Nuclear Reactors Chapter 4

1. Nuclear ReactorsChapter 4

2. Nuclear Reactors • Categories • Breeder or Converter or Burner • Coolant {water, heavy water, gases, liquid metal} • Moderator {water, heavy water, graphite} • Boiling water or pressurized water systems

3. Nuclear Reactors Notes: 63 – 5 = 58 n produce Pu-239 78-63 = 15 n produce U- 236 59-32 – 17 n produce Pu-240 In steady – state ~ 32% of energy is produced by Pu fissions. • Neutron Balance in Thermal Reactors

4. Nuclear Reactors Notes: Fuel is typically 20% Pu and 80% depleted U. Bulk of fissions from Pu Pu created > Pu consumed since: 116 n absorbed in Pu, but: 134 – 13 = 121 produce Pu-239. 4. Thus Rx “breeds” fissile Pu 239 • Neutron Balance in Fast Reactors

5. Nuclear ReactorsLight Water Reactors The most widely used electricity producing reactors in the world today are thermal reactors that are moderated, reflected and cooled by ordinary (light) water (H20). 2. Two Main Types -Boiling water reactors (BWR) -Pressurized water reactors (PWR)

6. Nuclear ReactorsLight Water Reactors Advantages - Abundant supply of water. - well known properties - cheap cost Disadvantages - Water has high vapor pressure, requiring high pressure operation. - Water has a large cross- section of absorption for neutrons. Therefore it is not possible to fuel a light water reactor with natural uranium. The fuel must always be enriched to some extent.

7. Nuclear Reactors Pressurized Water Reactors

8. Nuclear ReactorsPressurized Water Reactors- Coolant Path

9. Nuclear ReactorsPressurized Water Reactors- Coolant Path • Components

10. Nuclear ReactorsPressurized Water Reactors- Reactor compartment

11. Nuclear ReactorsPressurized Water Reactors • Flow paths • Components One of 1st reactor designs Standard for Naval vessels 3. Requires steam generator (next slide)to produce steam for turbines.

12. Nuclear ReactorsPressurized Water Reactors- The Steam Generator • Steam flow path • Feed water path • General chemistry • (pH, phosphates) • Components

13. Nuclear ReactorsPressurized Water Reactors- The Pressurizer • Components • Down powerTc (I), Coolant Expands, Pzr level (I), Pzr Pressure (I), Spray valve opens, Steam condenses, Pzr Pressure (d), limiting the Pressure surge. • Up Power Tc (d), coolant contracts, Pzr level (d), Pzr pressure (d), water flashes to steam, Pzr pressure (I)

14. Nuclear ReactorsPressurized Water Reactors- The Pressurizer

15. Nuclear ReactorsPressurized Water Reactors- The Fuel -Slightly enriched 2-5 w/o U-235 -Fuel pellets of UO2 black ceramic looking -Pellets 1cm x 2cm -Loaded into Zircaloy tubes (low Σa) -Zircaloy is alloy of zirconium + tin + iron + chromium -Rods arranged as cluster or assembly Other arrangements possible, such as fuel plates vice rods.

16. Nuclear Reactors AP 1000

17. Nuclear Reactors AP 1000 • Westinghouse Electric Design • Generation III + • PWR • 2 Loops • ~1000 MWe

18. Nuclear Reactors Boiling Water Reactors

19. Nuclear Reactors Boiling Water Reactors • BWR Advantages: • Direct cycle, no secondary loop • Less mass flow rate since coolant water is permitted to • absorb latent heat and sensible heat. • Can operate at lower pressure ~ 900 psi • {not zero/ atmospheric pressure since • 1. high temp required to drive turbines • 2. high pressure prevents wall dryout} • Lower pressure mean thinner pressure vessel • and less expensive components. • BWR Disadvantages: • Radioactive coolant throughout engine room • Shielding and containment larger • Lower power density – need larger core and PV then PWR

20. Boiling Water Reactors Nuclear Reactors

21. Nuclear Reactors Heavy Water Reactors A heavy water (D2O) where D = H2 , can operate on natural uranium because the absorption cross section of deuterium for thermal neutrons is very small. D2O is also less effective in moderating neutrons than H2O. Therefore neutrons loss less energy per collision and travel farther before reaching thermal energies. The core of a heavy water reactor is considerably larger than a LWR.

22. Nuclear Reactors Heavy Water Reactors H2 is a rare isotope of H. ~ 150 ppm It contains 1 n and 1 p, instead of just 1 p. 3. D2O was discovered by American Harold Urey in 1931, for which he received the 1934 Nobel prize in chemistry. D2O is 10% heavier than H2O It is 3x worse at slowing neutrons It is 600x worse at absorbing neutrons Can extract D20 from H20 is a multistage process.

23. Nuclear Reactors Heavy Water Reactors CANDU Reactor – Canada Deuterium Uranium reactor

24. Nuclear Reactors Heavy Water Reactors CANDU Reactor – Canada Deuterium Uranium reactor

25. Nuclear Reactors Heavy Water Reactors CANDU Reactor – Canada Deuterium Uranium reactor

26. Nuclear Reactors Heavy Water Reactors CANDU Reactor – Canada Deuterium Uranium reactor • To avoid a large pressure vessel it uses pressurized tube concept. • Rx consists of a large tank called a calandria filled with D2O • moderator at atmospheric pressure. • Tank is penetrated by hundreds of horizontal tubes containing • the fuel. The D2O coolant flows through the tubes at high pressure • (~1500 psi) and does NOT boil. • Thus by pressurizing the coolant rather than the whole reactor • a large pressure vessel is avoided. • Because of lower pressure (than conventional PV), the tubes • can not be raised to high enough temperature to steam at same • temp as light water reactors. The result is plants are less efficient • (~28-30%). • Plant reactivity is controlled by absorber rods, and light water • compartments.

27. Nuclear Reactors Breeder Reactors Four Types • Liquid Metal Cooled Fast Breeder Reactor. LMFBR • Gas Cooled FBR • Molten Salt BR • Light Water BR • Only LMFBR is only one significantly commercialized anywhere • in world. • Operates on U -> Pu fuel cycle • fueled with Pu isotopes • blanketed with U 238 natural or depleted. • No moderator since we want fast neutrons

28. Nuclear Reactors Breeder Reactors Four Types • Use sodium coolant because • Excellent heat transfer • Non corrosive • High plant temperatures • High power density • Disadvantage of Sodium coolant • Sodium reacts violently with water • High melting point (98 C) need heated piping. • Forms β and ϒ emitters (radioactive) most plants • use two loop system to prevent radioactive sodium • from entering S/Gs.

29. Nuclear Reactors Breeder Reactors