguards inspectors to verify the operator's declaration of reactor shutdown periods. (Hint: These methods are based on gamma and neutron radiation measurements of a spent fuel). Use sketches/plots to substantiate your answers. 4

5 Power Engineering Problem 13 of 15 (25 points) Consider a TRIGA fuel rod as in the drawing below. Compute the centerline temperature. Neglect axial heat conduction effects. Data: Zr rod radius= m Outer fuel meat radius = m Inner clad radius = about same as fuel outer radius Outer clad radius = m Coolant temperature: 50 o C Convective heat transfer coefficient: 1612 W/m 2 -C Fuel-clad gap conductance: W/m 2 -C Problem 14 of 15 (25 points) Generation IV nuclear energy systems are intended to improve over earlier designs by addressing (in part) the following principal goals: (1) increase in safety and reliability, (2) improve sustainability in energy generation, and (3) reduce the long-term stewardship burden for high level waste. Consider and respond to the following: Provide a simple diagram depicting a Generation IV system (your choice) and identify its key features. Name and describe three particular design features in current or earlier reactor designs that caused challenges in each of the stated goal topics. State (technically) why the challenges existed. Note: only describe one feature per goal and your selected features may come from different reactor systems. From the various Generation IV systems being developed around the world, describe a specific design feature that improves upon each of the stated goals. Explain (technically) why each improvement meets its goal. Note: only describe one feature per goal and your selected features may come from different Generation IV systems. Problem 15 of 15 (25 points) Consider horizontal flow through either an evaporator or a condenser. For the evaporator scenario, assume the flow enters at saturated conditions (x = 0) and leaves at x = For the condenser scenario, assume the flow enters at x = 0.20 and exits at saturated conditions (x = 0). Qualitatively compare the pressure gradients of these two scenarios. Be sure to discuss the magnitude of the pressure gradient (e.g., is dp/dz for one always larger or smaller than the other? Can the two ever be equal?). In either case, assume homogeneous flow. 5