Nuclear Physics

Question 1

Describe how a nuclear power plant generates electricity

Answer 1

Nuclear fission creates heat, which is then used to generate steam. The steam rotates the rurbines, which turn the generator to produce electricity.
(L1 p. 4)

Question 2

Describe the differences in reactivity control between the two types of reactor systems (PWR vs BWR)

Answer 2

Both - Insertion of control rods (PWR - top, BWR - bottom)
PWR - using boric acid (chemical shim) added to or diulted from the moterator before the moderator enters the reactor. Boric acid absorbs neutrons.
BWR - increasing recirculation pump speed - sweeps steam bubbles from the core more rapidly. This increases neutron moderation, and thus increases the population density of available slow neutrons.
(L1 p.7)

Question 3

Describe the design differences between the two types of reactor systems (PWR vs BWR)

Answer 3

PWR - indirect-cycle with a liquid primary loop that flows through the reactor and a liquid/vapor secondary side where the steam used to turn the turbines is produced in the steam generators. Since none of the primary fluid reaches the turbine, shielding of the steam cycle is not required.
BWR - direct-cycle, they require shielding of the steam cycle (including the turbine) against potential radiation hazards present in the water and steam. (L1 p.10)

Question 4

Describe the differences in steam generation between the two types of reactor systems (PWR vs BWR)

Answer 4

BWR - boiling of the water occurs in the reactor. The steam produced in the reactor flows through moisture separators and dryers inside the reactor vessel to remove moisture prior to exiting the vessel. The dry steam flows to the turbine, where the energy of the steam is used to turn the turbine and generator. The steam is then condensed and the water is pumped back to the reactor.
PWR - high pressure keeps the water in the reactor from boiling. The pressure is controlled by a pressurizer. This highly pressurized and heated primary coolant (water) is pumped to a heat exchanger called a steam generator. The steam generator is a large, cylindrical steel vessel containing water at a lower pressure (called the secondary coolant). The high pressure primary coolant flows through tubes in the steam generator and heats up the surrounding lower-pressure secondary coolant, causing it to boil. After transferring heat energy to the secondary coolant, the primary coolant flows back into the reactor to be reheated. The steam produced in the steam generator is used to turn the turbines. The steam is then condensed and the water is pumped back to the steam generators.
(L1 - p.9)

Question 5

What is a neutron

Answer 5

a neutron is a sub-atomic particle with no charge that has about the same mass as a proton (L2 - p.4)

Question 6

What is a proton

Answer 6

a positively-charged particle that, along with neutrons, comprise a nucleus. Protons have a positive charge exactly equal in magnitude to the charge of an electron (L2 - p.4)

Question 7

What is an electron

Answer 7

a negatively-charged particle with little mass that orbits the nucleus. An atom with an equal number of electrons and protons has a neutral charge. (L2 - p.4)

Question 8

What is an isotope

Answer 8

atoms of the same element (same number of protons) that may have different numbers of neutrons. Ex. Hydrogen - 1 proton in the nucleus, 1 proton + 1 neutron, or 1 proton + 2 neutron (L2 - p.4)

Question 9

What is an atomic number

Answer 9

is the number of protons in an atom (L2 - p.5)

Question 10

What is an atomic mass

Answer 10

is the total number of protons and neutrons (L2 - p.5)

Question 11

What is an atomic mass unit

Answer 11

1/12 the mass of a carbon-12 atom or 1.66054 * 10^-24 grams (a proton and neutron both weigh 1 amu) (L2 - p.8)

Question 12

Describe the fission process

Answer 12

When a heavy nucleus is hit with a neutron at the right energy, it gains “excitation” energy from the neutron’s mass and kinetic energy. In certain heavy isotopes, this added excitation energy may put the “compound” nucleus over its critical energy threshold. This may induce the nucleus to split, or fission, into two lighter radioactive nuclei (also called fission fragments, fission products, or daughters). Two or three other (fission) neutrons are also released along with heat energy and radiation (beta particles, and gamma rays).

During fission, some of the total mass of the original heavy nucleus and absorbed neutron is lost in the reaction. In accordance with Einstein’s equation (E = mc2), this lost mass “defect” is converted to about 200 MeV of energy. One electron volt, eV, is a unit of energy equal to 1.6 x 10-19 joule and 1 MeV, or mega electron volt, is one million electron volts. Fission also releases two to three other (fission) neutrons along with heat energy and radiation.
(L2 - p.13)

Question 13

Describe the life cycle of a neutron

Answer 13

1) Fast neutrons are born from thermal fission, 2) they have the possibility of leaking out (escaping the reactor core before they slow down).
3) They may fall into energy resonance with the surrounding material and are captured.
4) If they reach thermal equilibrium with the fuel, they may still leak out of the core and escape the reactor.
5) The thermal neutrons may be absorbed into the fuel as opposed to any other material inside the core. (ex. control rods or control poisons)
6) these absorbed neutrons cause fission and will produce the next generation of two to three fission neutrons for every neutron absorbed by the fuel. (L2 - p.21-22)

Question 14

Describe criticality and reactivity

Answer 14

Criticality - When a fission reaction becomes self sustaining, we say the reactor is critical, k=1 and the average neutron population (or power) stays constant. When k is greater than 1, the number of neutrons grows exponentially with time and the reactor is supercritical. When k is less than 1, the number decreases exponentially with time and the reactor is subcritical.
Reactivity - measure of the departure of a reactor from criticality and is described as the % difference in the value of k from 1. It can be positive, zero, or negative. Positive reactivity - withdrawing control rods increase neutron multiplication and moves reactor towards supercritical.
Negative r - inserting control rods or depleting U-235 fuel over fuel life-cycle) takes away neutrons and moves the reactor towards subcriticality.
Temperature changes affect water density and changes the number of moderating atoms in a given volume. In BWRs, the control of water flow changes the amount of steam production and density of the coolant.
(L2 - p.19, 24)

Question 15

Describe how a chain reaction is controlled and maintained

Answer 15

1) Use of delayed neutrons (DN), produced by fission fragments a few seconds later, used to sustain criticality. Without them fission reaction will not be sustained. Use of DN provides inherent safety control mechanism.
2) Materials that absorb or capture neutrons are used in the reactor to control the chain reaction. Control rods made of neutron-absorbing materials (such as silver, cadmium, or hafnium) are inserted or withdrawn from the reactor to control the power level or shut down the reaction. Withdrawing the control rods reduces the amount of negative reactivity in the core, which increases neutron multiplication and moves the reactor towards k > 1, or supercritical. Conversely, inserting the control rods will add negative reactivity and take away neutrons. This moves the reactor towards k < 1, or subcriticality.
PWRs - soluble neutron posion, boron is dissolved throughout the moderator/coolant. Reducing fission.
BWRs - steam voids, amount of boiling directly affects the amount of water available to provide moderation of the neutrons. More boiling means more steam, which means less moderator density, which means less moderation. Without moderation, the fast neutrons do not slow down to a thermal energy, so the reactor has less positive reactivity. Controlling the flow rate of water through the core directly affects the quantity of steam in the core, thereby directly affecting reactivity.

(L2 - p.23, 25)

Question 16

Define radioactivity

Answer 16

Unstable nuclei naturally releasing energy by means of radioactive decay (L3 p.2)

Question 17

Define radioactive decay

Answer 17

Unstable atomic nucleus loses energy by emitting ionizing particles and radiation (L3 p.2)

Question 18

Define radiation

Answer 18

Sub-atomic particles and energy emitted by unstable isotopes to become stable (L3 p.2)

Question 19

Define radioisotope

Answer 19

A radioactive isotope of an element (L3 p.4)

Question 20

Define half-life

Answer 20

The average time it takes for half an existing population of radioactive isotopes to decay (L3 p.4)

Question 21

Describe alpha particle emission

Answer 21

Typically, heavy radioactive nuclei such as U-238 and Pu-239 emit alpha particles. Alpha particles resemble the nucleus of a Helium atom with two protons and two neutrons. They are relatively big with a lot of mass and energy. Because the alpha is such a relatively large particle, a lot of energy is released when heavy nuclei undergo alpha decay. However, because of their size, alpha particles are easily stopped by thin layers of shielding. A sheet of paper or the outer layers of dead skin will stop them. (L3 p.3)

Question 22

Describe beta particle emission

Answer 22

The mid-sized fission fragment nuclei typically decay by beta particle emission. The beta particle emission is the decay of a nucleon, either an electron or an anti-electron (positron) from the nucleus. Electron emission is known as beta minus decay and positron emission as beta plus decay. The daughter products of the beta decays are also usually radioactive, and thus lead to a decay chain that will terminate ultimately in a stable isotope. It is because of these radioactive fission products that used fuel removed from the reactor must be isolated and shielded in order to minimize radiation exposure to the public and environment. (L3 p.3)

Question 23

Describe gamma ray emission

Answer 23

A radioactive nucleus in an excited state can decay to a lower energy level by releasing gamma rays. The number of neutrons and protons in the nucleus does not change in the gamma decay process. Gamma radiation has much greater penetrating power than alpha or beta particles, so heavy shielding, such as lead, must be used to reduce exposure to gamma radiation. (L3 p.3)

Question 24

Using the half-life of a radioisotope and a specified decay time, determine the approximate amount of the radioisotope that has decayed and the approximate amount that is remaining

Answer 24

If an isotope has a half-life of 10 years and it is allowed to decay for 10 years, then only half of the isotope will remain. If the isotope decays for an additional 10 years, then only 1/4, or 25%, of the original isotope will remain. At this time, the other 75% of the isotope has decayed to a more stable configuration. (L3 p.4)

Question 25

Describe how temperature affects the fission process

Answer 25

During startup the control rods are withdrawn until the fuel temperature and moderator temperature begin to increase. The temp. increases will insert negative reactivity, which will offset the reactivity increase caused by the withdrawal of control rods, and will ultimately return the startup rate to zero (L4 p.2,6)

Question 26

Describe how pressure affects the fission process

Answer 26

The effect of the presence of steam bubbles, or voids, in BWRs is similar to the moderator temperature effect in that the moderator density is reduced. In a BWR, increasing pressure decreases the presence of voids. Conversely, decreasing pressure increases the presence of voids. In under-moderated systems, the presence of voids effectively removes moderation of neutrons and lowers the reactivity. In an over-moderated system, just the opposite is true. The presence of voids removes excess moderator, lowering absorption and increasing reactivity.
The flow rate of water through the core can affect the amount of voiding in the core. Increasing the water flow through the core will displace the steam with water, which will provide increased neutron moderation. The increased moderation will produce more thermal neutrons to cause fission of the U-235 atoms.
(L4 p10)

Question 27

Describe how voids affect the fission process

Answer 27

Recirculation flow in a BWR - This method provides up to 25% power changes. Increasing the flow will decrease the number of steam bubbles within the core (void fraction), which will increase reactivity via increased moderation, thereby increasing power. Decreasing the recirculation flow will increase the number of steam bubbles in the core, displacing the water that was used for neutron moderation. The decreased moderation will decrease the positive reactivity, causing the power to decrease.

Question 28

Describe how fuel constituents affect the fission process

Answer 28

Fuel enrichment is affected by U depletion and Pu production, which alters the number of fissile atoms in the fuel. U-235, initially loaded in the reactor fuel at the beginning of the cycle, is depleted by neutron absorption, resulting in removal of positive reactivity. Pu-239 is produced from neutron absorption in U-238 and increases positive reactivity.
(L4 p.10)

Question 29

Describe how control rods affect the fission process

Answer 29

Control rods contain materials that absorb neutrons. Inserting more control rods into the core is an addition of negative reactivity, while withdrawal of control rods from the core provides a removal of negative activity.
(L4 p.10)

Question 30

Describe how boron concentration affects the fission process

Answer 30

PWR - Soluble boric acid serves as a chemical shim (changing boron concentration) in the primary coolant to provide longer-term control. Increasing the boron concentration via boration or decreasing the boron concentration via dilution provides somewhat uniform control of reactivity, both vertically and radially, in the core.

Question 31

Describe how fission product poisons affect the fission process

Answer 31

XE-135 and SM-149 have a high neutron absorption cross-section they absorb thermal neutrons that would be absorbed in the fuel, they are a source of negative reactivity.

Question 32

Describe how reactor operation affects fission product poison

Answer 32

Fission products capture neutrons and lower reactivity. Many of these isotopes have fairly low absorption cross sections; however, two isotopes, Xenon (Xe-135) and Samarium (Sm-149), have very high neutron absorption cross sections. Their accumulation in the fuel rods can cause major changes in reactivity.

(L4 p.16)

Question 33

Describe how changes in various plant parameters affect reactor operations in a PWR and a BWR

Answer 33

In PWRs & BWRs:
- Feedwater temperature decrease increases moderator density (with a negative moderator temperature coefficient, the density increase inserted positive reactivity because more neutrons were being moderated. (More fissions occur, so reactor power is increased.)

• Feedwater flow increase also increases Reactivity.

(L4 p.14, 20)

Question 34

Define decay heat and describe its affect on plant operations

Answer 34

is when fuel continues to generate some heat because of the decay of fission products after the reactor is shut down and the fission chain reaction has stopped.
The amount of decay heat decreases with time since it originates from radioactive decay processes. Still, over the first several days following a reactor shutdown, adequate cooling must be available to remove the decay heat to reduce the potential for core damage. Reactor designs have specially engineered safeguards to provide such cooling.
Since much of this early decay heat will be from short-lived fission products, the heat production rate decreases sharply in the first few hours. For example, about 16 MWt is generated after one day of reactor shutdown, and it drops more slowly to about 9 MWt after the first five days. Without cooling, such thermal power is sufficient to boil the core dry and melt the fuel.
(L4 p.22)

Question 35

Identify the types of reactivity control accidents

Answer 35

Reactivity control accidents are positive reactivity additions that typically result in rapid power increases and potentially damaging fuel temperatures.
The Chernobyl accident,, started as a reactivity control accident. In normal reactor operation, a combination of prompt neutrons, which are neutrons produced at the moment of fission, and delayed neutrons, which are produced at some time after the fission, maintain reactor criticality. There are not enough prompt neutrons to maintain criticality. At Chernobyl, due to human errors and design weaknesses, the reactor achieved prompt criticality, in which a supercritical condition was achieved solely by prompt neutrons without the need for the contribution from delayed neutrons.
A reactivity control accident in an LWR is highly improbable due to negative reactivity feedbacks, shutdown mechanisms, and strict regulatory and operational procedures. Control rods are not absolutely fail-safe; control rod release mechanisms can fail or the motion can be blocked. In such cases, redundancy is always provided to greatly reduce the risk of such events.

(L5 p4)

Question 36

List the causes of fuel failures

Answer 36

1) Debris-induced clad failure - Over the life of a plant, small pieces of hardware may break off or be left in piping during maintenance operations. They may be carried through the reactor coolant flow loop and become trapped and vibrate against the cladding, causing it to fret and open a crack. This occurrence is commonly known as a failure of the FME (foreign materials exclusion) practices.
2) Manufacturing defects and design limitations - Defects in fuel manufacture (such as end cap weld contamination causing weld corrosion) can result in failure of the cladding
3) Power-induced failures -
Inadvertent rapid power increases or control rod misalignment may cause localized power increases that can damage fuel rods.
Some fuel failures are caused by pellet-clad interaction (PCI). As power increases, fuel pellet temperature increases, and the fuel pellet expands. If the gap between the pellet and the cladding wall is too small, the pellet can contact the cladding and place excessive tensile stress on the cladding, causing a breach of the cladding.
4) Chemistry-induced failures - Cracking of cladding can result from unsatisfactory water chemistry controls or monitoring. Improper materials handling during outages can also introduce harmful chemicals into the NSSS.
In some PWRs, corrosion products have been deposited on the fuel cladding. In locations where sub-cooled nucleate boiling occurs, boric acid and LiOH accumulate in those deposits, shifting core power to other regions of the core. This can cause power-induced fuel failures.
(L5 p6)

Question 37

Identify the consequences of fuel failures

Answer 37

Radioactive materials in gaseous and/or solid form escape the fuel cladding and enter the reactor coolant, increasing the probability and magnitude of radioactivity releases to the containment and personnel who may be working there. Fuel failures contribute to increased personnel exposures, radioactive waste, and maintenance outage time, as well as decreased productivity. They also add cost because finding failed fuel during a refueling is a very time-consuming task requiring special equipment and perhaps even extra vendor expertise. This is necessary, so the assembly is not reloaded into the core. The chances of a “hot particle” exposure are also elevated when fuel defects occur.
(L5 p6)