Lesson 10 the reading module

Read the text: Passive Nuclear Safety.

Passive nuclear safety describes a safety feature of a nuclear reactor that does not require operator action or electronic feedback in order to shut down safely in the event of a particular type of emergency (usually overheating resulting from a loss of coolant or loss of coolant flow). Such reactors tend to rely more on the engineering of components such that their predicted behaviour according to known laws of physics would slow, rather than accelerate, the nuclear reaction in such circumstances. This is in contrast to some older reactor designs, where the natural tendency for the reaction was to accelerate rapidly from increased temperatures, such that either electronic feedback or operator triggered intervention was necessary to prevent damage to the reactor.

Terming a reactor 'passively safe' is more a description of the strategy used in maintaining a degree of safety, than it is a description of the level of safety. Whether a reactor employing passive safety systems is to be considered safe or dangerous will depend on the criteria used to evaluate the safety level. This said, modern reactor designs have focused on increasing the amount of passive safety, and thus most passively-safe designs incorporate both active and passive safety systems, making them substantially safer than older installations. They can be said to be "relatively safe" compared to previous designs.

The temperature coefficient of reactivity is a measure of how the reactor responds to increased temperature. A positive number denotes a trend of increasing power production as temperatures rise, whereas a negative number denotes a trend of decreased power production as temperature rises. For liquid cooled reactors (especially those that use water as coolant) the temperature coefficent is closely linked to the reactor's void coefficent.

If the coolant is a liquid, increasing temperatures can cause small gas bubbles to form, displacing the coolant. The void coefficient of reactivity is a number representing how the reactor responds to the formation of such bubbles. A positive number signifies a tendency for reactor activity to increase, whereas a negative number signifies a tendency for reactor activity to decrease. Ideally the void coefficient should be close to 0, such that neither a temperature increase or decrease will cause a power surge. Very large positive void coefficents are particularly undesirable since they could lead to a rapid uncontrollable growth in heat production, as happened during the Chernobyl disaster.

I. Reading Exercises:

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Exercise 1. Read and memorize using a dictionary:

Feedback, emergency, to rely, predicted behavior, trigged intervention, damage, to evaluate, to signify, void coefficient, a power surge.

Exercise 2. Answer the questions:

1. What does passive nuclear safety describe?

2. What is a measure of how the reactor responds to increased temperature?

3. What have a modern reactor focused on?

4. What is the void coefficient of reativity?

5. Why are very large void coefficients particularly undesirable?

Exercise 3. Match the left part with the right:

1. Passive nuclear safety describes	. a) small gas bubbles to form, displacing the coolant
2. A positive number denotes	b) a safety feature of a nuclear reactor
3. If the coolant is liquid, increasing temperature can cause	. c) a trend of increasing power production as temperature rise

Exercise 4. Open brackets choosing the right word:

Such reactors tend (to rely/ to depend) more on the engineering of components such that their predicted behaviour according to known laws of (physics/chemistry) would slow, rather than accelerate, the nuclear reaction in such circumstances. This is in contrast to some (newer/older) reactor designs, where the natural tendency for the reaction was to accelerate (rapidly/slowly) from (increased/decreased) temperatures, such that either (electric/electronic) feedback or operator triggered intervention was Whether a reactor employing passive safety systems is to be considered safe or dangerous will depend on the criteria used to evaluate the safety levelnecessary to prevent damage to the reactor.