CHF(critical heat flux),MCHFR(minimum critical heat flux

[matt222]

CHF(critical heat flux),
MCHFR(minimum critical heat flux ratio)

how we will find the MCHFR for vs time for coast down transient by drawing channel operating channel and CHF setpoint curves for several time?

I really confused about this can you help me about it

 

[Astronuc]

CHF(critical heat flux),
MCHFR(minimum critical heat flux ratio)

how we will find the MCHFR for vs time for coast down transient by drawing channel operating channel and CHF setpoint curves for several time?

I really confused about this can you help me about it

MCHFR depends on CHF, the enthalpy and flow rate of the coolant, and the local heat flux at the critical location.

What is the significance of CHF?

 

[matt222]

its iportant in the safety for heat transfer fro the cladding, its important for the design

 

[Astronuc]

its iportant in the safety for heat transfer fro the cladding, its important for the design

I mean - what is the physical significance of CHF? In other words, what physically happens at the location when the heat flux of the cladding reaches CHF on a BWR fuel rod? This is fundamental knowledge for a fuel designer, fuel performance engineer and reactor operator.

 

[matt222]

its like a limit so we can't go beyond it to aviod heat trnsfer in the cladding, but i am asking something else

 

[Astronuc]

its like a limit so we can't go beyond it to aviod heat trnsfer in the cladding, but i am asking something else

Think about the difference in heat transfer coefficient between water and steam, and the significance of heat transfer coefficient, heat flux, and delta-T between coolant and cladding surface.

What happens when the heat flux reaches CHF? What is the state of the coolant at the cladding surface at CHF? What is that word for that state?

 

[matt222]

I have no idea about it at all that's why I asked <<<no information on the web and I don't have any supported material to read

 

[Astronuc]

I have no idea about it at all that's why I asked <<<no information on the web and I don't have any supported material to read

I would think that would be covered in the textbook that one is using.

There should be a curve that plots ln q'' vs T or delta T = T-Tsat, and which describes different Boiling Regimes.

See Fig. 11-9 in El-Wakil, Nuclear Power Engineering, 1962
Fig 11-8 in El-Wakil, Nuclear Heat Transport, 1971, 1978
Fig 4-12 in Lahey and Moody, The Thermal-Hydraulics of a Boiling Water Nuclear Reactor, 1975

In a PWR, the CHF coincides with departure from nucleate boiling (DNB), which is not allowed in PWR, although some nucleate boiling is permissible unless it promotes significant crud deposition.

In a BWR, CHF coincides with 'burnout' or a more commonly used term, 'dryout', which is not permssible in a BWR.

When the wall heat flux exceeds the CHF, then the heat transfer abruptly decreases, and the cladding temperature increases, such that local oxidation/corrosion can increase, which leads to wall thinning, as well as the cladding gets softer (yield and ultimate tensile strength decrease), so that the cladding could strain more than desirable. If the temperature gets too high, the cladding may balloon due to increase in internal pressure.

 

[matt222]

unfortunatly I don't have these book, I have the diagram only but i don't have explantion for it just slide, but thanks its very good inforation i understand to it, but what about as i said flow coastdown , i think in this case the mass flow rate will decrease resulting decrease in the heat flux and we will have big gap margin between it, is true what i said, because i have to show as it asked me find MCHF versus time?

 

[Astronuc]

unfortunatly I don't have these book, I have the diagram only but i don't have explantion for it just slide, but thanks its very good inforation i understand to it, but what about as i said flow coastdown , i think in this case the mass flow rate will decrease resulting decrease in the heat flux and we will have big gap margin between it, is true what i said, because i have to show as it asked me find MCHF versus time?

Two things happen with decreasing mass flow: 1) the same portion of coolant (differential volume) picks up more heat, thus at a given axial elevation (downstream location), the specific enthalpy is greater (i.e. it's hotter if liquid, or it has higher quality if two phase), and 2) the heat transfer decreases, which means at a given location, delta-T is greater, which means the cladding temperature is greater.

If one is also depressurizing the system, that too decreases the saturation temperature.

 

[BEngelage]

Please help

I am also looking for the answer. This a problem the professor said to get help from the internet for. I don't just want the answer, I want to understand it all to. What are there relevant channel operating curves (coolant mass flow, coolant enthalpy)? What are the axis for these curves? For CHF limit curves (many curves based on time) what are the axis of the graph? Any help would be much appretiated.

Describe how you would determine the minimum critical heat flux ratio versus time for a flow coastdown transient by drawing the relevant channel operating curves and the CHF limit curves for several time values.