Nuclear Fission

Q13. The core of a thermal nuclear reactor contains a number of components that are exposed to moving neutrons.

(a) State what happens to a neutron that is incident on the moderator.

The neutron undergoes an elastic collision with the moderator 'bouncing off' with a lower speed and therefore lower kinetic energy.

Any reference to absorption loses the mark.

[1 mark]

(b) State what happens to a neutron that is incident on a control rod.

The neutron is absorbed by the control rod.

Just saying it is ‘stopped by the control rod’ will not get the mark.

[1 mark]

(c) A slow-moving neutron is in collision with a nucleus of an atom of the fuel which causes fission. Describe what happens in the process.

The neutron is absorbed by the U-235 nucleus and U-236 is formed.

This causes the nucleus to split into two smaller daughter nuclei (alternative words for nuclei were not acceptable (eg saying 'daughter products' would not get you the mark) )

releasing several fast-moving neutrons (‘neutrons’ must be plural to get the mark.)

[3 marks]

(d) A thermal nuclear reactor produces radioactive waste.

State the source of this waste and discuss some of the problems faced in dealing with the waste at various stages of its treatment.

Your answer should include:

the main source of the most dangerous waste

a brief outline of how waste is treated

problems faced in dealing with the waste, with suggestions for overcoming these problems.

The description expected in a competent answer should include:

the main source of the most dangerous waste The (highly radioactive/ most dangerous) waste is composed of fission fragments from the fission of uranium-235 or from (spent) fuel rods.

a brief outline of how waste is treated The waste is initially placed in cooling ponds/water (close to the reactor for a number of years) Plutonium/uranium is separated to be recycled high level waste is vitrified/made solid into (pyrex) glass then placed in (stainless) steel/lead/concrete cylinders/containers/bunkers to be stored deep underground. Simply stating buried/underground is not enough to get this mark

problems faced in dealing with the waste, with suggestions for overcoming these problems. To get the mark the problem and its solution must both be given, some examples are given below. The waste is initially very hot/generates heat so has to be placed in water/cooling ponds (to remove the heat energy). The waste (initially) is highly radioactive and needs to be screened in water/cooling ponds (to absorb the radiation). The waste (initially) is highly radioactive and needs to be remotely handled (to avoid human contact with the waste). In liquid form the (high level) waste may leak, hence the need to vitrify (and barrel in steel). The waste will be radioactive for hundreds/thousands of years so storage needs to be stable in a container hence the need to vitrify (and barrel in stainless steel). The waste will be radioactive for hundreds/thousands of years so long term storage needs to be in geologically stable areas (deep underground). Transporting waste presents a potential danger to the public so waste is transported enclosed in impact/crash resistant/extra thick and strong casings or processed onsite or nearby.

 

Making points is not enough - the style of presentation of your answer is also assessed:

High Level – Good to Excellent All three bullet points must be addressed. The source must be identified and two stages in the treatment sequence must be given. Finally three problems encountered in the treatment of waste and how the problems are overcome should be stated. The information presented as a whole should be well organised using appropriate specialist vocabulary. There should only be one or two spelling or grammatical errors for this mark.	To be in this top band communication skills must be good and the ideas easy to follow.
	6 marks At least 6 points made coming from all three of the bullet point headings. (note some written points may count as answers to bullet point headings 2 and 3)
	5 marks 5 points must made coming from all three of the bullet point headings.
Intermediate Level – Modest to Adequate All three bullet points must be addressed. The source must be identified as well a stage in the treatment along with a problem encountered in the treatment of waste and how it is dealt with. One additional piece of information must be made from any of the bullet points listed below to be at the top of this band. The grammar and spelling may have a few shortcomings but the ideas must be clear.	To be in this moderate band communication skills must good enough to understand the ideas easily even if the order is a little unclear.
	4 marks 4 points made coming from at least 2 bullet point headings.
	3 marks 3 points made coming from at least 2 bullet point headings.
Low Level – Poor to Limited To be at the top of this band two bullet points must be addressed which must include a problem encountered in the treatment of waste and how it is dealt with. A single mark is awarded if any of the information given in the bullet points listed below is given. There may be many grammatical and spelling errors and the information may be poorly organised.	2 marks Two points made from any bullet point heading.
	1 mark Any point made coming from any bullet point heading. OR The script as a whole shows some basic understanding of the issues.

[6 marks]

(Total 11 marks)