Fusion and fission are nuclear reactions in which large amounts of energy are released. (a) (i) In a fusion reaction, two hydrogen nuclei are forced together to form a helium nucleus. Explain why a very high temperature is needed for this reaction to happen. (ii) In a fusion reaction, the combined mass of the two small nuclei is greater than the mass of the resulting nucleus. This decrease in mass, m, appears as energy, E, according to the equation. E = mc² c is the speed of light = 3.0 × 10⁸ m/s. Calculate the decrease in mass. decrease in mass = ______________ kg (b) Nuclear fission is used in nuclear reactors in some power stations. In the reactor, a fission chain reaction is maintained and controlled to produce a supply of energy to generate electricity. Figure 14 is a diagram of a nuclear reactor. Explain how the graphite core and the movable rods are used to maintain and control the chain reaction.

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Fusion and fission are nuclear reactions in which large amounts of energy are released - Edexcel - GCSE Physics - Question 10 - 2018 - Paper 1

Question 10

Fusion and fission are nuclear reactions in which large amounts of energy are released. (a) (i) In a fusion reaction, two hydrogen nuclei are forced together to form... show full transcript

Worked Solution & Example Answer:Fusion and fission are nuclear reactions in which large amounts of energy are released - Edexcel - GCSE Physics - Question 10 - 2018 - Paper 1

Step 1

Explain why a very high temperature is needed for this reaction to happen.

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Answer

In fusion reactions, very high temperatures mean that the hydrogen nuclei possess high kinetic energy. This high energy is necessary to overcome the electrostatic repulsion that exists between the positively charged nuclei. As both hydrogen nuclei have the same positive charge, they repel each other due to the electrostatic force. Only when the temperature is sufficiently high can the nuclei have enough energy to overcome this repulsion and come close enough for the strong nuclear force to bind them together.

Step 2

Calculate the decrease in mass.

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Answer

Using the equation for the energy-mass relationship:

$E = mc^2$

Given:

Energy released per fusion reaction, $E = 4.5 × 10^{-12} ext{ J}$ .
Speed of light, $c = 3.0 × 10^{8} ext{ m/s}$ .

We can rearrange the equation to find the decrease in mass:

$m = rac{E}{c^2}$

Substituting the values:

$m = \frac{4.5 × 10^{-12}}{(3.0 × 10^{8})^2}$

Calculating:

$c^2 = (3.0 × 10^{8})^2 = 9.0 × 10^{16} ext{ m}^2/ ext{s}^2$ ,
Thus,

$m = \frac{4.5 × 10^{-12}}{9.0 × 10^{16}} = 5.0 × 10^{-29} ext{ kg}$ .

Step 3

Explain how the graphite core and the movable rods are used to maintain and control the chain reaction.

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Answer

The graphite core in a nuclear reactor acts as a moderator, slowing down the fast-moving neutrons produced during fission reactions. This slowing effect increases the likelihood of these neutrons colliding with other nuclear fuel (usually uranium), allowing for more fissions to occur.

Movable rods, made of materials that absorb neutrons (such as boron or cadmium), are inserted or removed from the reactor core to control the rate of the chain reaction. By adjusting the position of these rods, operators can either increase the number of neutrons available to sustain the fission process (by withdrawing the rods) or decrease it (by inserting the rods). This method allows for precise control over the reactor's output and safety.

Fusion and fission are nuclear reactions in which large amounts of energy are released - Edexcel - GCSE Physics - Question 10 - 2018 - Paper 1

Worked Solution & Example Answer:Fusion and fission are nuclear reactions in which large amounts of energy are released - Edexcel - GCSE Physics - Question 10 - 2018 - Paper 1

Explain why a very high temperature is needed for this reaction to happen.

Calculate the decrease in mass.

Explain how the graphite core and the movable rods are used to maintain and control the chain reaction.

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