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Controlled Fission

Controlled Fission. k  = f p ,. Fast from thermal, as defined in HW 11. Fast from fast, . Thermal from fast, p. Thermal available for fuel Thinking QUIZ

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Controlled Fission

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  1. Controlled Fission k = fp, • Fast from thermal, as defined in HW 11. • Fast from fast, . • Thermal from fast, p. • Thermal available for fuel • Thinking QUIZ • For each thermal neutron absorbed, how many fast neutrons are produced?Will need this when discuss two-group diffusion. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  2. Neutron reproduction factor keff = 1.000 Neutron Life Cycle x 0.9 Thermal utilization factor “f” x x 0.9 Resonance escape probability ”p” • What is: • Migration length? • Critical size? • How does the geometry affect the reproduction factor? x 1.03 Fast fission factor “” Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  3. Neutron Life Cycle Why should we worry about these? How? Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  4. Controlled Fission k = fp(1-lfast)(1-lthermal) Not fixed…! • Thermal utilization factor f can be changed, as an example, by adding absorber to coolant (PWR) (chemical shim, boric acid), or • by inserting movable control rods in & out. Poison. • • Reactors can also be controlled by altering neutron leakages using movable neutron reflectors. • f and pfactors change as fuel is burned. • • f, p, ηchange as fertile material is converted to fissile • material. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  5. Controlled Fission • Attention should be paid also to the fact that reactor power changes occur due to changes in resonance escape probability p. If Fuel T↑, p↓ due to Doppler broadeningof • resonance peaks. Under-moderation and over-moderation. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  6. Controlled Fission • Time scale for neutron multiplication • Time constant  includes moderation time (~10-6 s) and diffusion time of thermal neutrons (~10-3 s). • TimeAverage number of thermal neutrons • t n • t +  kn • t + 2 k2n • For a short time dt • Show that Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  7. Controlled Fission • k = 1 n is constant (Desired). • k < 1 n decays exponentially. • k > 1 n grows exponentially with time constant  / (k-1). • k = 1.01 (slightly supercritical..!)  e(0.01/0.001)t = e10 = 22026 in 1s. • Design the reactor to be slightly • subcritical for prompt neutrons. • The “few” “delayed” neutrons • will be used to achieve criticality, • allowing enough time to • manipulate the control • rods (or use shim or …). Reactivity. Dangerous Cd control rods Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  8. Fission Reactors • Essential elements: • Fuel [fissile (or fissionable) material]. • Moderator (not in reactors using fast neutrons). • Reflector (to reduce leakage and critical size). • Containment vessel (to prevent leakage of waste). • Shielding (for neutrons and ’s). • Coolant. • Control system. • Emergency systems (to prevent runaway during failure). Core Chapter 4 in Lamarsh Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  9. Fission Reactors • Types of reactors: • Used for what? • Power reactors: extract kinetic energy of fragments as heat  boil water  steam drives turbine  electricity. • Research reactors: low power (1-10 MW) to generate neutrons (~1013 n.cm-2.s-1 or higher) for research. • Converters and breeders: Convert non-thermally-fissionable material (non-fissile) to a thermally-fissionable material (fissile). • ADS. • Fusion. What are neutron generators? Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  10. Fission Reactors • What neutron energy? • Thermal, fast reactors. • Large, smaller but more fuel. • What fuel? • Natural uranium, enriched uranium, 233U, 239Pu, • Mixtures. How??? From converter or breeder reactor. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  11. Fission Reactors • What assembly? • Heterogeneous: moderator and fuel are lumped. • Homogeneous: moderator and fuel are mixed together. • In homogeneous systems, it is easier to calculate p and f for example, but a homogeneous natural uranium-graphite mixture (for example) can not go critical. Why? • What coolant? • Coolant prevents meltdown of the core. • It transfers heat in power reactors. • Why pressurized-water reactors. • Why liquid sodium? Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  12. More on Moderators • What moderator? • Cheap and abundant. • Chemically stable. • Low mass (high  logarithmic energy decrement). • High density. • High sand very low a. • Graphite (1,2,4,5) increase amount to compensate 3. • Water (1,2,3,4) but n + p d + enriched uranium. • D2O (heavy water) (1!) but has low capture cross section  natural uranium, but if capture occurs, produces tritium (more than a LWR). • ….. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  13. More on Moderators Moderating power For a compound? Never consider this only! Moderating ratio  Calculate both moderating power and ratio for water, heavy water, graphite, polyethylene and boron. Tabulate your results and comment. HW 12 10B Good absorber, bad moderator. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  14. More on Moderators HW 12 (continued) Calculate the moderating power and ratio for pure D2O as well as for D2O contaminated with a) 0.25% and b) 1% H2O. Comment on the results. In CANDU systems there is a need for heavy water upgradors. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  15. More on Moderators Recall  After n collisions After one collision f Total mean free path = n s Is it random walk or there is a preferred direction??? th Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  16. More on Moderators Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  17. More on Moderators After one collision. Recall (head-on). Then the maximum energy loss is (1-)E, or E  E\  E. For an s-wave collision: Flat-top probability Obviously Assumptions: Elastic scattering. E Target nucleus at rest. E Spherical symmetry in CM. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  18. More on Moderators • Scattering Kernel. • Slowing down density. • Migration length. • Fermi age and continuous fermi model. HW 13 (or 6\) (Re)-verify For doing so, you need to verify and use Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  19. More on Moderators HW 13 (or 6\) continued… • Forward scattering is preferred for “practical” moderators (small A). • If isotropic neutron scattering (spherically symmetric) in the laboratoryframe  average cosine of the scattering angle is zero. Show that Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  20. More on Moderators HW 13 (or 6\) continued… Spherically symmetric in CM Show that Try to sketch. • Neutron scattering is isotropic in the laboratory system?!  valid for neutron scattering with heavy nuclei, which is not true for usual thermal reactor moderators (corrections are applied). • Distinguish from • Angular neutron distribution. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  21. More on Moderators Self regulation. Moderator-to-fuel ratio  Nm/Nu. • Ratio  p a of the moderator  f  (leakage ). • Ratio  p  f  (leakage ). • T  ratio  (why). • Other factors also change. • Temperature coefficient of reactivity. • Moderator temperature coefficient of reactivity. Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

  22. One-Speed Interactions • Particular  general. • Recall: • Neutrons don’t have a chance to interact with each other (BAU 2007 review test!)  Simultaneous beams, different intensities, same energy: • Ft= t (IA + IB + IC + …) =t (nA + nB + nC + …)v • In a reactor, if neutrons are moving in all directions • n = nA + nB + nC + … •  • Rt= t nv = t  Nuclear Reactor Theory, JU, First Semester, 2010-2011 (Saed Dababneh).

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