7.2 Nuclear Reactions

When a uranium–235 nucleus undergoes fission, one of the possible reactions is: 

The binding energy per nucleon, E, is given in the table below: 

Nuclide	E/MeV
	7.60
	8.39
	8.74

A 1500 MW nuclear reactor, operating at 27% efficiency, uses enriched fuel containing 2% uranium–235 and 98% uranium–238. The molar mass of uranium−235 is 0.235 kg/mol.

The average energy released by the various modes of fission of uranium–235 is 200 MeV. 

In the research into nuclear fusion, scientists are working with 1.5 kg of Lithium. One of the most promising reactions is between deuterons, , and tritium nuclei,, in a gaseous plasma. Although deuterons can be relatively easily extracted from sea water, tritium is more difficult to produce. It can, however, be produced by bombarding lithium−6,  , with neutrons. 

These reactions can be represented in the following nuclear equations:

The masses of the nuclei involved are given in the following table:

This is a synoptic question and will need knowledge from previous IB topics.

Plasma is superheated matter. It is so hot that the electrons are stripped from their atoms, forming an ionised gas. 

The Sun is made up of gas and plasma and can be thought of as a giant fusion reactor. At its core where fusion takes place, the plasma is (mainly) protons with a temperature of about 1.5 × 10⁶ K.

Near the Sun's surface, however, protons have a mean kinetic energy of 0.75 eV, which is too low for fusion to take place.

The energy produced by the Sun comes from a cycle of hydrogen fusion, during which the net effect is the fusion of 3 protons to a helium nucleus. One of the steps in the cycle is:

The amount of energy radiated away in this step is 5.49 MeV. 

The following data are available:

• Mass of nucleus = 2.01355 u
• Mass of proton = 1.00728 u 

(i)

Calculate the mass of the helium nucleus, in standard units

 [3]

One possible fission reaction of uranium-235 is

The following data are available:

• Mass of one atom of = 235u
• Binding energy per nucleon for = 7.59 MeV
• Binding energy per nucleon for = 8.29 MeV
• Binding energy per nucleon for = 8.59 MeV

A nuclear power station uses the uranium-235 as fuel. The useful power output of the power station is 1.4 GW and it has an efficiency of 30%.

(i)

Show that the specific energy of  is about 7.5 × 10¹³ J kg⁻¹.

[3]

(ii)

Determine the mass of which undergoes fission in one day.

[2]

One of the waste products of the reaction is xenon−140, . Xenon−140 is radioactive, decaying through decay.

The graph shows the variation with time of the mass of 1kg of xenon−140 remaining in the sample.

An alternative nuclear fuel to the traditionally used uranium-235 is thorium-232. When thorium-232 is exposed to neutrons, it will undergo a series of nuclear reactions until it eventually emerges as an isotope of uranium-233, which will readily split and release energy the next time it absorbs a neutron.

Part of the thorium fuel cycle is shown below.

Once the uranium-233 nucleus absorbs a neutron, it undergoes fission, releasing energy and two neutrons and forming the fission products Xenon and Strontium as in parts a-c. Any isotopes of uranium-233 which do not undergo fission decay through a chain ending with a stable nucleus of thallium-205 .