Nuclear Fission

Question 1

Peak maxima for fission products of 235U.

Answer 1

A = 95, A = 138

Question 2

On average, how many neutrons do 6 fission reactions produce?

Answer 2

15
6 => chain reaction
5 => absorbed by 238U
4 => lost by leakage/control rods

Question 3

What is the function of using a moderator to slow neutrons down to 0.025 eV?

Answer 3

Allows for selective activation of 235U (not 238U) to increase the likelihood of fission

Question 4

Fuel rods

Answer 4

= small UO2 pellets placed inside rods
12 ft long
1 cm diameter
made of Zr alloy (Zr is strong, stable, has low neutron absorption)
arranged into an array (= “assembly”) - the core of a nuclear reaction usually contains 180 assemblies

Question 5

Coolants

Answer 5

Absorb heat from the fission reaction and transport the heat from the core to the turbine
Also play in a minor role in slowing neutrons down to 0.025 eV (by absorbing their energy)
The same as the moderator in many plants

Question 6

Fission of 235U releases 202 MeV of energy in the form of…

Answer 6

Hot fission products (170 MeV)
(Delayed) beta radiation from fission products (20 MeV)
Gamma radiation (7 MeV)
Fast (hot) neutrons (5 MeV)

Question 7

How many neutrons are produced per fission reaction?

Answer 7

2.5

Question 8

Moderators

Answer 8

Slow neutrons down by causing them to collide
The same as the coolant in many plants
** Higher levels of moderation mean lower fuel needs because more uranium nuclei undergo fission **

Question 9

Properties of effective moderators

Answer 9

Of similar mass to neutron for efficient energy transfer - if the atom is too big, the neutron will just bounce off it as fast as it collided with it

Question 10

Examples of moderators

Answer 10

2H (in D2O) (but also water itself) - deuterium has a smaller likelihood of absorbing the neutron than 1H because tritium is less stable than deuterium

12C (as pure graphite of CO2)
12C and 16O are very stable
Cheaper than D2O

Question 11

Control rods

Answer 11

= “the brakes!”
Regulate the distribution of / control the power in the reactor during operation by absorbing all neutrons
Most important function = STOP nuclear fission process when required
Can be inserted into assembly in 2 secs

Question 12

Examples of control rods

Answer 12

Boron carbide
Silver
Indium
Cadmium
Hafnium

Question 13

Gas diffusion method of uranium enrichment

Answer 13

Utilises the different diffusion coefficients of 235- and 238UF6
UF6 is pumped through 1000s of filter barriers (‘pinholes’) - 235UF6 passes through 0.4 % faster due to its slightly smaller size

Question 14

What are the 3 reactor types?

Answer 14

1. Gas-cooled
2. Pressurised water
3. Boiling water

Question 15

Gas-cooled reactors

Answer 15

Coolant = CO2
Moderator = graphite (allows use of natural U)
Hot CO2 causes water to boil into stream that drives the turbine

Question 16

Pressurised-water reactors

Answer 16

Water = coolant and moderator
There is a second cycle of water away from the core of the reactor that is not involved in producing steam - this increases safety
Most common type of reactor

Question 17

Boiling water reactors

Answer 17

Water = coolant and moderator
Water boils in the reactor and the stream drives the turbine
Bulk boiling of water in the reactor
Controls rods inserted from the bottom

Question 18

What does THORP stand for?

Answer 18

Thermal Oxide Reprocessing Plant

Question 19

Why are the products of nuclear fission radioactive?

Answer 19

They have a high n/p ratio

Question 20

Critical mass of 235U

Answer 20

50kg of pure 235U

Question 21

Critical mass of 239Pu

Answer 21

10kg of pure 239Pu

Question 22

Disadvantages of/problems associated with nuclear energy

Answer 22

Environmental pollution
Decommissioning of old reactors
Reprocessing/storage of spent nuclear fuel

Question 23

UF6 isotope separation equipment

Answer 23

Made of stainless steel/an Al alloy coated in an impervious layer of metal fluorides
(Because UF6 is corrosive and very reactive)

Question 24

Safety systems in nuclear power plants

Answer 24

Contained within large steel buildings approx. 3ft thick to sustain the high internal pressures
The containment pressure is controlled by containment spray systems/fan cooler systems/ice condensers
Emergency feed water will continue to supply water to the steam generator to continue heat removal from the core after shutdown

Question 25

LFTR

Answer 25

Liquid Fluoride Thorium Reactors
232Th is 100% abundant so no enrichment is required
232Th is 4x more abundant than U and is available in most countries
Most fissions products have intermediate half lives so radioactive waste storage is < 500 years
Requires a neutron flow for operation meaning there is no risk of an uncontrolled accident
6kg of 232Th produce the same energy as 300kg of enriched U

Question 26

Main challenge of nuclear processing

Answer 26

Separating U and Pu from the fission products and from each other

Question 27

Radioactive waste

Answer 27

Can be divided into 3 groups

1. Highly radioactive, short-lived isotopes - will decay under controlled conditions within a few years
2. Isotopes of intermediate half lives (<100 years)
3. Long-lived isotopes (t1/2 > 100 years) - must be stored for 100s of 1000s of years until their radioactivity has dropped to natural levels

Question 28

Transmutation

Answer 28

Exposing long-lived nuclear waste to neutrons to give isotopes with much shorter half lives, resulting in nuclear waste with t1/2 < 100 years
This may eliminate all very long-lived waste

Question 29

PUREX process

Answer 29

1. Fuel rods are dissolved in 7M HNO3 and filtered to remove the insoluble Zr casing. This yields an aqueous solution of UO2(NO3)2(H2O) and Pu(NO3)4
2. U and Pu are extracted into the organic phase on addition of TBP/kerosene. This yields UO2(NO3)2(TBP)2 and Pu(NO3)4(TBP)2. Other fission products do to form strong TBP complexes so remain in the aqueous phase
3. Aq/org extraction is carried out using ‘pulsed’ columns - the aq phase falls and the org phase rises, the phases are mixed by ‘pulses’ of compressed air
4. Pu(IV) is more readily reduced than U(VI) - U(IV) is added as a reducing agent, alongside N2H4 to prevent oxidation of U4+ by HNO3
5. Pu(IV) is reduced to Pu(III) (Pu(NO3)3), which can be extracted into the aq layer, leaving U(VI) in the org layer

Question 30

Pu processing

Answer 30

Pu3+ is oxidised to Pu4+ and precipitated as the oxalate Pu(C2O4).6H2O
The oxalate thermally decomposes to PuO2
PuO2 stored in stainless steel cans under Ar

Question 31

U processing

Answer 31

UO2(NO3)2(TBP)2 is moved into the aqueous layer by ‘stripping’
The aqueous layer is then evaporated to give UO2(NO3)2.6H2O
This thermally decomposes to give UO3

Question 32

MOX fuel

Answer 32

PuO2 mixed with UO3