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Sorption Membranes for Waste Solidification

#1595


The Development of Nonconventional Sorption - Membrane Technology of Rendering Liquid R/A Wastes from NPP and Nuclear Submarines Harmless and Solidifying Them into Water - Resistant Mineral Geocement Stone

Tech Area / Field

  • ENV-RWT/Radioactive Waste Treatment/Environment

Status
8 Project completed

Registration date
29.08.1999

Completion date
05.03.2004

Senior Project Manager
Mitina L M

Leading Institute
FEI (IPPE), Russia, Kaluga reg., Obninsk

Collaborators

  • Argonne National Laboratory (ANL) / West, USA, ID, Idaho Falls

Project summary

The objective is to develop an alternative unconventional technology and to construct a pilot facility in order to reprocess NPP and submarine LRW into water resistant mineral-like materials.

Tasks:


- to simplify considerably the technological process of reprocessing medium-active (105-109 Bq/L) medium-salt LRW into a solid state;
- to reduce the volume of solid radioactive waste produced in the result of LRW reprocessing which is subject to a long-term storage or disposal;
- to prevent the accumulation of radioactive still bottoms and to reduce their collected volume;
- to use cheap and accessible natural mineral raw materials applicable for the subsequent immobilization into the mineral-like materials for radionuclide sorption from LRW;
- to use easily - recoverable ultraporous membrane filters for separation of mineral raw material suspension;
- to use metallurgical production wastes with the aim to immobilize mineral raw materials which contain radionuclides into the rock-forming mineral, i.e. alkaline and alkali earth hydroalumosilicate;
- to generate a mineral-like stone with a significantly improved water resistance (by 2-3 orders of magnitude) and mechanical strength;
- to facilitate significantly further water-salt solution management after Cs-137 and Sr-90 have been sorbed, as these radionuclides make the principal contribution into the LRW radioactivity.

The Analysis of Technologies Being Applied

1. Conventional technology

The NPPs liquid radioactive wastes (LRW, liquid RAW) refer to the wastes of low and medium level of activity. Their specific activity does not exceed 3,7ґ1010 Bq/L.

All the NPPs LRWs are approximately the same in terms of their chemical composition and so the flow-sheets of their reprocessing are actually very similar. The technology consists of the following main stages:


- the homogeneous waste is supplied to input tanks with the subsequent mixing, averaging, pH correction (up to pH=10), filtration (in case of a precipitate formation).
- next comes boiling down which is performed in two steps (the first step is carried out in order to purity water, the second one - to concentrate the still bottoms LRW of the boiler used at the first step and to send concentrates to the LRW storage facility).

The LWR averaging in the input tank is usually aimed at producing solutions with their salt concentration of 1 g/L and specific activity up to 106 Bq/L. After the first step of boiling down the solution salt concentration can increase up to 25 g/L. Additional boiling down at the second step results in concentrates with their salt concentration up to 650 g/L [1,2].

Concentrated radioactive solutions are stored in the tanks of LRW intermediate storage facility for a rather short period of time as there is a potential risk for the structural materials to lose their tightness because of the corrosion, thus resulting in the environment contamination.

The final stage of LRW reprocessing is to transform concentrates into the solid state with the subsequent disposal.

There are several groups of problems encountered in the course of implementation of indicated technology for reprocessing NPP LRW of low and medium activity:


- to perform a multistep concentrate purification of the volatile impurities of cesuim, iodine, ruthenium radionuclides as well as oils and surface-active substances, filter recovery included;
- to develop special conditions of boiling down and additional boiling in order to reduce the deposition rate on the boiler heating surfaces at the first (scale deposits) and second stages («encrustation» with partially soluble sodium salts) and on the still bottoms supplying pipelines coming from the additional boilers; as well as in order to prevent concentrate foaming and high-active foam ingress into the condensate in the course of reprocessing radioactive water which contains surface-active substances; to develop the procedure of washing the indicated equipment of the scale deposits.
- the equipment is complex and expensive.

So the introduction of those sorption methods which have not been extensively applied yet because of their high cost and sorbent deficiency and which are meant for the preliminary RAW purification of radionuclides will make it possible to decrease total labour cost due to simplification of NPP LRW reprocessing technology and equipment.

2. Prototype (an alternative option described in literature)

The method of LRW reprocessing by purification on selective sorbents which have been preliminarily treated with various oxidizers with the subsequent cementing of spent sorbents (4) is the technical solution which is the closest to the one proposed by the authors (3). The disadvantages of this technique consist in the following: a) it requires the use of additional equipment, chemical agents and materials in order to oxidize LRW and to capture radionuclides which are released together with oxidation products, b) it requires the use of expensive synthetic sorbents, c) the volume of generated solid radioactive waste is reduced rather insignificantly, d) the cement compounds demonstrate a relatively low water resistance.

In order to overcome the listed drawbacks certain proposals are put forward in order to simplify the technology and make it cheaper due to the use of some natural materials as sorbents as well as to improve the LRW reprocessing safety at the sacrifice of LRW oxidation stage elimination. This task can be solved by means of the proposed method of LRW reprocessing with radionuclide sorption on natural sorbents under static (steady-state) conditions. The filtration sorbent is supposed to be separated through the ultraporous membrane filters with the subsequent cementing of the spent sorbent with the use of dry components of the binding system which contain the blast furnace granulated slag and clay material, and sodium silicate solution. This composition of the binding system allows a new type of cement, i.e. slag-alkaline cement (geocement), to be produced.

3. The essence of the proposed technology

The technology proposed can result in a comprehensive solution of the problem: a sorption of radionuclides by means of unconventional accessible and cheap natural fine-dispersed minerals without any preliminary LRW treatment; the sorbents are separated by filtration through easily-recoverable ultraporous membrane filters; the sorbents are cemented together with slag by means of unconventional slag-alkaline binding system into mineral-like materials, i.e. hydroalumosilicates which have increased water resistance and mechanical strength.

Introduction (implementation) of this technology will solve the above-mentioned problems in the field of LRW management and its long-term storage or disposal including ecologically safe isolation from the environment.

4. Competence of Project participants

The experts involved in the work under the project have at least 10-15 year experience in the following fields:


- development of sorption methods of liquid media purification;
- development of dispersion media partitioning and purification;
- LRW solidification by means of cementing
- radioactive waste reprocessing and storage control.

5. Expected results and their application

To construct two demonstration facilities with the aim to reprocess LRW with the volume of 1 m3 and to allow these facilities to be tested in Russian Federation (special water purification IPPE shop). These facilities will be the basis for developing production capacities for the alternative standard reprocessing of LRW at NPPs of Russian Federation and CIS countries.

6. Fulfilment of ISTC targets and tasks

This project is in a complete agreement with the fulfilment of ISTC principal targets and tasks:


- it offers the possibility for scientists and experts of Russian Federation whose activity was related to weapons and especially those who have knowledge and experience in the field of mass distraction weapons and missile delivery systems to use their abilities for the peaceful activity;
- it supports fundamental and applied investigations and technology development in peaceful purposes especially in the field of environment protection, energy generation and safety assurance in nuclear power;
- it helps to solve national and international technical problems (besides those indicated above);
- it supports the conversion to market economy, which meets social needs.

7. Scope of Work

In order to develop the technology and to construct the demonstration model the following scope of work should be performed:


- to test experimentally the optimum parameters of Cs-137 and Sr-90 sorption, as these radionuclides make the main contribution to the LRW radioactivity. Sorption should be carried out under the static (steady-state) conditions up to the level required by the Regulatory Document of Russian Federation (NRS-96, Norms of Radiation Safety) with the application of natural accessible and cheap fine-dispersed materials used as sorbents. They include mineral silicate rock whose deposit is available in Kaluga region (tripoli, clinoptilolite);
- to develop the technology of membrane filter fabrication based on a porous material. A fine-porous filtering membrane is applied on to this material by means of a plasma-chemical synthesis technique. The field of application: water - salt solution purification.
- to test the optimum parameters of partitioning a sorbent and a water-salt solution by means of filtering membranes;
- to test the parameters of generating a stable sorbent suspension in solution;
- to test the parameters of filter recovery and its life-time;
- to test the parameters of sorbent transfer from the filtering part to the cementing vessel;
- to develop the formula of a binding system in order to form geocement;
- to develop the parameters of sorbent immobilization into geocement;
- to examine geocement properties;
- to design, fabricate and test working sections for sorption-membrane purification of water-salt solution with the volume up to 0,5 L and cementing;
- to develop a task order and to design a demonstration model for LRW reprocessing (with the volume of 1m3);
- to construct, assemble and commission the demonstration model;
- to test two demonstration models;
- to develop general recommendations on water-salt solution management.

8. Role of foreign collaborators


- to exchange information in the course of project implementation;
- to make comments to technical reports submitted by the project participants;
- to participate in technical inspections of the activity under the project;
- to render the assistance and financial support to the project participants in terms of their participation in international meetings;
- to assess the technology developed under the project.


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