What are the safety requirements for BWR and PWR core margin and boiling?

[matt222]

What is the difference between BWR and PWR in the case of SEFs?

 

[NUCENG]

What is the difference between BWR and PWR in the case of SEFs?

There are two very basic differences one in terms of coolability design and the second in terms of margin to fuel limits.

A PWR has steam generators which contain large volumes of water which is separate from the primary coolant loop. This volume will cool the primary loop as auxiliary feedwater systems pump additional water into the steam generators. They can also be vented to atmosphere to cool and reduce pressure.

In a BWR pressure relief is from safety valves to the suppression pool containing the reactor coolant. Emergency Core Cooling Systems pump coolant directly into the reactor vessel.

In terms of core margin. A PWR operates at higher pressures and temperatures and is noit designed for any boiling (ONB or onset of nucleate boiling is bad for a PWR.

A BWR operates at saturation pressure and is intended to boil. (No bubbles no dollars). The BWR is concerned with dryout conditions or departure from nucleate boiling (DNB).

Other differences include PWR use of soluble boron for reactivity control during normal operation, control rods on top for PWRs and below for BWRs. And many more... But they have in common that the safety systems must control temperature, pressure, and reactivity to protect the public.

Beyond that we can argue, as designers and physicists have for years, as to which is better or worse, simpler or more complex, riskier or less risky, expensive or cheaper. But for now, that is a nutshell version as I see it. That is not a simple question with a two paragraph answer.

 

[gmax137]

...
In terms of core margin. A PWR operates at higher pressures and temperatures and is noit designed for any boiling (ONB or onset of nucleate boiling is bad for a PWR...

There is some amount of nucleate boiling during operation of a PWR, and it's not necessarily bad. It is the departure from nucleate boiling that leads to increased fuel and clad temperatures in a PWR, just as in a BWR.

... That is not a simple question with a two paragraph answer.

You got that right, NUCENG.

To the OP, you may wish to read through the US NRC General Design Criteria (available on the NRC website, look for Regulations -- 10CFR50 -- Appendix A). The GDCs can give you a pretty good idea of what is required from the safety systems. Most of the GDCs are applicable to both BWRs and PWRs.