While shutdown, the core accidentally went critical and the reactor pressure vessel rose in temperature and was hot for two hours, but all instrumentation has failed and no one is sure what the temperature was. Explain how examination of the materials in retracted instruments (that had been in the core and are now extracted via thimble tubes) could be used to estimate the temperature. Specifically, use the TTT curve for 1.13% C steel (Figure 10.15) and the observation that what had been a pure gamma phase was now only 50% gamma phase to estimate what the temperature had been (assume a constant temperature for the two hour period).