NEACRP-A-746
Analysis of Nuclear Characteristics of Chernobyl Reactor by WIMS-ATR Code
T. Wakabayashi and N. Fukumura
September 1986
Power Reactor and Nuclear Fuel Development Corporation Oarai Engineering Center 4002 Narita, Oarai-machi, Ibaraki-ken JAPAN I. Introduction
The RBilK-1000 nuclear power reactor is a graphite-moderated,boiling-light- water-cooled, pressure-tube-type thermal reactor in which uranium oxide slightly enriched in 235U is used as fuel.
An accident occured at the fourth unit of the Chernobyl’ nuclear power plant
with damageof the reactor core on April 26, 1986. The USSRState Commitee on the Utilization of Atomic Energy reported the
causes of the accident in the IAEA experts’ meeting on 25-29 August 1986 in Vinenna”. In the present study, nuclear characteristics ~of Chernobyl reactor, especially
coolant void coefficient of reactivity, temperature coefficient of graphite and isotopic composition of spent fuel, were analyzed using WIK-ATR code improved by
Fugen operation data and critical experiments in DCA (Deuterium Critical Assembly).
a
-t- II. CALCULATIONALMETHOD
The WI%-D” , and the CITATIONa’ codes were used to calculate coolant void coefficient of reactivity, temperature coefficient of graphite and isotopic composition of fuel burned to 20 #d/Kg.
The WIMS-Dcode is a general lattice cell program that uses transport theory to calculate flux as a function of energy and position in a cell. The basic cross-section library is in 69 groups with 14 fast, 13 resonance and 42 thermal groups. The transpot equation is solved by a collision probability method using up to 69 neutron energy groups. The WINS-D gives D, c a, c rem, C f and K-infinitive for the whole unit cell. The group constants are given in few energy groups suitable for use with other computer codes such as the CITATION.
Besides producing few group cell-averaged constants, point-by-point reaction rates over the entire energy range are calculated for detectors such as plutonium and uranium. - Evaluation and adjustment 4, to the WIBS nuclear data library for each uranium and plutonium isotope have been preformed using Fugen operation data and results of micro-parameter experiments in DCA. The code with new library was named the
WINS-ATR. The coolant void coefficient of reactivity and temperature coefficient of graphite were calculated in two-dimensional diffusion theory of two groups. The two dimensional (X-Y) model of Chernobyl reactor was employed for the present
calculation using the CITATION. Table I shows the reactor data of Chernobyl nuclear plant.
10160003 -2- IlI. Results Calculated results of coolant void coefficient of reactivity and temperature coefficient of graphite are shown in Figures 1 and 2. The results of WI&ATR and CITATION calculation agree well with the data in
Reference 1. Table II shows isotopic composition of uranium oxide fuel burned to 20 UWd/Kg.
The calculational result is in good agreement with the data in Reference 1. Figures 3 and 4 show neutron spectra in fuel and graphite as a function of neutron energy in RBMK-1000reactor. It is understood from the Figure 3 that increasing coolant void shifts neutron spectra harder.
REFERENCES 1) “The Accident of the Chernobyl’ Nuclear Power Plant and its Consequences”, Information Compiled for the IAEA Experts’ Meeting, 25-29 August 1986, Vienna.
2) J.R. Askew., et al., “A General Description of the Lattice Code WIllS’, J. of
British Nucl. SOC., 5, 564 (1966).
3) T.B. FOWLER,D.R. VONDYand G.W. CUNNINGAM,“Nuclear Reactor Core Analysis Code: CITATION”, ORNL-TM-2496Rev.2 USAECJuly(1971).
4) T. Wakabayashi., et al., “Characteristics of Plutonium Utilization in ATR,”
NEACRP,1985.
-3- 10160004 Table I Reactor Data of Chernobyl Nuclear Plant
Thermal power 32OO;W
Number of fuel assembly 1659
Xumber of fuel element in a fuel assembly 18
Diameter of fuel element 13.6mm
Fuel enrichment 2.0% 0
Diameter of Pressure tube 88mm
Lattice pitch 250mmX250mm
Fuel burn-up 20llWd/Kg
Mass of uranium in fuel assembly 114.i’Kg a Core average burn-up (25 April 1986) 10.3MWd/Kg
l0160005 -4- Table II Is~otopic Composition of Fuel Burned to 20.0 MWd/Kg of Chernobyl Reactor (Kg/t HM)
Nuclede WIMS-ATR Ref. 1
235 u 4. 8 4. 5 236 lJ 2. 5 2.4~ z39 P u 2. 4 2. 6 ZdO P u 1. 8 1. 8
24’ Pu 0. 6 0. 5 L
~0160006 -.5-- l
2.0 -
1.0 -
0.0 h I 0 i0 40 60 ab 0
Coolant Void (%I
Figure 1 Coolant Void Coefficient of Reactivity of Chernobyl Reactor
(Burn-up : 10.3 MKd/Kg)
10160007 -6 - .
1O.C
o.o- 9 450 Sk0 640 7 I
Graphi te Temperature (“C)
Temperature Coefficient of Graphite of Chernobyl Reactor (Burn-up : 10.3 Mkd/Kg)
- 7- 101600~8 VOID: 0% VOID: 40% - VOID:10096
0” -A 0’ ch ENERGY ( EV 1 0 0 Figure 3 Neutron Spectra in Fuel Pin of RIIMK-1000 versus Neutron Energy
(Burn-up : 10.3 MWd/Kg) VOID: 0% VOID: 40% N VOID:10096
ENERGY ( EV I
Figure 4 Neutron Spectra in Graphite of RBMK-1000versus Neutron Energy (Burn-up : 10.3 MWd/Kg)