The first completed experimental work at WWR-M reactor

 R.F.Konopleva
 S.R.Novikov


    Authors of the first experimental work at WWR-M reactor: S.R.Novikov and R.F.Konopleva, as well as professor Van Lint V.A.J. (USA) and WWR-M reactor Head K.A.Konoplev   (from the left to the right).

   Following physical start-up of WWR-M reactor in the end of December 1959, reactor was prepared for operation at 10 MW power rating. The power was raised gradually by steps with intermediate stops. Researchers of "Non-equilibrium electronic processes in semiconductors" laboratory of A.F.Ioffe Physical-Technical Institute proposed to carry out measurements of fast neutron flux distribution in reactor beam holes with the use of semiconductor transducers.

   For the first time, simple enough method was used, which was based on variable electric conductivity of semiconductors during irradiation by fast neutrons.

   Normally, a so-called method of "threshold indicators" is used for relative measurement of fast neutron flux, with an application of sulfur foils, phosphorus and aluminium [2], which energy thresholds fall within 1.5 to 6 MeV. In addition, a method based on variable electric conductivity of semiconductors during fast neutron bombardment may be used.

   It is well known that during irradiation of semiconductors fast neutrons give rise to defects of crystal lattice, concentration of which is in proportion to neutron fluence. Generated defects lead to variation of charge carriers' concentration, i.e. to variation of electric conductivity. Defects of crystal lattice occur during irradiation of semiconductors by fast neutrons with energy exceeding some critical one. The value of critical energy depends on specific semiconductor crystal structure. For example, with reference to germanium, c = 300 eV. Defects may be also generated by -quanta, however the number of defects generated by one -quantum (N= 1.810-4) [3] is substantially less than the number of defects generated by fast neutrons (Nn = 1.6) [4]. As in the reviewed experiment the flux of -quanta (reduced to 1 MeV) was of the same order as the flux of fast neutrons, the portion of defects caused by -quanta can be neglected. Moreover, the rate of variation of electric conductivity is proportional to the density of fast neutron flux and we used this for measurements of relative fast neutron flux distribution in WWR-M reactor channels.

   The ratio of electronic germanium electric conductivity to fast neutron fluence is given in Fig.1 [5]. As it is clear from this Figure, electric conductivity varies first linearly with the flux. The fluence limit value, up to which electric conductivity variation remains linear is defined by initial resistivity of the specimen and can reach 1018 n/cm2 . Besides, the rate of electric conductivity variation is in proportion to intensity of fast neutron flux.

   Germanium specimens of n-type with specific resistivity of 1 W and dimensions of 10x1x1 mm3 were used as neutron flux transducers.

   Specimens encapsulated in cadmium covers of 0.5 mm thickness were placed at equal spacing from each other in reflector vertical beam channel along the height of reactor core.

   Within this study, measurements were completed in beam holes, where thermal neutron /fast neutron flux ratio was 10 maximum, and cadmium shield of 0.5 mm thickness was sufficient to reduce contribution of thermal neutrons to electric conductivity variation up to a value of approximately 10%.

   Electric conductivity of specimens was measured in the process of irradiation using the change of the current, with dc voltage applied to specimen being constant.

   Fig.2 depicts relative distribution of fast neutron flux in one of reflector vertical beam holes. Fig.3 depicts relative distribution of fast neutron flux in all experimental beam holes at the level of reactor core center. Such a distribution is obtained for the lay-out of the reactor core shown schematically in Fig.4. For comparison, Fig.3 shows a curve of relative distribution for fluence of resonance neutrons obtained by activation of golden foils.

   Thus, the method described enabled us to use simple enough approach to measure relative distribution of fast neutron flux with energy over 300 eV.

   This is the first experimental work, which was completed at the WWR-M reactor in July 1960 and was published in December issue of "Atomic Energy" magazine in 1961 [6].

Fig.1. Dependence of electronic germanium electric conductivity on fluence of fast neutrons.Fig.2. Relative distribution of fast neutron fluence in reflector vertical beam hole.
  
Fig.3. Relative distribution of fast neutron fluence in reactor experimental beam holes at the level of reactor core center:
  -measurement by activation of golden foils;
  - measurement by germanium electric conductivity.
    Fig.4. Lay-out of beam holes:
    1 vertical beam hole;
    2 reactor core;
    3 water cavity;
    4 beryllium reflector.

Reference

  1. E.Alexandrovich, M.Bartenbakh "Atomic Energy", 8, issue 5, 451 (1960)
  2. V.A.Dulin, V.P.Mashkovich et al. "Atomic Energy", 9, issue 4, 318 (1960)
  3. N.A.Vitovsky, T.V.Mashovets, S.M.Rivkin "Solid body physics", 1, No.9, 1381 (1959)
  4. I.W.Cleland, J.N.Crawford, J.C.Pigg Phys.Rev., 98, No.6, 1742 (1955)
  5. K.Lark-Gorovits Success of physical science, 50, No.1, page 51 (1953)
  6. R.F.Konopleva, S.R.Novikov "Atomic Energy" 11, issue 6, 546(1961)

   GO TOP