FUEL PERFORMANCE ASSESSMENT WHEN MODELING GAMMA HEATING DURING STEADY-STATE AND TRANSIENT SCENARIOS

摘要

In today's light water reactors operating under steady-state conditions, it is understood that ～2.5-3.5% of the energy generated comes from gamma-ray heating. From a thermal-hydraulics viewpoint, the effects of gamma-ray heating are accounted for in the core coolant temperature rise. However, from a fuel performance standpoint, it is important to understand the heat generation within the fuel as it dictates the overall temperature distribution and thus safety margins. The U.S. Nuclear Regulatory Commission's (NRC) steady-state fuel performance code FRAPCON currently lacks the ability to model gamma-ray heating of the coolant, resulting in all of the energy from the user-supplied linear heat generation rate (LHGR) being deposited within the fuel and thermally transferred to the coolant. This work analyzes the effects on LWR steady-state fuel performance when accounting for gamma-ray heating with a modified version of FRAPCON-3.5. During a hypothetical loss of coolant accident (LOCA), the varying coolant conditions and fission product decay can result in different gamma-ray heating phenomena, namely where the energy is deposited and in what quantity. The NRC fuel performance code FRAPTRAN takes into account a 2% value of gamma-ray heating deposited into the coolant, based on the fuel rod's current LHGR. This work will include radiation transport calculations using Monte Carlo N-Particle (MCNP) and SCALE to determine where the gamma-ray energy is deposited with a 17×17 pressurized water reactor (PWR) fuel assembly and varying accident conditions, and a new gamma-ray heating model will be implemented into FRAPTRAN-1.5.