Safety in reactors Flashcards
Why must chain reactions be monitored
if a chain reaction in a nuclear reactor is left to continue unchecked, large amounts of energy are released in a very short time. Many new fissions will follow each fission causing a runway reaction which could lead to an explosion = atomic bomb
Critical mass =
amount of mass/fuel required so that the chain reaction continues at a steady rate where one fission follows another
The mass of the fissile material must be > than the critical mass (super-critical) because
some fission neutrons escape from the fissile material without causing fission and some are absorbed by the other nuclei without fusion.
Sub-critical mass causes
too many of the fission neutrons to escape because the surface area to mass ratio of the material is too high
Explain what type of mass is used in nuclear reactors
super-critical mass (> critical mass) of fuel is used (where several new fission’s normally follow each fission) and control rods control the rate of fission.
Role of control rods
Control rods control the exponential increase of neutrons/the chain reaction by limiting the number of neutrons in the reactor.
4 safety features
1) thick steel vessel
2) thick concrete walls
3) control rods
4) remote handling devices
Explain steel vessel
The reactor core is in a thick steel vessel designed to withstand the high pressure and temperature in the core. The thick steel vessel absorbs beta radiation and some of the gamma radiation and neutrons from the core
Explain thick concrete walls
The core is in a building with very thick concrete walls which absorb the neutrons and gamma radiation that escape from the reactor vessel
Explain control rods
In an emergency, the reactor will be shut down automatically by the release of the control rods which fall into the reactor under gravity (as placed vertically) straight down, stopping the reaction as quickly as possible reducing the risk of an explosion
Explain remote handling devices
The sealed fuel rods are inserted and removed from the reactor by remote handling devices.
Why are fuel rods much more radioactive after removal
Because the fuel cans initially contained only U-235 and U-238 which emit only alpha radiation which the fuel cans absorb whereas now emit beta and gamma radiation (due to many neutron-rich fission products that are formed) as well as now containing Pu-239 (U-238 + neutrons) which is a very active alpha emitter and if inhaled can cause lung cancer
How do control rods work
The control rods are made up of a material like boron that absorbs neutrons and can be inserted into the core automatically to keep the number of neutrons in the core constant so exactly one fission neutron per event on average goes on to produce further fission.
Why is it important to have exactly one fission neutron per event on average goes on to produce further fission.
It keeps the rate of release of fission energy constant.
What happens as the control rods are pushed further and further down
If the control rods are pushed in further, they absorb more neutrons so that the number of fission events per second and the rate of release of fission energy is reduced; they can be pushed in far enough to stop fission events entirely.
