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History of the KORSAR computer code development

In the 1990s, knowledge gained in the area of interest and fast progress in computer technology in Russia set the stage for developing new generation computer codes intended to model integral behavior of nuclear reactors at power plants. Specifically, the development of advanced codes was motivated by the following factors:

  • need for modernization and life extension of ageing NPPs;
  • construction of Russian design NPP projects abroad;
  • development of new generation NPP designs with passive safety features.

Solution to these factors involves an in-depth numerical analysis of operational transients, design-basis accidents, and beyond design-basis accidents at NPPs that should be performed to support safety assessment of nuclear reactors in accordance with both Russian and international standards.

Work on the development of the KORSAR new generation thermal-hydraulic computer code was started at FSUE “Alexandrov NITI” (hereinafter NITI) in 1996. The code is based on advanced physico-mathematical models and can be used in a wide area of applications. The predecessors of the new code were three computer programs developed at NITI in the early and middle 1990s. The DZHIP program was designed for calculation of transient thermal-hydraulic processes in coolant circuits. The program contained a two-fluid two-phase model (6 conservation equations) and had significant application background [1]. The PARNAS software package also contained a two-fluid model. It was certified by Russia’s Nuclear Supervision Service (Gosatomnadzor) in 1997 [2]. The NERA program was used for a long time to test a variable drift-flux model of two-phase flow in steam generating piping [3].

In addition, NITI has gained much experience in mathematically modeling the dynamic behavior of processes in reactor equipment components.

The KORSAR code development was divided into 3 steps. Each step was completed with the issue of a new version of the code. The main features of the code versions are given in Table below.

Code versionCharacteristic featureDevelopment schedule
KORSAR/V1 Two-fluid model of thermal-hydraulic processes in coolant system; point reactor neutron kinetics model;
calculation of variations in the soluble boron concentration;
models of cooling system components.
1996 – 2001
KORSAR/V2 In addition to KORSAR/V1 capabilities:
spatial reactor neutron kinetics model.
1997 – 2002
KORSAR/V3 In addition to KORSAR/V1 and KORSAR/V2 capabilities:
calculation of non-condensable gas effects;
models of steam turbine system components.
1999 – 2003

As seen from the schedule, development of the code versions was carried out on a concurrent basis: work on new versions started while verification and validation of previous versions was in progress.

The code developers had three main tasks to perform:

  • develop a user-code interface that would ensure solution to a variety of thermal-hydraulic problems in flexible topology mode. This task naturally involved selection of a platform for building the code functionality;
  • select physico-mathematical models and numerical methods;
  • verify, validate, and document the code (prepare and issue the code documentation and verification report).

All three tasks were accomplished successfully by 2001 and the KORSAR/V1.1 (approved version name) verification report was issued and submitted to Russian Gosatomnadzor for certification. The certification process was completed in 2003 and Certificate No. 168 was granted оn 23 December 2003.

Further, the VVER NPP designers (FSUE OKB “Gidropress” and FSUE “SPbAEP”) proposed that the code developers start working on a special version of KORSAR that would integrate KORSAR/V3 capabilities with Gidropress developed models (TVEL-2, TVEL-3, KAMERA-V2) and KUPOL-M program created at IPPE. This special code version was named KORSAR/GP and certified by Rostechnadzor (Environmental, Industrial and Nuclear Supervision Service of Russia) in 2009 (Certificate No. 263 of 23.09.2009).

A further evolution of the code was initiated by JSC “Africantov OKBM”, the designer of naval nuclear reactors. The KORSAR developers were invited to work out a code version for safety assessment of integral reactors that would be installed on nuclear powered icebreakers in Russia. This new version was named KORSAR/BR. It was certified in 2014 (Certificate No. 355 of 17.04.2014) and included all capabilities of KORSAR/V3 version plus KUPOL-MT containment processes model developed by IPPE. 

Work is underway on a new version, KORSAR/3D, that would allow simulation of neutronic and thermal-hydraulic processes with 3D approach.


  1. Yu.A., Chernov I.V., Yudov. Yu.V. Verification of DZHIP and RELAP5 codes in ISB-VVER test facility using ISP-1 and ISP-2 // Thermal Engineering.  No. 3, pp 8 – 13.
  2. ARNAS program. Summary. (Certificate No. 70 of 06.03.1997) // VANT. Series: Physics of nuclear reactors. 1999, 1, pp 97 – 99. (available in Russian only)
  3. Migrov Yu.A., Tokar O.V. Use of a phase drift model in reactor dynamics calculation programs // IFZH. 1994. V. 67, No. 3 – 4, pp 209 – 218. (available in Russian only)