Correction in Homogenization Methodology for Deformation of CANDU Fuel Channel

As the operational time increase, pressure tubes and calandria tubes in CANDU core encounter inevitably a geometrical deformation along the tube length. A pressure tube may be sagged downward within a calandria tube by creep from irradiation. Fuel pin bundle shape may also be changed by irradiation creep. The worst case from safety point of views must be the irregular deformation of fuel bundle in sagged tube. When a pressure tube at high temperature contacts with a calandria tube wall at low temperature, hydrogen spread and blister by gradient of temperature may also be formed. Then, a crack course damage of pressure tube can occur. This event can bring about a problem that is serious in integrity of pressure tube. A measurement of deflection state of in-service pressure tube is, therefore, very important for the safety of CANDU reactor. In this paper, evaluation of impacts on nuclear characteristic due to fuel channel deformation were aimed in order to improve nuclear design tools for concerning the local effects from abnormal deformations. Even though the rate of deformation may not be high, local effects can cause the loss of thermal margin in the aspect of operational and accidental safety. It was known that sagged pressure tube can cause the eccentric configuration of fuel bundles in pressure tube by 0.6cm maximum. In this case, adverse pin power distribution and reactivity balance can affect reactor safety under normal and accidental condition. Thermal and radiation-induced creep in pressure tube would expand a tube size. It was known that maximum expansion may be 5% in volume. In this case, more coolant make more moderation in the deformed channel resulting in the increase of reactivity. Nuclear impact from these two types of deformation mechanisms were analyzed by codes HELIOS and MCNP by comparing the K-infinite values of unit cell consisting of fuel bundle, pressure tube, calandria tube and moderator zone. Sagging of pressure tube did not cause considerable change in K-inf values. However, expansion of the pressure tube made relatively large change in K-inf. Modeling of eccentric and enlarged configuration is not easy in preparation of input geometry at both HELIOS and MCNP. On the other hand, there is no way to consider this deformation in one-dimensional homogenization tool such as WIMS code. The way of handling this deformation was suggested as the correction method of expansion effect by adjusting the number density of coolant. The number density of heavy water coolant was set to be increased as the rate of expansion increase. This correction was done in the normal intact channel without changing geometry. It was found that this correction was very effective in the prediction of K-inf values. In this study, further investigation was done in order to check whether this correction may be also effective in nuclear safety parameters such as coolant void reactivity worth, temperature feedback coefficients of fuel, moderator, and coolant. Pin power distribution in the eccentric fuel channel was also analyzed. HELIOS code was used to check the burnup effect up to the discharge rate of 7.2MWD/kg. CTC of deformed fuel channels increased depending on the degree of burn-up with positive values during the fuel lifetime. In comparison of CTC between the intact and deformed fuel channels, there was no significant difference. MTC and FTC also showed no significant change. Therefore, it is concluded that the deformation of fuel channels made no significant influence on the values of CTC, MTC, and FTC. In case of expansion deformation, the increased volume of coolant changed neutron spectrum with the change of reactivity. However, this effect may not cause any local disturbances in pin power distribution. On the contrary, eccentric movement of fuel bundle from sagging may increase the unbalanced increase of moderation at the upper part. It may be expected that local pin power peaking may happen at the outer ring fuel pins at upper location. Pin power distributions were compared under Hot Full Power condition at Beginning of Cycle. As a result, there was no significant difference between pin power values between intact and deformed fuel channels. Even in the case of deformation of sagging with expansion, it was shown that errors were less than 2%. The maximum pin peaking factors were changed from 1.152 to 1.142 by 0.95% difference without changing peaking pin locations. Expansion of the pressure tube in the CANDU fuel channels could cause local and high impact in reactivity measure in the aspect of safety evaluation. WIMS code could represent expansion of the fuel channel by changing the geometries. However, in the full core analysis it is not easy to check the concern local deformation of fuel channel. The suggested correction method is find feasible to be applied for