Deliverables

Executive summary: Severe accidents may pose a radiological threat to the environment as a result of the failure of the three engineering barriers considered in the “Defense-indepth” approach. The SOCRATES project aims at filling the understanding gaps of the liquid source term in those scenarios and at offering innovative solutions to mitigate and monitor the release of radionuclides into the environment. The present report collects the data from past accidents concerning liquid source term (leaching data, though, would be specifically addressed in D4.1 of the SOCRATES project): TMI-2, ChNPP Unit 4 and FDNPP Units 1-3. The data collected highlight that: FP trapping in aqueous ponds is independent of the reactor technology; accident scenarios make substantial differences in the radioactive compositions of aqueous solutions; nature of fission products (volatility and water solubility) is a key feature for aqueous ponds compositions; and, intervals of radioactive concentration in aqueous solutions are hard to be set. The data compiled in this report will be instrumental when setting up the experimental matrices in WP3 of the SOCRATES project.

Executive summary: The SOCRATES project was launched to address key knowledge gaps related to the characterization and prediction of the liquid source term during severe nuclear accidents. To this end, dedicated experimental programs and corresponding analytical investigations are being conducted, with the objective of transferring the generated insights to plant scale analyses.

The present document provides an assessment of the state of the art in severe accident simulation tools ASTEC, AC² and MELCOR with regard to their applicability to liquid source term analyses.

These integral severe accident simulation codes are used to model the progression of accidents in nuclear power plants, from core degradation to source term release. They couple thermal-hydraulics, physicochemical phenomena, and fission product behavior to provide plant level predictions for safety evaluation and regulatory analysis.

These codes are primarily focused on corium progression until stabilization and on atmospheric source term to assess radiological consequences from short-term venting or in case of containment leaks. Therefore, the evolution of the liquid source term—necessary for mitigating and monitoring radionuclide releases into the environment, as well as for managing contaminated water requiring treatment—remains insufficiently understood and modelled.

In addition, the chemical composition of the melt remains highly uncertain during the long term of an accident. Consequently, these codes are less applicable in a predictive manner regarding the chemical boundary conditions of the cooling water. The water chemistry strongly depends on operator actions and on the chosen make-up systems for both short- and long-term cooling.

Based on these facts, long-run liquid source term modeling with the traditional severe accident codes does not seem a viable option. Instead, an ad-hoc modeling is recommended. It is noteworthy, though, that input decks for those specific models might be informed by severe accident codes in aspects like geometry and chemical composition of solidified molten materials and cooling water. Of course, final settings of initial and boundary conditions will be user's matter. This approach of giving starting conditions of the melt and the cooling water is also needed to simulate the experiments conducted in the SOCRATES project.

Executive summary: In operating conditions in a Nuclear Power Plant, strict control of water chemistry is vital to prevent corrosion and maintain component integrity. In case of a severe accident, contaminated water present in the lower parts of the containment or in spent fuel pools can harm system operations (resulting from oxidation, corrosion or clogging processes) and may alter post-accident cooling capacities.

Within the SOCRATES program, the objective of work package 3 is to investigate by laboratory scale tests, the intricacies of liquid chemistry in such complex situation and to delving deep into how common materials in nuclear facilities react with fission products when immersed in liquids. One objective of these experiments is to provide key insight into cesium and strontium long term fate.

The present document summarizes the main liquid boundary conditions that will be considered to build up the test matrix for the experimental work. These boundary conditions designate both the composition of the contaminated liquids as well as their physicochemical parameters (temperature, pressure and irradiation) that can be expected in severe accident situations. Synthesis of WP2 outputs and literature review on sump reactivity in LOCA situation allows us to define the more relevant conditions that will be considered.

The document provides the detailed test matrix of the experimental work that will be undertaken within WP3, with a focus on the first experimental steps. First, debris dissolution tests will be performed to supplement the knowledge acquired in LOCA situations by investigating conditions relevant to SA conditions (air/water radiolysis, long duration tests). Special attention will be brought to solvent radiolysis and how to reproduce this reactivity without irradiation.

In a second step, it is proposed to investigate reactivity in bulk solution considering nature of buffer, elements issued from debris dissolution (Si, Ca, Al, Zn, Fe) and key FPs (Cs and Sr) with increasing chemical complexity. The formation of precipitates will be one of the major aspects that will be investigated- with a focus on FPs reactivity. The main parameters that will be investigated are the pH level, element concentration, temperature and pressure (without radiolysis).

These two experimental stages are expected to last 23 months and will involve both VTT and ASNR facilities. A MSc Student will be hosted at ASNR in the first semester of 2026 to support part of the experiments.

Second part of experimental work will consider more representative situation involving complex chemical systems (including presence of non-soluble debris) and explore the influence of solvent/air radiolysis. Given the limited number of experiments that can be performed with irradiation (3 max.), experiments with simulated radiolysis will be considered as well.

Detailed conditions of these experiments will be discussed during 2026 SOCRATES annual meeting based on the results obtained in the first stages of this study.

Executive summary: The HORIZON-EURATOM SOCRATES project addresses critical gaps in our understanding of the liquid source term during severe nuclear accidents. In addition, it offers innovative solutions to mitigate and better assess the release of radionuclides into the environment. This topic has gained significant interest after the Fukushima Daiichi nuclear power plant accident because of the large volumes of contaminated water to be treated, and the behaviour of radionuclides in the water. The aim of SOCRATES is to directly contribute to the mid-to-long-term accident management of nuclear power plants, enhancing safety, environmental protection, and safe waste management.

The main objective of the Work Package 4 is to investigate the source and formation of long-term liquid phase contamination. The first tasks are to identify boundary conditions and key parameters for the leaching of radionuclides from the fuel, and to define a test matrix for the experimental research. For this, a literature survey has been carried out and documented. In the next phase, leaching experiments with different materials will be carried out step-by-step, from “simple to more complex experiments”. The materials used in the experimental work are: i) inactive ceramic material models with representative fission product species, ii) simulated fuel pellets, and iii) prototypic, uranium-containing corium debris and TMI-2 corium.

This report summarizes the review made on source and formation of liquid phase contamination by leaching based on earlier experiments as well as water contamination data from past accidents. The review aims to identify the key processes to assess the relevant variables/factors determining the behaviour of corium debris leaching. The report is based on work by the following organizations: PSI (WP coordinator), ASNR, ETHZ, JRCKarlsruhe, KU, RUB, SSTC and VTT.

In the first section, background of the water contamination is given, including the origin of contaminated water. The main chemical elements are described, and the relevant accident scenarios described. Of the accident scenarios leading to water contamination, this review focuses on severe core melt accidents.

Experimental data for leaching of radionuclides in water are reviewed in chapter 2. These data include leaching tests of simulant materials for both in-vessel and ex-vessel corium as well as leaching tests with corium collected from the TMI-2 and Chornobyl plants. Leaching of simulant material for in-vessel conditions was typically investigated using UO2 and ZrO2 mixed with tracers. The results show that the atmosphere under which the simulant debris is synthesized has a decisive effect on the leaching behaviour of actinides, oxidizing conditions during synthesis resulting in increased leaching. Similarly, leaching in seawater increases leaching as compared to distilled or pure water. Volatile fission products, e.g., Cs, have a much higher leachability than actinides. However, their content in debris is much lower than that of actinides.

For tests at ex-vessel conditions, simulant debris were synthesized by mixing UO2-ZrO2 with concrete constituents CaCO3 and SiO2. For the leaching tests, different tracers were doped into the material. Similar to in-vessel debris, the atmosphere of the synthesis had a significant effect on leaching, oxidizing conditions during synthesis promoting higher leaching fractions. In addition, it was observed that the presence of calcium compounds significantly enhanced the leaching of all fission products except for Zr. At high synthesis temperature of 1600°C, a glass-like coating was formed which reduced leaching rate, possibly due to reduced surface area, surface area being an important parameter affecting leaching behaviour. It is also feasible that some of the elements, e.g., Cs, would have been incorporated in the glass matrix thereby making them less prone for leaching in the water.

In tests with real corium from TMI-2, corium samples collected from two different locations, i.e., molten core and lower crust, were leached in deionized and borated water. The results showed no major differences between the samples or water composition. An exception was Ag which showed much higher release from the core than from the crust sample. This difference was attributed to the possibility that Ag in the core sample was released from the Ag precipitates originating from the control rods. Comparison of leaching behaviour of TMI-2 corium with that of spent nuclear fuel revealed that the U dissolution rate from both TMI samples was two orders of magnitude higher than dissolution from spent fuel in deionized water. Dissolution of Cs did not vary much between different samples (spent fuel or TMI-2 corium) or the liquid composition as it is expected to be mobilized from the fuel matrix due to irradiation and high temperature both during normal operation and during the accident.

Leaching tests of two types of Chornobyl “lava”, black and brown, were also reviewed. Brown colour appears to be caused by numerous tiny inclusions of uranium oxide phases containing up to several wt% zirconium. Black ‘‘lava” contains fewer inclusions, and its colour can be related to uranium dissolved in the glass matrix and to radiation defects. Leaching tests with distilled water, seawater, and sodium carbonate solution were carried out. Leaching tests in seawater were carried out at two temperatures, 25 and 90°C. There did not appear to be major differences between leaching of actinides, Cs and Eu from the two lava types. The water composition, i.e., distilled or seawater, did not have a major effect on leaching behaviour. The main difference in leaching was seen by the water temperature, Cs leaching rate being more than an order of magnitude higher at 90°C than at 25°C.

Section 3 describes water contamination issues and analysis of the contaminated water composition from the three major accidents, TMI-2, Chronobyl, and Fukushima Daiichi. The data can be used to estimate the approximate concentrations of different radionuclides in the contaminated water. At both TMI-2 and Fukushima Daiichi, water treatment systems were installed to remove radionuclides from the contaminated water. Water composition values are given for both untreated and treated water. For all three plants, a rough estimate of the fractions of major fission products in the contaminated water are given. According to the available data, large fractions of Cs and I were found in contaminated water in TMI-2 (up to 50 % of the initial inventory) and Fukushima Daiichi (20-70 % for units 2 and 3 after a few weeks from the start of the accident), whereas significantly smaller fractions of 137Cs and 90Sr were analysed as expected in the water in Chronobyl plant (3-10 % of initial inventory).

The data given in this document together with data in D3.1. can be used to define the test conditions in WP4. It is observed that the gas atmosphere during corium formation is of decisive importance for leaching of corium. In addition, especially for ex-vessel corium which includes concrete compounds SiO2 and CaCO3, a very high synthesis temperature affects leaching behaviour as an amorphous glass can be formed on the corium surface suppressing leaching. These results emphasize the importance of using representative synthesis temperatures, gas atmospheres, as well as representative cooling rates in corium synthesis as these parameters have a decisive impact on the corium leaching properties. Water composition has an effect on leaching, but this appears to be of second effect importance as compared to the influence of water temperature. Measurements of concentrations of different radionuclides in the contaminated water from past accidents can be used to estimate the presence of different impurities in the water. Also, the fraction of different fission products in contaminated water can be used to guide development of boundary conditions for the experiments.

Executive summary: This report concentrates on radioactive contamination associated with civilian nuclear activities. Rather than being about the analysis and management of artificial radioactivity which has been released into the environment, this report is about the management of radioactivity within nuclear sites. A need exists to manage radioactivity at nuclear sites under normal conditions, during an accident and also after an accident has occurred. Different absorbent materials namely for radiotoxic cesium and strontium (e.g. due their bioaccumulation) in liquid phase were reviewed.

For cesium clays, cyanoferrates and zeolites are able to remove cesium from waste water. These reagents are preferred as they are less combustible than the ion exchange resins and are likely to be able to form waste forms which are suitable for long term storage in a disposal site. Within the cyanoferrates both the Czech potassium nickel ferrocyanide on PAN and the Finnish CsTreat® have clear potential for use on a large scale.

For the removal of strontium zeolites, titanium oxide materials such as the Finnish SrTreat® have clear potential to be able to condition strontium containing waste waters into solid wastes suitable for disposal. When a cyclic system is desired then a weak acid resin such as a crosslinked polyacrylic acid or crosslinked polymethacrylic acid would be suitable.

While the metal organic frameworks (MOFs) have been shown in some laboratory tests to be able to adsorb cesium or strontium, the technology associated with these solids is far too immature. The synthesis of many of the MOFs is associated with a toxic solvent (DMF) and many of them require expensive and exotic reagents. As a result this report recommends that within SOCRATES that we do not concentrate on the MOFs, instead we should concentrate in the production, formulation, testing and uses of cyanoferrates, zeolites, clays, titanium oxide based materials (such as sodium titanate) and for special cases the organic ion exchange resins.