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Sorbents NIKET and UNIKET - statement of the problem of creating a sorption module for NPP

Development of sorption module for mitigation of severe accident of NPP

(The text of the report is significantly abbreviated, since the data obtained were subsequently fully included in the work "Technical Report of Experimental Results in SevAcc project", given below)

M. Skala, P. Kus - Centrum vyzkumu Rez, Czech Republic;
V. Havlova - UJV Rez a.s., Czech Republic;
M. Cernik - Technical University of Liberec, Czech Republic.


Abstract

This contribution is giving the insight into the current situation of sorption material suitable for liquid radioactive waste treatment. It is a crucial responsibility of NPP operators to ensure the high level of nuclear safety. Broadening the scope of readiness for all possible scenarios contributes to the improvement of nuclear safety and influences the public image of nuclear energy. The background of presented project is readiness of NPP operators for non-project scenarios. However, the results are applicable in the whole scope of liquid radioactive waste treatment.


The topic of the presented project is an efficient treatment of a large volume of liquid radioactive waste resulted from a severe accident of NPP. After the reactor cool-down a large volume (10,000 m3) of radioactive waste would need to be cleaned.
The presented work describes evaluation of both commercial and novel materials and approaches to address the topic of large volume of radioactive waste treatment. The emphasis was given mainly to the sorption process. However, the membrane technologies were investigated as well. Various sorption materials were compared based on their properties, commercial availability and the results of stability and sorption efficiency tests. The equilibrium and kinetic parameters were evaluated in order to rank the sorption materials. The samples of sorbents were exposed to the model solution of boric acid simulating the post-accident solution. Sorption efficiency was tested on radioisotopes of Cs and Sr.

Introduction

The nature of severe accident type in case of this project rests in the scenario when the main pipeline of the primary coolant is ruptured so the coolant is flowing from both sides. In that case the container sprinkler emergency system is activated. Six months after the accident the reactor is supposed to cool down enough so the post accidental solution can be treated. The composition of the solution can be derived according the accident scenario (primary solution composition, emergency system solution composition). In scope of this project, which is primarily dealing with sorbent selection (from a large number of sorbents), only the simplified solution matrix composition was used. Influence of the substances eluted from the concrete to the solution during the reactor cool-down was neglected. As a representative radioactive contaminants Cs and Sr was used.

The aim of this work is to compare various sorbents for application in this solution. The sorbents are chosen from different categories. The commercial sorption materials with references in field of nuclear waste treatment are tested together with sorption products from different industries and newly synthetized materials. Based on the set of tests the most suitable sorbent is to be selected.

Sorption Materials

The number of sorption materials is available on the market as the actual sorption products intended to be used for Cs or Sr capture from liquid radioactive waste (CsTreat, SrTreat, Ionsiv R9160-G, Ionsiv R9515-G, UNIKET, NIKET etc.). Furthermore, there are sorption products well established in other industries (e.g. fresh water production – Bayoxide E33, Titansorb, Everzit As, ZeoCem ECO etc.) which can be tested in order to bring them to the new application. The third group encompasses new materials and substances which exhibit interesting sorption properties but they haven’t been used commercially for the sorption cleaning in any form or are even still in stage of laboratory synthesis.

The tested sorbents can be, regardless their origin, divided based on their composition, the type of the sorption site respectively. For purpose of this work the following groups were proposed: zeolites, titanates, iron oxides, iron hydroxides, cyanoferrates, aluminium oxide.

Model solution

 

Table 1 Quality parameters of the model solution

Substance/parameter

Value

Unit

H3BO3

15

g/L

KOH

3.3

g/L

N2H4

0.8

g/L

pH

8.62

-

conductivity

5.57

mS/cm

Cs

1×10-3 *

mol/L

Sr

1×10-3 *

mol/L

* in case of the sorption test

Results

Sorbent stability in model solution


The statistical evaluation of measured data gives us insight on the whole group of tested materials. As a measure of the sorbent impact on the model solution quality the difference between initial and final values of pH and conductivity were used (see Figure 3, Figure 4). From the evaluation it is apparent that the majority of the tested sorbents doesn’t alter the solution pH value. However, by most of the sorbents the decrease in treated solution conductivity was observed. That’s likely due to the fact that sorbents are not selective for one particular cation and besides the target ions (Cs, Sr) exhibit as well affinity towards potassium ion, K+, (which contributes significantly to the solution conductivity due to its great specific conductivity). This may have an impact on decontamination efficiency due to the rather high potassium content and possible competition between K+ and Cs+ ions. 

Conclusions

Forty samples of sorption materials were tested for capture of caesium and strontium from model post-accidental solution of VVER reactor type. For all samples Kd value was determined for caesium and strontium respectively. Moreover, the chemical stability of sorbents was tested by one-week contact test with model solution. Sorbents were ranked based on their performance in Cs (respectively Sr) capture and based on their stability in the model solution.

Яндекс.Метрика