Modification and application of the system analysis code ATHLET to trans-critical simulations

During the loss of coolant accident (LOCA) of supercritical water cooled reactor (SCWR), the pressure in the reactor system will undergo a rapid decrease from supercritical to subcritical condition. This process is called trans-critical transients, which is of crucial importance for the LOCA analysis of SCWR. Using the current version of system code (e.g. ATHLET, REALP), calculation will be terminated due to the abrupt change of void fraction across the critical point (22.064 MPa). To solve this problem, a pseudo two-phase method is proposed by introducing a fictitious region of latent heat (enthalpy of vaporization hfg∗) at pseudo-critical temperatures. A smooth transition of void fraction can be realized by using liquid-field conservation equations at temperatures lower than the pseudo-critical temperature, and vapor-field conservation equations at temperatures higher than the pseudo-critical temperature. Adopting this method, the system code ATHLET is modified to ATHLET-SC mod 2 on the basic of the previous version ATHLET-SC mod 1 modified by Shanghai Jiao Tong University. When the fictitious region of latent heat is kept as a small region, the code can achieve an acceptable accuracy. Moreover, the ATHLET-SC mod 2 code is applied to simulate the blowdown process of a simplified model. The results achieved so far indicate a good applicability of the new modified code for the trans-critical transient.

► The pseudo two-phase method is proposed and utilized to modify ATHLET code.
► A smooth transition of void fraction under trans-critical transient can be realized by this method.
► The newly developed ATHLET-SC code can be adopted to simulate the blowdown process of a simplified model.