Abstract: Plutonium-238 (²³⁸Pu) is an ideal isotopic heat source commonly used in the production of radioisotope thermoelectric generators (RTGs). It has broad applications in fields such as space exploration and polar meteorological research. Reactor irradiation remains the primary method for ²³⁸Pu production. A theoretical model was derived for the maximum conversion ratio of ²³⁸Pu production from ²³⁷Np and formulas was provided for calculating the maximum conversion ratio and the corresponding conversion time in the paper. Theoretical calculations for high-temperature gas-cooled reactors (HTGR) and pressurized water reactors (PWR) were presented. Additionally, the NUIT code for burnup calculation, developed by the Institute of Nuclear and New Energy Technology (INET) at Tsinghua University, was employed to calculate the conversion curves for ²³⁸Pu production from ²³⁷Np in HTGR and PWR. The effects of neutron flux levels and neutron spectrum differences on the maximum conversion ratio and production rate of ²³⁸Pu were examined. A comparison indicates that the simulation results of the NUIT code align well with the theoretical analysis. The results demonstrate that increasing the neutron flux level effectively enhances the production rate of ²³⁸Pu. During the early stages of irradiation, the production rate is primarily influenced by the neutron capture reactions of ²³⁷Np. Furthermore, under the same total neutron flux level, although the maximum conversion ratio of ²³⁸Pu in HTGR is lower than that in PWR, the conversion rate in HTGR is significantly higher. Building upon this analysis, the Monte Carlo code OpenMC and the NUIT code were used to analyze ²³⁸Pu production in HTGR and PWR under varying temperatures and moderator volume ratios. The analysis reveals why HTGR achieves a higher ²³⁸Pu production rate than PWR under the same total neutron flux level: HTGR has a higher moderator volume ratio, leading to more effective neutron moderation, higher thermal neutron flux, and a larger one-group capture cross-section for ²³⁷Np, thereby increasing the ²³⁸Pu conversion rate; HTGR operates at a higher temperature. Although elevated temperatures have minimal impact on the one-group capture cross-section of ²³⁷Np, they significantly reduce the one-group absorption cross-section of ²³⁸Pu, resulting in lower neutron consumption by ²³⁸Pu and consequently higher production yields. This study explores ²³⁸Pu production through irradiation in HTGR, elucidating the underlying factors contributing to their higher production rate. Moreover, the capability of certain pebble-bed HTGR for continuous fuel replacement enables flexible control over irradiation duration, a distinct advantage over conventional PWR. A comparative analysis of ²³⁸Pu production in HTGR and PWR reveals that although the purity of ²³⁸Pu produced by HTGR is lower than that from PWR, HTGR exhibits higher production efficiency. Moreover, the content of the byproduct ²³⁶Pu in the ²³⁸Pu produced by HTGR is significantly lower than that in PWR. These findings indicate that, under the condition of meeting the required purity of ²³⁸Pu, HTGR possesses a certain advantage over PWR in terms of production performance.