Inside the reactor core

11. 5. 2019

Temelín performed an outage at its Units 1 and 2 in March and April. Maintenance includes 15,000 operations.

The Temelín nuclear power plant ranks among the installations with the toughest security measures in the country. Not everyone can access its heart, the nuclear reactor. MF DNES had an opportunity to look inside during the Spring outage of Unit 1.

V jádru reaktoru

Once a person has been cleared, they need to pass through several checks. The first one is at the gate to the power plant. While this is a routine, “airport-type” security check, a radioactivity reading is taken to make sure people do not bring it inside. In other words, this is a measure to see whether the visitors have brought it with them and did not “pick it up” inside the plant.

They say it is quite uncommon for the meter to beep here; however, Marek Sviták, the plant’s spokesperson, remembers a rather quaint situation when it happened with a member of a contractor company. The instruments read values which slightly exceeded the standard natural dose of radioactivity.

The person would undergo several inspection scans in which plant personnel identified specific radionuclides in his body. The man later stated he had eaten some boar meat from an animal which may have presumably consumed food in a location affected by the Chernobyl radioactive cloud 33 years prior.

Before admission into the security area of the power plant, everyone is issued bright yellow overalls, special boots, and a hard hat. The color is said to have been selected on purpose. All clothes must be returned upon leaving the plant and should someone forget, the bright yellow would remind them. The next check station issues a dosimeter which continually monitors the radiation dose.

36.6 meters above ground

The circular reactor hall is at an elevation of 36.6 meters above ground and is accessed via an elevator. It is a busy place during the outage: fuel is being changed and thousands of inspections are carried out, including those of safety systems. Some of the tasks consist of repairs or modernizations – for example, this year’s outage at Unit 1 tested new spent fuel casks. These casks will be used in live practice during the summer outage at Unit 2.

Fuel is replaced once a year during the outage; the best timing is in the spring and summer when the grid has a lower consumption of power. Unit 1 was put into outage in March and April; the reactor in Unit 2 is scheduled for the same in mid-June. Usually one-fourth of the fuel is replaced. On this occasion, the replacement in Unit 1 is slightly higher. The engineers have replaced 48 of a total of 163 fuel assemblies.

Together with fuel manufactured by TVEL in Russia, six new fuel assemblies made by Westinghouse Electric Sweden were installed. ČEZ wants to put these to the test before it organizes a tender for fuel supplies after 2023, when the current contract for fuel from the existing supplier expires. The engineers from Temelín like to compare uranium to coal. In the past year of continuous operation before the outage, Unit 1 produced 9.4 TWh of electricity. It used 20 tons of uranium. To deliver the same amount of electricity, coal power plants would have consumed 8.5 million tons of coal.

The fuel assemblies are kept in the reactor for four years, but they are relocated inside the pressure vessel during the outage. Fuel replacement is conducted under water. Sviták explains: “We change the assembly positions so that the heat as well as neutrons inside the reactor are distributed evenly.”

The fuel loading process takes four days to complete. The engineers at Temelín NPP use a special, remotely-controlled fuel handling machine. Its maximum speed borders on 30 centimeters per second. However, the actual speed during fuel loading is much lower. The handling machine inserts each fuel assembly into its position inside the reactor at a speed of 3 centimeters per second. Every movement is observed by operators with a special camera.

After four years inside the reactor, spent fuel is put to “rest” and left to cool for up to 10 years in the “pool” in the reactor hall, then it is placed in special casks and put into intermediate storage. The casks for spent fuel are painted blue and manufactured by Škoda JS in Pilsen. Each cask weighs 105 tons and holds 19 fuel assemblies for up to 60 years.
It is Škoda JS who conducts most of the maintenance activities in the primary circuits of the country’s nuclear power plants. Using special tools, the engineers participate in dismantling of the reactor, a task which takes four days to complete with all tests and inspections. This is performed after the fission has stopped and the reactor has cooled down.

Reactor assembly after the insertion of fuel also takes from four to five days; the reactor head is held shut by 54 studs. A giant tensioner is used to tighten them. Everything is assisted by a massive yellow polar crane which is installed inside the reactor hall. The crane is also used for transporting assemblies with new nuclear fuel into the reactor hall and to remove assemblies filled with spent fuel.

Pressure tests

Hundredths of millimeters matter. “The final stage of reactor assembly when joints are sealed is critical for accuracy and human error. All sealed joints are carefully inspected. Whenever leakage is indicated, the seals must be reinstalled and all tests run again,” says Petr Zapletal, Manager of the Temelín NPP Center at Škoda JS. However, he says this rarely happens.

Tightness of the primary circuit, including the reactor, is tested under pressure. “The first series of tests are cold pressure tests which use a pressure of 3 MPa. In the next stage, hot pressure tests are performed using pressures over 17 MPa, which is higher than the operating pressure,” says Sviták.

This year, after the fuel was replaced, the engineers also tested the tightness of the containment, the protective building that surrounds the reactor. The containment must withstand the effects of a tornado or an earthquake. In layman’s terms, the test is performed by pressurizing the containment with air and measuring the pressure leaks. The containment is fitted with fifty sensors; the sum of all leaks must not be bigger than the equivalent of a pin hole, Sviták explains.

“The outage schedule consists of nearly 15,000 operations, which we must perform in under two months. Nearly a thousand people from some sixty contracting companies participate in the project.

Getting out of the reactor hall takes perhaps even longer than getting in. Any potential contamination is checked at a total of four stations. This may have become a routine procedure for the employees, but it takes visitors a while to find a good posture for the measurement to be performed successfully. All’s well that ends well; we are not carrying any radioactivity away from the power plant.

By Martin Petříček, MF DNES reporter, Source: Mladá fronta Dnes