Dear Li,
the IEEE_UNDERFLOW_FLAG and IEEE_DENORMAL messages are normal, and can be ignored.
The meaning of the message is explained on Wikipedia:
In a normal floating-point value, there are no leading zeros in the significand; rather, leading zeros are removed by adjusting the exponent (for example, the number 0.0123 would be written as 1.23 × 10−2). Denormal numbers are numbers where this representation would result in an exponent that is below the smallest representable exponent (the exponent usually having a limited range). Such numbers are represented using leading zeros in the significand.
The unexpected STOP of FLUKA is unrelated to these messages.
As @cesc pointed out, and possibly the error message at the end of the .out file suggests, you run out of the memory usable for scoring.
Cheers,
David