Опасная зона

Опасная зона

Sunday, February 5, 2012

Visiting a Nuclear Research Reactor

This is a continuation of the list "10 things a physicist should have done during his career", which started with the very first post of this blog.

  1. Split an atom
  2. see Cherenkov light

    Now point 3:
  3. operate a nuclear reactor
    (Note, doing this mostly results in fulfilling 1) and 2) )

I had this once-in-a-lifetime chance at our collaborators at the Department of Nuclear Chemistry at the Johannes Gutenberg University in Mainz, where they operate a 100 kW TRIGA mark II research reactor by General Atomics. They have ample amounts of experiments taking place at the reactor and at the neutron beam lines, such as Boron neutron capture therapy (BNCT), research of ultracold neutrons, do trace element analysis of mineral water and even wine.

The institute also offers a hands-on nuclear reactor training courses for students in nuclear chemistry and engineers from various companies (e.g. Areva). August 2011 I had a chance to realize this unique opportunity! I signed up for the 1-week practical course, which introduced me and 9 other (much younger) students to various topics around the nuclear reactor:
  • lectures on reactor physics and radioprotection
  • starting and shutting down check lists
  • calibration of control rods
  • fuel element inspection
  • power regulation
  • reactivity measurements
  • and various neutron activation experiments
It was awesome!

The TRIGA research reactor at Mainz.

First part of the course was to get confident with the infrastructure around the reactor core. Every morning a long check list must be completed before start-up of the reactor. The check list involved a thorough inspection of the cooling circuits, radiation monitors and ventilation system. (It took us 1-2 hours to complete it.)

First page of the check-list which has to be completed and duly signed every morning before the reactor is put into operation.
We checked the power supply and the emergency power supply, double redundant ventilation, primary and secondary cooling circuit (double redundant too), radiation monitors, quality of water, warning systems...

Primary cooling circuit, double redundant. If one pumping chain fails, another takes over.
I was a bit surprised how inconspicuous the heat exchanger was, it's a rather small device. However the reactor does operate with merely 100 kW at maximum, so the amount of heat is really limited.

The heat exchanger is seen to the left. It interfaces the primary (marked with red labels) and the secondary cooling circuit (marked with blue labels).

Apart of the regular checks, the course also involved some other tests, such as quality control and inspection of fuel elements, several types of sample irradiation as well as calibration of neutron detectors and other reactor parameters.

The reactor core viewed from the platform on top of the reactor structure. Water is about 6 meters deep.
In the picture above, several tubes and rods can be seen sticking into the core. There are three control rods, two which adjust the power of the reactor and a third rod which is used for pulsed operation of the reactor. Several additional tubes are used for sample transport. Four horizontal beam lines (also visible in the picture above) are available for neutron extraction, and a special channel provides thermal neutrons for e.g. the BNCT experiments.
In between the long rods the fuel elements can be seen. A 30 cm thick graphite reflector surrounds the core.

Gone fishing. Here I try to put an Americium-Beryllium neutron source next to one of the neutron counters for a calibration test.
The water in the reactor pool has extremely high purity, and does not contain any trace elements which can become activated. Therefore no gloves are necessary when messing around with reactor pool water.

Using a waterproof video camera, we could visually inspect details of the reactor core, while it was shut down. In the picture below the top of some fuel elements can be seen and one of the control rods to the left.
Fuel elements are categorized as low-enriched or high-enrighed uranium (LEU or HEU, respectively). High enriched material is difficult to handle bureaucratically, since it is potentially weapons grade quality.  The fuel elements at TRIGA reactors are all LEU, but enriched to the maximum allowed ~20% Uranium 235. As a comparison, fuel elements for normal power production are enriched to about 5% U-235. A few grams of weapon grade uranium are still used for the neutron detectors.

We were allowed at some point to hold an unused fuel element, which are about 70 cm long, diameter of 4 cm. They are not "hot" in either sense, but pretty heavy for their size. When tilting them you can hear the Uranium elements moving inside. Some of the fuel elements they store were from the decommissioned neutron cancer therapy project at the German Cancer Research Center in Heidelberg (DKFZ), I remember I had my office just next to the reactor building while I worked there. As the fuel elements deteriorate, they increase in length which is carefully logged. When the length increase is goes beyond a threshold, or if any other defects are visible, the fuel elements will be replaced. (The price of one fuel element is comparable with that of a sports car.)

Visual inspection of the reactor core with a waterproof video camera mounted on a 6 meter long stick which is immersed into the reactor pool. The top of four fuel elements are visible in the foreground. Note the sporadic white pixels in the screen which is caused by gamma photons from the radiation from the decay of the activated metal structure.

We all learned how to start and shut down the reactor. Operation happens from a control panel with very nice analog dials and buttons. I just love that!

In particular I found it interesting to get a feeling of how the power and neutron fluxes change as you adjust the control rods. It is a very non linear system with hysteresis in the sense that a state at a given configuration depends on how the reactor was operated before. Power drops due to reactor poisoning as a function of time, which again is compensated by extracting the control rods. Eventually an equilibrium is established.

Samples to be irradiated are transported with a pneumatic system to the reactor core.

Once the reactor is operating in continuous mode, samples can be inserted into the core, which are irradiated with high neutron fluences. Afterwards the produced nuclei can be measured by gamma-ray spectroscopy.

Control panel for the reactor. The three control rods are operated by the three button groups
in the center of the panel. Left is the pulse rod, then there are two additional rods for coarse and fine tuning the desired reactor power. Here both the coarse and fine tuning control rods are extracted half way. (Or half inserted, depending on what kind of person you are.)

Prior pule operation, the reactor is put to 50 W level (i.e. producing less heat than a conventional light bulb). Pulse operation involves that one of the control rods (the “pulse rod”, specially designed for this purpose), is shot out of the core. The reactor becomes promptly critical, reaching a few megawatt for a split second, but due to the negative temperature coefficient of the reactor, the heat immediately makes the reactor sub-critical again. The result is a short intense spike of neutrons which is used for various experiments, and beautiful flash of Cerenkov radiation.

The negative temperature coefficient is a special feature of TRIGA reactors, which make them particularly safe to work with, since they simply cannot  meltdown. Any nuclear reaction simply stops if the temperature of the fuel elements goes above a few 100 degrees C.

Anyway, this was a very interesting course, I framed in the diploma I got, its hanging in my office. :)