Boron carbide as control rods for nuclear reactors

Question 1

How is nuclear power generated?

Answer 1

~ a U-235 atom is struck by an incident neutron, causing the atom to fission into two smaller atoms (krypton and barium)

~ significant heat is generated, and ~2-3 new neutrons are released

~ the fission products remain in the UO2 fuel

~ the reactor core resides inside a steel pressure vessel, so the water inside remains liquid, even though operating temp is 320C

~ steam is formed via heat exchange with water which boils in a separate vessel

~ to maintain efficient reactor performance, 1/3 to 1/2 of the fuel is replaced every 1-2 years

Question 2

How is steady state achieved?

Answer 2

~ each individual fission event should trigger exactly one subsequent fission event (critical state)

Question 3

What are control rods?

Answer 3

~ aka moderators, are neutron-absorbing

~ they are inserted or withdrawn from the reactor core while the fission process is occurring, to achieve the critical state

~ by using a large number of evenly-spaced, relatively thin control rods, interdispersed with fissile fuel rods (individually made of stacked pellets), uniform densities of fissions can be achieved
–> a typical power reactor might contain 50 clusters with 20 rods each

Question 4

What is shut down?

Answer 4

~ full insertion of the control rod to reduce fission by 90% in 2 seconds

Question 5

How is neutron absorption measured?

Answer 5

~ measured by it absorption cross-section, measured in barnes (unit of area)

Question 6

How is boron in terms of control rod applications?

Answer 6

~ Boron B-10 is one of the best neutron absorbers. However, B-11 makes up the majority of natural boron and has negligible absorption: B-10 enrichment is required

~ boron’s mechanical properties are less than desirable for building a control rod structure

~ boron carbide is boron-rich (B4C) and mechanically durable. It is typically sintered to modest relative density to permit space for volume expansion as boron transforms to other elements after absorbing neutrons