Joint Executed research activities on Source Term evaluation : From EURSAFE to SARNET

J.C. Micaelli, P. Giordano
Symposium on Off-site Nuclear Emergency Management - Rhodes Island, Greece, 21 to 24 September 2004

In spite of the accomplishments reached within the 4th and 5th Framework Programmes, a limited number of specific issues remain where research activities are still necessary in order to reduce uncertainties and to consolidate SA management plans. These remaining issues were pointed out by the Phenomena Identification and Ranking Table (PIRT) action conducted within the EURSAFE thematic network of the 5th Framework Programme [1].

Aiming at solving these remaining issues identified, and facing the inevitable reduction of the national budgets, many organizations felt necessary to better coordinate the national efforts to optimize the use of the available expertise and experimental facilities. About 50 European organizations, including technical supports of safety authorities, industry, utilities and universities decided to seize the opportunity offered by the EC in the framework of FP6 to join their efforts and to network in SARNET (Severe Accident Research and management NETwork, [2]) their capacities of research in the SA area in a durable way. In SARNET, project started in April 2004 and coordinated by the French “Institut de Radioprotection et de Sûreté Nucléaire”, all the organizations networked contribute to a Joint Programme of Activities (JPA) ; in particular, joint research activities are executed to solve remaining outstanding source term issues. Four selected topics are studied in a dedicated Source Term (ST) group : effect of air ingress; iodine volatility in the Reactor Coolant System (RCS); aerosol behaviour in reactor and iodine behaviour in containment.

The impact of an air oxidising environment in contact with UO2 irradiated fuel will be studied through different experimental tests (SET, such as MAESTRO, RUSET, VTT tests). In these conditions, some FPs which are known to be hardly released during SA progression in a reactor core under steam oxidizing conditions, may form highly volatile oxide species under air oxidizing conditions. This is particularly the case for ruthenium, whose behaviour varies widely with the oxygen potential. These volatile oxide species may reach the reactor containment under gaseous forms, and then may potentially be released to the outside environment. Interpretation of experiments will be also conducted, leading to models proposals.

In addition, the evolution of fuel management towards higher burn-up and the use of MOX fuel, make necessary to assess the possible consequences relative to these fuels, as most European Safety authorities require that SA be considered for power plants.

The impact of high temperature on behaviour of FP in the RCS, especially on the iodine one, will be also investigated in the ST topic of the network. The objective is to improve the predictability of iodine species exiting the RCS to provide the best estimate of the source into the containment. It is known that such behaviour is difficult to predict in some transients due to non-equilibrium chemistry phenomena. Experimental and theoretical works to analyse the species formed in the gas phase above the core and in the RCS (VERCORS, CHIP), analysis of FP speciation in the integral test Phébus FPT2, and particularly the analysis of control rod material release, will be performed.

Aerosol behaviour in the reactor will be investigated and the main objective, as recommended by the EURSAFE experts, is to quantify the source term especially in the case of steam generator tube ruptures which leads to a reactor containment building by-pass. Activities will consist of experimental (ARTIST, PSAEro, HORIZON, RADSOL…) and theoretical work. Deposited FP interaction with the substrate and revaporization phenomena will also be studied.

Concerning iodine chemistry, the main objective is to improve the predictability of the various chemical and physical processes which control the iodine behaviour both in the gas phase and water phase inside the containment. Various phenomena affecting the iodine chemistry in these phases will be experimentally investigated in SET (EPICUR, SISYPHE…).

The ultimate goal of most of these jointly executed research activities is to provide physical models to be integrated in the existing integral SA analysis code ASTEC, developed by IRSN and GRS, so as to make it the European reference for any safety studies. This code describes the behaviour of a whole NPP under severe accident conditions including SAM engineering systems and procedures.

[1] EURSAFE thematic network - contract FIKS CT 2001-20147
[2] SARNET Network of Work - contract FI6O CT 2004-509065