Name: NE 495 Elements of Nuclear Engineering Open Books and Notes Final Examination, Spring 2009 1. Indicate whether the following statements are true (T) or false (F). [50 ( ) The energy of a photon is inversely proportional to its wavelength. ( ) Radionuclides with too many neutrons for stability decay by both electron capture and positron emission. ( ) The concentration of uranium in sea water is less than that of iron. ( ) Of all isotopes with a mass number A = 235, 235 U is the least massive. ( ) 26 is a magic number. ( ) All nuclei of atoms contain at least one proton. ( ) In all exoergic binary nuclear reactions, the reaction can occur even if the incident projectile has negligible incident kinetic energy. ( ) In a fission reaction, the lighter fission fragment has a higher initial kinetic energy than the heavier fission fragment. ( ) A 3000 MW(t) reactor with new fuel fissions less than 3.5 kg of 235 U per day. ( ) A radioisotope source with a half life of 1 hour has an activity of 1 mci. Three hours earlier its activity was more than five time greater. ( ) Most naturally occuring cases of hereditary illness are a result of background radiation. ( ) A primordial decay chain is a necklace made by humans during the stone age. ( ) Red giants are stars in which iron is being formed in the stellar core. ( ) A beam of 1-MeV gamma rays with an intensity of 10 5 cm 2 s 1 uniformly irradiates a 1-gram sample of water. More that 5000 photons are scattered each second by the sample. ( ) A five-year old male is more susceptible to radiogenic leukemia than is a five-year old female. ( ) For the same absorbed dose, 5-MeV neutrons are a greater biological hazard than are 5-MeV protons. ( ) For a given dose, the dose equivalent in sieverts can sometimes be smaller than the absorbed dose in greys. ( ) The greatest component of background radiation exposure is from cosmic rays. ( ) The thermal utilization factor refers to the thermodynamic efficiency with which the thermal power of a nuclear power plant is converted to electrical power. ( ) The pressurizer in a PWR is needed to increase the pressure of the primary cooling water to prevent it from boiling.
2. Circle the designator in front of the most appropriate answer(s) from the list after each question. (a) In the current theory of element formation, elements with atomic numbers higher than 26 were and are made in which of the following way? [5 i. by nuclear fission ii. through the action of neutrinos on elements of atomic numbers less than 26 iii. by neutron absorption during supernova explosions and subsequent beta decay iv. by neutron absorption during supernova explosions and subsequent positron decay v. in fusion interactions within stars like our sun (b) Which expression will yield the activity of a uniform sample with atom density N, atomic umber Z, density ρ, half life T 1/2, decay constant λ, and interaction coefficient µ? [5 i. A = N/Z. ii. A = µ/ρ. iii. A = N ln 2/T 1/2. iv. A = ZT 1/2 /ρ 0.25 v. A = Nλ. (c) A 1-MeV photon has a mean free path in particular material of 1.325 cm. What is the tenth thickness of this material for 1-MeV photons? [5 i. about 1.738 cm 1 ii. about 1.738 cm iii. about 0.755 cm iv. about 3.051 cm v. about 3.051 cm 1 (d) The uncollided flux density of gamma rays at a distance of 20 cm from a 3.2 mci point source of 60 Co in air is approximately? [5 i. 3.9 10 5 cm 2 s 1 ii. 2.2 10 4 cm 2 s 1 iii. 4.4 10 4 cm 2 s 1 iv. 2.2 10 7 cm 2 s 1 v. 1.3 10 7 cm 2 s 1 (e) Uranium fuel 20 atom-% enriched in 235 U has a N 235 /N 238 atomic density ratio of [5 i. 0.20 ii. 0.25 iii. 0.3 iv. 0.4 v. 0.987
(f) Consider a bare, homogeneous, spherical, source-free, critical reactor operating at a steady-state power P o. Which of the following will happen if a neutron beam is directed on the core? [5 i. nothing ii. the reactor becomes supercritical iii. k eff remains unchanged but the power increases iv. the power increases exponentially v. the power will level out at a new steady-state power vi. the power decreases (g) A 30-year old medical technician is exposed to 0.35 Gy gamma radiation as he carelessly walks in front of the beam of an x-ray tube. You are the health physicist who is called in to talk to him. After explaining that he should be more careful in future, you tell him that his increased risk of cancer as a result of this single exposure is about [5 i. 3.7 10 8 % ii. 2.4 10 2 % iii. 1.3 % iv. 2.4 % v. 3.7 % (h) Boiling water reactors use jet pumps to adjust recirculation flow of water around the core. Which one of the following is a major purpose of this recirculation flow? [5 i. to replace water lost through steam leaks in the primary coolant ii. to pick up heat from the boiling water and carry it to the low-pressure turbines iii. to keep the control rods cool iv. to adjust the reactivity of the core v. to cool the jets of the reactor (i) A 1-MeV photon beam irradiates a lead sample. radiations are not produced. [5 i. characteristic fluorescent x rays ii. bremmstrahlung iii. Auger electrons iv. photoelectrons v. Compton scattered photons vi. annihilation photons Which, of the following secondary (j) The range of a 10-MeV proton in aluminum is approximately [5 i. 0.17 cm ii. 3.0 cm iii. 0.3 cm iv. 0.001 mm v..6 mm (k) A 1-gram sample of 235 U is uniformly irradiated by a flux of thermal neutrons φ =10 8 cm 2 s 1. The fission power generated in this sample is [5 i. 1.5 10 8 MeV/s ii. 3.0 10 10 MeV/s iii. 5.9 10 10 MeV/s iv. 1.2 10 13 MeV/s v. 3.0 10 32 MeV/s
3. A broad monoenergetic parallel beam of γ rays with an energy of 1 MeV normally incident on a concrete wall 10 cm thick. The transmitted uncollided photons have an intensity of 3.5 10 5 cm 2 s 1. (a) What is the incident intensity of the beam illuminating the incident side of the wall? [10 (b) What is the tissue absorbed dose rate in rad/h due to uncollided photons on the two sides of the wall? [15 4. A group of 4,000 male graduates of some colleges on the east coast move west a few years after graduation. Assume they are all 25 years old. They separate into two equal groups. One group decides to live in Kansas and the other group goes to Colorado. Due to differences in elevation (which affects cosmic background exposure) and home radon levels, each member of the Colorado group receives, on average, 550 mrad more radiation dose in the year following the move than the members of the Kansas group. How many more cancer deaths are expected in the Colorado group as a result of this one-year excess dose? [25
5. Iron piston rings in a motor have 60 Co embedded as a label with a concentration of 5.0 nci/g(fe). The engine is lubricated by recirculation of 8 kg of oil. After 1,000 hours of operation, the oil is found to have a 60 Co concentration of 2.7 fci/g(oil). How much iron has been worn from the piston rings? [25 6. Consider a beam of protons bombarding a sample of magnesium. One very useful reaction 25 12Mg(p,α) 22 11Na, which produces the positron emmiter 22 Na, used in medical diagnosis. (a) Calculate the Q-value for this reaction. [10 (b) Calculate the kinematic threshold for the reaction. [5 (c) Calculate the Coulombic threshold for the reaction. [5 (d) What is the minimum energy that a proton must have in order for the reaction to occur? [5 (e) What is the minimum kinetic energies of the products of the reaction? [5