ISSN (0970-2083)

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National Research Tomsk Polytechnic University (TPU), 634050, Tomsk, Russian Federation

- *Corresponding Author:
- A. Naymushin

National Research Tomsk Polytechnic University (TPU), 634050, Tomsk, Russian Federation

**E-mail:**[email protected]

**Received date:** July 06, 2016; **Accepted date:** August 19, 2016

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Journal of Industrial Pollution Control

Thorium, Thermal-neutron reactors, Neutron-physical computations, Nuclear fuel cycle, Neutronic code.

The explored resources of thorium in the earth crust are several times higher than the reserves of uranium and this significantly increases nuclear energy’s resource base in case of implementing closed nuclear fuel cycle.

30 isotopes of thorium are known as well as 3 excited
metastable states of its nuclides. Only one isotope
of thorium (thorium-232) has sufficiently long
half-life with respect to the Earth’s age, therefore,
almost all of natural thorium consists of this nuclide
only. Th^{232} itself is not fission by thermal neutrons,
but absorption of neutron by thorium-232 leads
to generation of uranium-233, which has high
probability to emit neutrons as a result of fission
by thermal and intermediate neutron fluxes.
Therefore, it plays the same role in a nuclear reactor
as U^{238}: nuclides absorbing neutrons transform into
secondary nuclides, which can fission by thermal
neutrons.

Thorium-232 is the best “resource” isotope compared with uranium-238 for the reactors with a thermal neutron spectrum. Uranium-233 emits more than two neutrons per capture of one primary neutron for wide range of reactors with a thermal neutron spectrum.

Thorium dioxide has greater chemical and radiation resistance compared with uranium dioxide, as well as better thermal-physical properties (thermal conductivity, linear expansion coefficient).

Secondary nuclear fuel in thermal-neutron reactors
is two isotopes of plutonium: Pu^{239} and Pu^{241}. The
former is generated as a result of absorption of
thermal and resonance neutrons by U^{238} nuclei, the
latter is generated as a result of double radiative
capture of neutrons by Pu^{239} nuclei. The scheme of
this process is as follows:

Uranium isotope U^{233} is the secondary nuclear fuel
in thorium thermal-neutron reactors. The scheme of
this process is as follows:

U^{233} has big value of η_{eff} coefficient, which represents
the number of secondary neutrons per one neutron
absorbed by nuclear fuel:

When transitioning from uranium to thorium fuel
cycle, production rate of long-lived minor actinides
significantly decreases in a nuclear reactor. If thorium
reactor operates exclusively in a U^{232}-Th fuel cycle,
actinides with masses higher 237 will accumulate in
negligible amounts in the reactor.

Isotope U^{232} draws special attention in thorium-based
cycle. It is generated by means of (n,2n) reaction
taking place on Th^{232}, Pa^{233} and U^{233} isotopes. Half-life
of U^{232} is 69 years. Among its daughter products are
Tl^{208} – isotope with very short lifetime emitting hard
gamma particles (2.6 MeV) (Belle and Berman, 1984).

Dose rates in thorium fuel will rise due to
accumulation of U^{232}. This creates additional
problems when dealing with spent nuclear fuel of
thorium reactors, in particular, when re-cyclization
of uranium takes place. However, at the same time,
presence of U^{232} is spent fuel increases proliferation
security of this nuclear cycle. Besides, thorium fuel
cycle is more preferable with respect to utilizing
weapon-grade plutonium since it does not lead to its
reproduction as in case of using U-Pu fuel cycle.

The “classic” drawback of thorium fuel cycle is
relatively high half-life of its intermediate product
Pa^{233} (27 days), which is an order of magnitude higher
than of Np^{239} (2.36 days). As a result, a significant
equilibrium concentration of Pa^{233} is generated in
thorium-based reactors and due to radiative capture
on its nuclides, further neutron losses take place.

A protactinium effect shall surely be observed
in thorium-based reactors. This effect is similar
to the neptunium effect in fast-neutron reactors
with uranium or uranium-plutonium fuel but is
worse in terms of control. Rise of reactivity during
extended shutdowns caused by fission of Pa^{233} and its transformation into U^{233} should be taken into
consideration when designing projects of thoriumbased
reactors.

Works on exploring the possibilities of using thorium in nuclear fuel cycle are based on either high reserves of thorium (India), or desire to reduce the consumption of nature uranium (Norway), or nuclear energy technology capable of taking advantage of thorium fuel cycle (Canada, Russia).

**Compution Model of the vver-1000 Reactor Cell**

Computation model is an elementary cell of the
VVER-1000 reactor with infinite height, consisting of
a fuel element surrounded by water coolant. During
the computation, real hexagon cell is replaced by
equivalent cylindrical cell with cross-section of a real
one (**Fig. 1**-**4**).

Fuel element’s core (zone 2) with a radius of 0.39 cm has inner hole with a diameter of 0.07 cm (zone 1). Zone 3 represents fuel element’s cladding; zone 4 represents water coolant (and moderator).

Fuel element’s cladding of the VVER-1000 reactor is made from zirconium alloy with the outer diameter of 0.91 cm and thickness of 0.65 cm. Fuel pellet made from uranium dioxide has the outer diameter of 0.78 cm and axial hole with the diameter of 0.14 cm. Placement step of fuel elements is 1.275 cm.

Material composition of the basic description of fuel
elements is represented by uranium dioxide with
density of 10.5 g/cm^{3} and 4.5 % U^{235} enrichment.
Cladding is made from zirconium alloy with 1 %
niobium. Water density corresponds to pressure of
16 MPa and temperature of 300°С.

**Comparision of Software for carrying out
Compution**

Major aspect of carrying out neutron-physical
computations of thorium-based fuel assemblies is
difference of values of interaction cross-sections in
evaluated nuclear databases; therefore, it is required
to carry out comparative analysis of different
software resources suitable for the computation,
which use different databases (Chadwick *et al*., 2006).

To carry out neutron-physical computations the following software resources were chosen:

• One-dimensional cell software WIMS-ANL
with 69-group ANL library obtained on the
basis of ENDF/B-VI evaluated nuclear data
(Deen *et al*., 2004);

• One-dimensional cell software WIMSD- 5B with the library based on ENDF/B-VI.7 evaluated nuclear data (Halsall, 1996);

• Three-dimensional precision software MCUPTR with MDBPT50 library based on ENDF/ B-VII.0 evaluated nuclear data (Alekseev and Gomin, 2011).

To determine special features of the computation in
different software, a model of the VVER fuel element
with UO_{2} based fuel with 20 % enrichment was
chosen.

**Fig. 5** shows reactivity margin vs. a reactor operating
cycle obtained in different software.

It can be noted that all three software resources show results of high convergence when estimating length of reactor operating cycle. Differences between the results obtained in MCU-PTR and WIMS-ANL does not vary more than 1.5%; in MCU-PTR and WIMSD- 5B – no more than 2%.

During computation of thorium-based fuel compositions besides determining the length of reactor operating cycle, major aspect is the correct computation of concentration of the nuclides involved in nuclear chains of reproducing materials transforming into fissionable materials.

**Fig. 6** and **7** show concentration (obtained from
different software) of the following nuclides during
the operating cycle: U^{235}, U^{238}, Np^{239}, Pu^{239}.

It can be noted that all three software quite precisely determine concentration of elements of the fuel composition. However, it is observed that the results obtained by means of WIMSD5B software significantly deviate from the results obtained by means of MCU-PTR software. At the value of fuel burn-up of 100 GW*days/tU and higher, WIMSD-5B software overstates the concentration of Pu-239 by 20-25%.

Main problem of exploring research of using thorium in fuel cycles is accuracy of library based on evaluated nuclear data. For example, error of microscopic cross-section for some reactions can reach about 50 %.

Our calculations show that, more suitable software for neutronic calculations of thorium-based fuel cycles is the three-dimensional precision software MCU-PTR with MDBPT50 library based on ENDF/ B-VII.0 evaluated nuclear data. High precision results also shown by one-dimensional cell software WIMS-ANL with 69-group ANL library obtained on the basis of ENDF/B-VI evaluated nuclear data. So, neutron-physical computations were carried out by means of one-dimensional cell WIMS-ANL software with 69-group ANL library. Specifying computations were carried out using MCU-PTR precision software.

This work was performed on the unique scientific IRT-T equipment and financially supported by Government represented by the Ministry of Education and Science of the Russian Federation (RFMEFI59114X0001).

Belle J, Berman R. 1984

Chadwick MB, Obložinský P, Herman M, Greene NM, McKnight RD, Smith DL, Van der Marck SC. 2006. ENDF/B-VII.0: Next Generation Evaluated Nuclear Data Library for Nuclear Science and Technology. Nuclear data sheets

Deen JR, Woodruff WL, Costescu CI, Leopando LS. 2004. WIMS-ANL User Manual.Rev

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