1. Boric acid

Boric acid (molecular formula: H3BO3), an inorganic acid, is a white powder or transparent crystallized substance that is soluble in water. In nuclear power reactors, boric acid (BA) is dissolved in the reactor coolant and used as a soluble reactive control agent.

2. Functions of boric acid in reactors

BA in nuclear power plants is to control the speed of nuclear fission. In general, the amount of neutrons determines the speed of fission as the boron absorbs the neutrons. B-10 (one of boron’s isotope) is a good neutron absorber. Normally, B-10, in the form of boric acid, exists in the reactor to control the speed of fission.

Boric acid (BA), after being prepared in the REA system, enters the CVCS system through the RCV loop. Then CVCS controls the reactivity by regulating the boric acid’s concentration. In such a way, CVCS controls the reactor capacity and maintains its safety. 

2.1 REA

In REA, Two different amounts of BA, 7000mg/kg and 12000mg/kg, are made by mixing crystallized boron with the desalted but not deoxidized water that comes from the demineralized water system on the nuclear island. 7000mg/kg BA will flow to the RCV tank, while 12000mg/kg BA will flow to the boron injection tank (BIT) where BA can shut down reactor in case of accident.

2.2 RCV

In RCV, three tanks contain 7000mg/kg BA. BA in this loop will 1) flow to CVCS after it has been degassed and desalted and 2) flow to the containment spray system where BA aids in decreasing the temperature and pressure in containment. 

2.3 BA controls reactivity

In CVCS, BA functions as a chemical shim. At the beginning, reactor’s positive reactivity is strong, so BA’s concentration is high. As reaction proceeds, the concentration of BA goes down and reactor is in criticality. If there is an accident, high-concentration BA can be injected and reactor will start the negative reaction.

BA’s concentration influences the temperature coefficient (T.C.) of moderator. When BA’s concentration is high, T.C. is positive, which is not good for the operation safety. In order to make sure the reactor’s safety, the maximum concentration of BA in coolant is 1400ppm when reactor works.

Advantages of using BA to control reactivity

As BA is dissoluble in water, no extra space is needed for BA to absorb neutrons. It simplifies the reactor core’s configuration and the top structure of reactor pressure vessel (RPV).

BA is evenly soluble in moderator and avoids the unevenness of neutron-flux density in the reactor core due to the use of control rods.

The use of BA allows drawing out all control rods when reactor operates so that reactor core power can be evenly distributed.

Disadvantages of using BA to control reactivity

Normally, it takes several minutes to regulate the concentration of boric acid via pouring concentrated BA or pure water into the primary loop. So this method is slow in controlling reactivity. 

3. EBA and its application

Natural boron contains two stable isotopes, namely B-10 and B-11 with respective enrichment of 19.78% and 80.22%. B-10’s absorption of neutron is much larger than B-11. B-10’s cross section for thermal neutron absorption is 3837 barns while the B-11’s is only 0.005 barns. B-11 is of no use as a neutron absorber. Hence, the enriched boric acid (EBA) with higher B-10 will be more popular. Currently, EBA, replacing natural boric acid (NBA) to apply to EPR rectors, has been used in many French, Finnish and Chinese EPR plants.

Compared with NBA, EBA can control the rector better because it increases B-10’s concentration in reactor coolant system, but decreases the amount of boric acid utilized. On one hand, higher concentration allows stronger reactivity and higher enrichment of uranium-235. In this case the burning of MOX fuel will increase and fuel’s cycle will be extended. On the other hand, the decrease of boric acid will lower down sedimentation and crystallization, increase heat exchange, controls the pH value more effectively, relieve corrosion, reduce radiation exposures to employees, improve the safety of NPP and optimize NPP’s operational cycle.

Even though EBA has distinctive advantages, its high costs hinder its application in nuclear power plants. At present, the price for EBA with 92% B-10 is between 2-3 dollars/gram, while that for the natural boric acid is only 0.001 dollar/gram. The reason why EBA is so expensive rests with the costly concentration process of B-10. So how to concentrate B-10 at a low cost will be the focus of future research.