Re: Thermal cycle

Mike,

Here is a summary of my notes regarding the thermal cycling which I hope captures our combined concerns to help more clearly understand them as well as our guidance.

Once you and I learned of the method your sub, D.L. Rickie, would use to heat the vessel – 6” wide strips with three strips would ganged together with one of them monitored to heat the vessel at a programmed ramp rate to a set point – we discussed mutual concerns about what the various failure modes were.  We then spoke with the D.L. Rickie foreman and learned that it is common to lose one or two channels during a heat cycle; sometimes even a whole zone. With this information, we discussed the potential gap between working heaters.  I said that I could ask our analyst to see what kind of temperature differential could occur.  You requested that I use a gap of 12”.

Kevin Freudenberg provided the result that with only a modest amount of heat (50 watts) with a 4 hour ramp rate to 150C that the temperature would reach a difference across a 6” gap by the 50C allowable.  The foreman then said that 40 amps of current run through each heating pad.  I calculated the wattage of each power supply (480 volts times 100 amps) 48,000 watts divided by 27 individual pads yields just under 1800 watts per pad.  SO the rate of heat is much higher than what Kevin used.  This means that the metal immediately under the pads will be much hotter than what he used and therefore the potential differential would be much greater.   Also there are a large number of thermocouples so there is also a potential for delay in noticing a problem and you want to allow time for the operator to react to a growing difference. 

Again, you and I are both are concerned about a fault scenario that 1) we have been told is a common occurrence 2) could easily occur undetected if one or two unmonitored pads of the three fail.

The criteria for +/- 25 C applies to the entire thermal cycle due to analysis that shows that localized heating could easily result in localized plastic deformation which would manifest as warping of the VVSA..  This is especially true in light of the fact that the VVSA consists of 20 plates which are in various states of anneal or work hardening.

We offer this analysis to prevent potentially disastrous deformation of the VVSA during the required thermal cycle.  We highly recommend applying the heat cautiously.  Recalling that NCSX originally advised a 10C per hour rate and is now saying that with 50 watt heaters 30C (~50F) per hour reaches the maximum allowable differential at only 150C; I think it is prudent to use no more than 20C per hour.  I really think that any saving of time in ramping up faster will be short compared to the bakeout soak time (48 hours total) and the time to cool back down to 100C between cycles.  As always, please feel free to ask any questions regarding our analysis or to discuss this further.

Mike Viola