Activity AP3.20 Uncontrolled chain reaction
Reducing the risk of producing an excessive flame
Setting up the arrangement
All through these supplementary notes, remember that you must carry out your own risk assessment and take suitable precautions.
Conduct trials before doing a class demonstration.
Description of experiment
This demonstration models the chain reaction that occurs when uranium atoms undergo nuclear fission. It involves constructing two boards to hold arrays of matches.
A single match from the centre of the bottom row of the array is lit, first using just one board; a few neighbouring matches may be ignited, but the flame soon goes out.
The second board is then placed such that the match-heads on one board are spaced evenly in relation to those on the other board; this increases the density of match-heads. The centre match from the bottom row is lit.
A chain reaction takes place with many matches being ignited, producing a dramatic flame.
This activity was trialled and has been used successfully in at least one school for many years, as well as by other schools following this scheme of work.
However, one school has now reported a larger than expected flame (around 4 feet high) from the matches on the combined boards. Technicians needed to extinguish the flames with a fire extinguisher to protect the ceiling of the room, before more than one-third of the matches had ignited. CLEAPSS would be interested to hear from schools with experience of this demonstration.
The composition of woods and match-head mixtures may vary, even between batches from the same source. CLEAPSS has not yet trialled this activity and it is difficult to standardise conditions in schools, but the following points may reduce the risks of producing an excessive flame.
Basic safety precautions
Place the demonstration on heatproof mats.
Exclude combustible materials from the vicinity of the demonstration.
Ensure there is a reasonable distance between the top of the board and the ceiling or any equipment or materials hanging from it.
Ventilation should be good but free from draughts.
A fire blanket should be available, and the teacher should know how to use it correctly to treat a fire without overturning delicate equipment.
If flames threaten to spread, open the fire blanket fully. Protecting your hands from the fire, approach towards the open ends of the demonstration. Carefully lay the fire blanket over the apparatus. Leave the fire blanket in place until the apparatus has cooled.
Trial before doing a class demonstration
Set up a trial demonstration using a smaller number of matches. Test this before scaling up to the demonstration level. It may be necessary to adjust the spacing of the matches or the number used.
Do trials using matches like the ones you will use on the day: use matches from a large enough stock purchased from the same supplier at the same time. This is to enable matches from exactly the same batch to be used for the class demonstration.
Insert matches securely, so that they are unlikely to shift from their positions or fall out.
Light a match from the centre of the bottom line of the array, not from one edge. The school reporting the incident had started the flame at an edge of the array, which may have affected the resulting reaction.
Before repeating the demonstration in front of a class, check that the apparatus is still sturdy and that fresh matches can be inserted safely.
Outdoor operation has been suggested. This should only be carried out if the weather is calm.
David Ellis, Castle Hall School, Mirfield, West Yorks writes:
We have been carrying out this demo for about two years with no problem. The flame does get quite high, but we soon learned to be careful in the sighting of the demo. We have always used 'Swan Vesta' matches merely because there are more in a box. On reflection though, they are short and will have less wood to burn. For the boards we have used cake boards which are fairly flame resistant and can fit in safety screen stands. March 2009.