Question

A) A 3000MWth LWR can be critical (k=1) at what power? Explain.

1000MWe 1000MW 3000MWth Any power level

B) Which of these reactor types has the weakest moderating coolant? Explain.

LMFBR BWR PWR AGR

Answer 1

A) For a nuclear reactor, ‘critical ‘means the chain reaction is self-sustaining & the reactor is operating at a constant power.

A factor known as reactivity represented by K. K is the ratio of the number of neutrons causing a fission event in the current generation to the number of neutrons causing a fission event in the next generation.

So for K <1 then the number of neutrons reacting is decreasing from one second to the next. And power produced is also decreasing. This is known as sub-critical.

For K=1 then the number of neutrons reacting from second to second stable so power level is also stable. This is when a reactor is critical. A critical reactor is the desired condition for producing power. .

For K>1 then the number of neutrons reacting is increasing every second. This means power production is increasing. This condition is known as super-critical. If no action is done, then reactor would rapidly exceed its design power limits & automatic safety shutdown will happen.

Hence answer is a 3000 MWth LWR can be critical (k =1) at 3000 MWth only.

B)

1) LMFBR – Liquid Metal Fast Breeder Reactor uses liquid metal as primary coolant.

2) BWR- Boiling Water Reactor uses water similar to PWR as moderating coolant but at very low pressure (@75 times atmospheric pressure). It consists of a single circuit.

3) PWR – Pressurized Water Reactor – uses ordinary water as both coolant as well as moderator.

The primary circuit consists of water flowing through the core of the reactor at very high pressure & secondary circuit consists of generated steam.

4)AGR – Advanced gas cooled Reactor – It uses moderator - graphite & primary coolant -carbon dioxide.

Thus we can see BWR – Boiling Water reactor are using the weakest moderating coolant – water at very low temperature