R2.30/94: Severe Transient Analysis RBM

The key purpose of Project R2.30/94 is to assess the adequacy of the computational codes used in the reference analysis of the following severe accidents:

Reactivity-induced accident (RIA) resulting from spontaneous Control and Protection System (CPS) rod withdrawal.
Flow blockage in the distribution group header (DGH).
Ultimate design-basis accident (pressure header rupture).
Anticipated transient without scram (ATWS) (loss of feed water).
The project also sought to verify the general safety concept of the RBMK plants using Smolensk NPP Unit 3 (third generation) as a reference plant.

1) Spontaneous withdrawal of CPS rod from the core

The main finding of the study was that SADCO code has been considered the best neutronic transport code. Its performance is comparable with the best modern Western codes in this field. At the same time it was recommended to continue the work on the improvement of the main cross-section libraries. The analysis also showed that all safety parameters were well below the acceptance criteria. It was concluded, however, that it would be useful to perform further studies on the sensitivity of the calculation results to the variations in power, burn up and coolant flow distributions, and to locations of CPS rods. Considering that the thermal hydraulic feedbacks play an insignificant role in the conditions of spontaneous withdrawal of a CPS rod, the most efficient way to perform a detailed analysis of the margins available to the thermal hydraulic instability is by step-by-step calculations using 3-D neutronic codes and best-estimate thermal hydraulic codes.

2) Flowblockage in Distribution Group Header (DGH)

Following flow blockage in DGH, the degraded cooling provided by ECCS bypass flow is demonstrated to be insufficient to prevent FC rupture. With greater bypass flow, FC ruptures can be avoided at all. The minimum value of ECCS bypass flow which precludes FC break can be determined from calculations.
The consequences of DGH flow blockage may depend on the number of high-rated channels connected to the affected DGH.
To preclude FC ruptures in case of DGH blockage, it is necessary to add a trip signal on low flow in several FC connected to each DGH (for all DGHs). Activation of higher bypass flow on demand will provide for sufficient cooling.
For better understanding of the safety issues associated with DGH flow blockage, which is a beyond-design-basis accident, it is necessary to perform additional sensitivity calculations.

3) Ultimate Design Basis Accident (UDBA)

RELAP5/MOD3.1 code can be used to perform the analysis to support safety protective measures and improvements in operating procedures. During a design-basis UDBA scenario, fuel claddings and pressure tubes parameters stay within the acceptance criteria. In case of reactor scram on low SD level (which is a key factor in the beyond-design-basis scenario), fuel claddings in most of the rated channels may lose their leak-tightness. Model uncertainties did not affect calculation results. It is recommended to use the same approach in subsequent assessments.

4) Anticipated transients without SCRAM (ATWS)

This study provided information on the behaviour of various parameters and on the time available for taking protective measures in case of such accidents. The study identified the signals that would be indicative of accident progression.
The findings of the study can be used for the development of the second shutdown system for RBMK reactors.
The project resulted in four Final Reports with conclusions agreed by the Russian and Western experts. As an additional output an executive summary report with common conclusions was prepared and approved by the parties.

Remark:

The specific project objectives were achieved. The project results show that the Russian codes, applied for the accidents analysis, are adequate for assessing the relevant physical phenomena for RBMK licensing purposes. The minor deviations between the results given by Western and Russian codes, when analysing the reference cases, were clarified and the reasons for them were well understood. A specific issue in the project implementation was the Quality Assurance both for the code development and the code application. The Russian awareness of Western practice was an important achievement in that respect.