Nuclear physics - Nuclear Energy

Question 1

For a spontaneous (nuclear/subnuclear) reaction in which no energy is supplied, the energy released eq?

Answer 1

Q = Δmc^2

Question 2

In any change where energy is released, eg radioactive decay, the total mass after the change is always —- than the total energy before bc —-

Answer 2

always less than bc some mass converted toe energy ( released).

Tips: when calc Q in β decay, assume mass of neutrino negligible.
if given mass of each atom rather than nucleus, calc mass of each nucleus by subtracting mass of e-s in atom.

Question 3

Strength of strong force can be estimated by?

Answer 3

by working out force of repulsion between two protons at a separation of 1fm (10^-15) using F=kQQ/r^2 ≈ strong nuclear force

Question 4

Define binding energy

Answer 4

Binding energy = the work that must be done to separate a nucleus into its constituent nucleons.

Question 5

When nucleus forms from separate nucleons, energy is released as strong force does work pulling the nucleons together. This E is equal to binding energy of nucleus. Bc energy is released when nucleus forms from separate nucleons, mass of nucleus is —–

Answer 5

mass of nucleus is less than nucleons.

Question 6

Whats the mass defect?

Answer 6

The mass defect, Δm, of a nucleus = m nucleons - m nucleus.

Question 7

Binding energy of nucleus eq?

Answer 7

binding energy of nucleus = Δmc^2

Question 8

If 2p and 2n inside nucleus bind together as a ‘cluster’, they may be emitted as α particle, bc ?

Answer 8

bc binding energy of α very large vs other clusters ∴ α particle gains sufficient Ek (equal to/from release of binding energy of cluster) to give small probability of “quantum tunneling” from nucleus, through the coulomb barrier.

Question 9

Nuclear stability:
define binding energy per nucleon
relate to nuclear stability

Answer 9

Binding energy per nucleon of a nucleus = the average work done per nucleon to remove all the nucleons from the nucleus.

It is therefore a measure of stability of a nucleus . The more binding energy per nucleon, the more stable the nucleus.

Question 10

draw out graph of binding energy per nucleon (MeV) against mass number A, inc numerical values.

Answer 10

see saved pic
graph shows max at 8.7MeV between A = 50-60.

Question 11

What’s nuclear fission?
does binding energy increase or decrease?

Answer 11

The process in which a large unstable nucleus splits into two fragments which are more stable than the original nucleus.
The binding energy per nucleon increases in this process.

Question 12

What’s nuclear fusion?
does binding energy increase or decrease?

Answer 12

The process of making smaller nuclei fuse together to form a larger nucleus.
Product nucleus has more binding energy per nucleon than the smaller nuclei ∴ binding energy per nucleon also increases in this process, provided nucleon number of product nucleus no greater than about 50.

Question 13

Induced fission:
1) If each fission event releases 2 n on average, after n generations of fission events the number of fission neutrons would be…
2) energy is —- when a fission event occurs bc —
this energy is equal to —-

Answer 13

1) 2^n
2) released bc the fragments repel each other (as they’re both +vely charged) with sufficient force to overcome the strong nuclear force trying to hold them together. the energy released is equal to the change of binding energy.

Question 14

Nuclear fusion:
can only take place if..
why is this necessary?

Answer 14

if the two nuclei that are to be combined collide at high speed. this is necessary to overcome the electrostatic repulsion between the two nuclei so they can become close enough to interact through the strong nuclear force.

Question 15

1H + 1H —>

Answer 15

1H + 1H —> 2H + β+ + ve

Question 16

1H + 2H —>

Answer 16

1H + 2H —> 3He

Question 17

3He + 3He —>

Answer 17

3He + 3He —> 4He + 2 1H

Question 18

Solar energy:
Solar energy is produced as a result of

Answer 18

fusion reactions inside the sun.

Question 19

Solar energy:
Temp at sun centre ≈ 10^8K. At such temps, atoms are stripped of their e-s. Matter in this state is referred to as — and they move —

Answer 19

plasma. nuclei of plasma move very fast bc of enormous temp.

Question 20

Solar energy:
fusions above (the fusion eqs) occur to form

Answer 20

4He , releasing 25 MeV of energy per 4He

Question 21

Fusion power:
prototype fusion reaction JET releases energy by (eq)?
Where did we get reactants from?

Answer 21

2H + 3H —> 4He + 1n + energy
The neutrons are absorbed by a blanket of lithium surrounding the reactor vessel. The reaction between the neutrons and the lithium nuclei produces 3H which is then used in the main reaction. 2H is naturally occurring in water.
6Li + 1n —> 4He + 3H + energy

Question 22

The thermal nuclear reactor:
Describe setup

Answer 22

Fuel rods spaced evenly in a steel vessel called reactor core. Also contains control rods and coolant (eg water, CO2 gas), and is connected by steel pipes to a heat exchanger.

Question 23

The thermal nuclear reactor:
Fuel rods contain?

Answer 23

enriched uranium (mainly 238U, 2-3% 235U)

Question 24

The thermal nuclear reactor:
Control rods absorb neutrons - what do we control ( +how many n) and why?

Answer 24

Depth of rods in cure adjusted to keep number of n in core constant so exactly one fission n per fission event goes on to produce further fission ∴ constant rate of release of energy.

Question 25

The thermal nuclear reactor:
Moderator eg? role?

Answer 25

eg water or graphite
slow the fastmoving n to thermal n by repeated collisions with the moderator atoms.

Question 26

The thermal nuclear reactor:
For a chain reaction to occur, the mass of —-
why??

Answer 26

the mass of the fissile material must be greater than a min mass, called critical mass. This is bc some fission n escape from fissile material w/o causing fission, and some are absorbed by other nuclei w/o fission. if mass < critical mass, too many fission n escape bc SA to mass ratio of material is too high.

Question 27

Safety features:
reactor core is thick steel vessel

Answer 27

withstands high P and T in core. absorbs β, some γ and n from core

Question 28

Safety features:
core in building with very thick concrete walls

Answer 28

absorbs n and γ that escape from reactor vessel

Question 29

Safety features:
emergency shut-down system

Answer 29

inserts control rods fully into core to stop fission completely.

Question 30

Safety features:
sealed fuel rods inserted and removed from reactor by means of remove handling devices

Answer 30

rods much more radioactive after removal bc fuel cans before contain 235U and 238U which emit only α but after use emit β and γ due to many n-rich fission products that form. In addition, spent fuel rods contain 239Pu - very active α emitter that causes lung cancer if inhaled.

Question 31

Radioactive waste:
High-level radioactive waste

Answer 31

removed by remote control, stored underwater in cooling ponds, as they continue to release heat due to radioactive decay, transferred to steel casks, stored in sealed containers in deep trenches for centuries.

Question 32

Radioactive waste:
Intermediate-level waste

Answer 32

sealed in concrete and stored in concrete buiildings

Question 33

Radioactive waste:
Low-level waste

Answer 33

sealed in metal drums and buried in large trenches