LIU Jingjing, ZHANG Weijiang, XU Jiao

(School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China)

Abstract: With the progress of modern science and technology, the demand for enriched 10B is becoming greater. Particularly, nuclear power reactors require large amounts of enriched boron-10 acid. However, currently the use of enriched boron-10 acid in China depends on import, thus limiting the development of advance materials in China. In this paper, the role of boric acid in the nuclear power plant and the important significance of recovering boric acid from nuclear waste are introduced. This article studies the principles on liquid-liquid extraction, ion exchange, membrane separation and evaporation as well as their respective advantages and disadvantages in the process of boric acid, recovery boric acid, and the recycling process of boric acid. Several feasibility programs and their applications were also elaborated.

Key words: boric acid; nuclear power p ants; liquid waste; recovery

Boron has two stable isotopes, namely 10B and 11B. 10B’s absorbing capacity is much stronger than 11B. So normally PWR power plants prefer 10B.

Advantages of using boron to regulate concentration in PWR are as follows: ① boron acid (BA) can be equably soluble in coolants (water) and it has no impact on neutron-flux distribution; ② chemical (containing BA) system and control rod system can be well matched to achieve the same results as a “rod-less” operation; ③ BA is stable enough to be compatible with other materials in the reactor; ④ it is convenient to add boron to the primary loop to shut down the reactor so as to ensure the safety in refueling. 

A 106kW nuclear power plant will discharge 132 m3 BA waste (BA’s concentration is 7 000 X 10-6) or 23 m3 preprocessed concentrated liquid waste (BA’s concentration is 40 000×10-6) every year. As it is important to dispose the radioactive liquid waste safely the solidification methods of radioactive wastes are utilized. Specifically, the cement solidification is a mature technology and widely accepted by nuclear power plants, nuclear departments and nuclear research centers in many countries.

But boric acid waste, dispersed in cement, hinders cement’s hydration reaction and restricts the amount of boric acid that can be contained in cement. For example, 200L solidified cement contains at most 10 kg boric acids.

In order to reduce the size of cement and lower down NPP’s operation costs, every country is eager to recover boric acid from liquid waste and reuse it.

1. Isolating technologies

1.1 Liquid-liquid extraction

Liquid-liquid extraction is a method to separate compounds based on their relative solubility in two different immiscible liquids. It is an extraction of a substance from one liquid phase into another liquid phase.

BA’s distribution ratio in organic phase is different from that in aqueous phase. Following steps can be taken: 1) to add an extracting agent to BA waste; 2) to extract BA to an organic phase, then 3) to extract the extracting agent by steaming or adding alkali lye. 

1.2 Ion exchange adsorption

Ion exchange adsorption is a method to separate boron from mixture by the adsorption of boron and iron adsorbent. Currently, ion exchange resin and dynamic ion exchange are widely used. But this method is only fit for disposing dilute solution because of adsorbent’s small capacity, low velocity and long cycle.

1.3 Membrane separation

Membrane separation is the process whereby mixtures are separated by solid membrane. Currently, in the nuclear industry, reverse osmosis and electro osmosis are extensively used to dispose radioactive liquids. Ultrafiltration membrane is used for the cooling of reactors. This method’s advantages include operation in normal temperature, no phase changes, high efficiency, no pollutions, etc. But due to the contradiction between the separating efficiency and the infiltration capacity, infiltration capacity is low if the separating efficiency is high and vice versa. 

1.4 Evaporation separation

BA becomes volatile at a certain temperature. The evaporation method is to evaporate BA from liquid according to the different distribution coefficient D (D=mole fraction at the vapor phase/mole fraction at the liquid phase) at different temperature and pressure.

2. Methods of recovering boric acid from nuclear power plant’s liquid waste

2.1 Crystallization

The BA waste in the reactor will be evaporated in an evaporator and some boric acid will be crystallized. The liquid with crystal then needs to be filtered. The filtered crystal will be acidized. The acidification is conducted at a temperature lower than 10℃ in an environment with pH value less than 2 (the less acidity, the less BA solubility). The acidized liquid will finally crystallized to pure BA (recycling). Recovering method is as shown in Chart 1. 

In the acidification process, lithium chloride can be added to increase the dissolve rate of boric acid according to the salting-out effect. It is very effective as 99.9% radioactive waste can be separated from BA. Additionally, the operation is not harmful and requires only simple equipment.

2.2 Extraction

BA liquid waste in NPP first evaporates some water in evaporator (the evaporation temperature is lower than 54.4 ℃) and gets the concentrated solution (mass fraction of BA is between 4% and 12%) which reacts with methyl alcohol and turns into boric acid ester compound. The boric acid ester and alcohol is azeotropic. So azeotrope can be fractioned. The boric acid ester mixed with water turns into BA. Hence, BA and alcohol are separated in the evaporator. The pure boric acid and alcohol can be used again and again.

For this method, it is better to choose methyl alcohol (mass fraction of methyl alcohol is 32%, tributyl borate is 68%, and the azeotropic point is 54.6℃) because the azeotropic point is lower and it is easy to operate. But related safety measures should be taken in operation because alcohol, although easy to evaporate and burn, is harmful to operators’ health and the surrounding environment.

2.3 Evaporation

BA waste, after pretreatment, BA evaporates to boric vapor under an appropriate temperature in the evaporator. The boric vapor enters distillation tower, then vapor is condensed at the top of the tower while BA flows out from the bottom of the tower. See chart 3.

Tritium is the only radioactive matter left in the boric vapor when this method is used. BA can undergo repeated evaporation and condensation till it reaches the requirements of nuclear boric acid. This method is simple, but the pipe orifice becomes easily blocked because the BA can easily be crystallized. Hence, this method requires an advance and efficient evaporator.

2.4 Gas stripping

BA waste, after pretreatment, enters the gas stripping tower. Inert gas enters from the bottom of the tower and brings out BA. The gas together with BA, after it has defoamed, can be separated in the washing tower. The inert gas flows out from the top of the tower and goes back into the gas stripping tower. The pure BA flows out from the bottom of the washing tower. See Chart 4.

3. Conclusion

As NPP take on different methods of recovering boric acid from nuclear power plant’s liquid waste, the requirement on recovering BA varies. The current methods of recovering BA are mainly extraction, adsorption, membrane separation and evaporation and perhaps new methods will be generated in the future. And as each NPP have varying conditions to consider, they should choose the most suitable recovering method favorable to them.